Design Of Transformerless Photovoltaic Power Generation System Based On Multi-level Technology

With the aggravating of energy consumption and environment pollution, solar energy photovoltaic generation technology attracts researchers'attentions for its pollution-free, reserves rich and distributed widely. Nowadays, most photovoltaic inverters include an isolating transformer, which has large volume, high cost, and especially great loss, thus the efficiency of the whole system is decreased. One of the trends of the photovoltaic power system development is transformerless one. However, in transformerless photovoltaic generation system, there are leakage currents, because of the removal of the transformer.A new high-efficiency transformerless photovoltaic generation system, which is based on single-phase hybrid clamped three-level inverter, is proposed in this paper. The leakage current problem in transformerless photovoltaic generation systems is solved. Because the AC output terminal voltage is synthesized by three voltage levels, it has low harmonic distortion and thus reduces filter requirements. As a result, it reduces cost, size and weight again. Comparing with diode-clamped multilevel inverter, the hybrid clamped three-level inverter doesn't have the problems of DC capacitor voltage unbalance, so it decreases the complexity of the control strategy greatly, and is beneficial to the level number expansion.The first part of the paper describes the development of the solar photovoltaic technology, introduces the research background and significance of this item, and then reviews several topological structures of transformerless photovoltaic generation system, and proposed a new high-efficiency transformerless photovoltaic generation system, which is based on single-phase hybrid clamped three-level inverter.The second part detailed analysis the causes and effects factors of the leakage current. And then leakage current of hybrid clamped three-level inverter is analyzed by theoretical analysis method and equivalent circuit method. Both theoretical analysis method and equivalent circuit method verify the total common-mode voltage generated is constant,so the proposed topology solved the problem of the leakage current. In the third part, a 5kVA prototype based on the hybrid clamped there-level inverter is built. The prototype in stand-alone mode is used LC filter and in filter network method is used in LCL filter, and suitable filter parameter is design. TMS320F2812 is made as the core of the main board and M57959L is made as the core of the driver board. The detection circuit and protection circuit are also design for the prototype run good.The fourth part the software development environment of DSP is introduced; the program based on phase shifted carrier PWM modulation and the main programs are design; the voltage feedback dual closed-loop control is introduces and design; the attentive points in programming are concluded.Finally, the simulation experiments of the single-phase hybrid clamped three-level inverter based photovoltaic system based on MATLAB/SIMULINK is design. In the simulation experiments phase shifted carrier PWM modulation and the voltage feedback dual closed-loop control is used. The result of simulation experiments verifies the proposed topology solved the problem of the leakage current. Based on the simulation experiments, a 5kVA prototype which based on the hybrid clamped there-level inverter is built, and a good result is achieved, experimental waveforms are given.