interview questions electrical engg

7/29/2019 interview questions electrical engg

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Q1. What do you mean by harmonics?

Definition for a harmonic is-a sinusoidal component of a periodic wave or quantity having a

frequency that is an integral multiple of the fundamental frequency.

The main sources of harmonic component are the phase angle controlled rectifiers and inverters

due to the fast switching action and firing scheme.

Filters are used to eliminate these harmonics as they may cause overheating of capacitors and

nearby generators and interfere with telecommunication systems.

Q2.Hunting of a synchronous motor:

When the mechanical load is constant, the rotor settles down to an absolutely constant speed with

the torque angle fixed by the particular delivered power.

When there is a change in load the rotor speed changes momentarily until the torque angle

adjusts itself to the new power.

If the load increases, the rotor slips backwards to an increased torque angle, while reduction

causes the rotor to advance a smaller torque angle.

Due to the M.I of the rotor either it overshoots or undershoots the mean position.

During this overshooting and undershooting some K.E. is stored in the rotor which causes

oscillation of the rotor periodically.

This rapid forward and backward motion of the rotor as it resolves at the average constant speed

is called HUNTING.


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DISADVANTAGES:

Hunting is an objectionable characteristic of a synch motor as it produces severe mechanical

stress as well as great variation in current and power taken by the motor.

In order to prevent hunting, dampers (or) damping grids are employed. Damper windings areshort circuited; copper bars are embedded in the faces of the field poles of the motor.

Question: what is "p.u." in electrical engg and what is the advantage of p.u. quantities?

Answer: p.u. stands for per unit (p.u=actual value/base value).

Advantages:

wide variation of electrical parameters gets eliminated by p.u. representation

Since the p.u. impedances of any component remains the same on either side of a

transformer, a power system can be analyzed on a single voltage level as the different

voltage level vanishes.

Question: Give two basic speed control scheme of DC shunt motor?

Answer: 1. By using flux control method: in this method a rheostat is connected across the field

wdg to control the field current. So by changing the current the flux produced by the field wdg

can be changed, and since speed is inversely proportional to flux speed can be controlled.

2. Armature control method: in this method a rheostat is connected across armature wdg and by

varying the resistance the value of resistive drop (IaRa) can be varied, and since speed is directly

proportional to Eb-IaRa the speed can be controlled.

Question: what is meant by armature reaction?Answer: The action of armature m.m.f. on the main m.m.f. is known as armature reaction.

Question: Enlist the types of dc machines?

Answer: D.C. machines are classified into two types 1) separately excited 2) self excited which

is further classified into 1)series 2)shunt and 3)compound(which is further classified into

cumulative and differential).

Question: why, when birds sit on transmission lines doesn't get shock?

Answer: Its true that if birds touch the single one line (phase or neutral) they don't get electrical

shock... if birds touch 2 lines than the circuit is closed and they get electrical shock... so if a

human being touches single one line (phase) then he doesn't get shock .if he is standing on the

ground then touching the line (phase) he will get a shock because the ground on what we are

standing is like line (ground bed - like neutral) that means that human who touches the line

closes the circuit between phase and neutral.


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Question: what is ACSR cable?

Answer: ACSR means Aluminum conductor, steel reinforced, e.g. 7/24 ACSR conductor has 7

steel strands forming a central core, around which there are two layers of aluminum strands(24).

Question: what is electric traction?

Answer: Traction means transport system i.e. transport of men and material from one place to

other. Electric traction means use of the electricity for all these .electric traction services are

broadly classified to railways, trams, trolleys, battery driven and solar powered vehicles.

Question: Why is the starting current high in a DC motor?

Answer: In DC motors, Voltage equation is V=Eb-IaRa

(V = Terminal voltage, Eb = Back emf in Motor, Ia = Armature current, Ra = Armature

resistance).At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra, where Ra is very less value and

Ia becomes very high.

Question: What are the transformer losses?

Answer: TRANSFORMER LOSSES

There are mainly two kinds of losses in a transformer namely

1. Copper loss (ohmic loss) and 2.magnetic loss (core loss).

Copper losses are caused by the resistance of the wire (I2R).

Magnetic losses are caused by eddy currents and hysteresis in the core.

In addition to the core loss and ohmic loss the following two losses are also present in the

transformer stray load loss and dielectric loss.

Question: what is Ferranti effect?Answer: when receiving end voltage is greater than the sending end voltage due to no load or

light loading, this effect is known as Ferranti effect.

Question: types of cooling system in transformers?

