power system interview question

7/28/2019 Power System Interview Question

1/10

DEPARTMENT OF ELECTRICAL AND ELECTRONICS

ENGINEERING

POWER SYSTEM INTERVIEW QUESTION

Q1. What is the Power Station ?

A1. Power Station is a place where Power is Generated and

Transmitted , eg. Tharmal power station, Hydel power station,

Nuclear power station etc.

Q2. What is Slack Bus ?

A2. Slack Bus is the responsible for the delivering losses in the power

system. in this bus the magnitude of voltage and phase angle are

specified. This bus is first to respond to a changing load condition.

Q3. what is critical disruptive voltage.

A3. The minimum voltage at which the breakdown of the insulating

properties of air occurs and corona start is called D C V .

Q4. How many types of fault occurs in the 3 P power system ?

A4. Faults in the 3 P system can be classified under the following :-

a. single line to ground (LG) fault

b.line to line (LL) fault

c. Double line to ground (LLG) fault

d. Thee Phase short circuit (LLL) fault

e. three phase to ground (LLLG) fault

Q5. How many methods uses for the load flow solution ?

A5.


2/10

1. Gauss-seidel method using Ybus2. Newton- Raphson method (NR)3. Power flow through line and slack bus power4. Decouple load flow method5. Fast Decouple load flow method.

Q6. What is skin effect ?

A6. When the AC passed through the conductor the density of the

current is not uniform over the cross-section of the conductor, the

density of the current is very high near the surface of the conductor

and very low at middle this effect is called skin effect. this effect isonly in the ac.

Q7. What is the Bundle conductor and why is is used ?

A7. A bundle conductor consists of two more parallel subconductors

at a spacing of several diameters.this is use to reduce the corona and

skin effectQ8.What is meant by Tine Graded Protection?

A8.Time Graded protection is a scheme of over current protection, in

which the discrimination is incorporated i.e, the time setting of the

relays is so graded that in the event of the fault, the smallest part of

the system is disconnected

Q9What are the main elements of current carrier protection?

A9.The main elements of the carrier current protection are:Transmitter, Receiver, Coupling equipment and Line trap

Q10.Why Merz-Price Protection is not suitable beyond 33kV?

A10.:Difficulties are experienced in balancing the secondaries of the

CTs and this is why Merz-Price protection is not employed beyond

33kV


3/10

Q11.What types of relays are suitable for the protection of radial

feeders?

A11.Induction type IDMT relays are more suitable for protection of

the radial feeders because the time current characteristics are similar

in shape and in no case they cross each other at any time

Q12.What is the purpose of Line trap and Coupling capacitors in

carrier channels?

A12.Line trap is an LC network inserted between the busbar and

connection of coupling capacitors to the line and tuned to resonance at

the high frequency and are used to confine the carrier currents to the

protected section so as to avoid interference with or from adjacent

carrier current channel.Coupling capacitors is used to connect the high frequency (carrier)

equipment to one of the line conductors and simultaneously serves to

isolate the carrier equipment from high power line voltage

Q13.What is Unit Protection?

A13. Unit system of protection is one in which the protection

responds to the faults in the protected zone alone and it does not

respond to through faults (faults beyond the protected zone). Noneunit systems does not have zone boundry

Q14.What is the minimum time interval set for two adjacent circuit

breakers to operate in radial feeders?

A14.It will be usually around 0.4 sec

Q15.Why Over Current Protection is not necessary for modern

generators?A15. Over Current protection is not considered necessary for modern

alternators because these are capable of withstanding a complete short

circuit at their terminals for sufficient time without much over heating

and damage

Q16.What type of protection is provided for the generators against

over heating of the generator stator?

A16. Resistance temperature detector


4/10

Q17.Which type of relays are used for the Merz-Price protection

system for alternator?

A17.Merz-Price protection is differential protection provided for the

alternator. The relays used in the Merz-Price protection system of

alternator are instantaneous electro-magnetic type protection

Q18.Why large alternator is grounded with large resistance?

A18. Large capacity of alternators are typically provided with

resistance grounding. High value of resistor is connected to the neural

path to the ground. If the generator is delta connected, then it is

grounded with the help of zig-zag transformer or (Delta-Star)

transformer such that a high resistor is connected between the neutralpoint and the ground. Resistance grounding is provided so as to limit

the short circuit current flowing during earth fault to stator winding in

order to provide protection against mechanical stresses and melting of

winding during Line to Ground short circuit. In large generators fault

current is limited as low as 10 to 15 amperes during short circuit.

Q19.Why it is not necessary to provide protection for turn to turn fault

in alternator?A19. The coils of the modern alternators are single turn and therefore

it is not necessary to provide protection for turn to turn faults

Q20Why it is necessary to suppress field immediately after

disconnection of faulty alternator from the system?

A20. In the event of fault on the generator windings even though the

generator circuit breaker is tripped, the fault is continuous to fed as

long as the excitation will exist because the emf is induced in thegenerator. Hence it is necessary to suppress the field immediately

after disconnecting the faulty generator from the system

Q 21 Why it not necessary to provide over-voltage protection in

turbo-alternator?


5/10

A21. The over voltage occurs when the prime mover speed increases

due to sudden loss of the load on the generator and the speed control

governors in case of turbo-generators are very sensitive to the speed

variations and therefore generator over voltage of significant duration

or magnitude does not generally occur. This is the reason why

generators are not provided with over voltage protection

Q22.Where does Negative phase sequence relay is employed?

A22Negative sequence relay is employed for the protection of

generators and motors against unbalanced loading that may arise due

to phase to phase faults

Q23.What is the operation principle of differential relay?A23.A differential relay operates when the phasor difference of two

or more similar electrical quantities exceeds a pre-determined amount

Q24.Why distance protection is preferred as primary protection

compared to over current protection for transmission lines?

