Power Systems : Important Interview Questions Part-1

1. What are the advantages of using bundled conductors?

Ans. In the bundled conductor GMR increases.

due to increases in GMR, the overall inductance decreases.
overall capacitance increases.
line surge impedance decreases.
line power handling capability increases.
Reduced corona loss
Reduced radio interference
Reduced voltage gradient

2. what are the problems associated with HVAC transmission which are not present HVDC?

Ans.

Due to the large charging current in HVAC, the long-distance transmission is prohibited.
Further for the long line, line inductance increases which reduce steady-state stability limit.

3. What is the problem associated with HVDC circuit breaking?

Ans. In the AC there are neutral current zero exists in every cycle, but in the case of DC, there are no neutral current zero exists. So in the case of DC low resistance switching is not possible. Although transient faults can be cleared by grid control permanent faults require a combination of grid control, fault locators and isolating switches. The lack of dc breakers has inhibited the networks of d.c. lines.

4. What are the factors affecting corona loss?

Ans. The corona loss is given by :-

The electric field around the conductor.
Atmospheric Factors: pressure and the temperature is low the corona loss is low.
Dust, rain and snow increase corona loss.
diameter of conductor
Corona loss increases with frequency.
A smooth surface of conductor reduces corona loss.

5. What is radio interference?

Ans. When the power line and communication line are in parallel and closed to each other, then radio interference is seen between these lines. Radio interference is the adverse effect introduced by corona on wireless broadcasting. The corona discharge emits radiation which may introduce noise signals in the communication lines, radio and television receivers.

6. Why overhead transmission is better than cable transmission?

Ans. Overhead transmission is better than cable transmission is:

In the overhead transmission line, the line charging current is less as compared to cable transmission. The large charging current on HV cables limits the use of cable for long length transmission.
Size of the conductor in overhead lines is smaller than cable for same power rating due to better heat dissipation capability of overhead lines.
Insulation cost is more in case of cables than the over headlines.

7. Why synchronous condenser is better than shunt capacitors?

Ans. A synchronous condenser is better than shunt capacitors are :

A synchronous condenser can operate at both leading and lagging power factor.so it can both consume and supply reactive VARs so it serves the purpose of both capacitor and inductor.
In the case of shunt capacitors, correction VARs fall with fall of voltage i.e. its effectiveness falls when it is most needed.
There is a smooth variation of VARs by synchronous condenser while step by step variation of static capacitors.

8. What is the difference between the voltage boost provided by the series capacitor and shunt capacitor?

Ans.

By installing Series capacitor line reactance reduces and improves steady-state stability limit.
Voltage boost provided by the series capacitor is = IX_C sin Φ_r
Shunt capacitor provides reactive power to the line and hence reduces loss in the transmission line.

9. Why neutral is connected to ground?

Ans. If neutral is not grounded than in case of fault the phase voltage increases and get equal to the line voltage.

The high voltages due to the arcing ground or transient line of ground faults are eliminated.
The overvoltages due to lighting are discharged to ground.
Sensitive protective relays against the line to ground faults can be used.

10. What is the effectively grounded system? What is its advantage?

Ans.

A system can be said to be effectively grounded when for all point on the system,
It is also called a solidly grounded system.
The effective grounded systems are less expensive than any other type of grounding for all operating voltage because for such a system the maximum line to ground voltage during a fault doesn’t exceed 80% of the line voltage of the healthy phases rise to about 100% of the line to line voltage.

11. How arcing on the transmission line is prevented?

Ans. Arcing on transmission lines is prevented by resonant grounding. A suitable inductor in the neutral causes the fault current I_L to flow, Which exactly balances the charging current. The value of that inductor is given by

12. Define TMS and PSM in case of relays?

Ans. TMS (Time Multiplier setting): The operating time of the relay depends upon the distance between the moving contact and fixed contact of the relay. By adjusting TSM we can adjust the operating time of relay. The higher the TMS the greater is the operating time.

PSM (Plug setting Multiplier): The minimum torque required for the movement of the disc is fixed for a particular design, i.e. the ampere-turns required are fixed. Therefore, for different pick up current settings, the number of turns is changed effectively so as to keep the same ampere-turns.By adjusting PSM we can adjust the pick current limit for the relay.

13. Which relay is most affected by arc resistance?

Ans. Mho relay

14. Which relay is most affected by synchronizing power surges?

Ans. Reactance relay

15. How the generator is protected against overload?

Ans. As in the case of overload, the current will be higher and losses in the alternator will also be high, so the temperature in the generator is increased due to copper losses. In the generator thermistors or thermocouples embedded at various points in the stator winding to give an indication of the temperature condition.