Answer: 1. ONAN (oil natural, air natural)

2. ONAF (oil natural, air forced)

3. OFAF (oil forced, air forced)

4. ODWF (oil direct, water forced)

5. OFAN (oil forced, air forced)

Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:

Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence

it causes less losses.

Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3


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times, that is why current also reduces up to 3 times and hence less motor burning is caused.

In addition, starting torque is increased and it prevents the damage of motor winding.

State the difference between generator and alternator

Generator and alternator are two devices, which converts mechanical energy into electricalenergy. Both have the same principle of electromagnetic induction, the only difference is that

their construction. Generator persists stationary magnetic field and rotating conductor which rolls

on the armature with slip rings and brushes riding against each other, hence it converts the

induced emf into dc current for external load whereas an alternator has a stationary armature and

rotating magnetic field for high voltages but for low voltage output rotating armature and

stationary magnetic field is used.

Why AC systems are preferred over DC systems?

Due to following reasons, AC systems are preferred over DC systems:

a. It is easy to maintain and change the voltage of AC electricity for transmission and

distribution.

b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the

equivalent DC transmission

c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.

d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine

wave current will naturally tend to zero at some point making the current easier to interrupt.

How can you relate power engineering with electrical engineering?

Power engineering is a sub division of electrical engineering. It deals with generation,

transmission and distribution of energy in electrical form. Design of all power equipments also

comes under power engineering. Power engineers may work on the design and maintenance of

the power grid i.e. called on grid systems and they might work on off grid systems that are not

connected to the system.

What are the various kind of cables used for transmission?

Cables, which are used for transmitting power, can be categorized in three forms:

Low-tension cables, which can transmit voltage up to 1000 volts.

High-tension cables can transmit voltage up to 23000 volts.

Super tension cables can transmit voltage 66 kV to 132 kV.

Why back emf used for a dc motor? Highlight its significance.


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The induced emf developed when the rotating conductors of the armature between the poles of

magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the

conductor, when the armature rotates, is called back emf. Its value depends upon the speed of

rotation of the armature conductors. In starting, the value of back emf is zero.

What is slip in an induction motor?

Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed

of the rotor of an induction motor is always less than its synchronous speed. It is usually

expressed as a percentage of synchronous speed (Ns) and represented by the symbol S.

Explain the application of storage batteries.

Storage batteries are used for various purposes, some of the applications are mentioned below:

For the operation of protective devices and for emergency lighting at generating stations andsubstations.

For starting, ignition and lighting of automobiles, aircrafts etc.

For lighting on steam and diesel railways trains.

As a supply power source in telephone exchange, laboratories and broad casting stations.

For emergency lighting at hospitals, banks, rural areas where electricity supplies are not

possible.

Explain advantages of storage batteries

Few advantages of storage batteries are mentioned below:

Most efficient form of storing energy portably.

Stored energy is available immediately because there is no lag of time for delivering the stored

energy.

Reliable source for supply of energy.

The energy can be drawn at a fairly constant rate.

What are the different methods for the starting of a synchronous motor?

Starting methods: Synchronous motor can be started by the following two methods:

By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary

motor. Then rotor poles are excited due to which the rotor field is locked with the stator-

revolving field and continuous rotation is obtained.

By providing damperwinding: Here, bar conductors are embedded in the outer periphery of the

rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is

started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the


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rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving

field.

Name the types of motors used in vacuum cleaners, phonographic appliances, vending

machines, refrigerators, rolling mills, lathes, power factor improvement and cranes.

Following motors are used: -

Vacuum cleaners- Universal motor.

Phonographic appliances Hysteresis motor.

Vending machines Shaded pole motor.

Refrigerators Capacitor split phase motors.

Rolling mills Cumulative motors.

Lathes DC shunt motors.

Power factor improvement Synchronous motors.

State Thevenins Theorem:

According to thevenins theorem, the current flowing through a load resistance

Connected across any two terminals of a linear active bilateral network is the ratio open circuit

voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load

resistance and internal resistance of the network. It is given by Voc / (Ri + RL).

State Nortons Theorem

The Nortons theorem explains the fact that there are two terminals and they are as follows:

One is terminal active network containing voltage sources

Another is the resistance that is viewed from the output terminals. The output terminals are

equivalent to the constant source of current and it allows giving the parallel resistance.

The Nortons theorem also explains about the constant current that is equal to the current of the

short circuit placed across the terminals. The parallel resistance of the network can be viewed

from the open circuit terminals when all the voltage and current sources are removed and

replaced by the internal resistance.

