Capacitor voltage ripple reduction in MMC-HVDC system using flat bottom current method

Abstract

Modular-multilevel converters (MMCs) are considered a useful technology in high-voltage direct current (hvdc) transmission systems. Since it offers many advantages like modular configuration with scalable output voltage. Voltage ripples on sub-module (SM) capacitors in MMC and circulating currents are two major challenges, which can affect the converter stability and performance. Conventionally, direct modulation method is used to insert or bypass SMs in MMC producing circulating current and SMs capacitor voltage oscillations. The closed loop and open loop control methods are used to overcome drawbacks in direct modulation method. However, these methods rely on the accurate measurements and estimation of SM capacitor voltages. A flat bottom current method is proposed in this paper to minimize the amplitude of ripple voltages on SM capacitors in MMC-based HVDC systems. Limited capacitor energy changes leading to reduce capacitor voltage ripples are observed by the proposed methodology using direct-modulation method without using circulating current suppression controller. The proposed method is tested on a two-terminal MMC-based HVDC system implemented by vector control approach in PSCAD/EMTDC software. Analytical results are verified by simulation results in reducing capacitor voltage ripples based on the direct modulation approach.