Capacitor Energy Variations in MMC Using Harmonics Injection

Modular multilevel converter (MMC) is considered as a good technology for high-voltage direct current (HVDC) transmission systems. It can generate a good quality staircase output voltage waveform due to the cascaded connection of large number of identical sub-modules (SMs). Energy variations in SM capacitor lead to capacitor voltage fluctuations which affect the size and stability of MMC. Traditionally MMC arm voltages are controlled using direct modulation method. The closed loop and open loop compensated modulation methods are implemented to overcome the drawbacks created in direct modulation method. However, these methods rely on the accurate measurements or estimation of SM capacitor voltages. In this paper, capacitor energy variations in MMC based on direct modulation method under three different categories using proportional-resonant (PR) controllers are discussed as (a) pure sinusoidal arm current (b) sinusoidal arm current with addition of large 2nd harmonic (c) sinusoidal arm current with addition of 2nd +4th harmonics. Analytical and simulation results show reduction in capacitor energy variations in MMC while injecting harmonics in the arm currents using direct modulation method.