Study on Harmonic Transfer in a Two-terminal MMC-Based HVDC System Considering AC Background Harmonics

In recent years, the modular multilevel converter (MMC) has become the most attractive multilevel converter technology for medium and high power applications, especially for high voltage direct current (HVDC) transmission systems and distributed power plants due to its outstanding advantages. However, MMC-based applications have some technical challenges such as circulating current, SM capacitor voltage balance, DC-side fault handling, etc that need further investigation. In the HVDC systems, harmonic issues receive considerable concerns from researchers and designers. This paper presents the analysis and simulation of the two-terminal MMC-HVDC system considering AC background harmonics. Harmonic transfer mechanism of the MMC-HVDC system under background harmonics is analyzed by using the average switching functions. The simulation results in PSCAD/EMTDC are agreeable to the theoretical analysis: the existence of background harmonics on the AC side of an MMC will generate the harmonic current on the DC lines and this harmonic in turn produces new negative- and positive-sequence harmonics on the AC side of the other MMC in the system. This gives an insight on the mechanism of harmonic propagation in MMC-HVDC systems, which is valuable to propose suitable solutions for mitigating harmonics or preventing potential resonance in these systems.