Research On Single-phase Two Stage PV Grid Connected Control System

The photovoltaic grid-connected control system is an important part in the research of new energy today.The research on the maximum power point tracking(MPPT),grid control strategy and filter in the system is conducive to the system to produce high quality grid-connected current,the development and utilization of new energy has important theoretical and practical significance.Because control method of the two-stage inverter topology is simple,the single-phase two-stage grid-connected inverter system was studied.Boost-type DC/DC booster circuit was selected at the previous stage,and the full-bridge transformer non-isolated grid-connected circuit was used at the latter stage.The LCL filter was used as the filter,and its parameters were selected.Aiming at the problem of oscillation prone to near maximum power point(MPP)in traditional photovoltaic power generation system,a disturbance observing method based on fuzzy control(P&O)was proposed to realize MPPT.The model of solar cell was established and the fuzzy controller was designed.According to the change of output power of solar cell,the perturbation step was adjusted in time to make the solar cell work near MPP.The simulation analysis shows that this method has better effect on reducing the oscillation near MPP than MPPT process under traditional P&O.Aiming at the problem that PV grid-connected system is easily affected by external disturbances and nonlinear uncertainties under the traditional double-closed-loop PI control strategy,a closed-loop control strategy with ADRC and PI control was proposed.The inner loop of current was controlled by PI to ensure that the inverter output meets the requirements of grid-connected current,the outer loop of voltage was ADRC,and the external disturbances and non-linear uncertainties were regarded as comprehensive disturbance terms,ESO was used to observe and determine the current fed compensation nonlinear feedback control law(NLSEF),and ultimately achieved the purpose of stabilizing the DC voltage.The simulation results show that this method can track the target trajectory better than the double closed-loop PI control method,which can effectively reduce the total harmonic distortion(THD)of the input current and stabilize the system quickly after the external disturbance was added.In order to solve the problem of resonance spikes when LCL filter was applied to PV gridconnected inverter system,the traditional LCL filter and split capacitor damping method were respectively used to improve the LCL filter,and the two different passive damping methods were compared from the damping effect and the power loss.The simulation results show that the splitcapacitor damping method can effectively reduce the resonance caused by the LCL filter and reduce the total harmonic distortion of the grid-connected current,which confirmed that the analysis and design rationality.