Experience on Stray Flux Interference of CT (Case Study of a Power Plant in Thailand)

A current differential protection protects the winding of a generator. The relay must not operate on the occurrence of external fault. There is a power plant in Thailand where the generators were tripped by external fault. The tripping of generator results in severe stability problem. The case study shows single line to ground external fault on a transmission line. The fault current wave shapes were distorted from sinusoidal. This causes generator current differential relay mis-operate. The paper presents the investigation of external faults tripping of the differential relay. By looking into the current signal, the investigation shows that CT saturation might not be the cause of the distortion. The study also simulates a simple magnetic field model using finite element software to investigate the possibility of stray flux interference between CTs.