State Maximum power transfer theorem

The Maximum power transfer theorem explains about the load that a resistance will extract from

the network. This includes the maximum power from the network and in this case the load

resistance is being is equal to the resistance of the network and it also allows the resistance to be

equal to the resistance of the network. This resistance can be viewed by the output terminals and

the energy sources can be removed by leaving the internal resistance behind

15. Explain different losses in a transformer.


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There are two types of losses occurring in transformer:

Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron

losses. Two types of iron losses are:

o eddy current loss

o Hysteresis loss.

These losses depend upon the supply voltage, frequency, core material and its construction. As

long as supply voltage and frequency is constant, these losses remain the same whether the

transformer is loaded or not. These are also known as constant losses.

Variable losses or copper losses: when the transformer is loaded, current flows in primary and

secondary windings, there is loss of electrical energy due to the resistance of the primary

winding, and secondary winding and they are called variable losses. These losses depend upon

the loading conditions of the transformers. Therefore, these losses are also called as variable

losses.

16. Explain different types of D.C motors? Give their applications

Different type of DC motors and their applications are as follows:-

Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is

suitable for constant speed drive, where high starting torque is not required such as pumps,

blowers, fan, lathe machines, tools, belt or chain conveyor etc.

Service motors: It has high starting torque & its speed is inversely proportional to the loading

conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low.

Therefore, motor is used in lifts, cranes, traction work, coal loader and coal cutter in coalmines

etc.

Compound motors: It also has high starting torque and variable speed. Its advantage is, it canrun at NIL loads without any danger. This motor will therefore find its application in loads

having high inertia load or requiring high intermittent torque such as elevators, conveyor, rolling

mill, planes, presses, shears and punches, coal cutter and winding machines etc.

Explain the process of commutation in a dc machine. Explain what are inter-poles and why they

are required in a dc machine.

Commutation: It is phenomenon when an armature coil moves under the influence of one pole-

pair; it carries constant current in one direction. As the coil moves into the influence of the next

pole- pair, the current in it must reverse. This reversal of current in a coil is called commutation.Several coils undergo commutation simultaneously. The reversal of current is opposed by the

static coil emf and therefore must be aided in some fashion for smooth current reversal, which

otherwise would result in sparking at the brushes. The aiding emf is dynamically induced into the

coils undergoing commutation by means of compoles or interpoles, which are series excited by

the armature current. These are located in the interpolar region of the main poles and therefore

influence the armature coils only when these undergo commutation.


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Comment on the working principle of operation of a single-phase transformer.

Working principle of operation of a single-phase transformer can be explained as

An AC supply passes through the primary winding, a current will start flowing in the primary

winding. As a result, the flux is set. This flux is linked with primary and secondary windings.

Hence, voltage is induced in both the windings. Now, when the load is connected to the

secondary side, the current will start flowing in the load in the secondary winding, resulting in

the flow of additional current in the secondary winding. Hence, according to Faradays laws of

electromagnetic induction, emf will be induced in both the windings. The voltage induced in the

primary winding is due to its self inductance and known as self induced emf and according to

Lenzs law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage

induced in secondary coil is known as mutually induced voltage. Hence, transformer works on

the principle of electromagnetic induction.

19. Define the following terms:-

Reliability,

Maximum demand,

Reserve-generating capacity,

Availability (operational).

Reliability: It is the capacity of the power system to serve all power demands without failure

over long periods.

Maximum Demand: It is maximum load demand required in a power station during a given

period.Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled

downtimes for preventive maintenance is called reserve-generating capacity.

Availability: As the percentage of the time a unit is available to produce power whether needed

by the system or not.

20. Mention the disadvantages of low power factor? How can it be improved?

Disadvantages of low power factor:

Line losses are 1.57 times unity power factor.

Larger generators and transformers are required. Low lagging power factor causes a large voltage drop, hence extra regulation equipment is

required to keep voltage drop within prescribed limits.

Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the

conductors will have to carry more current at low power factor. This requires a large conductor

size


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21. State the methods of improving power factor?

Methods of improving power factor:

By connecting static capacitors in parallel with the load operating at lagging power factor.

A synchronous motor takes a leading current when over excited and therefore behaves like a

capacitor.

By using phase advancers to improve the power factor ofinduction motors. It provides exciting

ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the

induction motor can be made to operate on leading power factor like an overexcited synchronous

motor.

22. State the factors, for the choice of electrical system for an aero turbine.

The choice of electrical system for an aero turbine is guided by three factors:

Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.

Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed

with simpler pitch- changing mechanism or variable speed with fixed pitch blades.

Utilization of electrical energy output: in conjunction with battery or other form of storage, or

interconnection with power grid.

23. What are the advantages of VSCF wind electrical system?

Advantages of VSCF wind electrical system are:

No complex pitch changing mechanism is needed.

Aero turbine always operates at maximum efficiency point.

Extra energy in the high wind speed region of the speedduration curve can be extracted

Significant reduction in aerodynamic stresses, which are associated with constant speed

operation.

24. Explain the terms real power, apparent power and reactive power for ac circuits and also the

units used.

Real Power: It is the product of voltage, current and power factor i.e. P = V I cos j and basic

unit of real power is watt. i.e. Expressed as W or kW.

Apparent power: It is the product of voltage and current. Apparent power = V I and basic unitof apparent power is volt- ampere. Expressed as VA or KVA.

Reactive Power: It is the product of voltage, current and sine of angle between the voltage and

current i.e. Reactive power = voltage X current X sinj or Reactive power = V I sin j and has no

other unit but expressed in VAR or KVAR.

Define the following: Average demand, Maximum demand, Demand factor, Load factor.


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Average Demand: the average power requirement during some specified period of time of

considerable duration is called the average demand of installation.

Maximum Demand: The maximum demand of an installation is defined as the greatest of all

the demand, which have occurred during a given period. It is measured accordingly to

specifications, over a prescribed time interval during a certain period.

Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the

rating of the connected load.

Load Factor: It is defined as the ratio of the average power to the maximum demand.

26. Explain forward resistance, static resistance and dynamic resistance of a pn junction diode.

Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called

forward-resistance.

DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across

the diode to the direct current flowing through it.

AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward

characteristic of the diode. It is the resistance offered by a diode to the changing forward current.

27. How does Zener phenomenon differ from Avalanche breakdown?

The phenomenon when the depletion region expands and the potential barrier increases leading

to a very high electric field across the junction, due to which suddenly the reverse current

increases under a very high reverse voltage is called Zener effect. Zener-breakdown or

Avalanche breakdown may occur independently or both of these may occur simultaneously.

Diode junctions that breakdown below 5v are caused by Zener Effect. Junctions that experiencebreakdown above 5v are caused by avalanche-effect. The Zener-breakdown occurs in heavily

doped junctions, which produce narrow depletion layers. The avalanche breakdown occurs in

lightly doped junctions, which produce wide depletion layers.

28. Compare JFETs and MOSFETs.

Comparison of JFETs and MOSFETs:

JFETs can only be operated in the depletion mode whereas MOSFETs can be operated in

either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier

injunction occurs and the gate-current is substantial. MOSFETs have input impedance much higher than that of JFETs. Thus is due to negligible

small leakage current.

JFETs have characteristic curves more flat than that of MOSFET is indicating a higher drain

resistance.

When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it

would be in a comparable MOSFET.


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30. Explain thin film resistors and wire-wound resistors

a. Thin film resistors- It is constructed as a thin film of resistive material is deposited on an

insulating substrate. Desired results are obtained by either trimming the layer thickness or by

cutting helical grooves of suitable pitch along its length. During this process, the value of the

resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of

resistance is obtained.

b. Wire wound resistorslength of wire wound around an insulating cylindrical core are known

as wire wound resistors. These wires are made of materials such as Constantan and Manganin

because of their high resistivity, and low temperature coefficients. The complete wire wound

resistor is coated with an insulating material such as baked enamel

31. What is a differential amplifier? Also, explain CMRR.

Differential Amplifier: The amplifier, which is used to amplify the voltage difference between

two input-lines neither of which is grounded, is called differential amplifier. This reduces the

amount of noise injected into the amplifier, because any noise appearing simultaneously on both

the input-terminals as the amplifying circuitry rejects it being a common mode signal.

CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain.

If a differential amplifier is perfect, CMRR would be infinite because in that case common mode

voltage gain would be zero.

3. Explain Marx circuit.

It is used with generators for charging a number of capacitor in parallel and discharging them in

series. It is used when voltage required for testing is higher than the available.

What is Resonant peak?

Answer:Resonant peak is defined as the maximum value of the closed loop transfer function.A large

resonant peak corresponds to large overshoot in the transient respose

What is Resonant Frequency?

Answer:The frequency at which resonant peak occurs is called the resonant frequency. Resonance

frequency explains about the speed of the transient response.

