Transformerless Three Phase Three Level Photovoltaic Grid-connected Inverters

With the development of photovoltaic (PV) power generating system, the demand of middleand high power level PV grid-connected inverter is increasing. On the other hand, in order todecrease the cost, size, weight and increase the efficiency, depth study of transformerless inverterhas been explored by many scholars. This thesis is meant to research on the high efficiency andmiddle power level three phase transfomerless grid connected inverter.Firstly, the operating principle of three phase diode clamped three level inverter is analyzed,the common mode equivalent model of three phase PV grid-connected inverter is deduced, and theconclusion points out the most important factor that influenced the leakage current of three phasephotovoltaic grid-connected inverter is the common mode voltage pulsation amplitude andfrequency, the influence of basic vectors to common mode voltage is analyzed, a modifiedSVPWM arithmetic which can suppress the leakage current is given, and the implementation stepsis given in detail. Secondly, the mathematical model of three phase three level grid-connectedinverter is analyzed in different coordinate system, the parameters of current loop and voltage loopis calculated based on the description of the relationship between the current loop and voltage loop.In order to build a three phase three level photovoltaic grid-connected inverter prototype rated at17kW, the design method of power circuit is given, including the switching devices, dc-linkcapacitor and LCL filter of ac side. The design method of control circuit is presented, including thesample conditioning circuit and the protection circuit.The grid connected system simulation model is built in PSIM, and the design method ofhardware parameters and software parameters is verified. The experimental results of soft starting,steady state and dynamic state are given based on the three phase transformerless photovoltaicgrid-connected inverter prototype platform. The results validate the high efficiency and lowleakage current characteristic of the three phase transformerless photovoltaic grid-connectedinverter.