Combined switching state of model predictive control for unbalance compensation strategy under three-phase four-wire system

Abstract

This paper proposed a method based on MPC strategy with optimal combined switching states, which enhance the natural voltage balancing capability of the NPC inverter under the three-phase four-wire grid. Natural balancing refers to maintain half of the dc-link voltage without external circuitry or an imposed control action designed to enforce voltage balance. We analyzed how the zero-sequence currents affect the neutral-point voltage at each type of vector and evaluated the index weight of different types of the vector by Analytic Hierarchy Process. Based on this conclusion, a combined state consists of zero-, long- and short- vectors applied to switching states selection compared to the unselected 27 switching sates. The proposed combination finds a trade-off between zero-sequence currents compensation and the neutral-point voltage balance. Simulation verifies the effectiveness and accuracy of combined switching state theory and control algorithm.