Distributed Control for the Modular Multilevel Matrix Converter

The modular multilevel matrix converter (M3C) is a direct ac/ac topology and is promising in the medium-voltage high-power application. However, the current M3C prototypes mainly adopt the centralized control structure that may hinder its modularity and flexibility. Based on the new distributed control architecture, the corresponding design principles for the M3C are proposed to realize both the distribution of control tasks and improve the expandability. And in order to reduce the dependence of a real-time communication network, a time-dimension decoupling strategy based on a voltage decoupling and current coupling control algorithm is employed, in which the voltage loop is divided into the input side, output side, and inner circulating decoupling models that are executed in the system controller, while the real-time current loop is established on the coupling abc coordinate frame in each local controller. In order to reduce the influence of communication delay, a novel scheme to replace the direct ac current reference with dc power value and double synchronization phase information is adopted. The controller parameters of the current loop are also designed carefully after frequency decomposition. Finally, an experimental prototype is constructed to verify the feasibility of the proposed control scheme after a series of tests in steady-state and dynamic operations.

I. Introduction

During recent years, the modular multilevel converter (MMC) family has been attracting attention in the industry and showing considerable promise in medium- or high-voltage industrial applications [1]. The modular multilevel matrix converter (M3C), also called the triple-star bridge cells converter [2], is also a member of MMCs. Fig. 1 shows the main circuit configuration of the M3C, which connects a three-phase load system to a three-phase supply system using nine branches. It can be seen that the M3C is a direct ac/ac converter without a dc-link capacitor, which is common in two-stage ac/dc/ac converters. Therefore, it enables the bidirectional power conversion and controls the sinusoidal current waveform of the input side with an arbitrary power factor at the same time [3]. Compared with the MMC, the M3C does not suffer a large capacitor voltage fluctuation at low-frequency operation. Therefore, it is more suitable for the high-torque low-speed drive applications [4]–[6], and is especially promising in an offshore wind power low-frequency ac transmission system [7], [8]. Fig. 1.

Circuit configuration of the M3C.