IoTOGRAPHY

Reading Time: 5 minutes

IoT Overview 

We are living in a world where technology is developing exponentially. You might have heard the word IoT, Internet of Things. You might have heard about driverless cars, smart homes, wearables.

The Internet of things is a system of interrelated computing devices, mechanical and digital machines provided with unique identifiers and the ability to transfer data over a network without requiring human-to-human or human-to-computer interaction

IoT is also used in many places such as farms, hospitals, industries. You might have heard about smart city projects too (in India). We are using lots of sensors, embedded systems, microcontrollers and lots of other devices connecting them to the internet to use those data and improve our current technology.

Our sensors will capture lots of data and it will be used further depending on the user or owner. But what if I say this technology can be harmful too? It may or may not be safe to use it. How?

These data transferring from using IoT from source to its destination can be intercepted in between and can be altered too. It can be harmful if the data is very important, For ex. Reports of a patient generated using IoT can be intercepted and altered so the doctor can not give the correct treatment to the patient. Also, some IoT devices can be used by the Army transferring very secret data. If it can get leaked, then it can create trouble for the whole country.

The Information-technology Promotion Agency of Japan (IPA) has ranked “Exteriorization of the vulnerability of IoT devices” as 8th in its report entitled “The 10 Major Security Threats”.

So, can we just stop using IoT? No, we can’t. We have to secure our data or encrypt our data so the eavesdropper can never know what we are transferring.

Cryptography Overview :

Cryptography is a method of Protecting information and communications through the use of codes, so that only those for whom the information is intended can read and process it.

There are mainly two types of encryption methods.

  1. Symmetric key
  2. Asymmetric key 

Symmetric key uses the same secret key to encrypt or decrypt data while Asymmetric key has one public key and one private key. A public key is used to encrypt data and it is not a secret, anyone can have it and use it to encrypt data but only a private key (of the same person whose public key was used) can be used to decrypt that plaintext.

In Cryptography, We usually have a plaintext and we use some functions, tables and keys to generate ciphertext depending on our encryption method. Also In order to make our data exchange totally secure, we need a good block cypher, secure key exchange algorithm, hash algorithm and a message authentication code.

IoTOGRAPHY

Block cipher – It is a computable algorithm to encrypt a plaintext block-wise using a symmetric key. 

Key Exchange Algorithm – It is a method to share a secret key between two parties in order to allow the use of a cryptography algorithm. 

Hash Algorithm – It is a function that converts a data string into a numeric string output of fixed length. The hash data is much much smaller than the original data. This can be used to produce message authentication schemes.

Message Authentication Code (MAC) – It is a piece of information used to authenticate the message. Or in simple words, to check that the message came from the expected sender and the message has not been changed by any eavesdropper.   

NOTE: you might wonder why we don’t just send data using key exchange algorithms when it is reliable to share secret keys. You can search for it or tell you in short. It is neither reliable nor secure to share data using key exchange algorithms.

LightWeight Cryptography:

Encryption is already applied at the data link layer of communication systems such as the cellphone. Even in such a case, encryption in the application layer is effective in providing end-to-end data protection from the device to the server and to ensure security independently from the communication system. Then encryption must be applied at the processor processing the application and on unused resources and hence should desirably be as lightweight as possible.

There are several constraints required to achieve encryption in IoT.

  1. Power Consumption
  2. Size of RAM / ROM
  3. Size of the device
  4. Throughput, Delay

Embedded systems are available in the market with 8bit, 16-bit or 32-bit processors. They have their own uses. Suppose we have implemented a system of Automated doors which open and close automatically at a bank. Which also counts how many people entered or left the bank. We want to keep this record secret and store it on the cloud. Using a 1GB RAM, 32bit / 64bit processor with a very good ROM just to ensure the privacy of data doesn’t make sense here. Because we will need a good space to install our setup, we will need to spend a lot more money than we should while this thing can be achieved with cheaper RAM, ROM and processor.

Keeping the above points in mind, implementing conventional cryptography in IoT which are used for Mobile Phones, Tablet, Laptop / PC, Server is not possible. We have to develop a separate field “Lightweight Cryptography” which can be used in Sensor networks, Embedded systems etc.

Applying encryption to sensor devices means the implementation of data protection for confidentiality and integrity, which can be an effective countermeasure against the threats. Lightweight cryptography has the function of enabling the application of secure encryption, even for devices with limited resources.

IoTOGRAPHY

Talking about AES, It usually takes 128bit long keys with 128 lock size. It uses 10 rounds of different steps like subbytes, shift rows, mix columns and add round keys. Implementing this requires a good amount of space, processing speed and power. We can implement it in IoT with reduced length of key or length of the blocksize but then it will take less than 30 minutes to break AES. 

IoTOGRAPHY

There are many Lightweight cryptography algorithms developed like TWINE, PRESENT, HEIGHT etc. Discussing all of them requires a series of blogs but I am adding a table showing a comparison of some Lightweight Cryptography.  You can observe changes in block size from 64 to 96 can create a huge difference in power consumption and area requirement. 

Lightweight cryptography has received increasing attention from both academic and industry in the past two decades. There is no standard lightweight cryptosystem like we have AES in conventional cryptography. Research is still going on. You can get updates of the progress at https://csrc.nist.gov/Projects/lightweight-cryptography.  

The whole idea behind this blog is to discuss lightweight cryptography and overview of it. 🙂

Author: Aman Gondaliya

Keep reading, keep learning!

TEAM CEV

A Power Module

Reading Time: 17 minutes

Special Thanks to

  1. Prof Varsha Shah, EE Dept, SVNIT
  2. Mr Anand Aggarwal’s 6.002 MIT OCW Course 

Above is a team of thousands of different power modules, better known as the control room of a power plant.🦾

Are you ready to know one of them???

INTRODUCTION

Control and measurement of system parameters is a crucial facet for reliable and safe operation of any electrical systems, particularly for real-time system. By real-time system we mean the system whose parameters like current, voltage, impedance, power, etc. changes with time, thus to maintain the parameters under threshold limits, we first require to monitor them i.e. take measurements in real-time.

For most of the electrical circuits, the voltages and currents are two parameters of greatest interest, as they are solely responsible for safe operation. When unchecked one leads to electrical breakdown and another a thermal breakdown.

Consider the following cases:

  1. A battery backup system, constant monitoring of battery terminal voltage is necessary to stop the battery from getting over-discharged. Also, current drawn has to be monitored to check that the battery doesn’t overheats and catch flames.
  2. In the power system, bus voltages and currents in line have to be maintained very precisely, which again requires first taking measurements.
  3. For metering of electrical energy consumed by a consumer, we need voltage, current and power factor measurement.
  4. Majority of control systems in industrial system employ a negative feedback technique which essentially requires sampling/monitoring of a particular output parameter, which is itself a form of measurement.

PREREQUISITES

KVL and KCL, 😅😅 rest leave on us!!!

THE PROJECT IN SHORT:

Under this project, we set out to build a dynamic power module for measurement of current and voltage in DC circuits and current, voltage, power factor and frequency in AC systems in real-time to constantly monitor them and trigger necessary safeguards.

It’s pretty obvious that real-time operations are best executed with the help of microcontrollers. Microcontrollers are equipped with a group of pins called ADCs which basically read analog voltage level and convert them to n-bit digital data. Problem is that these microcontrollers can hardly survive above 5 V pressure.

So, if we wish to measure higher AC/DC values then we are required to take proper samples of voltage and current, then do proper conditioning, and finally process the data to compute the parameters.

ARDUINO UNO and ADC

With first boards appearing in 2005, Italy based Arduino.cc is open-source software and hardware company which gives a range of affordable microcontroller. With a broad computational power range, they are easy to use platform for purposeful use in industry, education, art etc.

A Power Module

Atmel based Arduino UNO introduced in year 2010 with 32KB of flash memory is best suited to serve the purpose for this module.

Now we are interested in ADC function, for that Arduino UNO has following specifications:

A Power Module

The meaning of these specifications is:

“UNO contains 6, 10-bit channels for analog to digital conversion. It maps analog input voltage at these pins from 0-5 V into integer values between 0- 1023, yielding a resolution of 5V/1024 or 4.8828 mV/unit. It takes 100 usec to read one input, so max reading speed is 10000 times a second.”

Syntax assigned is analogRead(pinname). It reads pin “pinname” and returns 10-bit int accordingly.

Sample code:

A Power Module

ADC DISCRPTION AVIALABLE AT ARDUINO.CC

https://www.arduino.cc/reference/en/language/functions/analog-io/analogread/

VOLTAGE MEASUREMENT

A basic voltage divider with appropriate resistor values can be used to scale down higher voltages V to microcontroller compatible levels, Vs.

A Power Module

A Power Module

Suppose readings are to be made in 0-50 V range. We need to scale down this 0-50 V range to 0-5 V.

A Power Module

Now current has to maintained as minimum as possible, to reduce errors. Let current be 0.5 mA. R1 should be of 100K range (5/0.5m). So,

A Power Module

As already stated, UNO has 10-bit resolution so voltage from 0 to 5V would be mapped into integers value from 0 to 1023, which is 5/1023 =4.88 mV per unit, which is fairly good accuracy.

A Power Module

The digital data from ADC can be easily used to manipulated to get the actual voltage, as follows:

A Power Module

GETTING LARGER RANGE

For a range of 500 V:

A Power Module

A Power Module

For current to be 0.5 mA, R1 should be of 1M range (500/0.5m). Thus,

A Power Module

Which is not a standard value so let R2 be 10K.

New scale is:

A Power ModuleA Power Module

A Power Module

Putting the value of Vs:

A Power Module

DC CIRCUITS MEASUREMENTS

What we just did was for DC voltage measurement in 0-500V range.

AC CIRCUITS MEASUREMENTS

For AC measurement we have to make few modifications. As voltage range is only +ve (0 -5 V) so we need to either shift whole waveform above zero, or flip the negative cycle or simply chop it, and then take readings. Using suitable algorithms, AC values (RMS/ PEAK) could be found.

  1. READING RMS: Since UNO takes 100 usec for every reading. For a typical frequency of 50 Hz, a half-cycle consisting of 10 msec, so UNO can make 10m/100u or 100 readings. For these 100 readings, the formula to compute RMS can be applied as follows.

A Power Module

A Power Module

Sampling a half-sino, Image courtesy: Internet

  1. READING PEAK:
    1. Using sampling: Max value from the 100 readings from can be found out using some sorting algorithm and RMS can be simply computed from it.
    2. Using time-delay: Since peak occurs at t/4, so using a timer function to generate a time delay of 5 msec after zero crossing and then taking the reading would directly give the peak value.

Any of these three techniques can be used for AC measurement.

IMP: It might possibly be case that the waveform may not be crossing the zero when the ADC starts taking measurement case would result in wrong results or the ADC never captures the maxima case 2a will be faulty , to deal with this is to take a larger number of samples like 2000-3000 to reduce the probability of error occurring.

Sample code for case2a:

A Power Module

The above code can also be used to measure DC voltages.

The final circuit becomes:

A Power Module

Since the sample voltage can be directly fed to microcontroller so no conditioning is required. Let’s see is that the case for current measurement too???

CURRENT MEASUREMENTS

The underlying idea to measure current is to obtain proportional voltage samples for any given load current, read voltage value at ADC port and process the ADC output bit to get the current value.

A proportional voltage can be obtained simply by forcing the current through pure resistor. If the value of this resistor is extremely small compared to the load resistance the equivalent load impedance would hardly change thus load current remain the same and in turn a small but proportional voltage drop is obtained across the external resistance.

A Power Module

Assuming the load current range from 0-2 A. Keeping the external series resistance R, as small as 0.1Ω, the sample voltages will be in range of 0- 0.2 V i.e. (0 -200 mV).

Accuracy would be significantly compromised for small load current if this range of sample voltage is used at ADC.

GETTING LARGER RANGE

Here the sampled voltage requires a proper conditioning.

So, all we need to do is to boost up the sample voltage from 0-200 mV to 0-5 V range.

How would you do that???

Well this is a typical day-job in analog engineering. Technically this is called the signal amplification. Giving a signal a required gain to push the level to a higher value.

The Operational Amplifier

Let us just step back from the current project and take a dive to depths which is certainly not required as far as the project is concerned, but for the sake of spirit of learning more and better, in the name of love of subject. 🍹🍹

What are the Operational Amplifiers? 

This class of devices singly forms the backbone of the modern analog industry. Just as the gates in digital electronics, the induction motor in power systems, the IC engine in mechanical systems, the library functions in the computer engineering field, these operational amplifiers are the basal workhorses of the modern analog systems. These little beasts are characterized by a versatile application, which includes amplifier, voltage source, current source, filters, actuator driver, comparator, etc.

The very first need for amplifier circuits typically appeared in long telephone lines to obtain proper signal conditioning at the receiving ends. The problem of the available amplifier in those days were their highly undependable gain due to the inherent nature of the active components used, vacuum tubes in 1930s and transistors after 1947. The gain varied enormously for small changes in working temperature and supply voltage. External condition like season, weather, humidity all of them made the gain of amplifier almost uncontrollable.

Harold Black, an electrical engineer at the bell laboratories in 1927 came up with a revolutionary technique that has now became so ubiquitous in all electronic circuit for control applications, it is called the negative feedback concept.

THE BIG IDEA: Use an amplifier made of undependable active elements to get a very large gain, typically infinite, and then use dependable passive element to provide negative feedback to it to obtain any reduced desired gain or transfer function.

This remarkable concept is the underlining principle of all the practical operational amplifiers used today.

Now to understand op-amp, as known popularly, there are two standpoints. One is this:

A Power ModuleImage courtesy: Internet

And the other is this:

A Power Module

And we have no doubt that you would like to understand it through the second stand point.

Now recall the first part of the basic idea i.e. building infinite gain op-amp, more or less this comes under the domain of pure analog electronics but given the versatility of operational amplifier the second part of basic idea, the design of negative feedback circuit using passive elements (resistors, capacitors, inductors) comes under the realm of the electrical engineering. Also, it largely deals with core electrical circuit theories like KVL, KCL, Thevenin’s, etc.

The first standpoint leads to accomplishment of first part of the basic idea, and the second stand-point leads to the realization of second part of the idea.

Now you would wonder how can we execute second part without knowing the first part, and here comes a great powerful tool to do this for you, it is the hack of all the complicated system around us.

“THE ABSTRACTIONS”

Without a solid-thorough understanding of how that horrendously intricate mesh of transistor and resistances work to produce infinite gain, we can still build a perfect negative feedback circuit to obtain exact desired transfer function (gain), using the abstractions.

