Research On Transformerless Photovoltaic Grid-Connected Inverters

There is a strong trend in the photovoltaic inverter technology to use transformerless topologiesin order to acquire higher efficiency, lower cost, smaller size and weight. However, galvanicconnection between power grid and solar cell array in transformerless systems can introduceadditional ground currents, resulting in safety and electromagnetic interference problems. Hence,how to increase the efficiency and restrain the common mode leakage current in transformerlesssystems becomes a research focus. This thesis is meant to research on the high efficiency and lowleakage current transformerless PV grid-connected inverters.Firstly, a common-mode equivalent model for transformerless PV grid-connected inverters isanalyzed, and both the influences of different parameters on the common-mode current and theleakage current suppressing method are introduced. Then, in order to eliminate common-modecurrent, two new transformerless PV inverter topologies, a H6-type transformerless PVgrid-connected inverter and a neutral point clamped transformerless PV grid-connected inverter areresearched in this thesis. The H6-type topolpgy (named as H6) is based on the H5topology, and anew power processing path is formed with one additional switch. In the power processing period,the grid-tied current flows through three power switches during one of half line cycles, while flowsthrough only two power switches during another half line cycle. As a result, the conduction lossesreduced and heat-stress balance benefited with the H6topology. The neutral point clamped topology(named as H8) is composed of two basic switching cells, and the voltage stresses are reduced. Inaddition, the common-mode characteristic of H8only depends on the turn-on speed of theanti-parallel diodes. Thus, high efficiency and very low ground leakage current are achieved. Thecommon-mode performance of four existing topologies and the two new topolpgies are compared inthis thesis, and the total losses of power devices in these topologies are fairly calculated.Finally, a universal prototype inverter rated at1kW is built up. Both of the common-modeperformance and the conversion efficiency of the six topologies are compared under the sameexperimental conditions. The experimental results verify the validity and feasibility of H6and H8topologies applied for transformerless PV grid-connected generation system.