Two-leg Modular Multilevel Cascade Converter as Active Power Filter - Control Improvement

Abstract

This paper analyses the use of a simplified ac-side current control algorithm strategy, independent of the number of submodules, applied to a Modular Multilevel Cascade Converter acting as an Active Power Filter (MMCC-APF) with a reduced hardware structure. Here, Model Predictive Control (MPC) with a reduced number of states is used for both ac-side current control and submodules capacitor voltage balancing algorithm. Furthermore, analytical criteria are developed, to define the minimum number of states required for current control. This approach improves the control strategy, thus reducing computational effort for MMCC in APF applications. For the calculation of APF compensating currents, two main strategies are addressed, in order to assess ac-side currents control limitations: (i) total and (ii) specific frequencies harmonic compensation. The system analyzed in this paper consists of a nine-level MMCC connected to a medium voltage distribution grid, compensating the harmonics drained by a diode rectifier. All the analyses are conducted in the environment of circuit simulation PSIM.