Unwanted operation of differential protection function in T60 relays due to failures in CT/VT modules

T60 relay is one of the new digital relays has been designed to meet the requirements of high sensitivity, reliability and high speed. This development led to notable simplification in transformer differential protection 87T function by reducing cost, cabling and auxiliary equipment. However, still miss-operation cases in transformer differential protection can occur from time to time. This paper reports on two unwanted-operation cases of transformer differential protection happened in Garie power plant in Sudan. In the both cases, failures in the CT/VT module of T60 relays cause unwanted operation of the protection system. These failures could be a result of aging, problems in DC power supply or from the design of these modules. More investigations are needed to find the roots of these failures. In the both cases, the failures are hidden type in which the relay will not operate unless unusual event has happened. The failures could have been detected and removed, if the preventive maintenance was effective. In order to insure reliability and sensitivity of the relays, preventive maintenance should take place from time to time.