Modulation and Control Strategy of Three-Phase Three-Level Four-Leg NPC Inverter

In this paper, modulation and control strategies of a three-phase three-level four-leg neutral-point-clamped(3L4L-NPC) inverter are studied. A simplified space vector PWM method is proposed. Firstly, based on the modulation for the two-level four-leg neutral-point-clamped(2L4L-NPC) inverter, the modulation for 3L4L-NPC inverter becomes easier. Three-dimension space vector modulation(3D-SVM) for 3L4L-NPC inverter can be shifted to 3D-SVM for 2L4L-NPC inverter due to the modulation space for 3L4L-NPC inverter can be divided into 12 modulation spaces for the 2L4L-NPC inverter. Secondly, based on the rational arrangement of redundant vectors, an accurate compensation method for neutral point voltage balance is proposed. Since the fourth leg provides a path for the circulation of zero-sequence current, 3L4L-NPC inverter exhibits high performance for asymmetric or nonlinear loads. This paper provides a double closed-loop PI control to maintain the topology working reliably. The advantage of the proposed modulation strategy is that vectors, needed to be synthesized for the reference vector, can be found out easily, and duty cycles for each of them can be calculated in 3D-SVM for 2L4L-NPC inverter. In addition, the DC-Link capacitance voltages can work in a dynamic balancing mode with minimal vibration. The simulation is built to verify the correctness and the feasibility of the proposed modulation and control method.