This concept is so crucial and pervasive in building all modern perplexing systems the microcontrollers, computers, airplanes, particle accelerators, etc.

Consider this more striking example: the “printf” library function which we take for so granted, is an abstraction of the all the icky logics that goes into it to print a given string on some terminal or a display device. Try building your own function to print a string, you would be shocked at the complexity behind this little command.

The point is that we cannot keep on dwelling on basal stuffs if we wish to build something magnificent, if we do-we will never end up building an app, a website, a power converter, and so forth.

So, use of abstraction is a proven tool to reduce complexity, we can use this tool to derive some common results and build or understand large systems layer by layer.

What you see below is the abstraction of the mesh of transistor shown earlier.

A Power Module

A Power Module

WHAT DOES OP-AMP DO FOR US?

Well, they just simply produce output proportional to the difference in the voltage between the two input terminals. The proportionality constant is very high, order of 10^5, called the system gain, note Ed is in μV and o/p in V.

Mathematically,

A Power Module

The output characteristics for the device is as follows:

A Power Module

  • The magnitude of output voltage depends on the difference between in the input terminal voltage in active region and it saturates once output hits the supply voltage magnitude.
  • The polarity of output is same as the polarity of V+ wrt V-, thus V+ is called non-inverting terminal.

THE REAL OP-AMP

  • The variety of op-amps available are many LM324, LM339, LM258, etc. Most popular is IC 741. In our project we will use LM358, as it is single supply dual op-amp, so it will reduce complexity a bit.

A Power Module

A Power Module

      Image courtesy: ON Semiconductors

THE RULES OF OPERATION

  1. The difference in input voltage is very small (typically in μV) so the two input terminals can be assumed to be virtually shorted, i.e. at same voltage.
  2. The input impedance is very high, so both the input currents are zero.
  3. The gain is infinity.

This is all one need to know about op-amp, using these three rules op-amps can be used very easily to get required gain.

Let’s check it out.

ROUTINE EXAMPLES

Inverting configuration:

A Power Module

Just calmly apply the rules one by one.

  1. Input terminals at same voltage, so voltage at 2 is voltage at 1, i.e. zero.
  2. No current through input terminals. Apply KCL at terminal 2:

A Power Module

A Power Module

The characteristics become:

A Power Module

Non-inverting configuration:

A Power Module

Again, apply the same rules:

  1. Input terminals at same voltage, so voltage at 2 is voltage at 1, i.e. V1.
  2. No current through input terminals. Apply KCL at terminal 2:

A Power Module

A Power Module

The output characteristics become:

A Power Module

So, are you now able to appreciate the beauty of these curves we just obtained??? We began with an op amp with typically infinite gain (10^5), showing very creepy dependence on the temperature and external factors and here is a calm stable op-amp with desired finite gain by just using simple passive resistances.

New gain of system become (Rf/Rb) and (1+ Rf/Rb), which remains fairly constant for a wide temp range.

HOW THE NEGATIVE FEEDBACK WORKS?

Though equations obtained by reasonable mathematical approximations shows us independence of overall gain, but intuition is still lacking….

So, how does the results we just obtained is manifested actually????

So, lets simply heat an op-amp working in a non-inverting configuration, and see what happens.

As heating begins the gain begins to rise, and so does the output voltage. Corresponding to it there will be rise in voltage at terminal 2. Which would result in lowering the difference between the two input terminals, and consequently the output drops. This drop in output leads to drop in voltage at terminal 2 which results in increased differential voltage resulting in increased output. These oscillations die out soon and result is stable output displaying temperature independent gain. This is in general how a negative feedback principle works.

IMPORTANT

Ensure that the power is never off when the inputs and output are connected in the op-amp.

……………………………………….

So, it’s time to get back from where we left, the need to boost up the sample voltage from 0-200 mV to 0-5 V range.

Pretty cakewalk now, isn’t???

You would say it’s a lockdown. Where should I get the op-amp?

Cool, there are whole lot of gadgets and sensors where you can find it.

Say, for example, we extracted an op-amp from an IR sensor.

A Power Module

Due to unavailability of any solder iron, we just simply cut out the resistors, IR LEDs, POTs.

A Power ModuleUsing careful examination of the IC we traced out the whole IC circuit diagram.

A Power Module

And simply bought out the required terminals of op-amp by normal connectors.

A Power Module

To boost the sample voltage from 0-200 mV to 0-5 V range, we need a gain of (5/200m = 25), since inverting is not desired hence using the non-inverting configuration.

A Power Modulewhere,

A Power ModuleRequired gain is 25.

A Power Module

Let the resistances be: A Power Module

Finally, we used pair of resistors in combination to get the required ohms and made the op-amp circuit with a gain of 24.6 as follows:

DC CIRCUITS MEASUREMENTS

What we finally got is a 0-5 V scale that would end up giving us 0-1023 integer, we have to get all the way back to current, lets gooooo!!!!!🚀🚀🚀🚀🚀

Analog Voltage at the ADC:

A Power Module

Voltage input to the non-inverting amplifier:

A Power ModuleCurrent through the external resistance:

A Power ModuleThe overall conversion factor becomes:

A Power ModulePutting our design values:

A Power Module

AC CIRCUITS MEASUREMENTS

Contribute

FREQUENCY AND POWER FACTOR MEASUREMENT

Underdev

DISPLAY LCD

All about LCD interfacing with the Arduino can be very easily understood by referring this short 1 min read at Arduino.cc.

https://www.arduino.cc/en/Tutorial/LiquidCrystalDisplay

We tried the same, followed every step very accurately but unfortunately, results didn’t show up except this blank screen.

A Power Module

Help us to troubleshoot the problem by coming up with possible errors.

We build the icky circuit thrice from zero, and then checked and rechecked every connection, but failed.

MORAL:

  1. Life isn’t fair always, sometimes no matter how hard we try, no matter how dedicated our purpose is, we are destined to fail. We have realised this truth, and we hope to develop temperament to mindfully accept such failures in life.
  2. Connecting the circuit three times hadn’t yielded us the result, but surely planted in us the seed of perseverance to go through that nasty process. Surely, we raised our patience wall a little higher.

And it is these lessons and quality we wish to learn and develop by involving in these projects, not just simply putting things up.

THE FINAL CIRCUIT

Well do you think it’s done???

No, it’s not.

Since we are using same apparatus for the measurement of AC and DC. We hadn’t done anything to identify them. It can be done via program codes or by hardware.

By providing high or low manually on a digital pin we can indicate the microcontroller about it, say high for AC measurements and low for DC measurements.

THE FINAL CODES

#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
#define DCV_multiplier 0.499
#define DCC_multiplier 1.9531*10^-3
int read_voltage = A3;
int read_current = A4;
int select_pin = 7;
int voltage_adc_value = 0;
int current_adc_value = 0;
int voltage_peak_value = 0;
float dc_voltage= 0;
float ac_voltagerms= 0;
float dc_current = 0;
unsigned long sample_count = 0;
void setup()
{
  pinMode(A3, INPUT);
  pinMode (A4, INPUT);
  pinMode (7, INPUT);
  lcd.begin(16, 2);
  lcd.setCursor(0, 0);
  lcd.print("POWER MODULE");
  delay(3000);
  lcd.clear();
}
void loop()
{
  //Voltage Measurement//
  for(sample_count = 0; sample_count < 2000; sample_count ++)
  {
      voltage_adc_value = analogRead(read_voltage);
      if(voltage_peak_value < adc_value)
      voltage_peak_value = adc_value;
      else;
  }
      dc_voltage = voltage_peak_value * DCV_MULTIPLIER;
      ac_voltagerms = dc_voltage / 1.414;
  //Current measurement//
      current_adc_value = analogRead(read_current);
      dc_current = current_adc_value * DCC_MULTIPLIER;
  if ( select_pin==0 )
  {
      lcd.clear();
      lcd.setCursor(0, 0);
      lcd.print("DC SYSTEM");
      lcd.setCursor(0, 1);
      lcd.print(dc_voltage);
      lcd.print("V");
      lcd.print (" ");
      lcd.print (dc_current);
      lcd.print ("A");
 else
      lcd.print("AC SYSTEM");
      lcd.setCursor(0, 1);
      lcd.print(ac_voltagerms);
      lcd.print("V");
      delay(500);
  }
}

A Power Module

LOCKDOWN SPECIAL

We could arrange a low resistance of required power handling capacity, so current measurements cannot be made. Moreover, the resistors required for the voltage multiplier are also not available with us. So, we used op-amp to get the required gain, slight code modifications and obtained the results using the serial monitor. Battery voltage is 12.25 V power module shows 12.33 V, 0.65% error.

A Power ModuleA Power Module

CONCLUSION

These power modules can be custom build for battery monitoring for systems like drones, etc. by removing AC measuring components and using small uC like Arduino NANO. They could be used for real-time monitoring of some load. They could be used to trigger some protective measures like triggering a relay, blowing a buzzer, etc. when any parameter beyond a limit.

The development or building of measurement systems is less about dwelling on rigorous electrical concepts rather more appropriately it could be categorized as a form of art, which requires small intuition of some very basic electrical phenomenon and rest is all about creativity to obtain the desired result by arranging the already available elementary elements.

If we could imagine force exerted on a wire carrying current in a magnetic field, if we can imagine emf induced in a changing magnetic field, if we can imagine magnetic field due to a coil carrying current, we are ready to go, about learning, understanding, modifying and building interesting measurement systems.

🍹🍹🍹

Keep reading, keep learning

AANTARAK DIVISION,

TEAM CEV!

References

  1. Special thanks to Prof Varsha Shah
  2. MIT OCW 6.002 Circuits and Electronics
  3. Mr Anand Aggarwal’s Fundamental of Analog and Digital Electronics
  4. Arduino.cc

 

RADIOACTIVE FIRE: The Chernobyl Disaster

Reading Time: 8 minutes

On April 26, 1986, a sudden surge of power during a reactor systems test destroyed Unit 4 of the nuclear power station at Chernobyl, Ukraine, in the former Soviet Union.  A nuclear meltdown in one of the reactors caused a fire that sent a plume of radioactive fallout that eventually spread all over Europe. You know how dangerous radioactive materials involved in a fire can be and what they had done to Chernobyl. Let’s find out all these in detail.

RADIOACTIVE MATERIALS

Radioactive materials are any material which contains unstable atoms that emit ionizing radiations as it decays. Radioactive atoms have too much energy. When they spontaneously release their excess energy, they are said to decay. After releasing all their excess energy, the atoms become stable and are no longer radioactive. This radiation can be emitted in the form of positively charged alpha particles, negatively charged beta particles, gamma rays, or x-rays. Radiation is energy given off in the form of rays and high-speed particles.

Fires involving radioactive materials can result in widespread contamination. Radioactive particles can be carried easily by smoke plumes, ventilation systems. Fire in radioactive material is hazardous, and we know that Nuclear Power Plants contain lots of radioactive material. Fire in a Nuclear Power Plant is dangerous as it will result in the release of numerous radiations which is very dangerous for the environment. Most of the radiation released from the failed nuclear reactor is from fission products iodine-131, cesium-134, and cesium-137. Iodine-131 has a relatively short half-life of eight days, but is rapidly ingested through the air and tends to localize in the thyroid gland. Caesium isotopes have longer half-lives (cesium-137 has a half-life of 30 years) and are a concern for years after their release into the environment. Such incidents happened in this world, and the major one was the Chernobyl Nuclear Power Plant Incident. Let’s discover what happened there, and the steps taken.

THE INCIDENT

On April 26, 1986, the Chernobyl power plant located near the city of Pripyat in northern Ukraine became the site of the worst ever nuclear accident. A massive steam explosion destroyed the reactor hall of unit 4, and radioactive material was released, affecting large parts of Ukraine, Belarus and Russia, but also reaching western Europe. On the evening of April 25, 1986, a group of engineers, lacking knowledge in nuclear physics, planned an electrical-engineering experiment on reactor number 4. They thought of experimenting how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply.

CAUSE OF DISASTER

RADIOACTIVE FIRE: The Chernobyl DisasterRADIOACTIVE FIRE: The Chernobyl Disaster

Operators decided to conduct a safety test, which they have timed to coincide with a routine shutdown for maintenance. The test was to determine whether, in the event of power failure, the plant still-spinning turbines can produce enough electricity to keep coolant pumps running during the brief gap before the emergency generators kick in.

To conduct the test reactor number 4’s core cooling system was disabled to keep it from interacting with the test. The reactor had to be stabilized at about 700-1000 MW prior to shut down, but it fell to 5000 MW due to some operational phenomenon. Later the operator committed an error and caused the reactor to go into the near-shutdown state by inserting the reactor control rods, which resulted in the drop of power output to around 30 MW. This low power wasn’t adequate to make the test and will make the reactor unstable. They decided to extract the control rods to restore the power, which was the violation of safety rules due to the positive void co-efficiency of the reactor.  The positive void coefficient is the increasing number of reactivity in a reactor that changes into steam. Extraction of control rods made power to stabilize at 200 MW at which they carried out the test, but the reactors were highly unstable at the lower power level. Even though the engineers continued with the experiment and shut down the turbine engine to see if its inertial spinning would power the reactor’s water pumps, it did not adequately power the water pumps. Without the cooling water, the power level in the reactor surged.

The water pumps started pumping water at a slower rate and them together with the entry to the core of slightly warmer feed water, may have caused boiling (void formation) at the bottom of the core. The void formation, along with xenon burn out, might have increased the power level at the core. The power level was then increased to 530 MW and continued to rise. The fuel elements were ruptured and led to steam generation, which grew the positive void coefficient resulting in high power output.

The high power output alarmed the engineers who pressed the emergency shutdown button and tried to insert all the 200 control rods, which is a conventional procedure done to control the core temperature. But these rods got jammed half the way, because of their graphite tip design. So, before the control rods with their five-meter absorbent material could penetrate the core, 200 graphite tips simultaneously entered the core, which facilitated the reaction to increase.

This ended up in two explosions. The first explosion, to be quickly followed by at least one more, blows the 1,000-ton roof right off the reactor and shoots a fireball high into the night sky. A blackout roils the plant as the air fills with dust and graphite chunks, and radiation begins spewing out.

All the materials such as Fuel, Moderator and Structural materials got eject, starting several fires and the destroyed core got exposed to the atmosphere. In the explosion and ensuing fire, more than 50 tons of radioactive material got released into the atmosphere, where air currents carried it. The blast was 400 times the amount of radioactive materials released at the time of the Hiroshima bombing.

