A novel neutral point clamped transformerless inverter for grid-connected photovoltaic system

Abstract

A novel full-bridge single-phase neutral point clamped (NPC) transformerless inverter for grid-connected photovoltaic system is proposed in this paper with the intent to improve the reliability and reduce the common-mode leakage current. The proposed inverter incorporates two coupled inductors along with series connected switch on ac side that operate alternately in positive and negative half period of grid. This inverter eliminates shoot through problem which is common in NPC topologies, resulting improved system reliability. Therefore, dead time at both switching frequency commutations and grid current zero crossing instants is not required. The coupled inductor along with series connected switch will not allow current to flow through antiparallel diode of switches, avoiding reverse recovery issues. Switches such as cool MOSFETSs now can be used in the proposed inverter, which increases the converter efficiency. In addition to high reliability, the proposed inverter maintains common-mode voltage at a constant level resulting low common-mode leakage current. Derivation of the full-bridge NPC inverter from existing single-phase transformerless H6 topology is presented. Other alternative full-bridge NPC inverter topology is also illustrated. The principle of operation and leakage current analysis of proposed NPC inverter are described. Finally, performance of the NPC inverter is verified by simulation studies and results are presented for peak power rating of 2.2kW.