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A lossless single machine infinite bus power system is shown below.
The synchronous generator transfers 1.0 per unit of power to the infinite bus. Critical clearing time of circuit breaker is 0.28 s. If another identical synchronous generator is connected in parallel to the existing generator and each generator is scheduled to supply 0.5 per unit of power. Then the critical clearing time of the circuit breaker will
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- reduce to 0.14 s
- reduce but will be more than 0.20 s
- remain constant at 0.28 s
- increase beyond 0.28 s
Correct Option: B
δcr = critical clearing angle
δmax = π – δ0
By equal area criterion, A1 = A2
⇒ Pm [δcr – δ0] 
δcr = cos-1 [(π - 2δ0) sinδ0 - cosδ0]
and critical clearing time,
| tcr = |  |  | 1/2 | ..............(A) | |
| πƒPm |
when one alternator is connected to the bus,
Pm1 = Pe1 = 1p.u.
When two alternator are connected in parallel, we get
Pe2 = Pm2 = 0.5 p.u.
By equation (A),
| tcr ∝ | |
| √Pm |
| = √ | ||
| tcr2 | Pm1 |
| tcr1 = tcr2 | |
| Pm2 |
= 0.28&dadic;2 = 0.396 sec
