interview question explained-8

Hi folks.
Here we are going to talk about a very important practical aspect of power system engineering.

Q. Why secondary of the Current transformer is never kept open or should not be opened?

Let us first see what is current transformer. It has same working principle (self and mutual induction) as that of voltage transformer. I want to measure the AC current flowing through a conductor. For that purpose, I put a circular magnetic core around it. I wrap a wire on that core. Now the rate of change of flux in the core in responsible for the induced EMF in the wrapped wire. That flux will be produced by the straight conductor carrying current. So the current flowing through the wrapped wire is giving me the reflection of the current flowing through the straight conductor. This is nothing but current transformer.

To have the transformer action, the flux produced by primary (straight conductor) should be opposed by the flux produced by the secondary (wrapped conductor). If we open the secondary of the Voltage transformer (traditional transformer, you know from 12th), the primary current is very small (no load current). So the flux produced in the core is very small and will not cause any undesirable effects.

For a current transformer, The current in primary is not depending on the current in secondary. It is depending on the load connected across it. So when secondary is opened, The opposing flux produced by secondary is vanished. So the only flux present in core is flux due to primary and it is very high. This large flux induces extremely high voltages across open secondary. This voltages may rise to several KV that is enough to damage any equipment, or the insulation or in worst cases to kill the operator.

You can also understand this as CT always tries to maintain constant secondary current. When there is some resistance across its secondary, (typical relay coil resistance is 0.5 Ohm) Small voltage is enough to make flow rated current. When the secondary is opened, To make flow the same current through high air resistance, the CT develops high voltages across it.

So there is spacial care taken provide a shorting switches across the secondary of the transformer. Many relay constructions have a spring operated shorting switch that keeps secondary of CT shorted when the body of relay is removed form its plastic casing. Please keep this thing in mind. Just for information, The voltage developed by a protective CT should be greater then the voltage developed by metering CT. We will soon talk about Difference between these two as it is interesting to see how hardware construction changes to satisfy peculiar purpose.

Thank you for your time and please feel free to leave comments about your views. You can also mail me at dnachiketa1010@gmail.com

Transformers-2

Hi folks.
Here we are going to talk about the tertiary winding of the transformer.  To start with, lets revise the facts about tertiary winding and then we will understand why it is so.

1.Tertiary winding is always connected in delta.
2. It is never used on the transformer where any of the windings (primary or secondary) are connected on delta.
3. It is never used on the transformer with 3 limbed core (implies used on transformer with 5 limbed core or 3 single phase type construction).

To understand 1st and 2nd point,

• Due to non linearity of the core, we can’t generate the sinusoidal flux in the core by applying sinusoidal voltage.
•  Without sinusoidal flux in the core, the secondary induced EMF will not be sinusoidal. This will make transformer unsuitable to supply single phase loads on secondary.
• To make flux in the core sinusoidal, we need to apply EMF that has 3rd harmonic components in it.
• Star windings don’t help the 3rd harmonic currents (co-phasal nature). But delta winding does. 
• So if any of the windings are connected in delta, 3rd harmonic voltage will be available. 
• This voltage will induce sinusoidal flux in the core and secondary EMF will be sinusoidal which is most desirable.

To understand 3rd point,

• Like 3rd harmonic flux, 3rd harmonic currents are also co-phasal in nature. They need closed path within the phases.
• If you observe, the extra 2 limbs in the 5 limbed core provide that path. So 3rd harmonic flux exists and affects the induced secondary EMF.
• If the core is 3-limbed, the 3rd harmonic flux has no closed path within the phases. So it cant flow and affect the output.

Even though we are connecting source the gives output at fundamental frequency only, from where are we getting the 3rd harmonic EMF in delta is very interesting concept. We will soon talk about it.  Also if we don’t connect the tertiary winding in the star-star 5 limbed core, problem of floating neutral will happen. You can search for above on your own.

Thank you for your time and please feel free to comment about your doubts and suggestions. You can mail me at dnachiketa1010@gmail.com about your areas of interest.

interview question explained-7

Hi folks.

There are some areas in our branch where, if we apply basic concepts, we end up with mined blowing results. Let us talk about one of such concepts.So here comes one of such concepts.

Q. there is a delta-star connected power transformer. A L-G fault occurs on the secondary of the transformer. What will be the fault sensed on the primary side?

Answer: For sure it’s not L-G. Let us see what it is.

