Abstract
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.
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