HOW FIRE WAS CONTROLLED??

The firefighters present didn’t have any clue about what they were handling. They believed that they were tackling an ordinary blaze and were wearing no protective clothing. They turned off Reactor 3  immediately followed by reactor 1 and 2. By the next morning, all the fire was extinguished except for a blaze in the reactor 4.

Soviet authorities spooked by the political fallout tried to cover up the scale of the disaster. They even denied any knowledge of the nuclear disaster after Sweden reported radioactive particles in its airspace.

After continuous pumping of radiations into the air, authorities realized that they had to stop.

The radiations coming out were getting dangerous and were needed to stop. Hence, they used Boron because of its property to absorb neutrons so it would effectively end the fire by neutralizing the uranium atoms shot about at random. With the help of helicopters, they dumped more than 5,000 metric tons of sand, clay and Boron onto the burning, exposed reactor no. 4.

The helicopters used to dump the load struggled as they were not allowed to fly directly above the open reactor.

RADIOACTIVE FIRE: The Chernobyl Disaster

While the fire got suppressed, the authorities came up with a more significant problem of a nuclear meltdown due to overheating.

Half of Europe was in the danger of getting wiped out as the core was melted and was reacting with the groundwater underneath the plant, this would have caused a second, bigger explosion.

Three volunteer divers were sent into the depths of the power plant to open valves that would drain the water To prevent the second explosion. In this way, the big bang got restricted.

But 400 miners also had to be brought in to dig underneath the power plant and install a cooling system as the groundwater was still contaminated.

The heroes completed their work, knowing they were being exposed to radiation poisoning, in just six weeks despite a three-month project projection.

The efforts of all involved saved millions of lives.

WHY BORON AND SAND?

Exposing a burning nuclear core to the air is a problem on at least two levels.

First was an ongoing nuclear fission reaction. Uranium fires off neutrons which are slamming into other atoms and splitting them, releasing more energy yet and feeding the whole hot mess. The second problem was the presence of an assortment of types of relatively lightweight elements that form when uranium atoms split in the fire coming right out of a nuclear reactor which was very dangerous for the human body.

The sand was to smother the exposed reactor, squelching that deadly smoke plume.

Boron is one of the few elements to possess nuclear properties, which warrant its consideration as neutron absorber material. Due to its atomic structure, it’s sort of neutron-thirsty. So the plan was to dump enough boron on the exposed reactor, and it would absorb so many of those wildly firing neutrons that the reaction would stop.

In Chernobyl’s case, however, dumping the boron and other neutron absorbers onto the reactor turned out not to work as helicopters were not allowed to fly directly above the open nuclear reactor.

WHY NOT WATER?

As the molten metal was present inside the reactor, it oxidises in contact with water, stripping oxygen from the water molecule and leaving free hydrogen. Hydrogen could mix with air and explode. That’s why divers were sent into the depth of the power plant to open valves which drains out the underground water.

WHAT WOULD WE DO TODAY?

The modern-day nuclear reactors are much less likely than Chernobyl to encounter any sort of disaster. They will never run as hot and operate in sturdier vessels. The buildings themselves are designed to do much of the work to squelch a nuclear reactor fire and a radioactive plume. Modern-day reactors are equipped with chemical sprays that can flood the reactor buildings and will take the isotopes out of the air before they can escape. Unlike the Chernobyl, nuclear facilities are entirely enclosed in sealed structures of cement and rebar. These structures are so strong that even the jet crash won’t affect them, and it wouldn’t expose the core.

Emergency handbooks are present for each nuclear power plant laying out information of what responders should do in the events of all sorts of somewhat- plausible to highly unlikely emergencies. As soon as the reactor fails to shut down normally, lots of boron is to be shovelled into the core.

RADIOACTIVE FIRE: The Chernobyl Disaster

CONCLUSION

The accident at the Chernobyl nuclear power plant in 1986 was a tragic event for its victims, and those most affected suffered significant hardship. The leading cause of the disaster was the technical flaws in the process of Steam Turbine Test and low safety measures taken during the test. Due to radiation emission from the reactor, several problems related to human beings and the environment. Dumping boron was a good idea, but they were not able to find a better way to drop it. Fighting a fire on an exposed uranium core will always come down to more or less fancy versions of dumping boron and sand.

REFERENCES

  1.  International Journals of Advanced Research in Computer Science and Software Engineering ISSN: 2277-128X (Volume-8, Issue-2)
  2. https://www.livescience.com/65515-chernobyl-in-modern-times-nuclear-emergency.html
  3.  Mikhail Balonov, Malcolm Crick and Didier Louvat, Update of Impacts of the Chernobyl Accident: Assessments of the Chernobyl Forum (2003-2005) and UNSCEAR (2005-2008)
  4. INSAG-7, The Chernobyl Accident: Updating of INSAG-1, A report by the International Nuclear Safety Advisory Group, International Atomic Energy Agency, Safety Series No. 75-INSAG-7, 1992, (ISBN: 9201046928)
  5. https://www.ebrd.com/what-we-do/sectors/nuclear-safety/chernobyl-overview.html
  6. http://www.unscear.org/unscear/en/chernobyl.html
  7. United Nations Scientific Committee on the Effects of Atomic Radiation – Chernobyl
  8. https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/chernobyl-bg.html
  9. https://www.livescience.com/39961-chernobyl.html
  10. https://inis.iaea.org/collection/NCLCollectionStore/_Public/12/627/12627520.pdf

FIRE: The Perception

Reading Time: 7 minutes

“Fire breaks out in a building,” “Australia’s biggest forest fire ever rages.” We often hear about such devastative fire incidents in the media. We know that fire is dangerous and can cause severe damage and destruction and, at times, death. Since our earliest days, humans have sought to find out what fire is, how it starts, and what keeps it going.

Sometimes we might think that fire is a living thing! It moves, ‘eats’ things, and seems to breathe. The ancient Greeks thought it was one of four major elements, along with water, earth, and air. They could feel, see, and smell fire just like they could the earth, water, and air, but fire is something completely different.

Let us go on a journey to unveil the world of fire!

What is Fire? Which state is it? A solid or a liquid or a gas or plasma?

No, it is neither of them. Fire is just a perception of matter that is experienced by the eyes. Typically, fire results from a chemical reaction between oxygen in the atmosphere and a variety of fuels. When the volatile gases are hot enough, the compound molecules break apart, and the atoms recombine with the oxygen to form water, carbon dioxide, and other products. In other words, they burn, which results in a fire. The rising carbon atom is the reason for the production of light during the fire. Ignition temperature needs to be achieved for the combustion reaction to occur. During this reaction, the weak double bond of molecular oxygen gets converted into the stronger bonds of carbon dioxide and water, therefore, releasing energy, and this is the reason why fire is hot. The chemical reactions in a fire are self-perpetuating. The heat required by fuel is given by the heat of the flame itself, so as long as there is fuel and oxygen around it, the fire will continue.

FIRE TRIANGLE

FIRE: The Perception

The fire triangle is a triangle consisting of three components that help in the production of fire that are heat, oxygen, and fuel. Removal of any one of them will extinguish the fire. The alternative of the fire triangle is fire tetrahedron, which includes chemical reactions too, with all the other three components.

TYPES OF FIRE

  1. CLASS A- Fires involving ordinary combustibles such as wood, rubber, paper, cloth, and many plastics.
  2. CLASS B- Fires involving flammable gases such as gasoline, petroleum greases, tars, oils, oil-based paints, alcohols, solvents. It also includes combustible gases such as propane and butane.
  3. CLASS C- Fires involving energized electrical equipment such as computers, motors, transformers, and other appliances.
  4. CLASS D- Fires in combustible metals such as magnesium, titanium, zirconium, sodium, lithium, and potassium.
  5. CLASS K-Fires in cooking oils and greases such as animal and vegetable fats.

HOW FIRE SPREAD?

Once a fire has started, it grows through the transfer of heat energy from the flames. Heat energy transfers in three different ways-

  1. CONDUCTION- The heat from the fire spreads from molecule to molecule along the length of conducting materials. Materials that are good conductors absorb the heat from the fire and transfer it throughout the molecules of the substance.
  2. CONVECTION- It occurs in gases and liquids. It is the flow of fluid or gas from hot areas to colder areas. The heat of the fire raises the temperature of the air around it, which rises and spreads, which may burn the combustible materials.
  3. RADIATION- Heat of the Fire travels in the form of electromagnetic rays in air. Combustible materials can absorb the heat from the rays.

FIRE: The Perception

FIRE IN ZERO-GRAVITY

FIRE: The Perception

On earth, gravity determines how the flame burns. The product of combustion has more energy than the combustible substance and so moves around faster and takes up more space than the cooler air around them. Therefore, there is a buoyant force on them, which is higher than their weight. The hot gases in the flame are much warmer and less dense than the surrounding air, so they move upwards towards low pressure. This is why fire typically spreads upwards. However, in a zero-gravity region, there is no such thing as lighter or heavier air; thus, the fire heats the air, which just sits around the flame, causing it to burn slowly. This means the flame burns equally in all directions forming a globe instead of the flickering flame. Flames in the air can burn more slowly more coolly and with less oxygen because of which fire in space given the right conditions can expand in any direction as quickly as it can provide us to the nearby oxygen. The heat does not cause any rushing air or shockwaves. The cool thing found is that in space, combustion can happen with no visible flames. This phenomenon is demonstrated by the experiments conducted by NASA in the International Space Station, the Flame Extinguishment Experiment(FLEX). A more efficient combustion system that will not produce as much exhaust on earth can be made if these flames can be used as they burn cleaner.

FIRE: The Perception

FIRE SPREAD IN DIFFERENT SCENARIOS

  1. Fire in radioactive materials– Chernobyl incident was a nuclear accident in which radioactive material was present in the fire situation. Fire involving radioactive materials can result in widespread contamination. Radioactive particles can be carried easily by smoke plumes. Radiation includes alpha particles, which are extremely hazardous to people coming in contact with the fire because they can be inhaled and deposited in body tissues, where they can cause severe long term health effects.

FIRE: The Perception

  1. Fire in wood– Wood is a combustible material. Under the influence of heat, wood produces substances that react eagerly with oxygen, leading to the high propensity of timber to ignite and burn. Ignition and combustion of wood are mainly based on pyrolysis of cellulose and reactions of pyrolysis products with each other and with gases in the air, oxygen. When the temperature increases, cellulose starts to pyrolyze. The decomposition products either remain inside the material or are released as gases. Gaseous substances react with each other and oxygen, releasing a large amount of heat that further induces pyrolysis and combustion reactions.

Wood(C10H15O7)+heat —> Charred wood(C50H10O) + 10 CH2O(gas)

Forest fires include fire in a wood. Amazon forest fires and Bushfires in Australia are the major incidents, including the burning of wood. Fires in forests spread quickly due to the presence of combustible materials, which results in the realization of the fire triangle.

FIRE: The Perception 

  1. Fire in oil- Oils are flammable materials which are less denser than water, so floats on it. Disaster due to fire in oils includes oil well fires. Oil well fires are oil or gas wells that have caught on fire and burn. Oil well fires can be a result of human actions, resulting in accidents, which can be a result of arson or due to natural events, such as lightning. These fires are more difficult to extinguish than regular fires due to enormous fuel supply to the fire. The significant incidents include Kuwait Oil Fires and Deepwater Horizon Explosion.

FIRE: The Perception

Conclusion

Fire is a perception of our eyes to the exothermic combustion reaction. This is part of the Mini Analysis Project “Study and analysis of the phenomenon of Fire and it’s practical Implications through Case Studies”

This was an introductory blog describing the true nature of fire!

So ending this with a sneak peek of case studies we are going to elucidate in further blogs:

  1. Chernobyl Nuclear Disaster
  2. Australian BushFires
  3. Amazon Rain Forests

Intriguing blogs about the same coming soon…

Stay tuned until then.

REFERENCES

  1. “Glossary of Wildland Fire Terminology” (PDF). National Wildfire Coordinating Group. November 2009. Retrieved 2008-12-18.
  2. ^ Schmidt-Rohr, K (2015). “Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2“. Chem. Educ. 92
  3. “Iraq Fires erupt in large Iraqi oil field in south Compiled from Times wires © St. Petersburg Times published March 21, 2003”. Archived from the original on July 15, 2014.
  4. ^ “Hellfighters”. Archived from the original on 2014-07-14.
  5. https://science.howstuffworks.com/environmental/earth/geophysics/fire.htm
  6. https://science.howstuffworks.com/environmental/earth/geophysics/fire1.htm
  7. https://www.space.com/13766-international-space-station-flex-fire-research.html

Author: Hardik Khandelwal

TEAM CEV!!

BUILDING OUR OWN INVERTER

Reading Time: 13 minutes

Introduction

Multivibrators

Understanding the Circuit Elements

Battery

       Voltage

       Ampere

       Ampere-Hour

Electronic Switch MOSFET

      PINS

      Physics in one-line

      Circuit Diagram

      Datasheet: Max Ratings

 CD4047 

       PINS

       The Astable Mode

       Calculation of value of external R & C

       Circuit Diagram

       Datasheet: Max ratings

Three winding transformers

       Turn Ratio

Filters

       Choke coil

       Capacitors

Final Circuits and demonstrations                     

Conclusion

References

In the previous blog “Inverter Circuits: The Basics”, we have begun with the very raw idea of DC to AC conversion and methodically we developed our basic circuit for obtaining a typical square waveform having power frequency from a constant voltage DC source.

Inverter Circuits: The Basics

The circuit was like:

BUILDING OUR OWN INVERTER

We have understood the working of the circuit. Now for the purpose of practical implementation, all we require is the triggering circuit for the two MOSFETS at required frequency, proper dimensioning of the elements to check the reliable and safe operation, and also a filtering circuits to couple the load with our inverter circuit.

Let us introduce an exciting new class of electronic instrument…

Multivibrators

Multivibrators forms a wide class of electronic circuits and deals with two states (namely high and low) in different possible ways. Generalized diagram of a multivibrator:

BUILDING OUR OWN INVERTER

They are a total of three kinds:

  1. Monostable Multivibrators: this sub-class is stable only in one of states, say high (1). So, once triggered by some external signal then the circuit enters into its unstable state i.e. low (0) and returns back to its stable state 1 after some pre-fixed time-period. Example of these types of circuits are delay generator, timing circuits, etc.
  2. Bistable Multivibrators: these circuits are stable in both the states (1 & 0), so if multivibrator is in high states and if triggered by some external signal its state changes to 1 and remains 1 until the next trigger. In simple words an external signal just flips the current state of output. Example of these types of circuits is flip-flops.
  3. Astable Multivibrators: they are unstable in both states, thus if you give trigger/supply to them the keep on oscillating between the two states indefinitely. They find extensive use in timers and oscillators.

