The Research Of Three-phase Photovoltaic Grid-connected Inverter With LCL Filter

With the increasing exhaustion of tranditional fossil energy and deteriorating environment,the energy issue has become a chanllenge around the world.Vigorously development of renewable energy has become the trend of the future.Solar photovoltaic power generation with its advantages of clean and sustainable received more and more attention.The node voltage is affected by the non resisitance load.To reduce the system reactive power compensation devices and improve the utilization of photovoltaic power generation systems,it is of great significance to study the unified control strategy of photovoltaic power generation and reactive power compensation.Firstly,this paper introduces the commonly used circuit topologies of three-phase photovoltaic system.And considering the system structure,device cost and work efficiency,this paper chooses the two-stage non-isolated three phase photovoltaic grid connected inverter as the main topology.Compared with distribution static synchronous var compensator,they have a lots of commom points.Therefore consider to unified control of photovoltaic power generation and reactive power compensation.The overall control block diagram for the system are given to analysis the grid current active instructions and reactive instruction.Using power feedforward to speed up the response of the system.By comparing the traditional current tracking control method and synchronous reference frame quasi proportional intergral(SRFQPI)control method,using SRFQPI realize zero-steady-error tracking,to reduce the impact of grid frequency offset on the grid current and improve system robustness.Inverter output filter using LCL filter,it has good inhibition effect of the high order harmonic and small volume.But LCL filter has a resonance point,which can lead to grid current not stable,method should be adopted to suppress resonance.Through analysis and comparison to the traditional harmonic suppression strategy,passive damping and capacitor current feedback active damping control method is introduced.Due to the passive damping will increase the additional loss of system and the capacitor current feedback active damping methods could not test the capacitor current accurate,its application is limited.This article adopts the active damping of grid current feedback method to increase system damping,inhibit the LCL resonance,which is equivalent on both ends of capacitance in parallel RC branch in the case of not increase additional loss and inhibits LCL resonance peak.Then the parameters of the current feedback control method for parallel network are designed.Finally,a 30kW three-phase photovoltaic grid-connected inverter is finished.The main circuit parameters design and device selection of the experimental prototype are described in detail.And then,the hardware circuit of the control system is studied,especially the peripheral circuit.Last but not least,The control system software program is written,which inclued the main program,the interrupt program,the chart of the protection program etc.The effectiveness of the control method adopted in this paper is verified by experiment.