Duty Cycle-Based Differential Protection Scheme for Power Transformers

In general, a power transformer is equipped with differential protection relay with the second harmonic restraint criterion to improve reliability. However, this criterion may fail in some cases, resulting in incorrect operation of protection relay. Therefore, to further upgrade the performance of differential protection, this paper proposes a duty cycle-based scheme as complement. Firstly, the waveform of the differential current in one cycle and its two derived waveforms, constructed by translating and taking absolute, are transformed into corresponding square waves, respectively. Then, duty cycles of these square waves are calculated, and three criteria are presented to quantify the differences between the duty cycles of disturbances and faults. Finally, the criteria and the second harmonic restraint are combined through three logic gates to form the proposed scheme for protection. A large number of simulation experiments have verified the effectiveness of the proposed scheme, which outperforms the frequently used protection schemes.

I. Introduction

As an important primary equipment in the power system, the safe and stable operation of the transformer is of great significant. However, in the operation of a power transformer, it is inevitable to suffer from various disturbance currents, such as inrush current, the unbalanced current caused by different saturated degree of the current transformer (CT) at both ends of a transformer. Therefore, different from the differential protection installed on the transmission line, the protection device equipped in the transformer as the main protection also needs to be able to make a correct decision for various disturbances that may occur in the transformer.