SWITCHGEAR AND PROTECTION 3

31. A 3-phase 11kV  generator feeds power to a constant power unity power factor load of 100 MW through a 3-phase transmission line. The line-to-line voltage at the terminals of the machine is maintained constant at 11 kV. The per unit positive sequence impedance of the line based on 100 MVA and 11 KV is j0.2. The line-to-line voltage at the load terminals is measured to be less than 11kV. The total reactive power to be injected at the terminals of the load to the increase the line-to-line voltage at the load terminals to 11 kV is
a) 100 MVAR
b) 10.1 MVAR
c) -100 MVAR
d) -10.1 MVAR
ans. d

A generator feeds power to an infinite bus through a double circuit transmission line. A 3-phase fault occurs at the middle point of one of the lines. The infinite bus voltage is 1 pu, the transient internal voltage of the generator is 1.1 pu and the equivalent transfer admittance during fault is 0.8 pu. The 100 MVA generator has an inertia constant of 5 MJ/MVA and it was delivering 1.0 pu power prior of the fault with rotor power angle of 30⁰ .  The system frequency is 50 Hz.

32. The initial accelerating power (in pu) will be
a) 1.0
b) 0.6
c) 0.56
d) 0.4
ans. c
33. If the initial accelerating power is Xpu, the initial acceleration in electrical degrees/sec , and the inertial constant in MJ-sec/electrical degrees respectively will be
a) 31.4X,18
b) 1800X, 0.056
c) X/1800 , 0.056
d) 3/3.14, 18
ans. b

34. In load-flow analysis , the load connected at a bus is represented as
a) constant current drawn from the bus.
b) constant impedance connected at the bus.
c) voltage and frequency dependent source at the bus.
d) constant real and reactive power drawn from the bus.
ans. d

35. Gauss-Siedel interative method can be used for solving a set of
a) linear differential equations only.
b) linear algebraic equations only.
c) both linear and nonlinear algebraic equations only.
d) both linear and nonlinear differential equations only.
ans. b

36. The Gauss-Siedel load flow method has the following disadvantages. Tick the incorrect statements
a) Unreliable convergence
b) slow convergence
c) choice of slack bus affects convergence.
d) A good initial guess for voltages is essential for convergence.
ans. b

37. A power system consists of 300 buses out of which 20 buses are generator bus, 25 buses are ones with reactive power support and 15 buses are the ones with fixed shunt capacitors. All the other buses are load buses. It is proposed to perform a load flow analysis in the system using Newton-Rapson method. The size of the Newton-Rapson Jacobian matrix is
a) 553 X 553
b) 540 X 540
c) 555 X 555
d) 554 X 554
ans. b

38. If the reference bus is changed in two load flow buses with same system data and power obtained for reference bus taken as specified P and Q in the latter run
a) the system losses will be unchanged but complex bus voltages will change.
b) the system losses will change but complex bus voltages remain unchanged.
c) the system losses as well as complex bus voltages will change.
d) the system losses as well as complex bus voltages will be unchanged.
ans. a

SWITCH GEAR AND PROTECTION 2

21. A lightning stroke discharges impulse current of 10 kA (peak) on a 400 kV transmission line having surge impedance of 250 ohm. The magnitude of transient over voltage travelling waves in either direction assuming equal distribution from the point of lightning strike will be
a) 1250 kV
b) 1650 kV
c) 2500 kV
d) 2900 kV
ans. a

22. The insulation strength of an EHV transmission line is mainly governed by
a) load power factor
b) switching overvoltages
c) harmonics
d) corona
ans. b

23. Steady-state stability of a power system is the ability of the power system to
a) maintain voltage at the rated voltage level
b) maintain frequency exactly at 50Hz
c) maintain a spinning reserve margin at all times.
d) maintain synchronism between machines and on external tie lines.
ans. d

24. The load carrying capability of a long ac transmission line is
a) always limited by the conductor size
b) limited by stability considerations.
c) reduced at low ambient temperatures.
d) decreased by the use of bundled conductors of single conductors.
ans. b

25. An 800 kV transmission line is having per phase line inductance of 1.1 mH/km and per phase line capacitance of 11.68 nF/km. Ignoring the length of the line , its ideal power transfer capability in MW is
a) 1204 MW
b) 1504 MW
c) 2085 MW
d) 2606 MW
ans. c

26. An 800 kV transmission line has a maximum power transfer capacity of P. If it is operated at 400 kV with the series reactance unchanged, then maximum power transfer capacity is approximately
a) P
b) 2P
c) P/2
d) P/4
ans. d

27. A round rotor generator with internal voltage E1=2.0 pu and X=1.1 pu is connected to a round rotor sysnchronous motor with internal voltage E2= 1.3 pu and X= 1.2 pu. The reactance of the line connecting the generator supplies 0.5 pu power, the rotor angle difference between the machines will be
a) 57.42 degrees
b) 1 degree
c) 32.58 degrees
d) 122.58 degrees
ans. c

28. The pu parameters for a 500 MVA machine on its own base are: inertia , M= 20 pu ; reactance ,X= pu.
 The pu  values of  inertia and reactance on 100 MVA common base, respectively, are
a) 4, 0.4
b) 100, 10
c) 4, 10
d) 100,0.4
ans. d

29. A power station consists of two synchronous A and B of ratings 250 MVA and 500 MVA with inertia 1.6 and 1.0 pu respectively on their own base MVA ratings. The equivalent pu inertia constant for the system on 100 MVA common base is
a) 2.6
b) 6.15
c) 1.625
d) 9
ans. d

30. A shunt reactor of 100 MVAr is operated at 98% of its rated voltage and at 96% of its rated frequency. The reactive power absorbed by the reactor is
a) 98. MVAr
b) 10.402 MVAr
c) 96.04 MVAr
d) 100 MVAr
ans. d