The ________________'s function is to accept the high-voltage power supply (usually 110kV) of network/grid, and provide medium-voltage power supply (usually 35kV or 10kV) for traction substation and step-down substation through step-down.__________的功能是接受城市电网的高压电源，经降压为牵引变电所、降压变电所提供中压电源。