The challenge and solution of transformer protection under AC-DC hybrid transmission grid

In AC-DC hybrid transmission grid, Both DC magnetic bias and commutation failure are common circumstances and which have adverse impact on the behavior of transformer protection in adjacent AC side. Taking the ±500kV Bao-Dei and Gui-Guang II HVDC projects as model, and based on a large number of RTDS experiments, the performance of transformer differential protection under those two situations is discussed in detail. Quantitative analysis indicates that the second harmonic produced by DC magnetic bias may cause transformer protection fail to operate in minor internal faults. On the other hand, the AC system can be regarded as a harmonic source under the circumstance of commutation failure, and the high content of the second harmonic may lead to mal-restraint of transformer differential protection especially in slight internal faults. To solve those problems, a novel protection method based on excitation impedance is put forward in this paper. Simulation results show that the proposed scheme is high in sensitivity and reliability even under condition of minor internal fault along with DC magnetic bias or commutation failure.