In terms of the requirements of the load on the zero ground voltage, the power frequency machine is better than the high-frequency machine. The zero line of the high-power three-phase high-frequency machine will introduce the rectifier and act as the neutral point of the positive and negative buses. This structure will inevitably cause the high-frequency harmonics of the rectifier and inverter to couple on the zero line, raise the zero ground voltage, and cause the load end zero ground voltage to rise, which is difficult to meet the requirements of IBM, HP and other server manufacturers The voltage is less than 1V. In addition, when the utility and generator switch, the high-frequency generator often has to switch to the bypass because of the lack of zero line, which may cause the load flashover under certain conditions. Because the rectifier of power frequency machine does not need zero line to work, UPS can maintain normal power supply when the zero line is disconnected.