Thursday, November 1, 2012

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

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