Objective Questions on Electrical Transmission | 9

1. A Transformer costing Rs 90,000/- has a useful life of 20 years. Determine the annual depreciation charge using straight line method. Assume the salvage value of the equipment to be Rs 10,000/-.

Initial cost of Transformer(P) = Rs 90,000/-, Useful life (n) = 20 years, Salvage value(s) = Rs 10,000/- , Using straight line method, Annual depreciation charge = (P − S) ⁄ n = (90,000 − 10,000) ⁄ 20 = Rs 4,000/-.

2. Which distribution scheme is employed in metropolitan areas?

In metropolitan areas there are large nos .of industries present. Hence in order to maintain continuities of supply and quality of supply the scheme of connection is preferred is interconnected system. In inter connected system the feeder ring is energized by two or more substations or generating stations. Hence reliability increases.

3. A line which connects between substation and distribution transformer is

A conductor with no tapings is called feeder. Usually a conductor is connected is substation and distribution transformer is called primary feeder. A line is connected between distribution substation and load is called secondary feeder.

4. Which of the following is not the distribution system normally used?

3 phase 4 wire system means star with neutral connected system. 3 phase 3 wire system means delta connected or star without neutral. 1 phase 3 wire system means one phase, one ground, one neutral. Generally these all are uses at load side. 1 phase 5 wire system doesn't employ to any system.

5. In distribution system feeders feed power to

In distribution system power is delivered to distributors from a feeder. Feeder is a line which supplies a point of distribution network without being tapped at any intermediate point. In simple terms, the feeder may be defined as the line carrying electric current from the secondary substation to the distribution substations or as a primary distribution line. So, in distribution system feeders feed power to distributors.

6. There are how many types of dc distributors

4 types: Distributor fed at one end
Distributor fed at both ends
Distributor fed at centre
Ring distributor

7. Majority of distribution substations are type of

Majority of distribution substations are of pole mounted type

8. A power station consists of two synchronous generator A & B of rating 250 MVA & 500MVA with inertia of 1.6 p.u & 1.0 p.u. respectively on their own base MVA ratings .The equivalent p.u. inertia for the system for the system on 100 MVA common base will be

Equivalent p.u. = (250X1.6 + 500X1) ⁄ 100 = 9 p.u.

9. The combined frequency regulation of machines in area of capacity 1500 MW & operating at a normal frequency of 60 Hz is 0.1 p.u. on its own base capacity. The regulation Hz ⁄ Mw will be

Area capacity = 1500 Mw, Nominal Frequency = 60 Hz, Regulation = 0.1 = 10%. This implies that when the load of 1500 Mw is thrown off then Frequency rise = (60X10) ⁄ 100 = 6 Hz. So regulation is 6 ⁄ 1500 Hz ⁄ Mw.

10. A series generator is connected in series to a feeder having 0.5 Ohm resistance to boost the voltage. The induced voltage is given by Eg= 0.6Ia, Ra = 0.2 Ohm & Rs = 0.3 Ohm. Find the voltage boosted by the generator, when deleviring 400 Amp?

Eg=0.6*400=240 V, Vboost=Eg-Ia*(Rs+Ra)=240-400*(0.2+0.3)=240-200= 40V .

11. The sending end voltage of feeder with reactance 0.2 p.u.is 1.2 p.u. if the reactive power supplied at the receiving end of the feeder is 0.3 p.u. The approximate drop of volts in the feeder is

Drop of the feeder is =0.3⁄1.2*0.2=0.05 p.u.

12. The per unit impendence of a transmission line on a 50 MVA 132 kV base Is 0.5. the per unit impedance on 100 MVA base will be

Zpu(new) = Zpu(old) X (MVA new / MVA old ) X (kV old / kV new) = 0.5 X (100/500) X (1/1) = 0.1.

13. A power station consists of two synchronous generators A and B of rating 250 MVA and 500 MVA with inertia 1.6 p.u. and 1 p.u. respectively on their own base MVA ratings the equivalent p.u. inertia for the system on 100 MVA common base will be

H1 X S1 + H2 X S 2 = H X S
Equivalent p.u. = 250 X 1.6 + 500 X 1 / 100 = 9 p.u.

14. If the inductor & capacitance of a system are 1.0 H & 0.01 μF respectively & the instantaneous value of electric current interrupted is 10 amp. The voltage across the breaker contacts will be

v = ia√(L/C) = 10√(1 ⁄ 0.01X10 − 6) = 100 KV

15. Effect of gap between horns in horn gap arrestor

if gap increases arrestor will not work and in gap decreases then there is chance of short circuit . Ideal gap is 1.5 mm.

16. Power system is protected from lightning by use of

Lightning arrestor is a protective device that conducts high voltage surges on the power system to ground. It is used for protecting against lightning. There One end of the arrestor is connected to the terminal of the equipment to be protected and the other end is effectively grounded. Under normal operation, the lightning arrestor conducts no electric current to earth. On occurrence of over voltage, it provides a low resistive path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrestor to the ground instead of being sent back over the line.

17. In substations, Which is used most commonly arrestor?

Thyrite is a material obtained by a special type of clay mixed with carborundum (Silicon Carbide). Thyrite is used widely in lightning arresters. Thyrite is a non-linear resistor. i.e. it has high resistance at low voltages and low resistance at high voltages.

18. What are the types of lightning arrestors?

There are 5 types of lightning arrestors- horn gap, rod gap, multi gap, valve type, expulsion type.

19. Where is one & half breaker arrangement used?

One & half breaker arrangement is generally used in the extra high votage system for reducing the cost of circuit breakers.

20.  list 1 list 2 To Use a. Improve power factor 1.shunt capacitor b. Reduce electric current ripples 2. Series capacitor 3. Series reactor

shunt capacitors will improve power factor, series reactors (inductors) will reduce electric current ripples (because inductors doesnâ€™t allow sudden changes in current).