As we all know, L-G fault is associated with 3 symmetric components which are positive, negative and zero. These 3 components pass through star winding of the transformer and enter delta. In delta winding, the zero sequence components circulates and only positive and negative get chance to pass. So the device connected on primary will sense a fault that relates positive and negative components that is L-L fault.  So L-G fault on the primary of the transformer will be sensed as L-L fault on the secondary of the transformer.

The Basics:  Why did the zero sequence components pass through star winding but were blocked by delta winding?
Straight forward answer is, they are in the same phase (phase difference is zero).  If you carefully observe the 3 phase wave form, when 1 phase is at its maximum, the rest are at half of the negative maximum. It means one phase in incoming, and rest 2 combining acting as a return path. For zero sequence, as there is no phase difference, all the maxima will be happening at the same time instance. So they fail to flow through star connections (and line conductors). Yet in delta connection, they get a closed path in the phases (draw diagram of star and delta, you will get it). So they circulate within the phases and cause serious overheating issues.

There are some cases where zero sequence currents escape delta. Imagine a 3-phase arc furnace. You can’t assure that arc will be taking place between 3 electrodes all the time. If arc takes place between only 2 electrodes, there will be supply imbalance enabling zero sequence currents to come out of delta.

Thank you for your time and please feel free to comment on your views. You can mail me your doubts and suggestions on dnachiketa1010@gmail.com 

Behavior of inductor and capacitor with varying supply frequency

Hi folks.
For last two days I was in the clutches of the HV project and didn't get time to post. Yesterday a friend of mine asked me to post about working of the inductor and capacitor at very high and very low frequency. Thank you Sumit for this opportunity and helping me to make this whole thing a 2 way communication.:)

Well, Let us talk about ideal inductor and capacitor first. 
The ideal inductor behaves as short circuit at lower frequencies and open circuit at very high frequencies. It is obvious as the inductive reactance is directly proportional to the frequency of supply.  You can understand this as inductance is the property due to which element is able to oppose the change in current flowing through it. 
The ideal capacitor behaves as open circuit at lower frequencies and short circuit at very high frequencies. The capacitive reactance is inversely proportional to the frequency of supply. Let us go into the basics of this phenomenon.

Imagine that you have 2 metal plates separated by a dielectric. Say capacitor is charging from 0V. When 1st +ve charge comes on plate 1 it induces equal and opposite charge on plate 2. When 2nd +ve charge comes, it is opposed by the 1st charge and the charge induced by this 2nd will be opposed by charge already present on plate 2. Now 3rd charge will be opposed by 2 charges. So if you carefully observe, 

1. Capacitor is opposing unidirectional flow of charges. (This explains it is open for DC).
2.Capacitor tends to saturate for unidirectional flow of charges which is what we call charging of capacitor.
3.The first charge did not face any opposition which explains an uncharged capacitor is SC at t=0+.

The practical inductor has inter-turn capacitance that acts as SC and creates problems at high frequency operation.  It exists due to the potential gradient between the windings and air as dielectric. The Aryton windings are used to reduce this (using loosely wound turns- increase d and reduce capacitance).
The practical capacitor has inter-turn inductance due to the circular arrangement of the metal foil and dielectric. This inductance will keep quiet at lower frequencies but take over the capacitive properties at higher frequencies.

So there are complex construction constraints for power capacitors and inductors. In short, Low frequency gives power in hands of capacitor and high frequency gives power in the hands of inductor. In the middle somewhere, our poor champ resistor lies, where resonance takes place. We will talk about it soon.

Thank you for your time and please feel free to leave comments about your views or extra information.  You can mail me at dnachiketa1010@gmail.com

Inductors demystified.

Hi folks.
Let us talk about the dual of capacitor - inductor. Transmission line inductance plays a very vital role in terms of stability of the system.

Suppose you have a simple series circuit of a inductor, a switch, a DC source and a bulb. What happens when you close the switch, the bulb goes on burning brightly and then attains maxima. When you open the switch, the bulb burns very brightly and then quickly goes out.

The reason for this strange behavior is the inductor. When current first starts flowing in the coil, the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, current can flow normally through the wire. When the switch gets opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses (and energy gets dumped in bulb). This current keeps the bulb lit for a period of time even though the switch is open (mechanically). In other words, an inductor can store energy in its magnetic field, and an inductor tends to resist any change in the amount of current flowing through it.
(The application of above concept can be seen in the power electronic circuits where inductor forces current through the reverse biased switches and so they fail to turn off.)

