When two or more electrical generating sets or systems are paralleled to the same power distribution system, the power sources must be synchronized properly.
Without proper synchronization of the oncoming unit or system, power surges and mechanical and electrical stress will result when the tie or
generator breaker is closed.
Effect of synchronization failure.
Under the worst conditions, the voltages between the two systems can be twice the peak operating voltage of one of the systems, or one system can place a dead short on the other. Extremely high currents can result from this which put stress on both systems.
Practical Automatic Synchronization in a Power plant
How automatic synchronization is done in a power plant:
When the engine/turbine is started, it initially operates in CB open control mode. The speed accelerates up to idle speed, and thereafter ramps up to rated speed. When the engine speed reaches rated speed, an external device (synchronizer) activates the synchronization.
Commands from this synchroniser unit activate the two binary inputs Speed increase and Speed decrease to obtain the requested speed level thus to control frequency. The speed reference can be altered between a pre-determined min. and max. speed reference level by using these inputs, thus the internal speed
reference is in this way biased so that the generator frequency exactly will match the plant/grid frequency.
When the two frequencies are totally matched (in addition also the phase matching and the generator voltage level must match), the generator breaker can be closed. Alternatively an analogue synchronizer
can be used .