Grid active power filters using cascaded multilevel inverters with direct asymmetric switching angle control for grid support functions

This paper presents a new control technique for cascaded multilevel inverters, which can achieve harmonic and reactive power compensation and balance unbalanced load simultaneously. Unlike existing control strategies which work in the time domain and focus on reducing the overall harmonics and reactive power without differentiating individual harmonics, this new method works in the frequency domain and can efficiently compensate individual harmonics and reactive power apart from balancing unbalanced load. The principle of the proposed technique is to compensate harmonics and reactive power and balance unbalanced load with direct asymmetric firing angle and conduction angle control for each H-bridge of multilevel inverter. To verify the effectiveness of this new technique, genetic algorithm (GA) is used to solve transcendental equations and find switching angles of each H-bridge of multilevel inverter. From the simulation results, it can be seen that the proposed technique can not only achieve the same harmonic and reactive power compensation performance with a much lower switching frequency than traditional control methods of cascaded multilevel inverter based active power filter (APF), but also can balance unbalanced loads successfully at the same time.