(a) Three Devices for Protecting Parallel A.C. Generators:
- Overcurrent Protection Device (OCPD): This can be a circuit breaker or a combination of fuses and relays.
- Under-voltage Protection Relay:
- Reverse Power Protection Relay:
(b) Explanation for Each Device:
1. Overcurrent Protection Device (OCPD):
- Function: The OCPD protects the generator from damage caused by excessive current flowing through its windings. This can occur due to internal faults within the generator itself or overloading on the distribution system.
- Importance in Parallel Operation: When multiple generators are operating in parallel, a fault in one generator can cause a surge in current through the healthy generators. The OCPD in each parallel generator will trip, isolating the faulty unit and protecting the remaining generators from damage.
2. Under-voltage Protection Relay:
- Function: This relay monitors the generator’s output voltage. If the voltage falls below a preset threshold, the relay trips the generator breaker, disconnecting it from the parallel operation.
- Importance in Parallel Operation: A sudden drop in voltage from one generator can disrupt the synchronization and stability of the parallel system. The under-voltage protection relay ensures that a faulty generator with low voltage output is automatically disconnected, preventing it from affecting the entire parallel system.
3. Reverse Power Protection Relay:
- Function: This relay monitors the direction of power flow in the generator. In a healthy parallel operation, the generator should be supplying power to the distribution system. The reverse power protection relay trips the generator breaker if it detects power flowing back into the generator, indicating a potential problem.
- Importance in Parallel Operation: A generator experiencing internal issues or a failed governor might start motoring and drawing power from the system instead of supplying it. The reverse power protection relay detects this abnormal condition and disconnects the faulty generator, preventing it from overloading the other generators in the parallel system.
These three devices work together to ensure the safe and reliable operation of parallel AC generators. They protect individual generators from internal faults and overloading, maintain system stability by preventing issues with voltage and power flow, and ultimately contribute to the overall protection of the entire parallel generation system.