۱- Introduction

۲- Proposed new solutions

۳- Protection testing

۴- Summary and recommendations

References

Abstract

This paper presents a new approach to differential protection of power transformers with use of the integral principle. The required criteria signals are calculated directly from the operational and restraining currents in particular phases. Additional support of the second harmonic is also used. The proposed integral principle is simple and exhibits superior performance against other commonly applied algorithms. The theoretical investigations are followed by testing with simulation runs as well as recorded signals.

Introduction

The investigations related to differential protection of power transformers are presented in this paper, whereas special attention was put to relay performance for: – external fault cases with CT saturation, – magnetizing inrush due to transformer energization, – internal transformer faults. The problem of differential protection behavior for power transformer energization as well as for external faults with CT saturation is not new. There have been a lot of cases reported when transformer differential protection maloperated despite of the use of typical characteristics and stabilization approaches. Apart from the percentage differential characteristic, the commonly accepted solution for transformer protection stabilization is the second harmonic ratio principle, which is applied in products of most relay manufacturers, e.g. [1].