Exam Question

### (a) Conditions for Paralleling an AC Generator
Before an incoming AC generator (alternator) can be safely paralleled with live bus-bars, four critical conditions must be satisfied. Failure to meet these conditions will result in severe electrical and mechanical stresses, potentially causing catastrophic damage to the generator, prime mover, and switchgear.
1. **Equal Voltage:** The terminal voltage of the incoming generator must be equal to, or very slightly higher than, the bus-bar voltage.
* **Reasoning:** If a significant voltage difference exists when the breaker is closed, a large reactive circulating current will flow between the generators, limited only by their synchronous impedances ($$ I_{circulating} = \frac{V_{incoming} - V_{busbar}}{Z_{s1} + Z_{s2}} $$). This current causes excessive heat in the windings, potential magnetic stresses, and can trip protective relays. Keeping the incoming voltage slightly higher ensures that the new machine immediately exports reactive power (kVAR) and operates as a generator rather than motoring.