Analysis and design of grid-connected 3-phase 3-level AT-NPC inverter for low-voltage applications

The 3-level T-type neutral point clamped (T-NPC) inverter has become the most popular multilevel inverter used in low-voltage applications. However, the realization of a bidirectional switch located at the midpoint leg of the T-NPC inverter is achieved using two insulated gate bipolar transistors (IGBTs) with two antiparallel diodes. Power dissipation in these switches is high since there are two semiconductors (an IGBT and a diode) in the current path at the midpoint leg. Switch losses can be reduced and thus the highest conversion efficiency can be achieved if the bidirectional switches in the T-NPC inverter are replaced by highly efficient reverse-blocking IGBTs (RB-IGBTs). The objective of this paper is to assess the performance of a 3-phase 3-level grid-connected advanced T-NPC (AT-NPC) inverter with RBIGBT for low-voltage applications. This paper describes the operating principle of the grid-connected AT-NPC inverter, analyzes the power losses, and discusses the control strategy. A 1.7-kW laboratory prototype is designed and built to verify the feasibility and the effectiveness of the proposed 3-phase 3-level grid-connected AT-NPC inverter. Real-time control of the inverter is experimentally achieved using a dSPACE DS1103 controller. The experimental results show that the efficiency of the proposed grid-connected inverter is about 96.3%.