A new scheme for power transformer protection

In this paper, a new transformer protection scheme is proposed which is based on the combination of virtual 3/sup rd/ harmonic restrained and sampled value differential principles. The differential protection restrained by virtual 3/sup rd/ harmonic can identify asymmetric and symmetric inrush currents with semicycle data window and accelerate the protection tripping speed. The sampled value differential protection can overcome some disadvantages of harmonic restraining with a long delay in the case of high content of harmonic components during internal faults. The combination scheme takes advantages of these two methods and improves the protection performance effectively. Test results in electrical power dynamic laboratory (EPDL) illustrate the proposed scheme. Now the protection system has been put into trial operation in GeZouBa hydroelectric power plant.