Performance of current transformer operate under harmonic condition and their effects on transformer differential protection

This paper focused to determine the performance of Current Transformer (CT) operates under harmonics condition and their effects on transformer differential protection. A laboratory test has been implemented to determine the error produced by both CT and power transformer when operating under harmonic condition. The test was performing with the actual condition, where the power transformer is connected to an adjusted nonlinear load, so that the test can be conducted with several levels of total harmonic distortion current (THDi). The results shows, for THDi ranging from 16.70% to 40.88% the maximum errors occurred on CT at secondary power transformers is 27.21% and CT at primary power transformers is 8.12%. This error resulted in differential current flow 0.17A and relay trip without any fault. In this study it was found that the relays started to operate incorrectly on THDi 31.5%.