# GTU previous year question paper- BE- VIIth SEM -Power System Analysis -May/June- 2012

GTU previous year question paper

GUJARAT TECHNOLOGICAL UNIVERSITY

BE- VIIth SEMESTER–EXAMINATION – MAY/JUNE- 2012

Subject code: 1708

Subject Name: Power System Analysis

Instructions:

1. Attempt all questions.

2. Make suitable assumptions wherever necessary.

3. Figures to the right indicate full marks.

Q.1 (a) What is meant by per unit system? State advantages of per unit system.

(b) A synchronous generator is rated 645 MVA, 24 kV, 0.9 pf lagging. It has

a synchronous reactance 1.2 _. The generator is feeding full load at 0.9

pf lagging at rated voltage. Calculate:

(1) excitation emf (Ef) and power angle (_)

(2) reactive power drawn by the load

Carry out calculations in per unit form and convert the results to actual values.

Q.2 (a) With the simplifying assumptions explain the SC transient on a

transmission line and hence prove that;

Imm (max. possible) = 2[_2V/|z|], ie doubling effect.

Also draw necessary waveforms to explain doubling effect.

(b) Explain the short circuit of a synchronous machine on no-load.

OR

(b) Derive expression for sequence impedances of transmission line and

draw their sequence networks.

Q.3 (a) Explain in detail “phase shift in star delta transformer” for positive &

negative sequence voltages.

(b) Three, 6.6 kV, 3-phase, 10 MVA alternators are connected to a common

bus. Each alternator has a positive sequence reactance of 0.15 pu. The

negative and zero sequence reactances are 75% and 30% of positive

sequence reactance. A single line to ground fault occurs on the bus. Find

the fault current if (1) all the alternator neutrals are solidly grounded (2)

one alternator neutral is grounded through 0.3 _ resistance and the other two neutrals are isolated.

OR

Q.3 (a) With the help of sequence network derive expressions for sequential

components for an L-L-G fault on a power system.

(b) A 50 MVA, 11 kV, 3-phase alternator was subjected to different types of

faults. The fault currents were: 3-phase fault 1870 A, line to line fault

2590 A and single line to ground fault 4130 A.

The alternator neutral is solidly grounded. Find the per unit values of the

three sequence reactances of the alternator.

Q.4 (a) Derive the swing equation from the first principle.

(b) The impedances between various buses in a system are as follows

Each bus to reference j2.0 _

Bus 1 & 2 j0.8 _

Bus 1 & 3 j0.5 _

Bus 2 & 3 j1.0 _

OR

Q.4 (a) Explain the procedure of formulation of YBUS using singular

transformation. Derive the necessary equations.

Q.4 (b) A large 3-phase cylindrical rotor generator is delivering 1.0 pu power to

an infinite bus through a transmission network. The maximum power

which can be transferred for pre fault, during fault and post fault

conditions is 1.8 pu, 0.4 pu and 1.3 pu. Find the critical clearing angle

and sketch the power angle curves to show the equality of accelerating

and decelerating areas.

Q.5 (a) Explain the computational procedure of load flow by “Fast Decoupled

(b) A 6 MVA, 11 kV, 8 pole, 50 Hz synchronous generator having saturated

synchronous reactance of 0.5 pu is synchronized to 11 kV bus. Calculate

its synchronizing power and torque coefficient per degree mechanical

shift of rotor angle at no load.

OR

Q.5 (a) State and explain the conditions to be satisfied for successful parallel

operation of a 3-phase alternator with an infinite bus bar.

(b) Two alternators are operating in parallel. Explain the effect of increasing

excitation of one of the alternator.