Research On Series-connected Grid-tiedinverter System

Photovoltaic generation is one of the most widely used renewable energy applications. It is siginificant to explore a PV generation system with the combined features of high efficiency, high reliability and low cost. This paper investigates a new structure of series-connected grid-tied inverter system, which connects the AC-line frequency outputs of several inverters in series directly. This structure reduces the voltage-gain of inverters, so that lower the difficulty of inverter design and improve conversion efficiency. In addition, this system structure has advantages of high panel-level MPPT performance, no DC high-voltage, low cost, and flexible configuration.Firstly, this paper illustrates the structure of series-connected grid-tied inveter system and its working principle. The design of the series-connected system is optizmized with the efficiency improvement as the target. Both the power losses and costs are compared among four improved H-bridge topologies. The Heric topology with the highest efficiency is selected as the low voltage-gain inverter. According to the power loss analysis, the best series-connected number of both single-stage and two-stage inverter system is determined.Three control strategies of series-connected grid-tied inverter system are investigated including voltage/current collaborative control, distributed current control, and distributed voltage and current dual loop control. The steady and dynamic performance, reactive power regulation ability and distribution degree of three control strategies are fairly compared according to PSIM simulation results. The distributed voltage and current dual loop control strategy is most suitable for practical application.According to system specifications, the key parameter design process of both the power circuit and control loop is also introduced. To verify the series-connected grid-tied system and its corresponding control, a downscaled PV system prototype including three series-conncted inverters is built in the laboratory. Both the steady and dynamic experimental results show that the distributed voltage and current dual loop control strategy is practicable and effective.