The effects of harmonics on differential relay for a transformer

The use of nonlinear devices such as rectifiers, converters, power supplies, and other devices utilizing solid state switching have been increased in industry. These equipments have deteriorated the power quality. This deterioration causes the increase of power loss, resonance problem, insulation deterioration, and even safety problems for system apparatus. This paper aims at analyzing and probing into the influences of harmonics on a differential relay. First it probes the operation of a CT in the frequency domain. Then, it analyzes and compares the mathematical model, which is constructed by using EMTP, and the test results. There are many factors to be considered in differential protection design and application for a transformer. This paper only analyzes the influential phenomenon of harmonics to a differential relay. The results of this research can be valuable references for applying and designing differential relay protection