We are only interested in astable multivibrators, for generating the timing pulse for biasing of the MOSFETS. There is a great range of IC that gives this mode of operation of continuous oscillation on a single trigger, the most popular of all ICs is timer LM555, in 4000 IC series we have CD4047, CD4049, CD4093, etc.

Understanding the Circuit Elements

Understanding the working and principle of a system comes under physics or pure science domain but realizing those systems by considering the real-world parameters and the effects is what engineering is all about. Element dimensioning, suitability, economics and all those things which come into picture when we try to turn a diagram drawn piece of paper into a real system to transform lives, is an engineering task. For example, working of an Induction motor is a physical phenomenon, but to utilize these machines in such large numbers littered beyond the horizon is only made possible by what we call power system engineering. So, there is a line where the physics ends and the engineering begins.

BUILDING OUR OWN INVERTER

For an electrical system, it is the task of an engineer to take care of the parameter to be maintained under specified limits- voltage level to check dielectric breakdown and current level to check the thermal breakdown. With this background let us start exploring the circuit elements required in for our inverter.

Battery**

Voltage level, ampere-hour rating, max current rating

Electronic Switch MOSFET

The MOSFET we are going to use is the power MOSFET, modified specially to carry larger current, unlike usual low-power electronic circuits.

The device is identified technically as IRF540, n-channel 100V-0.055 Ohm, 22 A, Low-gate charge Power MOSFET, explained later.

PINS:

It is a three-terminal device numbered 1, 2 and 3 as given:

BUILDING OUR OWN INVERTER                        BUILDING OUR OWN INVERTER

The pins 1, 2, 3 are gate, drain and the source terminals.

BUILDING OUR OWN INVERTER

Physics in one line:

MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor, it is a voltage-controlled current device. The structure of the device is such that on the application of voltage at gate terminal a channel between the drain and source is formed which doesn’t exist earlier.

Without and with gate voltage:

BUILDING OUR OWN INVERTER         BUILDING OUR OWN INVERTER

If you apply voltage at the gate terminal then for some voltage across drain-source the current begins to flow from (2) to (3). The more the gate voltage larger is current at a given drain-source voltage, which saturate at some point. Graphically:

BUILDING OUR OWN INVERTER     BUILDING OUR OWN INVERTER

DATASHEET

As earlier stated, after the physics we now have to look at the engineering part. This MOSFET has a limit on   all of which can ve referred from the datasheet.

MAX RATINGS: BUILDING OUR OWN INVERTER

Did you noticed the things written along the name of device? Here is what all that mean….

BUILDING OUR OWN INVERTER

From the output and transfer characteristics it can be concluded that we should strictly restrict the gate source voltage between 4 V i.e. Threshold Voltage to 5 V for saturation current less than 22 A.

In the gate circuit let Vgg the supply voltage be 5V, Vgs be 4 V in on-state and Igs as 10 mA thus the value of resistance in the gate circuit for a given gate excitation voltage. (Refer datasheet)

BUILDING OUR OWN INVERTER

NOTE: Precise calculation for Ig is still not clear. The team thinks that maximum allowable gate charge and the pulse width has to be taken into consideration to calculate the maximum current that should be limited by the gate resistance. If you can contribute, please contact us.

CIRCUIT:

BUILDING OUR OWN INVERTERBUILDING OUR OWN INVERTER

In the circuit during on-state, the forward resistance between the drain-source is given, thus depending on the Vdd and load resistance the current will flow which should be less than 22 A at 25-degree Celsius.

FUN FACT: It is amazing to know that the device MOSFET is no 1 manufactured electronic device in our entire history with 13*10^12 billion units sold by 2018 since 1964. 🤐🤐🤐🤐

BUILDING OUR OWN INVERTER

Source: Wikipedia https://en.wikipedia.org/wiki/Electronics_industry#List_of_best-selling_electronic_devices

Check out which devices have been the top-selling product since their invention.

IMP HANDLING CARE:

Handling of MOSFET is little bit strenuous job, because it is a delicate device. If the gate, source and drain terminal are not shorted then there is possibility of static charge accumulation at the gate terminal hence forming enough electric field to puncture the ultra-thin silicon-dioxide layer, leading to permanent failure of MOSFET.

  • Ground yourself when handling MOSFET.
  • Keep the three-terminals shorted until plugged into the circuit.
  • Voltage should be applied only after all terminal are connected to the electrical circuit.

The next thing is the pulse generator circuit for proper switching of the MOSFET 1 and 2 to obtain square wave of 50 Hz.

CD4047B

Introduced in the multivibrator section, we are going to use CD4047BE CMOS Low-Power Monostable/ Astable Multivibrator for the generation of required gate pulse.

PINS:

BUILDING OUR OWN INVERTER      BUILDING OUR OWN INVERTER

Datasheet describes the 14-terminals of CD4047BE with nomenclature and function of device as listed below:

BUILDING OUR OWN INVERTER

Cleary we are interested in the Astable mode of operation as the gate pulse required is oscillatory with a 20 ms (1/50Hz) time-period.

The Astable Mode:

Again, datasheet according to the internal circuitry gives the terminal connection.

BUILDING OUR OWN INVERTER

BUILDING OUR OWN INVERTER

Calculation of value of external resistance and capacitors:

In the table above the time-period of pulse available at the pin 10, 11 is 4.40*RC seconds. This Q (pin 10) would ve the pulse input to the gate terminal of MOSFET M2 so that the positive square waveform appears across load. In the next half time-period Q will e low thus Q var (pin 11) connected to the gate of MOSFET M1 will e high and hence negative waveform appear across the load.

Assuming that we manage to get the sinusoidal output, the Q pulse and the load voltage waveform will look like:

BUILDING OUR OWN INVERTER

The required frequency for voltage is 50 Hz, time-period of which is 20 ms, which also becomes the time-period of the pulse Q.

Referring the datasheet, the typical values of resistance and capacitance at which the multivibrator CD4047 produces pulses with greater precision, the value we chose are 45 kΩ and 0.1 μF respectively.

Circuit Diagram

Considering the pin connections, the circuit for the gating of the MOSFET using CD4047E can be obtained as given. The output pins 10 and 11 should be given to the gate terminals of any of M1 and M2 device.

BUILDING OUR OWN INVERTERBUILDING OUR OWN INVERTER

CD4047 being operated in Astable mode, R = 45 kΩ, C= 0.1 uF, Vdd = 12V

NOTE: The voltmeter measures RMS value and the waveform is pulsating DC, peak value has to be considered while calculations.

A visual illustration of pulsating waveform for R and C as 330 kΩ and 0.1 μF:

DATASHEET: MAX RATINGS

For different supply voltages the sink current and source current ratings are indicated as follows. The pulse voltage can also be experimentally determined for given supply voltage.

BUILDING OUR OWN INVERTER

Thus, for obtaining required gate source voltage pulse, required value of resistor can be connected to interface CD4047 to the IRF540.

NOTE: The current limiting resistor calculation has still to be verified by considering the gate pulse requirement of MOSFET.

Three winding transformers

A three winding transformer is five terminals device as opposed to normal 4 terminal where we have two primary terminals and two terminals on the secondary side.

The basic transformation equation for two winding transformers, was:

BUILDING OUR OWN INVERTER

Where Vsec and N2 are voltage and turns respectively on the secondary side and Vpri and N1 for primary side.

In this case of three winding transformer this same equation is applicable on individual coil on primary as well as both coils combined.

BUILDING OUR OWN INVERTER

For individual coil:

BUILDING OUR OWN INVERTER

Here Vpri is voltage across any one of the primary coils. The transformer is rated as 12-0-12/240 V, 5A.

Hence putting the values, calculated value of turn ratio is:

BUILDING OUR OWN INVERTERBUILDING OUR OWN INVERTER     BUILDING OUR OWN INVERTER

a.) Measured Voltage across one coil for secondary being excited by mains supply

b.)Measured mains voltage on 21-03-2020 @18:30 HRS

So, actual turn ratio is:

BUILDING OUR OWN INVERTER

For both coils combined:

BUILDING OUR OWN INVERTER

According to the given ratings, calculated primary voltage is:

BUILDING OUR OWN INVERTER

BUILDING OUR OWN INVERTERMeasured voltage across both the primary coil

Using the turns ratio obtained in previous measurement, actual primary voltage is:

BUILDING OUR OWN INVERTER

Which is verified by the voltmeter readings. So actual turn ratio is 18.18.

Filters

            Choke coil

********

            Capacitors

Capacitors are among one of oldest device used so widely in electrical circuits. Their characteristics can be utilized in numerous ways. In DC circuits they act as voltage smother by filtering out ripples (rectifiers), in radio technology they are used for tuning, in industries like automobile and aviation- they are utilized as emergency energy storage banks, in power system they are utilized for power factor improvement resulting in power and voltage regulation, and here we will use it in AC circuits to the block DC voltage.

The equation governing the behavior of capacitor in DC/AC circuits can be easily understood using some textbooks, here we will consider some practical application points.

Now the most important parameters of capacitor are its capacitance, its maximum operating voltage ratings and maximum reactive power handling particularly for high-voltage and power applications.

Capacitance value of 1pf ceramic capacitors to 50,000 uf Electrolytic type to 10F supercapacitors, with different operating voltages are commercially available. Depending on the requirement different capacitor technology can be opted. Consider this insightful graph from Wikipedia which helps in determine the capacitor type for a given requirement.

BUILDING OUR OWN INVERTERPhoto courtesy: Wikipedia

These commercially available capacitors are broadly divided in two categories:

BUILDING OUR OWN INVERTER

  1. The polarized capacitors: These are polarity dependent and exclusively used for DC applications.
    1. Electrolytic types: gives advantage of small size and stability for relatively larger C (few uf to thousands uf) compared to unpolarized ceramics-type. Application in DC voltage smothering, etc. Also used in AC fan motors, how. Can you answer???👴👴👴 (0.1 uF – Thousands uF)

BUILDING OUR OWN INVERTER

IMP: The terminal marked negative should always e connected to the negative polarity of source.

2. Supercapacitors: most versatile category. With higher capacitance, they find applications in the field of electronics (to power memory during power cut-off), transportation, renewables, etc. Though the typical capacitance of single unit is higher but it’s working voltage is mere 2-5 V, so numerous cells are connected in series to obtain the required rating.

                      BUILDING OUR OWN INVERTER                  BUILDING OUR OWN INVERTER

Photo courtesy Internet

2. The unpolarized capacitors: These capacitors have no polarity specific terminals. Hence used in both AC and DC applications.

a.) Ceramic type: ideal for smaller capacitance and for wider frequency (specially high) spectrum. Oscillator tuning, HF applications. (1 pF – 0.1 uF).

BUILDING OUR OWN INVERTER

The meaning of 102 is 10*100 pf and 104 is 10*10000 pf. First two digits are the values and last is the multiplier and value come in pF.

  1. Film-type: More popular in high-voltage and high-power applications like snubber circuits, etc.

BUILDING OUR OWN INVERTERPhoto courtesy Internet

As far as general projects are considered either ceramic or electrolytic is preferred choice according the capacitance required.

Another important class of capacitors is the capacitors system employed in power systems, which goes far beyond off-topic to be discussed here.  However, for purpose of mere excitement and spark curiosity in the readers interested in electrical field this particular stuff, here is view of those massive capacitors systems:

BUILDING OUR OWN INVERTER

General Electric power factor compensation utility for power grids

Notice that the tiny ceramic capacitor and these megs-structures are also defined by same physical equation Q=CV and all others, and see how engineering has made them strikingly different. A physicist can only give us those equation but it’s the job of engineers to pump life in those dead equations to build some stunning things.

BUILDING OUR OWN INVERTER

The last circuit:

BUILDING OUR OWN INVERTER

The actual test circuit working at 50 Hz:

BUILDING OUR OWN INVERTER

Visuals of circuit operating with 330 kΩ resistance in CD4047:

The Inverter Family😇😇😇:

BUILDING OUR OWN INVERTER

Conclusion

Making the circuit work as expected was not the final aim of the project which we failed also as of now because of shortage of resources due to corona virus outbreak (low rating battery, choke coil, etc), thus not able to test our circuit on actual AC load.

But the core objective and higher purpose of the initiative which was to develop the critical thinking to build a concept of circuit to obtain desired results for a given set of initial conditions, honing skill to be able to select appropriated element from numerous choices available by considering the suitability, economics, etc., working out the circuit parameters by referring the standard datasheets and be able to work in team was surely achieved to satisfactory level.

References

Drive link to datasheets, important lectures videos and notes, etc:

https://drive.google.com/drive/folders/1fVLofMnjowNTGhpMEXaqG7IFt_WrT4v4?usp=sharing

Meanwhile the documentation of project was done in respect of the man Richard Feynman on whose vision CEV hang so tight:

BUILDING OUR OWN INVERTER

Science will do help us win over this corona thing, just as it has helped humanity fight influenza and whatnot, till then hold strong and keep believing.

Keep reading, keep learning!!!!!

TEAM AANTARAK, CEV

Anshumaan S Jhala | Chitturi Vamsi | Rakesh Dhadavi | Nitin Patel |Shayam | Rahul Kumar

Special Contributor: Vartik Srivastava

Inverter Circuits: The Basics

Reading Time: 9 minutes

INTRODUCTION

The Renewable energy is showing a great ramp up in these early decades of 21st century era. The trends and prediction show a promising future for solar, wind, tidal etc. The solar energy harnessed in the form of solar photovoltaics and heat are boosting ever since the early 2000s, and in the year 2019 it shot to gigantic 25% growth rate and increasing.

Inverter Circuits: The Basics

 

Note: A better-edited version of this blog is available in original MS Word format, link below:

https://drive.google.com/open?id=1a63QhjDmNMEEZI1iW9Ik_POmSW0UGre7

Follow for an eminent IEA report “World Energy Outlook 2019” to know all about our planet’s current energy scenario:

https://www.iea.org/reports/world-energy-outlook-2019/renewables#introduction

Inverter Circuits: The Basics

The problem with renewables is not of power scale but of irregular availability. For example, in a single hour, the amount of power from the sun that strikes the Earth is more than the entire human race consumes in a year, a hurricane every second releases the energy equivalent of 10 atomic bombs but last only a few minutes or hours.

