The Impact of System Power Angle on Power Frequency Variation Distance Protection of AC Transmission Lines Connected to the Receiving End Converter of MMC-HVDC System

Modular multilevel converter based high voltage direct current (MMC-HVDC) system connected to AC lines has become a typical scenario in the power system. Previous studies have proposed the concept of equivalent power angle for MMC-HVDC systems and explored the impact of changes in equivalent power angle on distance protection. In fact, changes in equivalent power angle of MMC-HVDC systems also have a significant impact on the adaptability of distance protection to power frequency changes. This study focuses on the adaptability of power frequency variation distance protection for AC transmission lines of AC transmission lines connected to the receiving end converter of MMC-HVDC system. Combining with the equivalent power angle analysis method of MMC-HVDC system, the adaptability of protection in the event of metallic faults in AC transmission lines is explored. In the case of phase to phase metallic short circuit faults, due to the increase of the equivalent power angle and the weak power characteristics of the MMC-HVDC source, there is a risk of rejection for internal short circuit fault and mis-operation for reverse short circuit fault; When the asymmetric grounding fault occurs, the protection has good adaptability due to the presence of zero sequence current. A model of renewable energy power MMC-HVDC system was established in PSCAD/EMTDC, and a large number of experiments verified the correctness of theoretical analysis. The theoretical analysis can provide a theoretical basis for the installation and setting of power frequency variation distance protection in engineering.