Breaker connection power supply on both sides of the closing operation known as the grid, and the grid in the following two situations: 1, difference frequency and network. Generators and systems connected to the grid and the two systems have been dissolved between the two interconnection lines and networks are poor frequency and network. When the grid is required, the voltages at both sides of the grid should be similar and the frequencies are similar. When the phase difference is 0o, the grid operation is completed. 2, the same frequency and network. Unresolved line between the two systems and the grid is the same frequency and network (or co-ring). This is because the frequencies of both sides of the juxtaposition point are the same, but there will be a power angle δ on both sides. The value of δ is proportional to the reactance of the other tie lines in the system on both sides of the juxtaposition point and the active power delivered. The condition of grid connection in this case is that the grid-connected operation can be implemented when the voltage difference and the power angle between the two sides of the circuit breakers in parallel are within a given range. The moment of grid connection, parallel point circuit breaker power angle disappear immediately, the system trend will be redistributed. Therefore, the allowable power angle setting of the same frequency grid depends on the redistribution of the system power flow, which will not cause the relay protection action of new input lines or cause the system out of step on both sides of the parallel point.