Research On The Control Strategy For High-Performance Transformless Grid-connected Inverter

Solar energy has been recognized as the most ideal renewable energy because it is rich andbasically no pollution. To reduce costs, Photovoltaic (PV) power supplied to the grid is gaining moreand more attenttion. But the power quality deterioration as well as safety and reliability caused by theconnection between the PV panels and grid are still remained to be improved. As the transmissionpath between PV panels and grid, the performance and efficiency of grid-connected inverters areimportant to the power generation system. And the transformerless grid-connected inverters becomefirst choice because they are characterized with higher efficiency, lower bulk, weight and costs, etc.First of all, the leakage current of transformerless PV grid-connect inverter needs to beeliminated. According to the common leakage current analytical model of full-bridge and half-bridgestructure inverters，the mechanism of leakage current elimination for the general used inverters hasbeen analyzed in detail. Then the optimized topologies of H5, Heric and a H6-type configuration withlower leakage current and their optimized modulation strategy to make inverters operate in any powerfactors have been studied.This paper focuses on the control strategies for grid-connected inverters. The capacitor currentand injected grid current feedbacks control with grid feedforward compensation has been analyzed. Adetailed design process of LCL filter, which not only promises smaller volume, weight, costs andlosses, limits switching frequency ripple of injected grid current to an acceptable range, but alsoeffectively restrains the harmonic distortion from the grid, has been given. Some non-ideal factors,including the turn-on time, turn-off time and the conduction voltage of power switches, dead-timeeffect and harmonic distortion of grid voltage, cause the injected grid current distortion. And theresonant harmonic compensators have been used to suppress this current distortion. In order toovercome the effects caused by wide range of grid impedance values, an adaptive current controlscheme based on grid impedance estimation is proposed. According to the estimated value, thevoltage signal used for feedforward compensation is modified, and the controller parameter is alsoadjusted so that good phase margin or high bandwidth is guaranteed. The impedance measurement byinjecting a harmonic current and measuring the voltage response and the islanding detection methodbased on this impedance measurement have been studied. Simulation and experimental results verifythe correctness and validity of the oH5topology and its control strategy.