A24. Distance relay is superior to over current protection for the

protection of transmission lines. Some the reasons are faster

protection, simpler coordination, simpler application, permanentsettings without need for readjustment, less effect of the amount of

generation and fault levels, fault current magnitude, permits the high

line loading

Q25.Why biased differenial protection is preferred over simpledifferential protection?

A25. Biased differential relay is preferred because its operation is not

affected by the trouble arising out of the difference in the CTs ratios

for high values of external short circuit currents

Q26.Where Impedance relay, Reactance relay and Mho relays are

employed?


6/10

A26. The Impedance relay is suitable for the phase faults relaying for

the lines of moderate lengths Reactance type relays are employed for

the ground faults while Mho type of relays are best suited for the long

transmission lines and particularly where synchronizing power surge

may occur

Q27.What is percentage differential relay?

A27.It is a differential relay where the operating current required to

trip can be expressed as a percentage of load current

Q28.What are the main circuits in the Thermal Power Plant?

A28.Thermal Power plant consists of four main circuits, they are:

Feed water and steam flow circuit Coal and ash circuit Air and gas circuit Cooling water circuit

Q29.Steam power plant works on which cycle?

A29. Steam power plant works on the principle of Rankine Cycle

Q30.What is the Thermal efficiency of steam power plant?

A30. Thermal efficiency of steam power plant is defined as the ratio

of heat equivalent of mechanical energy transmitted to the turbine

shaft to the heat of combustion. Generally Thermal efficiency of the

steam power plant will be in the range of 30-35%

Q30.What is the overall efficiency of the Thermal Power Plant or

Steam Power Plant?

A30. Overall efficiency of the system is defined as the ratio of heat

equivalent of electrical output to the heat of combustion. Generally

Overall efficiency of the steam plant will always be less than the

thermal efficiency of the steam plant, it will be of the order of 29-33%


7/10

Overall efficiency of steam plant is determined by multiplying the

thermal efficiency of the plant with efficiency of the generator

(electrical efficiency)

Q31.Why the Thermal efficiency of the steam power plant is quitelow?

A31. In Steam power station, more than 50% of the total heat of

combustion is lost as heat rejected to the condenser and the loss is

unavoidable as the heat energy cannot be converted in to mechanical

energy with out a drop in temperature. Steam in the condenser is at

lowest temperature. This is the reason that the thermal efficiency of

the power plant is quite low.

Q32.On what factors efficiency (thermal) of the steam plant depends?

A32. Efficiency of the thermal plant depends on three factors, they are

1.pressure of steam entering the turbine2. temperature of the steam entering the turbine3.pressure in the condenser

Thermal efficiency increases with increase in temperature and

pressure of the steam entering the turbine. For this reason high

temperature and pressure are used. Thermal efficiency is effectively

increased by decreasing the pressure in the condenser, so pressure in

the condenser is kept as low as possible.

Thermal efficiency also increases by reheating the steam between

turbine stages

Q33. What for series and shunt compensation provided in EHV

transmission lines?

A33. Series capacitance is provided in EHV lines to artificially reduce

the series reactance of the line so as to improve stability, voltage

regulation and transmission efficiency


8/10

Shunt compensation is provided to artificially reduce the line

susceptance so as to improve the voltage regulation under light load

condition

Q34. What is the material used for overhead transmission lines?

A34. ACSR conductors are employed.

Q35. What are the problems associated with EHV transmission?

A35: The problems associated with EHV transmission are corona loss

and radio interference, requirement of heavy supporting structures and

insulation requirments

Q36 Why does surge impedance loading (SIL) increase with increasein voltage level?

A36: SIL varies as the square of the operating voltage, so SIL

inceases with increase in voltage level

Q37. What are the factors that limit the maximum power transfer

capability in a transmission line?

A37: Some of the factors which limits the maximum power transfer

are:

1.Electrical phase shift2.Voltage drop3.Thermal effects in the line

Q38. Explain some of the methods to improve the strength of

transmission system?A38.

Introducing or adding of new transmission lines in to the systemto avoid the overloading of the existing lines

Application of devices such as series capacitors in the rightlocation of power system which helps in increase in the power

transfer capability

Up gradation of the existing transmission system


9/10

Adoption of multi-voltage levels an multi-circuits in the existingac transmission system

Q39. Why phase shift is kept low for transmission of power for largedistances?

A39.

E1 X E2

Power delivered P = --------- Sin

X

Generally is kept low (around 30o) because any disturbance can

affect the stability of the system if value is high.

Q40.How energy is generated in Nuclear power plant?

A40. In Nuclear power plant energy is released by nuclear fission.

Nuclear fission is the process in which heavy nucleus such as233

92U,235

92U,and239

94Pu when bombarded by certain particles ( protons,

neutrons, X-rays), the heavy nucleus will split into two or more

smaller nuclei, ejection of two or more neutrons and liberation of vast

amount of energy. Therefore this split of heavy nucleus into two or

more smaller nuclei is calledfission process.

Q41.What are the particles used in nuclear fission?

A41. High energy alpha particles, protons, deuterons, X-rays as well

as neutrons can be used for the fission process. However neutrons are

chosen because neutrons are electrically neutral in nature and thusthey do not require high kinetic energy to overcome electrical

repulsion from the positively charged nuclei and can participate in

fission reaction.

Q42.What is fertile material and fissile material?

A42.Not all the Uranium extracted from the ore can be used as fuel

for plant for carrying out the fission reaction. Natural uranium


10/10

power system interview question

Documents