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C6811 Pages: 3

Find the expression for three phase power in terms of symmetrical components.

A synchronous generator and motor are rated 30MVA, 13.2kV and both have sub-
transient reactances of 20%. The line connecting them has a reactance of 10% on the
base of the machine ratings. The motor is drawing 20,000kW at 0.85 power factor
lagging at a terminal voltage of 12.8 kV when a symmetrical three phase fault occurs
at the motor terminals. Find the sub-transient current in the generator, motor and
fault.

A three phase generator is connected to a star-delta transformer as shown in the
figure.

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07
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1 م26

The reactance values referred to a common base are :
Zi 22 20
Alternator 10.1 10.1 10.05
Transformer 10.05 10.05 10.05
Transmission line 10.4 10.4 10.8

Determine the fault current when a double line to ground fault occurs at point ‘P’ at
the mid-point of the line if the alternator neutral is grounded. Assume that the
generator is not loaded

Obtain the symmetrical components of the following set of unbalanced currents

I, = 1.61 250°, 1, = 1.0L. 180°, I, = 0.9L 132°

PART C
Answer any two full questions, each carries 10 marks.

Explain the computational procedure for load flow solution using fast decoupled load
flow method.
Figure shows a three bus power system. The impedance of each line is (0.026 + j0.11)

pu.
೦

bo

2
7

The bus details are given in the table below

| Bus | Po(pu) | Qe(pu)

Assuming a flat voltage start, find the voltages and bus angles at the buses at the end
of the first iteration using Gauss-Siedel method.

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