16. How C.T. should be connected for protecting of 3 phase transformers?

Ans. In star delta connection there is a phase difference of 30° is present. It is required that this phase displacement must be nullified by connecting the C.T.s .The C.T.s on the star side of the transformer is connected in delta and on the delta side, they are connected in the star as the line currents of star delta power transformer will be displaced in phase by 30°.

17. How the effect of magnetising inrush currents is prevented in the transformer?

Ans. To take into account the magnetising inrush currents, a high speed biased differential relay incorporating harmonic restraint feature is used. This is achieved by the use of a second harmonic filter which is arranged to inject an additional bias current in the relay circuit proportional to the second harmonic component.

18. How the generator is protected against loss of excitation?

Ans. An offset mho relay is used to protect the generator from loss of excitation. The relay setting is so arranged that the relay operates whenever the excitation goes below the certain value and the machine starts running asynchronously.

19. How can we protect the generators form overvoltages?

Ans. An a.c. overvoltage relay is used which has a pickup value of 110% of the normal and operates instantaneously at about 130% to 150% of the rated voltage.This protection is normally provided for hydroelectric and gas turbine generators and not for steam turbine generators.

20. Which factors affect the stability of arc in vacuum circuit breakers?

Ans. Factors decide the stability of arc in vacuum circuit breakers are :

Contact material and
vapour pressure.
Circuit parameters.

21. Give 4 Advantage of HVDC?

Ans.Advantage of HVDC-

1.No distance limitation due to charging current. For long-distance transmission, HVDC system suffers lower electrical losses.

Less corona losses.
HVDC promotes asynchronous connection.
No reactive power compensation is required.

22. How to reduce the losses in the transmission line? How much amount of electricity is lost while transmitting?

Ans. Losses in the transmission line is

Transmitting electricity at high voltage reduces the fraction of energy lost to resistance. Also by improving the power factor of the load.

In India, average transmission and distribution losses have been officially indicated as 23% of the electricity generated.

23. How zero-sequence current flows in delta connection?

Ans. Zero sequence current will not flow in lines of the delta but it will circulate inside delta.

24. Why chock is present in the tube light?

Ans. Normal operating voltage of tube light is about 110 V but in India, 240 V is available. Hence choke gives the output of 110V.

But for initiating light (to ionize the gas in a tube), the system requires 800-1000 V which is provided by the starter, which blocks the current flowing from the choke and the voltage builds up across the load. Now the starter closes the circuit. The function of the starter is to start the tube light.

A choke is nothing but an inductor which performs two functions:

In conjunction with starter, it causes the tube to ignite by using back emf

to create plasma.

It controls the current through the tube when it is ignited.

25. Why industries have to pay more than active power demand?

Ans. In a large industry, more reactive power is required. Due to this reactive power demand, they pay more than active power.

26. What is the slack bus?

Ans. A ‘Slack” bus (or ‘swing bus’) is defined as V, δ bus, that is used to balance the active power & reactive power |Q| in the system while performing Load flow studies in Electrical Power systems. We can adjust the active power of the slack bus while performing load flow study.

27. What is the difference between CT and PT?

Ans. In the power system voltage and current are in order of hundred and thousand. The current transformer is used to measure the current and potential transformer is used to measure the voltage in the power system. A CT has a large number of turns on its primary winding. The primary is connected in series with the load so that it carries full load current. A low voltage range ammeter (0-5A) is connected across the secondary winding terminal. Secondary of the CT is practically short-circuited since the ammeter resistance is very low. It should not be made open as it has drawn heavy current and damages the primary winding of the CT.

A PT has a large number of turns in the primary and fewer tuns in the secondary and hence it steps down the voltage. The primary winding is connected across the supply voltage and low range voltmeter (0-110V) is connected across the secondary winding terminals.

28. What is Grid Failure?

Ans. A power grid is an interconnected network of transmission lines for supplying electricity from power suppliers to consumers. Any disruptions in the network cause power outages. This type of failure is called grid failure.The northern electrical grid in India had collapsed in 2012.

29. What is Load shedding?

Ans. Load shedding is aimed at removing a load from the power system when there is an imbalance between the electricity available and the demand for electricity

30. Why transmission is done on high voltage?

Ans. Transmission is done on high voltage because of the following reasons :

1 transmission losses in the power system