Research On Control Strategies Of Mudular Multilevel Matrix Converter

For intelligent power system and medium or high voltage motor driving, the application of power electronic technology is becoming increasingly extensive, then modular multilevel converter (MMC) has become a research focus because of significant advantages of low harmonic, high modularization and no need of many clamping devices. However there are some limitations for MMC, such as the sub-modular (SM) structure needs to be changed for the DC fault problem in MMC and MMC needs middle link to make AC/AC conversion. Modular multilevel matrix converter (MMMC) has combined the superiority of MMC and matrix converter (MC), then it could directly achieve AC/AC transformation and has four-quadrant operation capability. The output could achieve arbitrary amplitude, frequency and phase. As a result, MMMC has a broad application prospect in fields of HVDC, motor speed control and power electronic transformer.The topology structure of MMMC has a strong openness, and MMC could be regarded as a special kind of MMMC. MMMC achieves AC/AC conversion, leading to more complicated frequency relationship and SM capacitor voltage control. This paper introduces basic topology structures and research status of MMMC, and takes single phase to three phases MMMC as the research object, and analyzed arm power and capacitor ripple voltage under steady-state. Switch state mathematical model and the small signal model are established from SM to arm of N-SMs. The transfer function could be obtained by simplifying models for close-loop system design. On basis of power analysis under steady-state, the operating of MMMC are divided into three conditions according to frequency range:normal frequency condition, low output frequency condition, similar input and output frequency condition, and corresponding control strategies are proposed.Under normal frequency condition, a control scheme for hierarchical capacitor voltage balancing and arm current control is established, without additional suppression for circulating currents. Under low output frequency condition, on basis of low frequency power fluctuation analysis, the method of voltage envelope is put forward and it uses PI controller to abstract fluctuating strongly double-output frequency voltage ripple among three phase units. The circulating currents following low frequency fluctuation power could be obtained and are injected into close-loop control as effective components, achieving zero steady-state error control without extra power. This method has the very strong generalization and universality, and great reference value for the other modular multilevel topology research. Under similar input and output frequency condition, a method of injecting high-frequency common-mode voltage and circulating currents of same frequency is proposed, and reconstructed arm power only contains high-frequency components so that ripple voltage is reduced greatly. In this paper, a control block of injected common-mode voltage is designed so that the voltage could follow fluctuating power without complex calculation. For above three conditions, the Matlab/Simulation models are built, verifying the effectiveness and great dynamic and static performance of proposed methods.