Henries
The capacity of an inductor is controlled by four factors:
The number of coils - More coils means more inductance.
The material that the coils are wrapped around (the core)
The cross-sectional area of the coil - More area means more inductance.
The length of the coil - A short coil means narrower (or overlapping) coils, which means more inductance.
Putting iron in the core of an inductor gives it much more inductance than air or any non-magnetic core would.
The standard unit of inductance is the henry. The equation for calculating the number of henries in an inductor is:
H = (4 * Pi * #Turns * #Turns * coil Area * mu) / (coil Length * 10,000,000)
The area and length of the coil are in meters. The term mu is the permeability of the core. Air has a permeability of 1, while steel might have a permeability of 2,000.

Inductor Application: Traffic Light Sensors
We know inductance depends on the core material used. The sensor constantly tests the inductance of the loop in the road, and when the inductance rises (due to metal parts of car)  it knows there is a car waiting.

You cannot afford to forget this:
(We will go quite parallel to capacitor if you notice.)
The current flowing through an inductor can never change instantaneously. It is a current stiff element.


(CAUTION-  It is important to understand this as 2 pure voltage stiff elements and 2 pure current stiff elements should never be connected. Look for yourself in the VSI and CSI design constraints or Can you imagine what will happen if pure capacitor is connected across voltage source?)

If the rate of change of current is high, inductor will generate a voltage impulse (high magnitude for short duration).
If we draw graph of voltage across inductor and time, positive area (positive volt-Sec) and negative area (Negative volt-Sec) should be equal for 1 cycle.  This is called as volt-Sec balance theory. If you notice carefully enough, it is nothing but flux balance (Faraday's law).
Whatever may be the current through inductor, the energy stored by inductor from starting time to time instant ‘t’ is equal to the energy stored by the capacitor at time instant ‘t’. (again a very classic concept!)
Inductor connected DC source charges linearly (and not exponentially(RL circuit) – very common misconception) with slope Vdc/L.

Thank you for your time and please feel free to leave comments.
you can mail me your opinions on dnachiketa1010@gmail.com

power electronics-1

Hi folks.
Here we are going to talk about one of the 3 pillars of electrical engineering – Power electronics (other two being electrical machines and power systems). 
Semiconductor switches play a very vital role in any power electronic circuit. Let us talk about them first.

There are switches we use in home (the mechanical ones) that we operate conveniently with hands. We can’t use them in electrical circuits where we need high precision and speed of operation (this where relays fail). Functionality would be same if you compare.
What do we exactly want from a switch?
It should have high break down voltage (voltage it can sustain without damaging when reverse biased).
It should have low conduction loss (ON state voltage drop across it should be minimum).
It should have fast transition from On to Off and vice versa to achieve higher switching frequency.

We have some switches that use lightly doped semiconductor on either side of the PN junction to achieve high break down strength (lesser the impurity, more the force needed to throw them across barrier). But lightly doped materials have higher resistance and higher ON state voltage drop (lesser number of charges available to flow). Let us call these devices as GROUP 1(G1).

So overcome this issue we (the electrical engineers) invented a phenomenon called conductivity modulation. (Understand it as some process and for those who have got extra interest- it a phenomenon where p and n- junction is there and hole injection takes place in n- causing voltage peak in turn on characteristics). So due to this process turn loss reduces. Turn of process is carried out by injecting charges and Turn off process is carried out by removal of charge. But this takes time and reduces the speed of operation. Let us call them GROUP 2 (G2).

G1 have no conductivity modulation and hence they can operate at higher switching frequency than G2. 
G1 have very high losses if operated for high power applications and hence most commonly used for low power applications. G2 have low power loss and hence used for high power applications.
G1 are majority carrier device and G2 are minority carrier devices (due to conductivity modulation). 

G1 devices = MOSFET
G2 devices = POWER DIODE, SCR, GTO (Notice the presence of p and n- junction as well as voltage peak in turn ON characteristics of these devices).

Comparing, MOSFETs are used for high frequency (in MHz) and low power applications. They have high ON state loss and are majority carrier devices. SCRs are used for high power (current rating in KA) and low frequency (less than 250 Hz). They have low ON state loss and are minority carrier devices.

Thank you for your time and please feel free to leave comments about your views. You can mail your doubts at dnachiketa1010@gmail.com

Capacitors demystified

Hi folks.
We all need to deal with capacitors every now and then. Whenever we get unexpected voltage boost ( say ferranti effect), 99% of times capacitance is responsible. To determine voltage rating of equipments and so many other reasons, study of capacitor is inevitable.

