Topologies And Control Of Single-Phase Transformerless PV Inverters

Recently, the global energy and environmental degradation problems are becoming more and more serious. Therefore, governments around the world are pushing the development of the renewable distributed generation from a strategic perspective. The solar power, which is rich, renewable and clean, has drawn world-wide attentions. With the development of the solar power industry, the price for the solar power system decreases and the newly installed photovoltaic (PV) plants in the world keep growing these years. And the solar power industry has started to move forward to a market-driven industry. However, the initial investment for the solar power generation is expensive, and it takes long time to profit from it. Thus, the solar power generation industry concerns a lot about the system cost as well as the efficiency and the reliability. The transformerless PV inverter has become a trend in residual PV generation systems for its low cost and high efficiency. Unfortunately, the parasitical capacitor of the PV panels would form a common-mode circuit with the inverter and the grid. If the common-mode voltage varies in high frequency, large common-mode leakage current can be generated, which is not allowed by the international standard for the grid-connected devices.The research in this thesis mainly focuses on the common-mode leakage current elimination in single phase transformerless PV systems. Two types of circuit configurations are derived based on the common-mode current suppression principle:inverters with symmetrical output filters and inverters with asymmetrical output filters. Based on these two configurations, the generation law of transformerless topologies and the optimization for the system common-mode behavior are explored.Firstly, a hybrid-bridge topology is derived from the highly efficient and reliable inverter concept (HERIC) topology. Based on the analysis of the operation mode, it can be concluded that the hybrid-bridge topology can suppress the common-mode current in transformerless PV system. A PWM strategy is given for the hybrid-bridge topology to show the performance of the circuit. Besides, the non-ideal factors in the circuit that is related with the leakage current are discussed. Also, the comparison between the hybrid-bridge topology and the other transformerless topologies is carried out to show the advantage of the hybrid-bridge topology in industrial applications. And a3kW prototype is built to test the effectiveness of the hybrid-bridge topology.Secondly, the influence of the circuit parasitics to the common-mode behavior in transformerless inverters with symmetrical output inductors is studied. The HERIC topology is taken as an example to explore the common-mode performance of the circuit. And an active clamping structure for the common-mode voltage is proposed to optimize the common-mode circuit behavior, which can be applied in transformerless inverters with symmetrical output inductors. To verify the effectiveness of the active clamping structure, an active clamping HERIC topology is derived and tested against the conventional HERIC topology.Then, a transformerless multi-level inverter with flying capacitor is proposed. The circuit is composed of a three-level buck converter and a line-frequency unfolding inverter. It can achieve high conversion efficiency and reduce the inductance of the circuit due to the multilevel output voltage. Also, the common-mode leakage current is suppressed because of the line-frequency unfolding circuit. A PWM strategy is proposed for the multi-level inverter based on the analysis of the operational stages of the circuit. Besides, the parameter design for the circuit is also discussed to further optimize the conversion efficiency and power density of the circuit.Finally, a3kW grid-connected inverter product is built based on the hybrid-bridge topology. The inverter can achieve both high reliability and high efficiency. And it is verified through both the experiment and certification. According to the output performance of the product, the inverter is rather competitive as the typical mainstream product.