Impedance Modeling and Analysis of Series-Connected Modular Multilevel Converter (MMC) and its Comparative Study With Conventional MMC for HVDC Applications
Can Wang, Jiexiong Xu, Xuewei Pan, Wenming Gong, Zhe Zhu, Shukai Xu
Abstract
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.