What is Cut-off rate?Answer:The slope of the log-magnitude curve near the cut-off frequency is called the cut-off rate.

The cut-off rate indicates the ability of the system to distinguish between the signal and the noise

What is Phase cross over frequency?

Answer:The frequency at which the phase of the open loop transfer function is 180o

is called the

phase cross over frequency.


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What is Gain Margin?

Answer:The Gain Margin is defined as the reciprocal of the magnitude of open loop transfer function

at phase cross over frequency. The gain margin indicates the amount by which the gain of the

system can be increased without affecting the stability of the system

Phase Margin?

Answer:The phase margin is the amount of additional phase lag at the gain cross over frequency

required to bring the system to the verge of instability.

Voltage and Frequency operating range (tolerance) of Grid Supply in India

Frequency variation in the power system exists due to the mismatch between the supply of power

and demand for the power. Voltage variations exist in the power grid is due to the mismatch in

the reactive power between demand (MVAR) and available.

In spite of all these variations there is certain limit for the operation limits (variations allowed)

for voltage and frequency parameters dictated by the Grid Code. Any variations in the

parameters (voltage and frequency) below operating limits considered as power grid is unhealthy

and restoration steps will be taken to make the power grid healthy.

In India according to Electricity Grid Code, the operation variation in the frequency and voltage

allowed is given as below.

Frequency Operating variation limits:

49Hz to 50.5 Hz

Voltage operating variation limit:

Voltage Rating Maximum Voltage Minimum Voltage

400 kV 420 kV 360 kV

220 kV 245 kV 200 kV

132 kV 145 kV 120 kV

Islanding operation in Electrical Grid

Frequency and Voltage are two important parameters which decides the healthiness of the

electrical grid at a particular instant of time.


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Frequency fluctuations occur in the electrical grid when there is mismatch between the amount

of power generation and amount of power demand. Both under frequency and over frequency

operation is harmful to the operation of power system.

In electrical grid over frequency operation is a rare phenomenon. Most of the cases we observe

frequency drop phenomenon due to lack of power generation supporting the demand. Hencefrequency in the power system drops below the rated.

When the frequency tends to drop governor systems connected to the Turbo-Generators in

electrical grid will act and allow more steam to flow to turbine and increase the speed. However

there is limitation for this band. Beyond certain limit of operation if grid frequency is drop

Islanding operation is employed in the grid.

Islanding Operation:

When the frequency of the overall electrical grid is declining below the rated at pre determined

frequency Electrical grid is divided into number of small islands which contains some generation

units and load centers come out of the grid and operate together until the normal frequency is

restored. This method of splitting the electrical grid in to small Islands is to protect the healthy

part of the grid.

Properties of Sulfur Hexafluoride (SF6) gasSulfur hexafluoride gas (SF6) gas properties: Heavy, chemically inert, non toxic

No poisonous effect on the human body but decomposition products are poisonous

Color less and odor less

It is gaseous at normal room temperature and pressure

Density is about 6.6g/l at 20oC (5 times denser than air)

Critical temperature is at 45.6oC and can be liquefied by compression

Very good insulant with high dielectric strength

SF6 gas is electo-negitive ( tends to attract the free electrons and has thearc quenching property). Because of this main reason SF6 gas is used for arc quenching andinsulation medium in circuit breakers.

The gas is highly stable

Unlike solid insulation materials, electrical breakdown of sf6 gas does not result inpermanent deterioration of its properties

Decomposition occurs on the exposure to the electric arc. (Disassociation products will beSF2 and SF4 lower order fluorides)

Why Surge Tank require in Hydro Power PlantSurge tank is a small storage tank or reservoir required in the hydro power plants for regulating the

water flow during load reduction and sudden increase in the load on the hydro generator (water flow

transients in penstock) and thus reducing the pressure on the penstock. This is the advantage of

surge tank.
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Water Hammering effect:When the load on the generator is reduced suddenly (load throw-off), governor closes the turbine

gases and thus create an increase in pressure on the penstock. This may result in water

hammering effect" and may require pipe of extra ordinary strength this pressure, otherwise the

penstock may burst. To avoid this positive water hammering pressure some means is provided to

take the rejected flow of water. This can be achieved by providing small storage reservoir or tank toaccommodate this rejected flow. This small storage reservoir is surge tank and located close to the

power station.

Sudden Increase and Decrease in Load:When load demand on the generator decreases, it leads to rise in the water level in the surge tank.

