Three-level NPC-based bidirectional PWM converter operation for high availability/power quality bipolar DC distribution networks

This work proposes an operation strategy for a four-legs neutral point clamped converter (NPC) to be used in future bipolar dc distribution networks. The operation of the fourth leg with a three-level modulation is proposed to reduce the required inductance and switching losses. In addition, a proper control scheme based on a state space averaged model for the fourth NPC leg is introduced. This provides ripple free and balanced NPC partial dc-link voltages. The system is able to provide increased redundancy and improved dc voltage quality. Simulation and experimental results are presented to illustrate the operation of the 4-legs NPC with the proposed control and modulation scheme.