Low output frequency operation of modular multilevel matrix converter

When the multilevel modular matrix converters (MMMC) operate at low frequency, sub-module (SM) capacitors voltage mainly contains doubling-output frequency component inversely proportion to output frequency, especially when MMMC operate at zero frequency, the voltage ripple of SM capacitor can be infinite in theory. This paper proposes two methods to solve the voltage ripple problem of MMMC operating at low frequency, even near zero. The first one injects high frequency zero-sequence voltage on the neutral point and injects high frequency negative-sequence currents to each phase. To suppress low frequency fluctuation, the magnitude and phase angle of the zero-sequence voltage changes according to low frequency power, without complicated calculation. The second method is called following feedback low frequency ripple envelope method. The low frequency ripple voltage is calculated by difference between the reference voltage value and the average voltage value of each phase, then got eliminated by use of feedback close-loop control. In this way, low frequency ripple can be detected and minimized. The Matlab/Simulink simulation is built and the results have con firmed the validity and effectiveness of these methods.