GATE 2025 EE Q.28: solution and answer

Official answer: A, CSource: Official answer key, GATE 2025 EE (IIT Roorkee)

1. Synchronizing conditions: to connect an alternator to the grid without transients, four conditions must be satisfied at the moment the breaker closes — (i) equal RMS voltage magnitude, (ii) equal frequency, (iii) same phase sequence, and (iv) zero phase-angle difference between the incoming machine and the bus.

2. Voltmeter: connected across the incoming alternator and the bus (directly, or via the double-voltmeter/synchronizing-lamp arrangement), a voltmeter is used to confirm condition (i), equal voltage magnitude, before attempting synchronization.

3. Synchroscope: connected across the breaker (fed from the incoming machine's and the bus's potential transformers), a synchroscope is an instrument whose pointer position and direction of rotation directly indicate the phase-angle difference and whether the incoming machine is running fast or slow relative to the bus. The operator closes the breaker exactly when the pointer crosses the 'in-phase' (12 o'clock) position, which is the standard way of satisfying conditions (ii)-(iv) together.

4. Wattmeter: measures real power flow, a quantity that is only meaningful once the machine is already connected to the bus; it gives no information needed to decide when to close the breaker, so option (B) is not a synchronizing instrument.

5. Stroboscope: used to measure or observe mechanical rotational speed via a flashing light against a marked rotating part; it is not part of the standard electrical synchronizing (breaker-closing) procedure for an alternator, so option (D) is not a synchronizing instrument.

6. Hence the instruments required to synchronize an alternator to the grid are the Voltmeter and the Synchroscope — options (A) and (C).