In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric. You can easily make a capacitor from two pieces of aluminium foil and a piece of paper. It won't be a particularly good capacitor in terms of its storage capacity (leaking charge issues), but it will work.
In theory, the dielectric can be any non-conductive substance. However, for practical applications, specific materials are used that best suit the capacitor's function. Mica, ceramic, cellulose, porcelain, Mylar, Teflon and even air are some of the non-conductive materials used. The dielectric dictates what kind of capacitor it is and for what it is best suited. NASA uses glass capacitors to power up the space shuttle's circuitry and helps deploy space probes.

Air - Often used in radio tuning circuits
Mylar - Most commonly used for timer circuits like clocks, alarms and counters
Glass - Good for high voltage applications
Ceramic - Used for high frequency purposes like antennas, X-ray and MRI machines
Oil- Used in fan and long-time rating devices
Paper and electrolytic- many electronic circuits
Super capacitor - Powers electric and hybrid cars
(How super capacitors work-)

Even nature shows the capacitor at work in the form of lightning. One plate is the cloud, the other plate is the ground and the lightning is the charge releasing between these two "plates" due to dielectric breakdown.

Farad
A capacitor's storage potential, or capacitance, is measured in units called farads. A 1-farad capacitor can store one coulomb of charge at 1 volt.

To get some idea storing ability of capacitor, think about this:
A standard alkaline AA battery holds about 2.8 amp-hours.
That means that a AA battery can produce 2.8 amps for an hour at 1.5 volts (about 4.2 watt-hours - a AA battery can light a 4-watt bulb for a little more than an hour).
Let's call it 1 volt to make the maths easier. To store one AA battery's energy in a capacitor, you would need 3,600 * 2.8 = 10,080 farads to hold it, because an amp-hour is 3,600 amp-seconds.
If it takes something the size of a can of tuna to hold a farad, then 10,080 farads is going to take up a LOT more space than a single AA battery! Obviously, it's impractical to use capacitors to store any significant amount of power unless you do it at a high voltage.

Applications
 Electronic flash on a camera uses a capacitor - the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly. This can make a large, charged capacitor extremely dangerous - flash units and TVs have warnings about opening them up for this reason. (Capacitor holds and stores atmospheric charges. So terminals of power capacitors are always shorted through a resistor and never kept open for safety). They contain big capacitors that can, potentially, kill you with the charge they contain.
Big lasers use this technique as well to get very bright, instantaneous flashes.
Capacitors can also eliminate ripples and filtering, coupling. If a line carrying DC voltage has ripples or spikes in it, a big capacitor can even out the voltage by absorbing the peaks and filling in the valleys.
A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges. However, any alternating current (AC) signal flows through a capacitor is allowed. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing.

Capacitive touch screens
One of the more futuristic applications of capacitors is the capacitive touch screen. These are glass screens that have a very thin, transparent metallic coating. A built-in electrode pattern charges the screen so when touched; a current is drawn to the finger and creates a voltage drop. This exact location of the voltage drop is picked up by a controller and transmitted to a computer. These touch screens are commonly found in interactive devices and smart Phones.

History of the Capacitor
The invention of the capacitor varies somewhat depending on who you ask. There are records that indicate a German scientist named Ewald Georg von Kleist invented the capacitor in November 1745. Several months later Pieter van Musschenbroek, a Dutch professor at the University of Leyden came up with a very similar device in the form of the Leyden jar, which is typically credited as the first capacitor. Since Kleist didn't have detailed records and notes, or the notoriety of his Dutch counterpart, he's often overlooked as a contributor to the capacitor's evolution. However, over the years, both have been given equal credit as it was established that their research was independent of each other and merely a scientific coincidence.

The Leyden jar was a very simple device. It consisted of a glass jar, half filled with water and lined inside and out with metal foil. The glass acted as the dielectric, although it was thought for a time that water was the key ingredient. There was usually a metal wire or chain driven through a cork in the top of the jar. The chain was then hooked to something that would deliver a charge, most likely a hand-cranked static generator. Once delivered, the jar would hold two equal but opposite charges in equilibrium until they were connected with a wire, producing a slight spark or shock.