This produces a retarding head and reduces water velocity in the penstock. The reduction in the

velocity to the desired levels, makes the water in the tank to rise and fall until oscillations are

damped out.

When load demand on the generator increases, governor opens the turbine gates in order to allow

more water flow through the penstock to supply the increased load demand thereby creating a

negative pressure or vacuum in the penstock. This negative pressure in the penstock createsnecessary acceleration force and is objectionable for very long conduits due to difficult turbine

regulation. Under this condition additional water flows from the surge tank. As a result the water level

in the surge tank falls, an acceleration head is created and flow of water in the penstock increases.

Thus surge tank helps in stabilizing the velocity and pressure in the penstock and protects penstock

from water hammering and negative pressure or vacuum.

Different Tests conducted on TransformersSome of the difficulties encountered in testing large transformers are:

For carrying out tests for large transformer huge amount of energy is wasted

For carrying out direct tests by loading the transformer large amount of load is to beavailable at the test center

Therefore the performance characteristics of the transformer can be computed from the

equivalent circuit parameters which in turn may be obtained by carrying out simple tests on the

transformer.

Some of the tests that are conducted on transformers are:

PolarityTest:Polarity test on the transformer helps to determine the same instantaneous polarity of the

transformer. The relative polarities of the primary and secondary terminals at any instant must

be known for connecting the windings of the same transformer in parallel
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Open Circuit Test:This test isalso called as no load test. This test helps to determine the core losses in the

transformer.

Short Circuit Test:

Short Circuit test of a transformer helps to determine the full load copper losses for a giventransformer.

These open circuit and short circuit tests are carried out not by directly loading on the

transformer. Core Losses and Copper Losses can be determined from the open circuit and

short circuit tests respectively. Therefore by carrying out these two indirect tests efficiency at

different loads and regulation of the transformer can be determined for different power factors.

Sumpner's Test:This is also called back to back test. In open circuit and short circuit tests no information is

obtained about the temperature rise. In sumpner's test, two identical transformers are load to

the full power like regenerative test in dc machines, transformers draw only the power requiredto supply iron and copper losses of the transformers.

Insulation Materials used in Transformers and Induction Motors

Insulation materials used in transformers and motors are classified based on the temperature

withstand capability.

Some of the insulation classes used in transformers and induction motors are: Class Y, Class A,

Class E, Class B, Class F, Class H and Class C.

Class Y:

Maximum hot spot temperature for this type of insulator is 90oC

Some of the materials used are cotton, silk, paper, wood without impregnated

Class A:

Maximum hot spot temperature for this type of insulator is 105oC

Cotton, Silk, and Paper when suitably impregnated are the type of materials used

Class E:

Hot spot temperature is 120oC

Materials possessing degree of thermal stability allowing them to be operated at a temperature

15oC above higher than Class A. Ex: wire, enamel etc

Class B:


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Maximum hotspot temperature is 130oC

Mica, Glass fibre, asbestos, etc with suitable impregnation or coating substance

Class F:

Hot spot temperature is 155oC

Mica, glass, asbestos etc with suitable binding impregnation or coating substances capable of

withstanding 25oC higher than Class B

Class H:

Maximum hotspot temperature of 180oC

Combination of materials such as mica, glass fibre, asbestos suitably bonded

Class C:

Hotspot temperature above 180oC

Mica, porcelain, glass, quartz and asbestos with and without inorganic binder

1) Job Question: What are the different operation regions of the SCR?

Answer: SCR or thyristor will have three regions of operations based on the mode in which the

device is connected in the circuit.

Reverse blocking region: When the cathode of the thyristor is made positive with respect to the

anode and no gate signal is applied. In this region scr exhibits the reverse blocking

characteristics similar to diode.

Forward blocking region: In this region the anode of the thyristor is made positive with respect to

the cathode and no gate signal is applied to the thyristor. A small leakage current flow in this

mode of operation of the thyristor

Forward conduction region: when the forward voltage applied between the anode and cathode

increases at particular break over voltage avalanche breakdown takes place and thyristor starts

conducting current in forward direction. By this type of triggering the device damages the scr.

Hence a gate signal is applied before the forward break over voltage to trigger the scr.

2)

Job Question: What is Latching current?

Answer:Gate signal is to be applied to the thyristor to trigger the thyristor ON in safe mode.

When the thyristor starts conducting the forward current above the minimum value, called

Latching current, the gate signal which is applied to trigger the device in no longer require to

keep the scr in ON position.


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3) Job Question:What is Holding current ?

