An Improved Modulation Strategy for Fast Capacitor Voltage Balancing of Three-Level NPC Inverters

This paper presents an improved pulsewidth modulation strategy in conjunction with an optimal compensator for fast capacitor voltage balancing in three-level neutral-point-clamped (NPC) inverter. The voltage balancing compensator is designed in such a way that it produces optimal unbalance compensation coefficient according to the inherent limitations related to the variability range of modulating signals. It generates maximum compensating neutral current for the full modulation depth extending into overmodulation region and throughout the entire range of load power factor angles, and thus improves the unbalance compensation ability for all the operating conditions of the inverter. The optimal compensation offset signal corresponding to each operating point is determined from the boundary limit of the auxiliary modulating signals. Particle swarm optimization is applied for such purpose. The performance of the proposed optimal compensator for different combinations of modulation index and load power factors are evaluated through extensive simulation study using Matlab/Simulink and validated in experimentation using a three-level NPC inverter prototype with induction motor load. Finally, the voltage balancing performances of the proposed compensator are compared with that available in literature to confirm the usefulness of the proposed concept.