Lights out method and merging line a

Combining the electricity from one generator
with the electricity from other generators (or grid system) to achieve joint operation is called parallel (or grid-connected) operation. Before the generators are put into parallel operation, they must meet
four conditions: the same phase sequence, the same or similar voltage, the same or similar frequency, and the same voltage phase.
    The light extinguishing method is also called the dark light method, which is a quasi-synchronous method. The circuit is shown in Figure 7-4. It uses three sets of light bulbs to indicate the frequency and phase sequence of
the two generators .