Research On The Control Method Of Multi-String Photovoltaic Grid-Connected PV System Based On DAB Structure

Building integrated photovoltaic(BIPV)is one of the main ways of urban photovoltaic utilization.Because of the complexity of urban buildings,the efficiency of traditional centralized grid-connected photovoltaic inverters is relatively low.Multi-string photovoltaic grid-connected inverter can be installed and maintained very conveniently.At the same time,each string can achieve the maximum power point trace(MPPT)independently,which effectively overcomes the power mismatch problem and makes it suitable for integrated photovoltaic in urban buildings.In this paper,the dual active bridge is employed as the front-end circuit of multi-string grid-connected photovoltaic inverter,and the control strategy of grid-connected photovoltaic inverters is mainly studied.The power injected into grid of single-phase photovoltaic grid-connected inverters contains double frequency component,which will cause secondary ripple in DC-side voltage.On the one hand,it will produce large harmonic component in grid-connected current,on the other hand,it will affect the maximum power point tracking efficiency of the front-stage.Therefore,large capacity capacitors are generally adopted as DC-link,which increases system loss and volume.According to the cause of secondary ripple,this paper proposes a new control method,which can effectively reduce the capacity of connecting capacitor and ensure the high quality of grid-connected current waveform.Considering the problem of large backflow power of dual active bridge under traditional single-phase-shift(SPS)control mode,extended phase-shift(EPS)control was employed in this paper.In order to reduce backflow power and ensure soft-switching,a new optimization method has been studied based on DAB's working principle and mathematical model.Then combining with the maximum power tracking control of photovoltaic,and considering eliminating the influence of secondary ripple voltage on the output side,a control strategy of DAB under extended phase-shifting mode was proposed,which effectively improves the efficiency of MPPT.Finally,a simulation model of multi-string photovoltaic grid-connected inverters based on DAB structure was built in PSCAD with the above control method.The simulation results show that the proposed control strategy is effective.When the secondary ripple amplitude of capacitor voltage on DC side is large,the grid-connected current with better waveform quality can still be obtained,and the photovoltaic MPPT can be effectively improved.