Impedance-based Approach for High Frequency Interaction Analysis of MMC Systems

Modular Multi-level Converter (MMC) and its Valve Base Controller (VBC) are generally provided by one manufacturer. MMC and VBC operate together with ac network, dc system, and Pole Controller and Protection (PCP) system. Therefore, the MMC system is potential to interact with any of these parts in a wideband frequency range. This paper focuses on the interaction between the MMC system and the PCP system. By using a frequency scanning approach for the CIGRE DCSI MMC one terminal case, the MMC system admittance characteristic is obtained firstly in RTDS simulation. Then the closed-loop stability of the MMC system and PCP control system is analyzed through Bode Plot. With the designed DCSI MMC system, the larger gains of the inner-loop PI controller are the root causes of high frequency interaction between the MMC system and PCP control system. Finally, the conclusion is validated in RTDS time domain simulation.