Benjamin Franklin worked with the Leyden jar in his experiments with electricity and soon found that a flat piece of glass worked as well as the jar model, prompting him to develop the flat capacitor, or Franklin square (Google it). Years later, English chemist Michael Faraday (only cool guy with 2 units named after him and you know them) would pioneer the first practical applications for the capacitor in trying to store unused electrons from his experiments. As a result of Faraday's achievements in the field of electricity, the unit of measurement for capacitors, or capacitance, became known as the farad.

You cannot afford to forget this:
The voltage across a capacitor can never change instantaneously. It is a voltage stiff element.
If the rate of change of voltage is high, capacitor will generate a current impulse (high magnitude for short duration).
If we draw graph of current through capacitor and time, positive area (positive Amp-Sec) and negative area (Negative Amp-Sec) should be equal for 1 cycle.  This is called as Amp-Sec balance theory. If you notice carefully enough, it is nothing but charge balance.
Whatever may be the voltage applied across capacitor, the energy stored by capacitor from starting time to time instant ‘t’ is equal to the energy stored by the capacitor at time instant ‘t’. (Very classic concept!)
Capacitor connected DC source charges linearly (and not exponentially(RC circuit) – very common misconception) with slope Idc/C.

Thank you for your time. Please feel free to comment about your views. You can mail me your doubts at dnachiketa1010@gmail.com

interview question explained-6

hi folks.
Let us talk about a question that will make us think - OMG! I never thought this way..!

Q. why all the electrical machines are based on magnetic field interacting with current and not electric field interacting with current/charged body? is it not possible or make or is there any other reason?

Answer - folks, it is possible to make machines that work on the principle of electrostatics. Few examples would be corona motor or Whimshrust generator we talked about. We can get useful energy output in the form of electrical energy (from 2nd). About 70% of the electrical energy we generate is converted back to mechanical so corona motor will be helpful there. So it is possible. Then why don't we replace the induction motor with corona motor?

The answer my friends lies in the values of epsilon and mu for air. Epsilon is permittivity of the air and indicates the ability of air to carry electric flux lines and is of the order 10^-12. Mu is permeability of air, indicating ability of air to carry magnetic flux and is of the order of 10^-7.  So ability of air to carry magnetic energy is far more (about 100000 times) than the ability to carry electric energy (flux).

The torque output produced by any machine is directly proportional to flux present in the air gap. So power output and energy efficiency of the machines based on magnetic fields will be high. So we go for those only.  The machines based on electric fields have specialized applications in various fields.

Thank you for your time and please feel free to comment about any extra information or doubts.
You can also mail me your doubts and suggestions about blog on dnachiketa1010@gmail.com

Interview question explained-5

Hi folks.
One of frequently asked interview question is-
Q. Why the symbol chosen for resistance is a zigzag line?

Answer- If same question were asked for inductor, you can answer it by inductor is basically a coiled conductor or for capacitor, It is two conducting plates separated by a dielectric- so we show plates.

Well, in network theory, we show straight line for a conductor with zero resistance. It basically indicates that current faces no problem for passing through it. To show the extra efforts put by current (that appear in the form of voltage drop), why not to increase the distance traveled by the current inside a resistor? So the resistor has a symbol where it indicates the opposition to the flow of current (as its basic definition says). 

Thank you for your time. Please feel free to comment about your views and doubts. 

Update about online courses

Hi folks.
Many of you might be aware of NPTEL website that has video (along with transcripts) and audio lectures for almost every engineering stream.
NPTEL (National Programme on Technology Enhanced Learning) is a joint initiative of the IITs and IISc. Through this initiative, they offer online courses and certification in various topics. 

ABOUT ONLINE COURSES-
Online courses are Free for all. 
There is Certification exam at the end For a nominal fee.
(THIS CERTIFICATION WILL BE EXTREMELY HELPFUL FOR YOU IN TERMS OF CAMPUS AND PLACEMENT)

HOW TO APPLY- 
Go to- https://onlinecourses.nptel.ac.in/explorer
Click on a course name to see more details about it.
This site uses your Google account for authentication. You will need to log in to register for any course.
After registration you will be able to see the course contents as and when they are made available by the course instructors.
Every course has weekly online assignments and video lectures. You can ask doubts to your course mentors directly. 
The certification exam is not mandatory.  You can learn the part you want and forget about exam!
The courses now available are -
1. Basic Electrical Circuits (very imp for us)
2. Introduction to programming in C (out of interest or for people wishing to work in software)

Thank you for your time and please feel free to comment about more useful websites.