Research On Control Stategies Of Three-phase Grid-connected Photovoltaic Inverter

With the coming up of the concept of “smart grids”, distributed power generationsystem attracts more and more attention. As an important part of distributed powergeneration system, grid-connected photovoltaic (PV) power generation system has gooddevelopment potential. The inverter is key technology of grid-connected PV powergeneration system. At present, the rating of inverter is increasing, and parallel inverter isan effective way to increase the rating of system. Thus, the control strategies ofgrid-connected parallel PV inverter were researched in this dissertation.First the mathematical model of grid-connected parallel PV inverter insynchronous coordinate system was build, and based on this, inner loop of current andouter loop of voltage were designed by adopting vector control oriented by grid voltage.In order to make the system operate around the maximum power point, the maximumpower point tracking strategy of fixed-voltage start and adaptive variable-step track wasadopted.The control of circulating current of grid-connected parallel PV inverter is asignificant content in this dissertation. By adopting grid-connected parallel PV inverter,the reliability and efficiency of system can be increased, and the cost of system can bedecreased, while it brings the issue of circulating current. Circulating current can bring aseries of bad influence to the system, such as increasing the power loss, distorting thethree-phase current, increasing the total harmonic distortion (THD) and decreasing thesystem efficiency. The dissertation proposed a feedback control strategy of circulatingcurrent based on traditional PI control strategy of circulating current by analyzing themathematical model of circulating current in synchronous coordinate system.Theoretical analysis and simulation and experimental results all show when the filterconductors or the reference currents of two inverters are different, the effect oftraditional PI control strategy is poor, while the feedback control strategy is not onlysuitable for the current-equalizing situation, but also the situation of different filterconductors or different reference currents.At present, more and more PV power generation devices are connected to the grid,while it will probably result in islanding effect when the electric network is out ofelectricity. Islanding effect can cause great damage to electrical loads and maintenancepersonnel of network, so it must be avoided. This paper put forward an improved AFDislanding detection algorithm based on traditional AFD algorithm. The algorithmrealizes current frequency drift by controlling reactive current, so as to achieve positivefeedback of voltage frequency of the point of common coupling (PCC) in islanding state and realize anti-islanding function. Simulation and experiment results show that theproposed algorithm can not only eliminate the non-detection zone of passive islandingdetection algorithms while the power of loads and the power of inverter match, but alsoovercome active current fluctuation of traditional AFD algorithms and improve thestability of DC bus voltage, so the efficiency of maximum power point tracking wouldnot be influenced.At last, a15kW parallel PV Laboratory simulation system was built. The platformis composed of PV array simulator, power converter, sampling system, control system,host computerized supervisory control system. Power control, grid-generation withoutimpact, maximum power point tracking (MPPT), anti-islanding and circulating currentcontrol experiments have been carried out on this platform.