An improved transformer percentage restraint method based on transmission line information

Transformer is one of the most important elements in power system, but the correct operation percentage of protection relay is far less than that for line protection. Current differential protection has been the main protection technique for transformer unit. This protection technique, however, suffers a low sensitivity or reliability due to the problems such as CT saturation and transformer magnetizing inrush. Based on digital substation system, studies have found that the fault generated electric quantities can not only be used by the relay to achieve the line protection but also used for developing transformer protection techniques. The protection system can effectively block transformer protection when inrush occurs. The results from simulation test show that the new method is with simple principle and good reliability.