Inverter Circuits: The Basics

However, it an obvious fact that we couldn’t tap tiny fraction of these mighty energy resources due to present technology constraints, but nothing could be so soothing, to hear of energy availability at such minimal cost to our environment when climate scientist predicts of human race extinction due to fossil-fuel induced climate crisis.

The supply and demand cannot be achieved using renewables without proper engineering. Energy storage provides the only feasible solution to link the two. Storing when excess availability and delivering when demand exceeds generation.

There is massive science community working in the area of energy storage technologies: mega batteries, large pumped hydro storage, thermal storage, chemical storage, etc.

MIT Lecture on future of Energy Storage: https://www.youtube.com/watch?v=E76q-9q7ZDg

Now the other face of story is that the modern power system is in transitional period, now DC power is again gaining ground on consumer side like the highly efficient BLDC motors, semiconductor applications like large database centers, home appliances, and jillion low power devices operating on DC. But still AC is major form of consumption and unfortunately major storage technologies (most importantly batteries) don’t store AC power directly.

And here comes the topic of the blog: The Inverter Circuits.

Inverter Circuits are systems designed for efficient conversion of DC electricity into AC electricity, and comes in wide range of power ratings from small solar charging LED lighting system to medium house emergency system to massive receiving stations of HVDC transmission lines using different conversion principles.

Under this project, team Aantarak aimed to look upon inverter technologies and tried to develop an insightful understanding of the operational principles of inverter circuits, with a vision to further improve the existing technology.

In this first blog intuitive explanation of inverter principle and mathematical analysis of system performance, typically available topologies, major circuit engineering challenges and scope of improvement are explored.

BASIC IDEA OF DC TO AC CONVERSION

Suppose we have V Volts DC battery source and we want to power an AC load whose input is specified as f Hz.

How would you do that, think for a second.

To get an AC supply from a DC source, all we have to do is to interchange the supply terminal across the load at the required rate. So, for 1/2f  seconds we are giving +V, then zero and then -V for next 1/2f  seconds.

Inverter Circuits: The Basics

Well that is a square wave.

Inverter Circuits: The Basics

If you recall the mathematical treatment of waves shapes in previous blogs, the Fourier series expansion of this wave give the component waveforms.

Let square wave be represented as:

Inverter Circuits: The Basics

Fourier series expansion of the waveform is:

Inverter Circuits: The Basics

Graphically plotting (in the time domain) the components of a square wave:

Inverter Circuits: The Basics

Plotting the components in the frequency domain:

Inverter Circuits: The Basics

But for now, we can say we have obtained a voltage waveform which has a sinusoidal component of f Hz in it along with its multiples.

We will look at implication of multiple frequency components later.

If we try to realize this physical interchanging of the terminals using some electrical/electronic device then we are half done for building an inverter circuit. 🙌🙌🙌

So, let us now use this concept to layout circuit basic idea using some imaginary god-switch which is capable of doing what we want here exactly.

Assume an electrical circuit that works in following manner:

Inverter Circuits: The Basics

Between load terminals: LT1 and A & D and between LT2 and C & B we have a single pole double throw switch whose operational characteristics could satisfy following operations as described below.

So, for the first half time-period, load should be supplied by +ve voltage, for that:

  1. A & LT1 and B and LT2 must be shorted.
  2. D & LT1 and C & LT2 must not conduct and should withstand voltage appearing across them.

Inverter Circuits: The Basics

Then for a very small-time instant voltage should across load must be zero.

For that:

  1. A & LT1 and C and LT2 must be shorted or,
  2. D & LT1 and B and LT2 must be shorted.

Inverter Circuits: The Basics

For another half time-period, load should be supplied by -ve voltage, for that:

  1. D & LT1 and C and LT2 must be shorted
  2. A & LT1 and B & LT2 must not conduct and should withstand voltage appearing across them.

Inverter Circuits: The Basics

Now the vague principle of interchanging supply terminals to obtain an AC voltage waveform across a load using a constant voltage battery has become much clearer. Before we go on laying out exact switches required at the load terminals, we have to give a thought on the load current waveform too.

Real-world AC load

When the load is purely resistive- things are simple, the load current will follow the voltage linearly and the direction of current would be from A to B I positive cycle, zero current at t=t/2, and reverse direction in next half cycle.

When you search for switches with such characteristics, then the 21st century Doremon pocket’s hobbyist electronics store (Easy Electronics for SVNITians😅😅😅) will give you a BJT.

With proper base supply, we can obtain the desired characteristics.

Inverter Circuits: The Basics

Inverter Circuits: The Basics

But when it comes to real-world, rarely you would find an AC load of unity power factor or we can say purely resistive load in another way.

For non-resistive loads, the current through it and the voltage across the terminals are not linear. For inductive load the current will lag and for capacitive load the current would lead.

Consider a case when the voltage across the load somehow obtained is sinusoidal, which will be obtained in advanced inverter circuits.

The load being inductive, will show the following characteristics.

Inverter Circuits: The Basics

As indicated in the graph at the onset of every -ve half-cycle due to lagging nature, load current won’t reverse its direction similarly for +ve cycle. The above circuitry would reverse the voltage across terminal but it would not be able to sustain the required load current of opposite polarity. This could be easily achieved if we connect a diode to conduct current in short time period at the starting of +ve or-ve voltage waveform.

So, the new circuit now becomes:

Inverter Circuits: The Basics

Let us try to analyze the circuit operating over one complete cycle.

Inverter Circuits: The Basics

+VE VOLTAGE WAVEFORM:

From t = 0 to t = T2, transistors T1 and T4 should be ON and transistors T3 and T4 must be in OFF state.

  • t = 0 to t = T1: Current is negative so diode D1 and D4 will conduct.
  • t = T1 to t = T2: Current becomes positive hence transistor T1 and T4 conducts.

VE VOLTAGE WAVEFORM:

From t = T2 to t = T4, transistor T2 and T3 should be ON and transistors T1 and T2 must be in OFF state.

  • t = T2 to t = T3: Current is positive so diode D3 and D2 will conduct.
  • t = T3 to t = T4: Current becomes negative hence transistor T2 and T3 conducts.

In the following table we have required states of transistor (T1, T2, T3 and T4) to obtain the AC waveform across the load and which diodes are conducting and not-conducting current during the particular time-period.

Inverter Circuits: The Basics

We can replace the transistor and diode combination by another power electronics device, called power MOSFET. The greatest advantage of using this device is its capability for conduction of current in both directions. Additionally, it gives more power handling and reliable operation. The device is known by name of IRF540.

Inverter Circuits: The Basics

Internal Schematic Diagram:

Inverter Circuits: The Basics

NOTE: The diode shown in the symbol is parasitic. To know more follow the notes or the datasheet of IRF540.  

Inverter Circuits: The Basics

A generalized statement can be made that all the four switches are capable of conducting current in both directions during ON state.

Now let us have a glance on most important parameters of an IRF540 taken for datasheet:

Inverter Circuits: The Basics

The drain source breakdown voltage is 100V, while most of AC load operate on 230 V RMS voltage i.e. peak value of 325 V. So, this H- bridge configuration is modified for obtaining relatively higher AC peak voltage for low DC battery voltages. This is done by use of a three winding step-up transformer.

Inverter Circuits: The Basics

In this circuit when M1 is OFF and M2 is ON, the battery injects current to the primary winding (marked 3+) and the voltage appear across the load is +ve. When M1 is ON and M2 is OFF the battery supply the primary winding (marked 2+) and the voltage induces on secondary is -ve (opposite polarity than the first cycle).

Since the MOSFETs can carry bidirectional current through the source and the drain terminal thus this circuit can ve used to power non-linear loads also.

Gate Triggering Circuit

The MOSFETs M1 and M2 should be turned ON and OFF alternatively for 10 ms each for obtaining an AC output voltage of 50 Hz frequency. Numerous IC can be used to design the triggering circuits like IC4047, IC4049, etc.

CONCLUSION

The output voltage wave obtained using the circuit drawn above is a square wave. The Harmonic content would be little more than required for optimal performance.

Using a minor modification to this circuit we can get a modified sine wave.

Inverter Circuits: The Basics

It resembles more closely to pure sine wave hence improved harmonic performance. In the next blog will see how the harmonic performance is computed, measured and improved in inverter circuits, some practical circuits, and much more.

NOTE: Other advanced technique used is PWM (Pulse Width Modulation) technique.

Keep reading, keep learning!

AANTARAK DIVISION

TEAM CEV!!

BUILDING OUR OWN RECTIFIER

Reading Time: 10 minutes

What would you find?

The Bird’s Eye

Real-world approach for the circuits

Voltage Transformation

Rectification

Smoothening Capacitor

Designing a DC source for LED load

          Calculating the Load

          What should the value of capacitor?

          IC 7805

Designing a DC source for motor

         LM317

         Trimpot

Conclusion

Note: A better-edited version of this blog is available in original MS Word fromat, link below:

https://drive.google.com/file/d/1zbWqzMz4f0d-WwujdY8IQ0DqDEA9RE9-/view?usp=sharing

The Bird’s Eye

We have seen different rectifier structures of Half-wave, full-wave with center tap transformer and the bridge type.

Now, just connect the load to the output terminals from these circuits, and you are ready to go!! Hold-on, real life is not that much plain-sailing!

What we obtained as output – is a DC, just not changing its polarity. For acceptable performance we will require smooth flat DC output else high ripple content and poor DC performance is evident.

Practically, rectifier circuits come in whole range of variety, employing different approaches according to the application.

In previous blog we only studied uncontrolled rectification using diodes. Other classes of rectifiers include Controlled Rectification using SCR, and Active Rectification using transistors. Moreover, rectifiers also use different filtering techniques to optimize output characteristics using electronic filters, voltage regulators, etc.

These selections are purely made on the basis of requirement. For example, sometimes compact size is utmost criteria like in chargers (PWM technique), sometimes massive power is to be handled like in HVDC systems (Thyristor controlled), sometimes quality of waveform is more critical like in particle accelerators, and many times simplicity of the circuitry is of prime importance like in hand held home-appliances hair-dyer (diode rectification), etc.

NOTE: Whenever we design an electrical/electronic circuit, voltage and current through all the element must be calculated with acceptable error margin. Exceeding the voltage rating of any element will cause excessive electric field which will lead to permanent electrical breakdown, on other-hand exceeding the current ratings would surpass the thermal limit and cause the element to melt, burn and catch flames.

In this blog we will see A-Z of process of building a circuit and understand the circuit engineering like 2+ 2=4, by building some rectifier circuits using some of simplest techniques employed for AC to DC conversion.

Real world approach to the circuits

Voltage transformation

In most simple rectification techniques stepping down the AC supply voltage to a low AC voltage is first step. A 230/12 V, 500 mA transformer is most commonly used for these purposes.

BUILDING OUR OWN RECTIFIER

Rectification

As we have discussed we are building an uncontrolled rectifier circuit thus we require diodes for obtaining the half-wave or full-wave structures.

BUILDING OUR OWN RECTIFIER

The series 1N400X (X: 1-7) shown very reliable and accurate performance.

Datasheet shows the individual IMP ratings, for safe purpose 1N4007 is selected as they hardly differ in price.

BUILDING OUR OWN RECTIFIER

Smoothening

When we set to hunt-up various electrical elements for the purpose of stabilizing a pulsating voltage, no other candidate other than capacitors seems more worthy. Capacitors are utilized in almost all rectifiers.

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A capacitor is a basic circuit element which stores charge and retain its potential when external voltage is removed.

Consider the following circuit:

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The capacitor is in series with diode and the circuit is fed by an AC supply. The diode will only conduct in the forward biased condition and will block the current when it is in reversed biased, thus the capacitor will get charged to maximum supply voltage and will remain charged forever.

Now lets us connect a resistive load across the capacitor.

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This time when the diode is reverse biased the capacitor will try to get discharged in RC circuit and the voltage across the capacitor will decrease exponentially, whose rate will depend upon the time constant of the RC circuit. The voltage across the capacitor will continue to drop until diode becomes conducting in the forward biased condition and again start charging the capacitor.

Addition of a capacitor will greatly smoothen the voltage waveform across the load and corresponding current, by filling the gaps in voltage waveform.

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But to use this theory for smoothening the pulsating DC, we need to look at two things:

  1. The value of capacitor for the acceptable performance
  2. The value of current in the circuit for safe operation

WHAT SHOULD THE VALUE OF CAPACITOR?

For optimal operation the drop in voltage ( ) should be minimum as possible. Which is possible only when the time-period of the voltage waveform is much smaller than the time constant of the RC circuit, so that the diode starts conducting after capacitor loses small voltage.

Circuit theory leads us to say that when capacitor begins to discharge the drop in voltage at the terminals of capacitor is given by:

BUILDING OUR OWN RECTIFIER

The time for which the capacitor discharges can be approximately taken as T (time-period of source waveform), so the voltage across capacitor drops to:

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Thus,

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Evidently, for minimal drop we must have RC >>> T.

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For practical purposes, acceptable performance is obtained when:

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Designing a DC source for LED load

To design any AC to DC converter first thing is we need to consider is the load requirement. We first we have to refer to the datasheets of a white LED, screen shot below (datasheets links at end):

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The most crucial parameters to consider are: Continuous forward current = 30 mA; Reverse Voltage = 5 V

So, assuming the current through LED as 20 mA and forward voltage to be 3.5 V for good operation, equivalent resistance came out to be 175 Ω.

We can get the exact voltage across LED if we use transformer of calculated turn ratio. However, only standard transformers are available in market, most viable is the 230/12 V transformer.

So, when 230 V mains is stepped down to 12 V and rectified using single diode the capacitor charges up to 12*1.414 V, which becomes quite high voltage to be used for lighting up a LED.

Resistance in series can be connected for getting required drop across the LED but it dissipates considerable heat.

We can use an Integrated Circuit chip series, IC LM78XX for getting the constant output voltage of XX V for a range of specified input voltage.

IC LM7805

The IC series LM78XX is a three-terminal positive voltage regulator device, with many variant available with output voltages of 5, 6, 8, 9, 10, 12, 15, 18 and 24 V.  7805 is most popular as most of TTL devices has 5 V as operating voltage.

Ease of use, low cost, and self-employed thermal shut-down and safe operating area operating area performance are some great features available with this package.

Pin-out diagrams:

BUILDING OUR OWN RECTIFIER

IMP Ratings:

Some of important specification taken for datasheets are:

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Coupling with the rectifier circuit:

According to the datasheets, the IC should be connected in following manner for optimal operation.

