Protection Devices for A.C. Generators in a Main Switchboard (Single/Parallel Operation)
(a) Five Key Devices:
A main distribution switchboard for protecting AC generators, especially those capable of single or parallel operation, will typically include the following five devices:
- Circuit Breaker: The main circuit breaker protects the generator from overload currents. It trips and isolates the generator from the distribution system if the current exceeds a pre-set safe limit.
- Undervoltage Protection Relay: This relay monitors the generator’s output voltage. If the voltage falls below a minimum acceptable level, the relay trips and disconnects the generator from the system. This prevents the generator from operating under abnormal conditions that could damage itself or connected equipment.
- Overvoltage Protection Relay: Conversely, this relay protects against excessively high voltage output from the generator. If the voltage exceeds a pre-set limit, the relay trips, disconnecting the generator to prevent damage to itself or downstream equipment.
- Synchronizing System (for Parallel Operation): When operating generators in parallel, a synchronizing system is essential. This system ensures the generators are synchronized in terms of frequency, voltage, and phase before paralleling. This prevents damaging surges and ensures smooth load sharing between generators.
- Reverse Power Protection Relay (Optional): In some installations, a reverse power protection relay might be included. This relay protects the generator from motoring (acting as a motor instead of a generator) if the prime mover fails or the connected load becomes a source of power feeding back into the generator.
(b) Explanation for Each Device:
- Circuit Breaker: Protects the generator from damage caused by excessive currents due to overloads or faults on the distribution system.
- Undervoltage Protection: Prevents the generator from operating under abnormal conditions that could lead to internal damage or instability. It also safeguards connected equipment from potentially harmful low voltage situations.
- Overvoltage Protection: Protects the generator and downstream equipment from damage caused by excessively high voltage output. This can occur due to malfunctioning voltage regulation or other internal issues.
- Synchronizing System: Ensures safe and stable parallel operation of generators by synchronizing their frequency, voltage, and phase. This prevents damaging current surges and allows for efficient load sharing.
- Reverse Power Protection (Optional): Protects the generator from potentially damaging reverse power flow if the prime mover fails or if the connected load feeds power back into the generator. This can occur in some system configurations.