GATE EE 2014 Set 1 — Question 49
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Electrical Machines → Synchronous Machines → Parallel Operation
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Question
A three phase synchronous generator is to be connected to the infinite bus. The lamps are connected as shown in the figure for the synchronization. The phase sequence of bus voltage is R-Y-B and that of incoming generator voltage is R'-Y'-B'.
It was found that the lamps are becoming dark in the sequence . It means that the phase sequence of incoming generator is
Correct answer
(A) opposite to infinite bus and its frequency is more than infinite bus
Solution
The lamps are connected between the bus and the incoming generator as follows:
- Lamp : between phase R and phase R'
- Lamp : between phase Y and phase B'
- Lamp : between phase B and phase Y'
1.Phase Sequence Analysis: If the phase sequence of the incoming generator were the same as the bus (R-Y-B), and the lamps were connected between corresponding phases (R-R', Y-Y', B-B'), all lamps would brighten and darken simultaneously. However, the connections are crossed ( to B', to Y').
2.Sequence of Darkening: The sequence indicates the relative rotation of the generator phasors with respect to the bus phasors.
3.Opposite Sequence: For the lamps to darken in a sequence when crossed as shown, the phase sequence of the incoming generator must be opposite to that of the bus (i.e., R'-B'-Y'). In this case, R' corresponds to R, B' corresponds to Y, and Y' corresponds to B.
4.Frequency Analysis: The sequence specifically occurs when the incoming generator frequency is higher than the bus frequency (), causing the generator phasors to 'overtake' the bus phasors in that specific order given the crossed connections.
Therefore, the phase sequence is opposite to the infinite bus and its frequency is more than the infinite bus.Continue learning with Success Tracker
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