Abstract: Automatic standby power supply switching device is applied in power system widely. The device can be realized by using PLC. The four cases of automatic standby power supply switching of internal bridge main connection were analyzed. Because of the difference between charge and discharge condition of the four cases, they can be realized by means of software programming of PLC. The device has merits of high reliability, flexible controlling, simple connection mode, convenient debugging and economical investment compared with the same conventional device constructed by using relays.

Key words: power system, internal bridge connection, automatic switching of standby power supply, programmable controller（PLC）