Scalable Control-Oriented Model of the Modular Multilevel Converter for Polyphase Systems

This article presents a new state-space model (SSM) to represent the four types of currents flowing through a modular multilevel converter (MMC). In line with the converter topology, the model itself is modular and scalable. In other words, its configuration adapts itself to the number of ac phases, to the active or passive nature of the ac-side grid, to the connection between the ac and dc neutral points, and to the number of SMs. The model describes the influence of the MMC arm voltages on the currents using an average behavior of the SMs. An analysis of the operating space for the currents flowing in the MMC is proposed. Simulations performed covering the region of operation and using a software dedicated to electrical engineering systems demonstrate the value and the accuracy of the proposed model with negligible differences observed for all test conditions. A computational time comparison between the proposed model and the commercial software shows that the model runs about 2 times faster. Hardware-in-the-loop (HIL) testing demonstrates the capability of the scalable model to be used in order to derive a current-control law on a five-phase ac-system.