Waveform-Similarity-Based Protection Scheme for AC Transmission Lines of MMC-HVDC System with Offshore Wind Farms

The modular multilevel converter (MMC) has become a very promising technology for long-distance and large-capacity transmission of offshore wind power. However, both sides of the AC transmission line at the sending end are controllable electronic power devices, resulting in difficulty in fault identification and inapplicability of traditional differential protection schemes. In order to solve this problem, a wave-similarity-based protection scheme is proposed for AC transmission line faults. Firstly, the symmetrical and asymmetrical fault current characteristics of the double-fed induction generator (DFIG) and MMC are studied, indicating that the fault current characteristics are obviously different from the synchronous units. Secondly, the wave-similarity-based protection scheme is proposed based on the different wave forms of the fault currents of the MMC and DFIG. When the similarity coefficient is less than the margin coefficient, there is a fault in this phase. Moreover, the proposed wave-similarity-based protection scheme can identify all types of short-circuit faults correctly and is not affected by the transition resistance. Finally, simulations of an MMC-HVDC system with offshore wind farms are conducted to validate the effectiveness and correctness of the proposed protection scheme.