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NOTE:

  1. The value of capacitor can be approximate.
  2. For obtaining an output voltage of XX V select the LM78XX device and input voltage should be at least 2.5 V greater than XX V. That is a minimum of 7.5 is required for LM7805 to obtain a o/p as 5V.

Also, it is always safe to connect a resistance in series with the LED to check it blowing off.

Using simple circuit theory, we can obtain the value of resistance to limit the current to 20 mA.

The equivalent circuit at load side becomes:

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Total current dram is 60 milliamps, which is in safe level as max output current from IC 7805 is 1mA and that of transformer is 500mA.

So, our final circuit become:

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Element ratings:

  1. Transformer: 230/12 V, 500 mA
  2. Diode: 1N4007
  3. C = 100 μF
  4. C1 = 33 μF
  5. R1 = R2 = R3 = 75 Ω
  6. C2 = 10 μF
  7. LED: 5V, 30mA

Using Full-wave Circuit:

As we have seen in previous blog that the performance characteristics are improved significantly by replacing half-wave with full-wave rectifier, better smoothening action is overserved for same capacitors.

Because in this case the time for which the capacitor discharges can be approximately T/2 (half of the time-period of source waveform).

BUILDING OUR OWN RECTIFIER

So, the voltage across capacitor drops to:

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Thus, drop in voltage here is:

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Which is even less than the half-wave case.

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Designing a DC source for DC motor:

A DC Motor

A permanent magnet DC motor more or less work in voltage range of 5V-12V, drawing current ~100 mA and rotor RPM in several thousands. The ratings can greatly vary for different motors so individual device ratings should be considered.

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It is a two terminal device and direction of rotation changes according to the polarity of supply given.

Typical datasheet:

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The battery eliminator we derived for LED can be used satisfactorily to run a DC permanent magnet motor, of specified voltage rating by using LM78XX (XX: 05, 08, 10, 12, 16 etc.), higher capacitor value and transformer of required turn ratio and current ratings.

The voltage obtained is constant and fixed for a given circuit, however voltage control is most desirable property for any motor driving circuit for obtaining variable rotor speed.

Here again comes another wonder of electronics world, the IC LM317.

IC LM317

The Integrated circuit LM317 is a three-terminal, adjustable positive voltage regulator and was designed by American electrical engineer Robert C Dobkin in 1976.

This IC performs functions of its counter-part (LM78XX series) of smoothening fluctuating voltage and is also capable of obtaining a wide range of output voltage for fixed input voltage with allowable load current up to 1.5 A.

Other operational advantages include ease of use, high-performance regulation and self-current limiting capability make it blow-off proof.

Pin-out diagram:

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Connections are made considering the three pins to obtain required outcome.

 IMP Ratings:

Some of important specification are:

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*Note: The device detects only difference in the input and output voltage so as long as the difference is maintained between 3V-40 V and upper limit of output is within limits, input can be anything.

Coupling with the rectifier circuit:

The datasheet provides the circuitry required for obtaining required voltage regulation. Output range 1.25 – 37 V is obtained by varying the potentiometer (R2), according to the equation:

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For a given input voltage, max output voltage (i.e. = Vin – 3 V, for minimal voltage differential od 3 V) is obtained for highest value of R2.

So, supposed we require output voltage in range of (XX V-YY V) then input voltage should be at least (YY+3) V and for according to the lower limit XX V, R2 and R1 must be selected, keeping in mind 1.25 is always constant for LM317 for all i/p.

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*The capacitors are added to optimize the transient response by filtering out ripples.

Variable Resistor R2:

The job of variable resistance is obtained by a miniature electrical component called the trimmer potentiometers, in short TrimPot.

Trimmer Potentiometer (TrimPot)

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They also come in range of rating from 10 kΩ to 100 kΩ.

Pin-out diagram:

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The-three terminal device has its maximum resistance between two fixed terminals and one sliding-terminal fitted with screw is for obtaining variable resistance of around 3% of fixed resistance.

Thus, resistance of 9.7 kΩ – 10 kΩ is obtainable for a 10 kΩ potentiometer.

IMP Ratings:

The device should not be operated at high voltages (< 50 V) and also rotational life is of around 20 cycles.

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So, our final circuit becomes:

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Element ratings:

We can achieve a fairly good control over the rotor speed (though not very-very precise) by using some standard rated circuit elements.

  1. Transformer: 230/12 V, 500 mA
  2. Diode: 1N4007
  3. C = 2200 μF
  4. C1 = 33 μF
  5. R1 = 250 kΩ
  6. R2 = 10 kΩ
  7. C2 = 10 μF
  8. DC Motor: 5V, 100 mA

Here is the massive database link for Rectifier Circuits:

https://drive.google.com/open?id=1YzbjGSF4ZAtZI9q3di0Zhj-uWQlhwUlH

Keep Reading, keep learning

AANTARAK DIVISION

TEAM CEV!!

Rectifier Circuits: The Basics

Reading Time: 14 minutes

What would you find?

Setting the Stage

Requirements

Performance Parameters

          The Output Waveform

          The Elements Ratings

Rectifier Structures

          Half-wave

                   Circuit

                   Output Waveform

                   Elements Ratings

          Full-wave: Centre-tap and Bridge type 

                   Circuit

                   Output Waveform

                   Elements Ratings

Summary

Note: A better-edited version of this blog is available in original MS Word fromat, link below:

https://drive.google.com/file/d/1bhu2vm5EoFnPZkygcx5I9lvZ2xvTvkoS/view?usp=sharing

Setting the Stage……

Nearly a century ago two big stars of electrical engineering world were on a war. Thomas A. Edison had probably the toughest competitor anybody could have ever, it was Nikola Tesla. Morally it was a war of ideas and technically it a war of currents. Well established Thomas A. Edison was obsessed with his idea of direct current, young passionate Nikola Tesla however was confident of his visionary ideas of alternating current. Any of could have won the battle, but Nikola Tesla came out to be champion. Tesla’s brainchild the induction motor, and development of transformers won him the title to be one of greatest inventor of modern world, and thus paved way for AC technology to every corner of globe.

At that time Edison had a very narrow escape to win, if he had himself went on developing the HVDC technology for the transmission of bulk electrical power he would have won the battle, given the transistors was not invented till then!

So, it was an impossible task to efficiently step-up DC voltages to high level for efficient transmission, on the other hand the simplicity of induction motor had earned it a title of industrial horse and concretely popularized AC. From then on, the AC and DC technology have followed a graph with opposite slopes, AC being positive.

But the past few decades have shown a different trend, DC have begun to find its place in many applications in modern world of “electronics”.

DC currents have begun to show their capability to do a work more efficiently when backed by modern electronics. Single phase induction motors are now replaced by the efficient BLDCs, tube-lights has been replaced by LEDs, and what not.

Today’s hybrid power system with majority generation, transmission and distribution been AC and consumption shifting to DC, requires a very efficient AC-DC conversion, at all points, else we would be wasting our precious electrical energy in form of thermal waste uselessly heating our environment.

The most massive DC system in our present modern world is HVDC stations, the featured image!!

Take a look around, our laptop, mobile phones, TV sets, LEDs, numerous home gadgets run on DC.

So, we need a constant DC output from a 50/60 Hz sinusoidal AC waveform.

REQUIREMENTS

A dc voltage is defined as a voltage whose polarity remains the same, more accurately polarity doesn’t change. Terminal “A” at any point of time is at higher potential than terminal “B”. So, all the waveform which never crosses the x- axis qualifies to be called DC Voltage by this definition.

#stick some DC wave graphs

So, are they all the same, do they have same DC power delivering capability? Does any DC device like a battery, a DC motor, a LED, etc. will have same performance characteristics when operated by these DC voltages?

Clearly, we need to define which parameters differentiate one DC waveform from another.

To understand this, we need to zoom in, we need to see something which is not so apparent from the current point of view.

The Fourier transform of all these waves could give the real insights. As we all know Fourier transform is powerful mathematical technique to breakdown any signal/waveform into its component fundamental signals. It reveals the greatest mathematical truth and beautifully summaries that all the signal can be expressed as summation of a constant and sine and cosine components.

Once we get the spectrum of components of a waveform, we can now very easily comment about the capability of a DC waveform to do the work.

HOW?

Consider the electrolysis experiment, in which we pass a dc current to deposit some desired product on a given electrode.Rectifier Circuits: The Basics

When we excite the circuit with these different DC waveforms, Fourier series backs us to say that we are actually giving a sum of all the components calculated by this tool.

Result follows from this experiment is that the amount of deposition by a DC waveform only corresponds to the constant component of waveform (current/voltage). It is also quite obvious to say that all sine or cosine waveform component will not contribute to net deposition in their one time-period (due to reversing nature).

For any DC device powered by a DC current/voltage waveform, only the constant component is utilized for doing the useful work. For the sinusoid components the charge, deposition, torque, etc. is always zero in DC devices.

Before we jump in to see the parameter, let’s have a look on: what is RMS value of a waveform?

The book definition is, the RMS value is constant DC voltage equivalent of an AC voltage (or pulsating DC voltage) which will produce same resistive heating effect for a given resistor. So, power developed by an AC voltage is calculated and equated to that of equivalent constant DC voltage, hence we get RMS value.

We will use this definition throughout.

PERFORMANCE PARAMETERS

Consider this general rectifier layout diagram:

Rectifier Circuits: The Basics

While analyzing the performance of any rectifier structure we have to consider two things, one is the characteristics of output for efficient conversion and second is the ratings of the elements used for the safety and economy purposes.

Let us see how and which parameters are used to analyze the output waveform?

The output waveform:

First using the Fourier series, we calculate the DC value of the output waveform.

Rectifier Circuits: The Basics

Considering the time-period of waveform to be T.

Where,Rectifier Circuits: The Basics

DC component is calculated as, which is also called average DC voltage:

Rectifier Circuits: The Basics

Now calculate the RMS value of this waveform, which is according to the definition;

Rectifier Circuits: The Basics

We define a term called Form factor as:

Rectifier Circuits: The Basics

Now, the efficiency of rectification by common sense is ratio of actual DC power developed to the maximum power that could have been developed if the voltage has been pure DC, (assuming load to be purely resistive):

Rectifier Circuits: The Basics

So, Form factor more or less gives the quantitative measure of rectification. The higher the DC content, lower the FF thus higher is the efficiency of rectification.

So, a good rectifier system must have low Form Factor, ideally 1!

But there exists a qualitative difference between two rectified waveforms having same form factor.

For example, consider a DC waveform from a rectifier as square wave and triangle wave.

The square wave of amplitude 1 and time-period T, then it can be calculated that Vdc= 0.5 V and Vrms= 0.707 V, Form factor is 1.414.

Rectifier Circuits: The Basics

Now consider another DC waveform, a triangular wave of peak amplitude 1 and t1 = T/1.499, in this case Vdc =0.333 V and Vrms = 0.471 V, and thus form factor is 1.414.

Rectifier Circuits: The Basics

Though they have same FF but we can see they vary greatly in terms of smoothness, and that is particularly due to different AC components, defined as ripples.

Intuition can lead us to say that the ripple voltage must be the effective AC component of voltage, so the RMS of ripple voltage:

Rectifier Circuits: The Basics

Ripple factor is defined to give the degree of smoothness of a rectified waveform, it is defined as ratio of RMS of ripple voltage to the RMS of DC voltage.

Rectifier Circuits: The Basics

So not just high DC content is desired but also a desired degree of smoothness is expected. The second case becomes a necessity in field where precision is utmost like in particle accelerators, etc.

So, the RF is lesser for a smooth rectified waveform, ideally zero.

The elements ratings:

All the circuits elements which we will see later (transformers, diodes, capacitors, etc.) in rectifier circuits should be operated within their permissible ratings.

Almost every rectifier circuit is aided by a transformer to obtain the required voltage transformation. The rating of transformer to handle the power and current is also a critical performance parameter. We have defined Transformer Utilization Factor (TUF) to account the same.

TUF is the ratio of DC power supplied to load to the total AC power at the secondary of transformer.

So,

Rectifier Circuits: The Basics

and

Rectifier Circuits: The Basics

Where Vrms and Irms are the rms value of voltage and current waveform at the secondary of the transformer.

Rectifier Circuits: The Basics

There are certain operational limits of the diode which must be taken care under operation.

The Voltage stress occurring across the diode under non-conducting period must be less than the maximum voltage to cause rupture or breakdown of the diode, also called Peak Inverse Voltage (PIV) rating.

The current through diode must never exceed the peak forward current/average forward current defined limit, to check that the thermal limit of diode is not exceeded.

Elements ratings would be understood more clearly later.

RECTIFIER STRUCTURES

To understand the basics of different rectifier circuits we will first analyze them considering the ideal case- transformer is lossless, diode has no resistance, and load being purely resistive.

Half wave rectifier: This basic circuit is only used for low power rating applications. A diode of suitable ratings is used in series with load, diode conducts only in forward biased mode i.e. only when the voltage polarity across it is maintained, say positive. In next half-cycle the diode is reversed biased and the load current is zero.

Rectifier Circuits: The Basics

THE OUTPUT WAVEFORM:

Voltage across load is:

Rectifier Circuits: The Basics

Waveform:

Rectifier Circuits: The Basics

Fourier series:

Rectifier Circuits: The Basics

The DC component in waveform or the average DC voltage is:

Rectifier Circuits: The Basics

Rectifier Circuits: The BasicsRectifier Circuits: The Basics

The RMS value of the output voltage is:

Rectifier Circuits: The Basics

Using the trigonometric formula to get suitable form for integration:

Rectifier Circuits: The Basics

Which on integrating and putting the limits simplifies to:

Rectifier Circuits: The Basics

Now the form factor can be calculated as:

Rectifier Circuits: The Basics

The ripple factor to get the qualitative index of the rectified output:

Rectifier Circuits: The Basics

The efficiency of the rectification is:

Rectifier Circuits: The Basics

THE ELEMENTS RATINGS:

Transformer Utilization factor:

We have calculated Pdc as:

Rectifier Circuits: The Basics

The voltage waveform at secondary is a sinusoidal transformed waveform of max amplitude as Vm.

Rectifier Circuits: The Basics

The rms value of current in secondary is same as the load current.

Rectifier Circuits: The Basics

We have:

Rectifier Circuits: The Basics

PIV (Peak Inverse Voltage): In negative cycle the diode sees a Vm drop across it.

