Synchronization is a process of matching the voltage, frequency, phase angle and sequence of two AC power sources and running them in parallel. Other quantities can supplement in the process of synchronization, namely, rate of change of frequency and the rate of change of voltage. When synchronization is done properly, the systems mesh together and continue running harmoniously thereafter. However, when done poorly, synchronization can result in large, damaging inrush currents in the stator of the generator and unacceptable mechanical stress on the rotor shaft.
Depending on the direction, the process of synchronization can be divided into two types, viz. forward and reverse Synchronization.
In Forward Synchronization, the voltage, frequency and phase angle of the incoming generator is synchronized to match the values of the bus bar. This is generally used when a generator needs to be synchronized with an already charged bus bar.
Reverse Synchronization or backward synchronization is generally done when the supply from a grid utility is needed to be synchronized with a bus bar in the factory. Since, it is not possible to alter the voltage, frequency, etc. of the incomer, in this case, the grid. The voltage, frequency, etc. of the bus bar are adjusted to match the incomer.