Impedance Modeling and Analysis of Series-Connected Modular Multilevel Converter (MMC) and its Comparative Study With Conventional MMC for HVDC Applications

Similar to the conventional three-phase parallel-connected MMC (PC-MMC), the three-phase series-connected MMC (SC-MMC) is a strong potential solution for high-voltage direct current (HVDC) applications. The study of SC-MMC focuses on the control strategy, neglecting its small-signal dynamics, not to mention its stability when interacting with the AC grid. Work on small-signal modeling of MMC is limited to parallel-connected configurations. SC-MMC differs from PC-MMC in structure and harmonic content; hence, it is necessary to establish an accurate model of SC-MMC according to its own characteristics. This paper introduces the circuit and control of SC-MMC and compares it with PC-MMC. Multi-harmonic linearization is used to establish the AC impedance model of SC-MMC over the whole control system, including the third harmonic suppression controller. We take a PC-MMC station of the Wudongde HVDC project as a benchmark to explore the potential application of SC-MMC when constructing new HVDC stations to extract power from an existing DC transmission corridor. The strengths and weaknesses of SC-MMC in HVDC applications are discussed, and it is compared to PC-MMC with regard to the impedance characteristic.