PFC (Peak Forward Current): It is the maximum instantaneous current through the diode in forward bias condition, considering resistive load R, we have:

Rectifier Circuits: The BasicsRectifier Circuits: The Basics

CONCLUSION:

Half wave rectifier gives has following performance parameters:

  1. FF as 1.57 and efficiency of rectification as 40.5%, which means in the output waveform only 40.5% power is DC rest is AC component.
  2. RF as 1.21, which indicates not very smooth waveform.
  3. TUF is 0.286, which means the transformer must be (1/0.286 = 3.49) times higher rating that the actual power delivered to the load, so bigger transformer is required.
  4. PIV and PFC are calculated as above for selecting the diode.

Full-wave rectifier- Using center tapped transformer:

Rectifier Circuits: The Basics

Voltage across load is:

Rectifier Circuits: The Basics

Waveform:

Rectifier Circuits: The Basics

Fourier series:

Rectifier Circuits: The Basics

The waveform can be considered to be periodic in T/2 or in T, the calculation of parameter won’t be affected.

The average DC voltage is:

Rectifier Circuits: The Basics

Rectifier Circuits: The Basics

The RMS value of the output voltage is:

Rectifier Circuits: The Basics

We can also directly find the RMS value, as it would be same as that of a sine wave, as no part of waveform is lost.

Rectifier Circuits: The Basics

So, here the form factor is:

Rectifier Circuits: The Basics

The ripple factor of the rectified output:

Rectifier Circuits: The Basics

The efficiency of the rectification is:

Rectifier Circuits: The Basics

THE ELEMENTS RATINGS:

Transformer Utilization factor:

We have calculated Pdc as:

Rectifier Circuits: The Basics

The total power is shared equally by two secondary windings of the center-tap transformer.

In each half winding, the voltage waveform at secondary is a sinusoidal transformed waveform of max amplitude as , so rms value of voltage in one winding is:

Rectifier Circuits: The Basics

The rms value of current in secondary for one winding is same as that of the half-wave transformer (same current waveform): –

Rectifier Circuits: The Basics

So, power rating of secondary is twice that of each winding:

Rectifier Circuits: The Basics

We have:

Rectifier Circuits: The Basics

PIV (Peak Inverse Voltage): In negative cycle the diode sees a Vm drop across it.

PFC (Peak Forward Current): It is the maximum instantaneous current through the diode in forward bias condition, considering resistive load R, we have:

Rectifier Circuits: The Basics

Rectifier Circuits: The Basics

Conclusion

Full-wave rectifier with center-tap transformer gives has following performance parameters:

  1. FF as 1.11 and efficiency of rectification as 81.16%, which means that the output waveform has 81.16 % of total power as DC rest as AC component.
  2. RF as 0.482, which indicates more smoother waveform.
  3. TUF is 0.573, which means the transformer must be (1/0.573 = 1.745) times higher rating that the actual power delivered to the load, so less big transformer is required.
  4. PIV and PFC for selecting the diode is same as that of half-wave rectifier.

Full-wave rectifier- Bridge type

Rectifier Circuits: The Basics

Waveform:

Rectifier Circuits: The Basics

Fourier series:

Rectifier Circuits: The Basics

Only the elements rating would be affected, the waveform characteristics remains same as that of the previous case of center-tap transformer.

Rectifier Circuits: The Basics

Clearly the form factor and the ripple factor will also remain the same:

Rectifier Circuits: The Basics

The efficiency of the rectification is:

Rectifier Circuits: The Basics

THE ELEMENTS RATINGS:

Transformer Utilization factor:

We have calculated Pdc as:

Rectifier Circuits: The Basics

The rms voltage developing at the secondary winding is:

Rectifier Circuits: The Basics

The rms value of current in secondary winding is same as the rms current through the load-

Rectifier Circuits: The Basics

So, power rating of secondary of transformer is:

Rectifier Circuits: The Basics

We have:

Rectifier Circuits: The Basics

PIV (Peak Inverse Voltage): In negative cycle the Vm drop is shared equally by two identical diodes in series thus PIV rating of each diode is Vm/2.

PFC (Peak Forward Current): It is the maximum instantaneous current through the diode in forward bias condition, considering resistive load R, we have:

Rectifier Circuits: The Basics
Rectifier Circuits: The Basics

 

 

Conclusion

Full-wave rectifier with bridge-type gives has the following performance parameters:

  1. FF as 1.11 and efficiency of rectification as 81.16%, which means that the output waveform has 81.16 % of total power as DC rest as AC component.
  2. RF as 0.482, which indicates a more smoother waveform.
  3. TUF is 0.810, which means the transformer must be (1/0.810 = 1.234) times higher rating that the actual power delivered to the load, so less big transformer than the previous type is required.
  4. PIV of the diode is halved.
  5. PFC and average current remain same as that of half-wave.
  6. The current in transformer winding reverses unlike in centre-tap or half-wave rectifier structures where the current direction is remaining same and the possibility of core saturation is there.

SUMMARY

Rectifier Circuits: The Basics

In the next blogs to come the following topics will be explored :

Real world approach for the circuits

Applications

Harmonics distortion on AC side

Study of different loads

Keep reading, keep learning!

AANTARAK DIVISION

TEAM CEV!!

ELECTRICAL CORONA

Reading Time: 18 minutes

WHAT WOULD YOU FIND?

INTRODUCTION

PHYSICS

EFFECTS

AC AND DC CORONA

MATHEMATICAL DESCRIPTION

REMEDIES

EVEN DEMONS CAN BE TAMED!

UNANSWERED QUESTIONS

REFERENCES

Note: A better-edited version of this blog is available in original MS Word fromat, link below:

https://drive.google.com/open?id=14-sLYDWD_Io-uCGAdmw8XkM1w3OsPTre

INTRODUCTION

We have got in our head what is electrical breakdown means actually, refer to the previous blog if not!

The puncturing of the porcelain or polymer insulators is quite rare however the air the major insulator in overhead line give up quite easily if care is not taken. The breakdown of air is a phenomenon called corona discharge. In High-Voltage transmission (HVDC and HVAC) this becomes a critical design consideration. Why?

We find description of this phenomenon as early as 1920s in Peek’s foundational work, electric utilities have been extensively researching on it for past 50 years, still we have not uncovered it much clearly and completely. However, modern system does manage to effectively tackle the corona problems and, in this blog, we will see all about it.

A visual introduction would be very helpful to begin the topic:

https://youtu.be/C0Fry6ktu4w

IEEE definition of the phenomenon is as follows:

A luminous discharge due to ionization of air surrounding a conductor due to electric flux density (or voltage gradient) exceeding a certain critical value is called corona discharge.

We had already seen the distribution of the dielectric field, and we also know that the points at which it exceeds the flux density limit (30 kV per cm for air) the air starts conducting.

PHYSICS OF PHENOMENON: WHAT IS GOING DOWN THERE?

In general, any type of corona can be explained in following layman language:

  • Ionization and excitation: Though the air is neutral fluid but there are many sources which don’t allow it to be. The UV radiations from Sun, the Cosmic rays from space, the gamma rays from radioactive decay in soil are major sources for air ionization. These natural ionizing events create 20 ion-electron pair/cubic cm per second.
  • Electron accelerates: As soon as the potential gradient threshold limit is surpassed at the surface, it becomes capable to impart enough kinetic energy to the electron, generated from the ionization.
  • Collision and Avalanche: The energetic electrons collide with neutral gas molecules and knocks out electrons and effect become cumulative, leading -to formation of a conducting air, the plasma.
  • Drift out: Soon the electrons move out of range of high potential gradient and become less energetic and avalanche fades.
  • Recombine: The electrons slowly find positive ions and recombine to emit light in visible or UV region, they also initiate chemical reactions leading to formation of ozone, etc.

Note: Not all recombination takes place far away from conductor, they also occur in plasma region.

Now, the polarity of charge on the conductor significantly changes the process.

POSITIVE CORONA

When conductor has positive change:

  1. The electrons generated in plasma region are immediately drifted inwards, thus electron density in vicinity of conductor is lesser.
  2. of electrons are less but majority of them are in region of high potential gradient, hence the average energy of electrons is high.
  3. These electrons cannot support (or initiate) low energy chemical reaction.
  4. The high energy level of electron gives the phenomenon its characteristics emission of blue light or UV radiation.

NEGATIVE CORONA

When conductor charge is negative:

  1. The electron generated in the plasma region is repelled outwards, so as a result, in this case, large no of electrons in vicinity of conductor is observed.
  2. However, more electrons are concentrated in region far way from conductor surface, in low potential gradient hence are in low energy state.
  3. Due to low average energy of electrons, inelastic collision with neutral molecule don’t contribute significantly in avalanche as do the knocking of electron by photons (photoelectric effect) emitted from the recombination taking place in high potential gradient region of plasma.
  4. Ozone production is a relatively low energy process, average energy electrons in negative corona are perfect initiator.
  5. The energy state is also responsible for the characteristic red color of discharge. (Note: Red is low energy radiation compared to violet or blue)
+VE CORONA-VE CORONA
Less no. of electronsMore electrons
Highly energetic electronsLow average energy state
Avalanche produced by inelastic collisionAvalanche produced by photoelectric effect
Blue-violet uniform glowRed glow
Less ozoneMore ozone

NOTE: Modes of corona have been further classified based on voltage level. Research paper on the same is listed in references.

EFFECTS

Now corona have observeed to produce the following effects:

Light emission:

The recombination of electrons with positive species emit characteristics electromagnetic-waves. More on that later.

Radio Noise:

Experimental data have shown that the corona current is pulsating in nature. We haven’t found out yet! The frequency of this current lies in wide range of order of MHz, large part of which lies in our defined radio frequency band.

ELECTRICAL CORONA

This current produces two unwanted effects:

  1. This high-frequency corona current, called harmonics adds to load current of transmission line and cause distorted current and voltage sine waveforms, highly undesirable. In layman terms, it is called power pollution.
  2. Bound by laws of physics they produce electromagnetic waves in rf band, which interfere with the telecommunication signals. Depending on the distance, orientation, and several other factors the effect ranges from negligible noise to complete distortion of the communication signal.

Note: The radio waves and light emission are occurring on the same physics rules. 

Audible Noise:

Energy discharge is occurring in form of fast-moving particles in air. And audible noise is nothing but pressure disturbance in air created by motion of air particles. Corona discharge sounds like a hissing, low amplitude and occurs over wide frequency spectral range.

4. Chemical effects:

The ionized electrons start a cumulative process of forming electrically charged species. Oxygen atoms are major neutral atoms to form radicals O, which then combine with O or O2 to form O2 or O3 respectively. The reactive nascent oxygen also combines with metals and organic matter. The reactive ozone under extreme electrical stress might also react with stable nitrogen to form oxides. The ozone also reacts with insulation material, corroding them slowly, leading to damage without a sign of warnings.

Moreover, over a certain concentration ozone has proved to be toxic to life.

All of these are confirmed by many shreds of evidence like the smell of ozone, deposition of white powder, and worn out insulators.

ELECTRICAL CORONA

The insulation degradation is additive effect of corona and other environmental factors

Power loss:

All of these effects which include light, radio waves, audible waves generation and also, heat produced, lead to a waste of energy. To calculate the corona power loss, we have some methods and empirical formula, none of them with error less than 30%. Which marks the complexity of corona discharge!

Conductor vibration:

The effects mentioned above are monitored to determine the corona performance of any transmission lines. However, a weird, not much considered effect is also there, mechanical vibration of conductors.

ELECTRICAL CORONA

The wires under tests for corona noticed to vibrate in fundamental and other harmonics.

AC and DC CORONA

**Contribute

HVACHVDC
Poor foul weather performanceBetter performance in foul weather
More radio interference and audible noiseReduced RI and AI in wet weather

MATHEMATICAL DESCRIPTION

We have already calculated the potential gradient distribution in space for parallel conductor case and we also know that it is maximum at the surface of conductor.

ELECTRICAL CORONA

And is given by,

ELECTRICAL CORONA

For a given conductor spacing, and given voltage level, the potential gradient obtained at the surface will be constant, independent of any other external factors.

Now, the maximum dielectric stress that could be handled by air is g0, 30 kV/cm.

How come?

We know the distribution of potential gradient:

ELECTRICAL CORONA

Maximum voltage that could be applied is:ELECTRICAL CORONALet us see that if it is true that the corona always begins as soon as the potential gradient, g0, i.e. 30 kV/cm is reached at surface? Or the air can withstand more or less?

The question is reasonable because it has been observed for same voltage level and conductor arrangement, system have shown different corona characteristics. Negligible corona in dry summer but effect multiplies 100X in foul weather/rainy season. 

Effect of conductor spacing:

Question is same like: does the elastic limit of a rod depends on its physical dimension like its length. Clearly the rod always breaks at the same unit stress, no matter how long or short it is, so does the air’s dielectric strength is independent of conductor spacing. (Assuming air as a rod) 

Effect of conductor diameter:

It’s a well-established experimental fact that the air is apparently stronger at surface of small conductor, for same conductor spacing the potential gradient at surface of small conductor for onset of corona is greater than 30 kV/cm.

So, air breakdown potential gradient at surface is gv not g0, so the equation becomes:

ELECTRICAL CORONA

Let us call gv as apparent dielectric strength of air.

The experimentally calculated formula for the minimum potential gradient at surface to start corona is:

ELECTRICAL CORONA

Now, let us suppose that  occur at  from the centre, so:

ELECTRICAL CORONA

ELECTRICAL CORONAIt comes out that g0 should always be occurring at distance of  0.301sqrt(r) from surface of conductor not at the surface of conductor. At surface there should be some higher value, gv.

These terms however become insignificant for larger diameter conductors.

People have tried to explain the phenomenon using different theories, among them best suited is the electron theory.

“The 30 kV/cm is limit just sufficient to accelerate the electron over its mean free path to acquire as much as energy to form ions by collisions. The electrons would require some finite distance to get sufficiently energetic, on the other hand, the potential gradient from the surface goes on decreasing hyperbolically. Thus, greater potential at the surface of conductor is required than the dielectric strength of air, so that electrons get enough time (distance) to get enough kinetic energy.

ELECTRICAL CORONA

Here rises a very intriguing question, and it is, would corona will not take place if we keep the conductors spacing less than 0.301sqrt(r).

Its is quite amazing to know the answer as big “YES”.

The experiments have shown that the same fragile air, in experimental setups of small air spacing has been made to withstand gradients as high as 200 kV/cm.🤐🤐

Then why we don’t hang our wires as close as possible? 🙃

Note 1: It should be noted here that the 0.301sqrt(r) distance is several times greater than the mean free path, so the electron undergoes many collisions before actual ionizing collision. It is only after accelerating for this distance the electron gain required K.E. To get real clear, a dive into the depths of Kinetic theory of gas is required.

