Comparison of a soft switched TCM T-Type inverter to hard switched inverters for a 3 phase PV grid interface

Abstract

Three phase photovoltaic (PV) grid interfaces become more important, as the amount of installed medium scale photovoltaic power plants grows. The use of a split DC-link with its midpoint connected to AC ground is the most widely used approach to provide a constant common mode voltage in transformerless 3 phase inverters used as PV grid interfaces. These inverters achieve peak efficiencies exceeding 99% using a hard switched T-Type topology and SiC transistors. This paper investigates, whether the efficiency can be further improved by soft switching concepts. A soft switched TCM T-Type inverter is compared to a hard switched T-Type and a hard switched full bridge inverter for a 22kW, 3 phase photovoltaic grid interface with the constraint, that all systems have the same power density of 5 kW/liter. The comparison is based on model based optimizations of each system and measured switching losses for all three topologies. The hard switched T-Type reaches a European efficiency of 99.0%. The soft switched TCM T-Type can improve the efficiency to 99.1%.