Answer:When scr is conducting current in forward conduction state, scr will return to forward

blocking state when the anode current or forward current falls below a low level called Holding

current

Note: Latching current and Holding current are not same. Latching current is associated with theturn on process of the scr whereas holding current is associated with the turn off process. In

general holding current will be slightly lesser than the latching current.

4) Job Question:Why thyristor is considered as Charge controlled device?

Answer:During the triggering process of the thyristor from forward blocking state to forward

conduction state through the gate signal, by applying the gate signal (voltage between gate and

cathode) increases the minority carrier density in the p-layer and thereby facilitate the reverse

break over

of the junction J2 and thyristor starts conducting. Higher the magnitude of the gate current

pulse, lesser is the time required to inject the charge and turning on the scr. By controlling theamount of charge we can control the turning on time of the scr.

5) Job Question: What is the relation between the gate signal and forward break over

voltage (VBO)?

Answer:Thyristor can be triggered by increasing the forward voltage between anode and

cathode, at forward break over voltage thyristor starts conducting. However this process may

damage the thyristor, so thyristor is advices to trigger on through the gate pulse. When a gate

signal is applied thyristor turns on before reaching the break over voltage. Forward voltage at

which the thyristor triggers on depends on the magnitude of the gate current. Higher is the gate

current lower is the forward break over voltage

6) Job Question: What are the different losses that occur in thyristor while operating?

Answer:Different losses that occur are

Forward conduction losses during conduction of the thyristor

Loss due to leakage current during forward and reverse blocking.

Power loss at gate or Gate triggering loss.

Switching losses at turn-on and turn-off.

Different Levels of SCADA?

Answer: Different levels are SCADA systems are:

Field level devices and instruments eg: Sensors

Remote Terminal Units (RTUs)

Communication system present between the RTUs and Main Station


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Main Control Station or Master Station

Computer systems pertained to processing the data for displaying to operator

Why SCADA systems are implemented?

Answer: Some of the reasons or advantages why SCADA systems are implemented are:

SCADA systems improves the performance of the operation of the plant

SCADA systems provides better protection to the equipment of the plant

SCADA systems improves productivity of the personnel

Information receives very fast, process the information and display it to opertor in graphs and

plots. Hence helps the operator to take the decisions fast.

Provides better energy savings and saves economy.

What is the capacity of CPU of Remote Terminal Unit (RTU)?

Answer: Remote Terminal Units (RTUs) contain generally microprocessor having 16 bit or 32

bit (eg: 68302 or 80386)

What is the Power Supply provided to the RTU?

Answer: Remote Terminal Units (RTUs) are operated from 110V/240V + or - 10% and 50Hz or

12/24/48V + or - 10% DC Voltage

What are advantages of PLCs over RTUs?

Answer: Advantages of Programmable Logic Controllers (PLCs) are they can be used for

general purpose can easily set for verity of functions. Some of the advantages of PLCs over

RTUs are: PLCs are reliable, cheap, flexible compared to RTUs, sophisticated control and

troubleshooting in PLC is easier.

Master Terminal Units (MTU) in SCADA systems

Master Terminal Units (MTU) role:

Master terminal units (MTU) in SCADA system is a device that issues the commands to the

Remote Terminal Unit (RTUs) which are located at remote places from the control, gathers the

required data, stores the information, and process the information and display the information in

the form of pictures, curves and tables to human interface and helps to take control decisions.

This is the operation of the Master Terminal Unit (MTU) located in the control center.


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Communication between the MTU and RTU is bidirectional, however the major difference is

RTU cannot initiate the conversation, an RTU simply collects the data from the field and stores

the data. Communication between the MTU and RTUs are initiated by the programs with in the

MTU which are triggered either by operator instructions or automatically triggered.99% of the

instructions and messages to the RTUs from MTUs are automatically triggered. When Master

Terminal Unit (MTU) asks the desired information RTU sends it. So MTU is considered as

master and RTU is the Slave. After receiving the data required MTU communicates to the

printers and CRTs which are operator interface through necessary protocols. At this level of

communication it will be of the form peer-to-peer communication rather than master slave

communication unlike communication between MTUs and RTUs. Thus in SCADA system

Master Terminal Unit (MTU) acts as heart of the system.

What happen When Synchronous Generator loss Excitation?

