Load Balancing and STATCOM Application of MMC by Using Parallel Processing Controllers

In this paper, an arm-voltage based control has been used on a three-phase grid-connected modular multilevel converter (MMC) for harmonic current suppression, load balancing and negative sequence current elimination of grid currents when non-linear and unbalanced loads are fed from the same bus. The effect of unbalance between two DC-link capacitor voltages is taken into consideration. The conventional model of single-phase grid-connected MMC has been modified to include this unbalance effect into the arm-voltage based control. The control technique is accomplished by using synchronously rotating reference frame (SRF), proportional resonant (PR) controller, sorting algorithm and phase-shift carrier pulse width modulation (PSC-PWM). The control of the MMC with a PSC-PWM and sorting algorithm causes a significant time delay on the PWM waveforms organized in the registers of DSP. Two DSP units are operated with parallel processing to control the entire system. A new additional hardware is proposed to avoid the time it takes to renew the decision in the register of the DSP when the submodule is kept in the same switching state in the following period of the carrier wave. The results of Matlab simulation and experimental work acquired on the three - phase five -level MMC are compared.