Coupling Effect Investigation of Series-Connected Converters in MMC-Based Ultra-HVDC Station

The modular multilevel converter (MMC) technology is being utilized in ultrahigh voltage direct current (UHVDC) transmission for much larger power transmission and a series of multiple MMC converters are necessary to achieve ultrahigh voltage. Based on the scheme of the Wudongde multiterminal UHVDC project in China, this article develops a small-signal model (SSM) for MMC-based UHVDC station under constant active power-control mode. The dynamics of the high-end converter (HEC) and low-end converter (LEC) are modeled independently considering the internal harmonics. Then, the dynamic model of an MMC-based UHVDC station is developed by interfacing the models of HEC and LEC, considering the characteristics of parallel connection at the ac side and series connection at the dc side. In addition, a voltage-balancing control approach, which is necessary to balance the voltage between HEC and LEC, is also included in the dynamic model. After verifying the accuracy of the derived SSM, this article studies the coupling effect between HEC and LEC. The results show that the vector current controllers of HEC and LEC closely interact with each other, and the voltage-balancing controller makes the coupling between HEC and LEC more complex. Moreover, the circulating current suppressing controllers (CCSCs) of HEC and LEC are relatively independent.