Research On Control Strategy For Photovoltaic Grid-connected Inverter During Unbalanced Grid

Because of the energy crisis, the development and utilization of new energy isextremely urgent. Solar energy has gained more and more widespread concern as a kindof ideal energy source with advantages such as inexhaustibility and cleanness.Photovoltaic (PV) generation is an important form of solar energy application, and hasgreat practical significance. Recently, PV power generation technology has graduallydrawn the attention of the international community: Installed capacity of PV powergeneration systems continue to increase, the voltage level of Photovoltaic power stationconnected to the grid is increasingly higher, etc. At this time, the impact of PV systemreceived from the unbalanced grid fault cannot be ignored; the control of PV inverterunder unbalanced grid conditions has been paid more and more attention. In this paper,the two stages three-phase L filter grid-connected PV system as the research object, thecontrol of PV inverter and the problem of grid current distortion, over current andasymmetry are discussed.Firstly, the cause and the adverse effects of unbalanced grid are expounded in thepaper; through the analysis of these adverse effects, illustrate the research of PV invertercontrol technology under unbalanced grid has important practical significance. What’smore, the thesis depth analyzes the control method of grid-connected inverter underunbalanced grid, according to the difference of implication mode classified thesemethods, and summarize their advantages and disadvantages, so that the contrast andselection between the methods, further study are carried out expediently.Currently, the grid-connected inverter control research is mainly to suppress gridpower fluctuation and the spread of2nd fluctuation in DC voltage under unbalancedgrid, grid-connected current appears negative sequence component, amplitudeasymmetry and overcurrent. But with the increasing of PV generation systems installedcapacity, overshooting of asymmetric current will cause new impact on Power Grid:over current will make the photovoltaic system disconnect from the power grid, the gridpower flow change greatly; Transformer, line loss increase, etc. Therefore, this papermainly focuses on a method to solve grid current distortion, over current and asymmetryduring unbalanced grid. Through the analysis know that, due to the existence of thenegative sequence component of grid-connected current, resulting in current asymmetry;the grid voltage dropped during unbalanced grid, but the power injected into the grid photovoltaic system has not changed, the grid-connected current amplitude will increase.According to the inherent characteristics of photovoltaic array (i.e., output poweradjustable characteristics), the paper presents a PV array constant power and maximumpower tracking double-mode control strategy: the constant power control plays a mainrole while the unbalanced grid fault occurs; under normal circumstances, the PV arrayin accordance with the MPPT (Maximum Power Point tracking) control output. Thedetailed relationship between the dq rotating coordinate system positive sequence andnegative sequence voltage component under unbalanced grid and the unbalanced degreeis derived; the existing positive control was improved, and presents a positivecompensation control method based on this relationship. Combined with PV arrayconstant power and MPPT double mode control strategy and positive compensationcontrol, both can achieve unity factor and suppress grid current overshoot, made thecurrent symmetry; The control mode is simple, and easy to meet the grid power qualityrequirements.Lastly, the paper designed a two stage L filter grid-connected PV system, usingdirect power control in dc link and PI controller with a low pass filter, current loop usedq decoupling control, and verified the system performance through simulation modelwhich is established in Matlab/simulink. The experiment is carried out in the purchasedPV grid-connected experimental system. Through experiment on the RT-LABexperimental platform, verify the correctness of the designed system and theeffectiveness of the proposed control method.