Research On Repetitive Control Strategy For Transform-less Photovoltaic Grid-Connected Inverter

Solar energy alleviates the energy shortage problems of traditional fossil energy due to it is clean,no pollution and rich.Photovoltaic(PV)grid-connected inverter system is the medium of transforming the solar energy into electric energy,which faces the great challenge when solve the problem.PV panels,as the input side of the system,are numerous and have wide coverage area.Parasitic capacitor between panels and the earth cause a path for common-mode current with transformer-less inverter.Such a common-mode current can deteriorate grid-connected current,damage the equipment and threat personal safety.Meanwhile,the grid-connected current with a large number of low-order harmonics will pollute the public power grid and increase the power loss.Suppressing the common-mode leakage current is one of the most important problems in PV grid-connected inverter.In the paper,according to analysis,the topologies are divided into three types: topologies derived from full-bridge inverter,half-bridge inverters and two stage topologies with low frequency unfolding inverter.But this paper mainly focuses on the type one.Among them contain topologies with or without clamping switches.Then,based on the topologies with clamping switches proposed already,this paper deduces the general solution and compares the advantages and disadvantages respectively.On the basis above,a new idea of providing an extra low impedance path for common-mode current is got,aiming at bypassing the high frequency component of leakage current.Next,detailed comparison of different methods is given by simulations and experiments.Based on the system above,the influence in grid-connected current applied to repetitive control(RC)is studied in detail.From the stability analysis,two problems of grid-connecting with RC is existed,self-stability and system stability.So,the scheme of constant and low-pass filter is used to improve the internal model of the RC and the effect on the open-loop gain of the system is given.According to the analysis,it is not enough to just use a RC.Therefore,combined with PI control scheme is adopted to improve the system stability.Then,the detailed parameters are designed with LCL filter.In order to enhance the dynamic performance,the scheme of an improved internal model is adopted,namely dual-mode structure RC(DMRC).Finally,the relative merits of several schemes and correctness of parameters design are compared in detail through simulation and experimental results.