When two generators operate in parallel, their controllers must synchronise the generators, share the electrical load correctly, control the circuit breakers safely and protect the entire system.
Before the second generator connects to the live busbar, its controller must match:
The controller adjusts the engine speed to match frequency and may control the AVR to match voltage. It must close the generator circuit breaker only when all synchronising conditions are within safe limits.
Once both generators are connected, the controllers must share the real power load between the engines.
Identical generators should normally share the kW load equally. Generators with different ratings should share it proportionally to their capacities. This is generally achieved by controlling the engine governors.
The controllers must also share the reactive load between the alternators. This affects:
Reactive load sharing is normally achieved by adjusting alternator excitation through the AVR. Poor kVAr sharing can overload one alternator even when the kW load appears to be shared correctly.
Each controller should be able to:
Reliable breaker auxiliary contacts are required to provide accurate position feedback.
The system should normally protect each generator against:
Reverse-power protection is particularly important because it prevents a generator from absorbing power and operating like a motor following a loss of engine power.
The controllers must monitor the common busbar for:
The system must also determine which generator is allowed to close first onto a dead busbar.
In automatic operation, the controllers should:
Start and stop settings should include suitable time delays to prevent repeated cycling when the load fluctuates.
The system should always maintain sufficient generating capacity. For example, the second generator may start when the first reaches 70–80% load and stop when one generator can safely carry the total demand.
The settings should consider:
The controllers normally require a reliable communication link to exchange information such as:
The design must also define how the system will respond if communication is lost. A communication failure must not cause unstable load sharing or unsafe breaker operation.
If the available generator capacity is insufficient, the controller or an external PLC should be able to:
The system should provide clearly defined modes, including:
Manual operation should not allow essential synchronising and protection interlocks to be bypassed accidentally.
A suitable paralleling controller must perform four essential functions:
Synchronise the generators before connection
Share kW and kVAr correctly after connection
Control and interlock the generator circuit breakers
Protect the generators and manage system operation
A controller designed only for automatic starting and stopping is not necessarily suitable for parallel operation. It must specifically support generator-to-generator synchronisation, active and reactive load sharing, breaker control and system sequencing.
