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Swing Equation; Multimachine System; Machines Swinging Coherently; Machines Swinging Non-Coherently

A power syste...

Question

A power system consists of two synchronous generators P and Q of rating 300 MVA and 450 MVA with inertia constant 1.8 pu and 1.0 pu respectively. On their own base MVA ratings, the value of equivalent per unit inertia constant for the parallel system on 200 MVA common base will be

A

4.95 pu

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B

1.50 pu

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C

3.45 pu

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D

2.05 pu

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Solution

The correct option is A 4.95 pu
We know for coherently swinging generators,
$G_{e q} H_{e q} = G_{1} H_{1} + G_{2} H_{2}$
$= 300 \times 1.8 + 450 \times 1$

Also given $G_{e q} =$ common MVA = 200 MVA
$∴ H_{e q} = \frac{300 \times 1.8 + 450 \times 1}{200} = 4.95 p u$

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