Improvement of Reliability of Protection by Applying Microprocessor-based Relays

In Tokyo Electric Power Company, we are advancing the digitalization of the protection relay to improve the performance of power system protection and to free us from troubulesome maintenance of protection relay. At present, around a half of the whole relays used in 500kV and 275kV power systems are microprocessor-based relays, so-called digital type relays, and in the other power systems around one third are digital type relays. Therefore we require high level reliability of digital type relay and it is now in higher level than the conventional solid state relay's.This paper describes the classification of the hardware failure of digital type relay and also shows the effective methods to improve its reliability. Methods for reducing the failure and the bad influence on the performance of the relay are introduced. It is found that aging test of the relay in higher temperature and heat cycle test in severer condition are very effective to be reappeared the failure that does not occur continuously and to find out the failure component. And as a result, we are able to reduce the failure rate of the components made from semiconductors such as digital type IC by applying these testing methods to screening test of the components.