Note 2: When we are talking of ionization of air in actual it is the oxygen which is being ionized to form highly chemically active O, (30 kV figure is respect to that only). The radical than combines with different ions to form ozone, oxides, etc. depending on its energy level.  

Air density: Effect of temperature and pressure

Does the temperature and the pressure also have influence on the g0 and gv?  

Yes!

Different high voltage transmission systems (HVDC/HVAC) have shown an increase in corona loss from 3 to 100 times in foul weather than in clear sunny day. This indicates that meteorological conditions affect the g0 that’s why corona intensifies for same system and same voltage level.

We know that the for decreasing air density the intermolecular spacing will widen, which also implies that mean free path of electrons will increase, thus potential gradient (g0) required decreases, with decreasing air density.

ELECTRICAL CORONAδ is the relative density.

Using, the ideal gas equations, the ratio of density is calculated at two different temperatures and pressure, reference taken as 25° C and 75cm pressure ( δ is taken as 1). (PM=dRT)

ELECTRICAL CORONA

But this proportionality relation seems not applicable for the gv, thus the equation was taken of form:

ELECTRICAL CORONA

Repeated experiments, and fitting the results led to revealing f(δ) as:

ELECTRICAL CORONA

So,ELECTRICAL CORONA

and the new accelerating distance becomes:

ELECTRICAL CORONA

Accelerating distance increases with decreasing density. WHY?

We at CEV also cannot figure out, share with us if you can.

Conclusion: For decreasing air density the dielectric strength of air decreases proportionally, whereas the apparent strength also shows a non-proportional decrease.

 

Effect of Conductor surface:

To consider the effect of surfaces on corona discharge becomes of crucial importance when designing any product like connectors, spacer dampers, markers, end lockers, etc for high voltage applications.

To get an intuition of how the surface would impact the corona discharge, consider a uniform sphere being excited by high-voltage, for any voltage level the electric lines of force in air (or dielectric lines) around the surface on conductor is uniformly distributed. Now if we excite a metal sample which has non-uniform sharp corners then would the field distribution remain uniform?

The answer is no.

Various techniques are employed to find the rough distribution of electric field around the products, most viable is the FEM technique. This Finite Element Method (FEM) is used more popularly in civil and mechanical engineering, in visualizing the stress, compression and tension and identify the weak and vulnerable points in their structures.

Those same FEM techniques are used to get computer models simulations of the product to identify high potential gradient points.

Let see the result of a study for a HV hardware manufacturing company which exactly proved these theories using the FEM techniques. (refer 123)

This specimen was specimen for the study
This specimen was specimen for the study
This was the computer-generated model
This was the computer-generated model

 

ELECTRICAL CORONA
The radius of curvatures at finite elements (mm)
ELECTRICAL CORONA
FEM simulation of potential gradient at the surface (kV/mm)

It comes out that the points with sharp edges (high radius of curvature) has high degree of concentration of field lines thus greater potential gradients and are points where dielectric strengths are first crossed.

When tested for corona performance of the connector the results were as expected.

ELECTRICAL CORONA
Conductor being excited until corona was observed

Visuals confirm that the points predicted by simulation came out to point sources of corona.

To take the effect of surface roughness there is an irregularity factor, m. So, the apparent dielectric strength becomes:ELECTRICAL CORONA

m is 1 for smooth surfaces 

Conclusion: If the conductors get weathered over time and develop rough surfaces, the corona will start at lower voltage levels, at the sharp edges.

Finally, the expression for voltage level at which corona will begin for parallel wire is:

ELECTRICAL CORONA

REMEDIES

Corona discharge is highly undesirable in transmission lines because of the effects it produces. An average yearly loss for ±500 kV HVDC line is estimated to be around 25W/m, which is considerably large if summed over a year for larger distances. Included with radio interferences in telecom signals, noise pollution, emission of ozone which degrade the insulation and have toxic environmental effects, the corona discharge becomes a costly affair!

The current majorly employed techniques to check corona are:

Bundled conductors:

Bundling of conductors is the most effective and widely used techniques, to decrease the potential gradient on conductor surface for a given voltage level or in other way to increases the corona inception voltage and thus improving the corona performance of a transmission line.

For increasing voltage levels two, four, six and sometimes eight stranded conductors per phase are bundled using spacer damper at regular intervals are employed. These dampers keep in check that the conductors maintain required distance during high-winds.

ELECTRICAL CORONA

Now the actual question to be answered is how the potential gradient at the conductor surface decreases for a system of bundle conductors. The computation of electric field for a bundled arrangement can be done using numerous available mathematical tools.

Various research papers have used techniques of conformal mappings, simulated charge methods, FEM, integral equations, and other methods to calculate the field distribution much accurately.

Here let us use basic superposition method to conclude that the gradient decreases for bundle conductors.

Consider a single conductor of radius r, at voltage V from a plane at relatively large distance D.

Equations of electric field in space, conductor voltage and charge are as follows:

ELECTRICAL CORONA

ELECTRICAL CORONA

Now if we take two conductors at same voltage separated by a spacing S, then charge redistribution takes place.

Let Q1 and Q2 be charge on conductor 1 and 2 respectively:

Potential at surface of conductor 2 is (by integrating electric fields along x-axis):

ELECTRICAL CORONA

Similarly, by geometry potential at the conductor 1 surface is:

ELECTRICAL CORONA

Now as we know-ELECTRICAL CORONA

Which leads to-

ELECTRICAL CORONA

So, the potential becomes:ELECTRICAL CORONA

Assuming S≈r,

ELECTRICAL CORONA

Since we are computing for same voltage level, clearly-

ELECTRICAL CORONA

If not accurately, we can surely say that:

ELECTRICAL CORONA

Now calculating the electric field at the surface of conductor 2:

ELECTRICAL CORONA

Solidly we can conclude from the above expressions that the potential gradient decreases with adding more conductors in for one phase.

More accurate methods of calculation are given in references xx and yy.

Here are graphic simulations of electric field calculated for single, two, three and four conductors.

ELECTRICAL CORONA          ELECTRICAL CORONA

Notice how the gradient decreases for increasing the no of conductors in a bundle.

For three and four conductors bundle field even decreases more:

ELECTRICAL CORONA        ELECTRICAL CORONA     ELECTRICAL CORONA

Another important point to be noted is that the dependence of potential gradient is not linear for a given no of conductors in bundle. The potential gradient is a function of bundle geometry and achieves minimum value only for a particular value of bundle radius (conductor spacing).

ELECTRICAL CORONA
Potential gradient at the surface of bundles different no of conductors(N) and different bundle radius

Note:

  1. All the corona performance parameters like loss, radio interference, audible noise, etc. seemed to improve considerably for bundled conductors.
  2. Further study would reveal other advantages, like:
    1. Better thermal properties, better cooling due to increased surface area
    2. Reduced line inductance
    3. Increased transmission capacity
  3. However, wind and ice loads are greater for this arrangement, and also complication in designing and placing spacers is there.

Corona Rings:

Another effective remedy has same underlining principle of redistribution of electric field lines such that the potential gradient limit is exceeded only at few points!

ELECTRICAL CORONA

Use of curved smooth metal rings (shown above) called as corona ring connected to the high voltage hardware reduces the gradient on conductor surface by redistributing field in more uniformly.

FEM simulation confirms the same:

ELECTRICAL CORONA

The red regions (relatively sharper) have high potential gradient as act at point source for corona discharge, when electrically connected with the corona ring the field lines become uniform in space, hence improving the corona performance. In this particular case, they also improve the voltage drop distribution in the insulator strings, when used also for this purpose they are technically called grading rings for insulators.

When used specially for conductors (HV apparatus) they are called corona rings.

Nearly all the hardware of high voltage transmission system uses corona rings:

1. Corona rings safeguarding the sharp junction points of conductors and insulator and also improving the string efficiency:

ELECTRICAL CORONA

2. Circular corona rings on the switch gears at the HV substation:

ELECTRICAL CORONA

Smoothening:

We have already seen in the that the electric field exceeds the threshold limit first at the sharpest points due to greater concentration of lines. So, transmission line components are designed to be free of sharp edges and rough surfaces.

 

DEMONS CAN ALWAYS BE TAMED!  JUST USE SCIENCE

Somewhere in the early 1900s, an experimental scientist Birkeland’s high voltage electric gun demonstration model failed due to short-circuited followed by an arc. But soon he realized that the arc generated could be utilized for some useful purpose. Backed by businessman S Eyde he went on discovering a process you all would have heard of- the Birkeland-Eyde’s process for manufacturing of fertilizers. Since then these gas discharges have been used for industrial processes.

Even though the mysteries of electrical coronas remain largely undiscovered yet this phenomenon have found its applications into many crucial fields. These days corona is used for industrial chemical synthesis, photocopy machines, surface treatment, air pollution control, bactericidal applications, in fact they have even proved to improve the insulation, and many others.

Let us divide this section into two parts, application already being used and applications in which research is going on.

PHOTOCOPIER: It is pretty weird to know that we, electrical engineering student’s whole semester visualized corona as devil and in the end comes out to be principle behind our lifeline. 😅😅😅

AIRCRAFTS

OZONE PRODUCTION

Under-research applications:

AIR POLLUTION CONTROL

ELECTRICAL INSULATION

CONCLUSION

The scientific community along the industrial partners have been exploring Corona discharge for a long time, however due to complexity of phenomenon it has largely remained undiscovered and untapped.

Extensive research for HVDCs:

It has now become an absolute necessity to tap the renewable energy from coastal, offshore, and other remote location to keep up with ever-growing energy demand. HVDC system not just provides a gateway to transfer that large power to large distances by integration of renewables on the grid but also comes with great operational benefits of better efficiency, increased stability (no need of synchronism), reduced right of way and what not.

And in these times when HVDC projects are popping up across the globe, not many high-voltage industry companies share high-expertise in HVDC technology as they have in HVAC. Corona remains a critical consideration in any product design and thus extensive research is required.

Usefulness:

Corona discharge have found application in a wide range of field and still continue to unfold in quite unintuitive ways. Air pollution control and improving the electrical insulation are few examples of that class.

So, corona discharge is not just important from HVDC point of view rather it could be an underlying principle of new innovation to come in future!    

UNANSWERED QUESTION

You can contribute to the blog by sharing your wisdom on the following questions:

  1. How to calculate the dielectric strength of air, which atoms will you consider for the equation of ionization?
  2. Differentiating AC and DC corona in detail, and thus understanding of HVAC and HVDC lines corona performance.
  3. Why accelerating distance increases with decreasing air density?

REFERENCES

  1. Corona rings: https://www.slideshare.net/sampengalavenkatesh/52introduction-effects-of-corona-ring-design-by-electric-field-intensity-using-3dcoulomb
  2. Drive Link includes:
    1. Dielectric Phenomenon: WF Peek
    2. EPRI 365 kV and above Transmission line reference book
    3. HVDC and HVAC lines corona performance
    4. FEM techniques for HV hardware
    5. FEM analysis of Bundled conductors
    6. Biological Effects of transmission lines

https://drive.google.com/open?id=16BCGlCe3XNldjjMVoQ8FgYfbE0Ai6HfJ 

CAN YOU NOW EXPLAIN THE PHYSICS OF FEATURED IMAGE OF THE BLOG?

Keep reading, keep learning

TEAM CEV! 

 

Dielectrics and Insulation

Reading Time: 11 minutes

WHAT WOULD YOU FIND?

INTRODUCTION

WHY INSULATION SHOULD BE CONSIDERED?

THE TRANSMISSION LINE

            BARE CONDUCTOR

            INSULATED CONDUCTOR-UNDERGROUND CABLES

            PARALLEL CONDUCTORS-OVERHEAD CABLES

ELECTRIC AND MAGNETIC CIRCUITS

TWO WIRE ELECTRIC FIELD

THE BREAKDOWN

CORONA (next blog)

NOTE: A BETTER EDITED VERSION OF THIS BLOG IS AVAILABLE IN ORIGINAL MS WORD FORMAT, LINK BELOW:

https://drive.google.com/open?id=1rvvbWubkvasvO_regM7QRSzcN5KWCW9g

INTRODUCTION

The generation, transmission, distribution and the consumption of electrical energy for the service of mankind is what in general the power system is supposed to do. If you had followed through the previous blogs in the electrical genre than it would be clear to you that in general, a power system comprises of sources of electricity i.e. the generators on one end and consumers of electricity like motors, heaters, and other appliances on another end, and between them is transmission and distribution system for efficient transfer of energy between components which are separated by huge physical distances.

  1. https://cevgroup.org/fault-analysis-and-related-technical-problems-in-power-system/
  2. https://cevgroup.org/blackouts-facing-the-outrageous/
  3. https://cevgroup.org/electrical-power-sytem-the-indian-frame/
  4. https://cevgroup.org/electrical-power-quality/
  5. https://cevgroup.org/the-electric-traction-indian-railways/
  6. https://cevgroup.org/menlo-parks-wizard-vs-the-serbian-american-inventor/

The aim of this blog is to understand the physics of dielectrics, explore the insulation systems of the high-voltage lines, and the associated phenomenon like corona discharge with them. And thereby imparting light on some of the problems of high-voltage engineering.

The transmission and distribution system has three very basic working parts: conductors, insulators and dielectrics. It seems that the conductors are real heroes, whereas insulators and dielectrics are not doing a great job here. However, in reality, the path of the flow of electrical energy is decided by the conductors, insulators and dielectrics equally. Also, to understand the difference between dielectrics and insulators reaching the end of the blog is necessary.

Why insulators and dielectrics should be equally considered?

The advantages of utilising high- voltage are many. Decreased line loss, efficient conductor use and better voltage regulation are names of few. Modern system uses a blood pressure as high as 132 kV, 440 kV, 765 kV, 1100 kV and the limit is rising. Increasing the voltage will indeed get you some cool results but it also has another facet to it.

“INSULATORS: I DON’T WANT TO BE INSULATOR ANY MORE”

Dielectrics and Insulation

Yes at these extra high voltages the insulators and dielectrics begin to give up their jobs and the flow of electric energy is no more defined and our power system goes in vain if required engineering is not performed.

THE TRANSMISSION LINE

Understanding the science of the insulation is all about decoding the distribution of electric field around a conductor.

A bare conductor:

Consider a conductor of infinite length, having Q charge per unit length. The electric field will be radially outward or inward depending on the polarity of charge. The field of lines go till infinity where we have assumed potential to be zero.

Dielectrics and Insulation

Applying the Gaussian law:Dielectrics and Insulation