Research On Control Strategy Of Grid-connected Photovoltaic Inverter Based On Improved Proportional Resonance

With the intensification of energy shortage and environmental pollution worldwide,the development and utilization of renewable energy has become a research hotspot.With the rapid development of photovoltaic power industry,grid-connected photovoltaic power generation has become one of the main forms of solar energy utilization.As the core of grid-connected photovoltaic system,the performance of grid-connected photovoltaic inverters directly affects the stability of the system and the quality of output power.In order to improve the performance of grid-connected inverters,the control strategy of photovoltaic grid-connected inverters is studied in this paper.1)The mathematical models of LCL grid-connected inverters in abc,αβ and dq coordinates have been established,and the decoupling model in dq coordinates has been deduced.In the αβ coordinate system,there is no coupling relationship between the two axes and independent control can be achieved.In the dq coordinate system,there are coupling variables between the coordinate axes,and after decoupling,too many state variables will be introduced.So PR control has been adopted in αβ coordinate system to realize error-free tracking of sinusoidal variables.2)For the resonance problem of LCL filter,passive damping and active damping control strategies have been used to suppress the resonance peak,and the damping effect has been analyzed.A current double-loop active damping control strategy with filter capacitor current as inner current loop and grid-connected current as outer current loop has been adopted,which has good damping effect,strong anti-interference ability and fast dynamic response.The parameters of LCL filter have been reasonably designed according to the parameter restriction conditions.The correctness of the parameters has been verified by the simulation.3)In the ideal power grid,in order to solve the problem of small bandwidth of PR controller and steady-state error when the frequency of power grid has been offset,the PR controller has been improved and quasi-PR controller has been adopted.The current double-loop control strategy based on quasi-PR has been designed.Through the analysis of open-loop and closed-loop bode diagrams of the system,it can be concluded that the control strategy can realize the error-free tracking of the system.The effectiveness and correctness of the control strategy have been verified by the simulation of steady-state performance and dynamic performance.4)In the case of non-ideal power grid,a harmonic compensator has been designed on the basis of quasi-PR controller.By analyzing the traditional parallel harmonic compensator,it has been concluded that the compensator can compensate the harmonics of the power grid,but the harmonics of the power grid will affect the reference current,so that there will be harmonics in the reference current,and the quality of the grid-connected current can not reach the standard.Therefore,an improved harmonic compensator has been designed,which can suppress the harmonics even if the reference current contains harmonics,so as to reduce the harmonic content of the output grid-connected current and achieve reliable grid-connected.The simulation in Chapter 5 proves that the improved harmonic compensator has good harmonic compensation effect.