Control Strategy for Four-Leg Nine-Switch Inverter under Unbalanced Loads

On account of that nine-switch inverter (NSI) cannot supply symmetrical voltages to unbalanced loads, based on the NSI, N-phase leg is added to the neutral points of the loads, which forms a four-leg nine-switch inverter (FLNSI). The neutral point voltages under unbalanced loads are analysed and derived by space vector pulse width modulation (SVPWM) strategy for calculating the duty ratio of N-phase leg. While the calculations of other three-phase legs duty ratios of SVPWM method remain unchanged. Although SVPWM method can realize FLNSI outputs symmetrical voltages for unbalanced loads, it needs trigonometric function calculation and sector judgements. The carrier-based pulse width modulation (CBPWM) strategy is proposed for the sake of simplifying SVPWM. All of the duty ratios are used to calculate modulation waves of FLNSI. Compare the modulation waves with a carrier, which can control the neutral point voltages as corresponding zero-sequence voltages. Therefore, two groups of three-phase symmetrical sinusoidal voltages are obtained under both balanced loads and unbalanced loads. Finally, the experimental results are given to show the advantages of FLNSI topology and to validate of the effectiveness of the applied modulation strategy.