At some particularly important power distribution sites, usually a transfer switching equipment (TSE) is used in combination with an Uninterrupted Power Supply (UPS) to ensure uninterrupted power supply. The transfer switching equipment is connected to mains supply and a generator (or another mains supply), and its load end is connected to a working load via the UPS to supply power to the working load. In the case that one mains supply is normal, the mains supply is communicated with the UPS to supply power to the load through the mains supply. When the mains supply fails, the transfer switching equipment is switched to disconnect from the mains supply and communicates the generator or another mains supply with the UPS so that the generator or said another mains supply continues to supply power to the load. During transfer of the transfer switching equipment, the UPS continues to supply power to the load by virtue of electrical energy stored therein and thereby ensure uninterruption of the power supply.
A neutral pole and a phase pole in an ordinary transfer switching equipment usually have the same structure, so during the switching of the power supply, opening and closing operations of the neutral pole and phase pole are performed completely simultaneously. As such, during switching, during a time period after the neural pole of the first power supply is disconnected from the neutral pole of the UPS until the neutral pole of the second power supply is connected to the neutral pole of the UPS, the neutral pole of the UPS is temporarily disconnected from the neutral pole of the power supply. The temporary disconnection of the neutral pole will cause a change of potential of the UPS neutral pole relative to the ground, usually up to tens of volts. This will cause adverse influence or loss to a part of electrical equipment. For example, in an application in which a combination of a transfer switching equipment and an uninterrupted power supply is used to provide uninterrupted power supply to a data center, the temporary disconnection of the neutral pole causes zero to earth voltage of the uninterrupted power supply to rise and causes restart of the server, and this will cause serious consequences such as data loss and even damages of the server.
At present, there exists a transfer switching equipment, wherein during the switching, there is a period of overlapping time in which the neutral pole of the mains supply and the neutral pole of the generator or another mains supply are closed at the same time to communicate with the neutral pole of the UPS at the same time. However, this kind of transfer switching equipment still has a conspicuous drawback. First, during the switching of the transfer switching equipment, there is a period of overlapping time in which a phase pole of one power supply (e.g., one mains supply) and the neural pole of another power supply (e.g., another mains supply) are closed at the same time, which exerts an adverse influence on the voltage on the load, e.g., generate burrs in the voltage waveform. Furthermore, this kind of transfer switching equipment is complicated in structure, very large is product size, and is not conducive to installation and the saving of space.