Research on the Influence of Frequency Deviation on Transformer Differential Protection

In some situations such as DC island operation with blocking faults or faults occurring in the island operation of small and medium-sized hydropower stations, the system frequency will have significant deviations. These frequency deviations can lead to electrical measurement errors. The paper studies the influence of frequency deviation on the operation behavior of transformer differential protection. Firstly, the characteristic coefficient of the recovery inrush current amplitude was theoretically calculated when the system frequency deviated. Then, the effect of the Fourier algorithm on calculating the second harmonic content caused by frequency deviation was derived under the condition of no inrush current. Finally, the influence of frequency deviation on transformer differential protection was analyzed under the two conditions of no inrush current and recovery inrush. The theoretical research and simulation analysis show that when the frequency increases but there is no inrush current, the frequency deviation maybe cause the action of the second harmonic braking element to lock the transformer differential protection. If an internal fault occurs, it will lead to refused operation of transformer differential protection. When the frequency decreases and recovery inrush current occur, the magnetic flux increases and the inrush current amplitude also increases, which may lead to mal-operation of the differential protection.