Topology and Control of Nine-Level Hybrid T-Type Nested NPC Inverter

In this paper, a novel nine-level hybrid T-type nested neutral-point-clamped (9L-HTNNPC) inverter is proposed. The topology is derived by incorporating an additional H-bridge (HB) cell into each phase leg of the four-level NNPC circuit. This configuration can eliminate the DC-link neutral point and reduce the number of active switches. However, the main drawback of the proposed topology is the capacitor voltage imbalance, caused by insufficient redundancy, particularly for the capacitors in the HB cells. To address this issue, a capacitor voltage balancing control strategy is developed based on redundant switching state selection and offset voltage injection under the level-shifted PWM (LSPWM) technique. The performance of the proposed topology and balancing control method has been verified through simulation results under various operating conditions.