Power Flow Control with Capacitor Voltage Control of Hexagonal Modular Multilevel Converter as a Loop Power Controller

抄録

This paper intensively discusses control of the transformer-less hexagonal modular multilevel converter (HMMC). The HMMC is applied as a loop power controller (LPC) for a three-phase power system in which the ends of two power feeders are connected to form a power loop. The LPC is an attractive power system that enables changing the power flow balance between two feeders to avoid exceeding the current capacity. HMMC is a modular multi-level converter (MMC) topology, and has extensive ability for multilevel output voltage. MMC represents an emerging topology for three-phase BTB applications such as wind turbine generation systems, STATCOM, etc. The transformer-less configuration of MMC facilitates weight and size reduction of the converter, but the capacitor voltages in cell modules need to be regulated properly. This paper proposes a control scheme based on the current control of HMMC for LPC applications, which includes active and reactive power control, capacitor voltage control, and some related controls. This article describes the current flow control between two feeders; the three-phase load on each feeder is connected to HMMC. Especially, in order to maintain constant capacitor voltages, a combined circulating current control strategy including independent circulating current control for the primary and secondary sides and an internal circulating current control is proposed. This method is effective for operation under different phases between two feeders or three-phase unbalanced load for LPC applications. Further, simulations and experiments in typical cases verify the feasibility and effectiveness of the proposed control strategies.