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Research on specific fault analysis and protection countermeasures of AC/DC hybrid system

The AC/DC hybrid grid is a new form of grid development. The world's largest and highest voltage hybrid grid has been built in China. Unlike the pure AC system, AC/DC hybrid system AC fault characteristics is the result of the superposition of faults in the AC and DC systems. During the fault, the AC and DC systems interact with each other. When a line fault occurs on the AC side, it is easy to cause a commutation failure on the inverter side, which in turn adversely affects the AC system, resulting in a frequency offset of the power frequency fault current. Therefore, the power frequency protection based on Fourier transform has an adaptive problem in the AC/DC hybrid system. When the serious commutation fails, the power will be reversed, and the traditional longitudinal protection may be misplaced. This paper analyzes in detail the commutation failure and power reversal mechanism during AC system failure, the variation of AC system fault characteristics and the applicability of longitudinal protection. On this basis, further explore the impact of fault location and transition resistance on commutation failure, frequency offset and power reversal. Finally, a new protection scheme for power reversal in AC/DC hybrid systems is proposed.