Research On Soft-switching Grid-connected Photovoltaic Inverter

In hard-switching inverter, power switch IGBT works with switching loss, which takes large proportion in the total system loss. However, soft-switching could reduce the loss and improve the efficiency. In this paper, ZVT is applied to control the voltage of IGBT, so that there is no switching loss due to the overlap of voltage and current. Because each bridge leg has an auxiliary resonant cell, and it is independent among three phases, so the operational reliability of the system is higher. The introduction of the auxiliary resonant cell has no effect on controlling strategy of the main circuit, and we only need to add control algorithm of auxiliary cells.Study on the working principle of zero voltage ARCPI, and analyze10working modes. The voltage and current’s mathematical formula is derived, then build the simulation circuit by Matlab/Simulink software, realize the zero voltage switching of the inverter.For the traditional controlling strategy can’t adapt to the changing of dc voltage in a large scale and also the imbalance of the dc bus capacitor voltage in photovoltaic power system, a new time-varying control strategy is provided to reduce the switching loss further. Especially in the stage of turning off, the turn-off loss is greatly reduced through grading using auxiliary resonant cell. When the dc voltage is changing and the dc bus capacitor voltage is imbalance, time-varying control strategy can make sure that invert circuit IGBT switch in ZVT, auxiliary IGBT and diodes switch in ZCT, all these work without switching loss.In order to build the250kW three-phase inverter system, we make selection of lord and auxiliary IGBT, fast recovery diode and dividing capacitor, give rigorous theory analysis and calculation of the value of resonant inductor and capacitor, on this basis select the most appropriate parameters. Then calculate the key parameters in the control strategy of inverter. In grid-connected LCL filter circuit and feed-forward decoupling current tracking is designed. After a great deal of commissioning work, the filtering inductor and capacitor and PI adjuster parameters are determined, in which case the harmonic content is low in output current, at the same time the dynamic response is good.Hard-switching inverter, time-fixed ZVT soft-switching inverter and grid-connected of time-varying ZVT soft-switching inverter are compared. From the analysis of IGBT voltage and current, resonant inductor current, harmonic content in the current of inverter and nets, we conclude that the soft switch technology can reduce switching loss,and improve the quality of output power. And also, time-varying control has more advantage than timie-fixed control. In time-varying control, the contrast simulation is done by changing resonant inductor and capacitor. The result is that the turn-off loss can be reduced more through reducing capacitance, but it will increase the harmonic content in output current as well as the precharge time. So we just need to select appropriate value of resonant inductance and capacitance.