Research On Topology And Control Of PV Inverter Based On DC-bus Capacitor Optimization

In the Grid-connected PV systems, PV inverter is a bridge connecting PV arraywith public power grid and its performance directly influence on the performance andefficiency of the whole PV power generation system. In conventional PV invertertopology, the DC side of PV inverter should be in parallel with an electrolyticcapacitor whose capacity is large enough to achieve power decoupling. However, theapplication of large-capacity electrolytic capacitor would lead to the power dissipation,low reliability, oversized volume and some other disadvantages. Most of all, the shortlife expectancy of the electrolytic capacitor makes the life expectancy of PV inverterand PV array seriously mismatched, which hinders the application and developmentof PV inverter, and then the promotion and development of the entire PV industry.In this article, the goal of the study is to prolong the life expectancy of the PVinverter. According to the short life expectancy of electrolytic capacitor, this articlestrives to improve the performance of the PV inverter by replace the large electrolyticcapacitor with film capacitor which has a relatively small value of the capacity. Thisarticle analyzes the reasons of the PV inverter decoupling problem in detail. Andthrough derivation, a conclusion is reached that if the output power of PV array andfrequency of the grid voltage remain constant, the capacity of the decouplingcapacitor in the DC bus side is only influenced by the average capacitor voltage andthe voltage ripple, but has nothing to do with any other condition. Based on the aboveconclusion, this article designs a multi-level transformer-isolated PV inverter based oncapacitance optimization. Besides, this article proposed a double-loop control strategybased on PR controller which includes a current inner loop and a capacitor voltageouter loop. Meanwhile, in order to prevent the DC component of the output currentinject into the grid, the virtual capacitor control strategy is applied.The simulation of the proposed PV inverter and its control system is conductedin the MATLAB/Simulink environment. Results show the correctness and rationalityof the design in this paper.