Abstract
Relay protection is developping along with the development of power supply system. The short circuit is inevitable. It requires of the selectivity ,quick disconnection,sensibility and reliability. As is known to all,because of the physical factor(such as lighting strike),the quality of electric component(such as transformer, generatrix,cable,power capacitor and motor),and the factor of maintenance. The fact of short circuit and abnormal running of the power supply system is inevitable. So the work of relay protection is to cut the component away from the system selectively and automaticly in order to minimize the losing and assure the system run freely maximaly. It also can reflect the abnormal state of the eletric component and give the warning signal,cut the load which depend on the real condition and the bearbility of the electirc component. So relay protection is an important tache of the system. So it is very significant.