During normal operation of the Synchronous Generator or Alternator excitation to the rotor is

provided by the small DC generator running on the same shaft along with main generator. When

synchronous generator delivering power to the grid or load it delivers both real power and

reactive power to the system. Reactive power is the magnetizing power which helps to deliver

the real power from source to the load and also reactive power helps to improve the voltage of

the system. Almost all the loads (except capacitance loads) in the power system draws reactive

power from system. Alternators, synchronous condensers and capacitor banks connected in the

power systems are the only sources of reactive power.Real power delivered by the generator

(MW output power) is governed by the prime mover and the Reactive power delivered (MVAR

output ) is governed by the field excitation. When field excitation to the synchronous generator is

lost then synchronous generator operates as induction generator and instead of delivering

reactive power it absorbs the reactive power from the system as much as 2 to 4 times the

generator's rated load. However the real power (MW output) delivered by the induction generator

will remain almost the same as this is controlled by the prime mover.But loss of generator's

reactive power in the system may cause instability to the system. Also, as alternator is operating

as induction generator, high currents are induced in to the rotor teeth and wedges and may

damage the rotor of the generator. However large alternators are designed to withstand this

induced currents.Immediate tripping of alternator in the case of loss of excitation is not necessary

unless the terminal voltage of the generator falls below desired limit due to voltage collapse.

Loss of excitation relay (40G) is used for sensing the loss of excitation.

1. Which type of motors are mostly used in industrial applications. Reasons for that?

2. What are the different speed control methods employed in dc motors?


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5. What is the principal of motor?

6. What is power factor?

7. Should power factor be low or high?

8. Difference between field rheostat and armature rheostat.

9. Why field rheostat is kept in minimum position?

10. Why armature rheostat is kept in maximum position?

11. What is meant by derating factor?

12. What is stiffness factor?

13. What is the dependency of stiffness on load angle??

14. What is 100% protection of generator? Why 100% is not used but 95% is generally

used?

15. What is the difference between a Verilog task and Verilog function?

16. What is the unit of magnetic flux density?

17. Why increase in current leads to increase in conductor temperature?

18. How can a equal potential zone be carried out in conductors?19. What is essential to prove safe isolation of electrical circuit?

20. What is the ratio of true power to apparent power in an AC circuit?

21. What is power relay?

22. Differentiate between power relay and reverse power relay.

23. What is the suitable transmission voltage?

24. What is the maximum operating temperature for a thermoplastic insulated cable?

25. Explain the working of variable frequency transformer?

26. Which oil can be used in transformer?

27. What is excitation in case of DC motor?

28. Why is the starting current high in DC motor?29. What is rotary phase converter?

30. Differentiate between digital phase converter and ordinary phase converter.

31. What is knee point voltage?

32. List the advantages of star-delta starter with induction motor?

33. Which type of transformer is used for lighting loads?

34. What is star-delta transformer?

5. Why we cannot generate power at high voltage?

6. Difference between alternators employed in thermal plants and hydro plants?

7. What is an equal area criterion?

8. What is the significance of ABCD parameters?

9. What is skin effect and methods to reduce skin effect?

10. Why busbars in EHV substations are hollow bars, what is the reason?


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11. Why stones are employed in the switchyard?

13. What is the use of salt while earthing a power system?

14. Different methods to turn on the SCR? Which method is best suited, why?

15. What is SCADA system?

16. What is the use of silica gel in the transformer?

15. Why we use test signals in control systems?

16. Difference between the open loop control system and closed loop control system?

17. What is Race around condition?

18. Why inductors are employed in power electronic circuitry?

20.What is Reactance relay, how they operate?

2. What is diversity factor in electric installations?

1. What are the different conditions for parallel operation of Transformers?

2. What are V and inverted V curves in Synchronous motor and their significance?

3. Explain electrical corona and methods to reduce the electrical corona?

4. Explain about P, PI and PID controller?Their advantages and disadvantages?

5. What are the different braking methods used in dc motors

6. What is water hammering phenomenon?

7. Advantages of DC motor over AC motors?

8. What is Load factor, Demand factor, Installed capacity, spinning reserve?9. Which power plants can be used as a peak load plants. Reason?

10. What is String efficiency in insulators?

11. What is Transient stability and methods to improve transient stability?

12. What is single line diagram and the assumptions required?

13. What is Step Potential and Touch potential in substations?

14. What are different switching schemes employed in substations?

15. How Speed of DC shunt motor is controlled?

16. What is the difference between Microprocessor and Microcontroller?

17. What is SIL and its significance

18. Realize Ex-OR using NAND gates19. What is Race around condition

20. Why single phase induction motor is not self starting and methods to start single phase

induction motor?

interview questions electrical engg

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