A New Compact Soft Open Point Based on Cascaded H-Bridge and Mixed-Frequency Modulation

As a power electronic device in place of normal open point, the soft open point (SOP) can improve the operating flexibility of a distribution network. Most of the existing SOP projects adopt the back-to-back modular multilevel converter (BTB-MMC) topology. However, the BTB-MMC requires a large number of submodules, which is costly and bulky. In this article, a mixed-frequency modulation based SOP (MFSOP) topology is proposed. The proposed MFSOP consists of a shunt cascaded H-bridge (CHBs), multiple series CHBs, and multiple <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> resonance branches. Each terminal is equipped with a Y-△ transformer to isolate the medium-frequency zero sequence voltage introduced by the mixed-frequency modulation from the power grid. Compared with the traditional BTB-MMC, the proposed MFSOP can reduce the number of submodules, which makes the SOP more compact in size. The circulating current models are derived, and two circulating current suppression strategies are proposed based on circulating current models. Then, the control strategies are described and the key parameters are designed. Finally, the correctness and validity of the proposed MFSOP are verified by simulation and experimental results.