Research On Control Strategy Of LCLCL Grid-connected Inverter System Under Weak Grid

At present,the new energy generation represented by solar energy and wind energy can not only alleviate the problem of energy shortage,but also reduce the environmental pollution.Therefore,it has been widely concerned and favored by domestic and foreign scholars.The grid-connected inverter,as an important component of the connection between the new energy power generation system and the power grid,its performance will directly affect the reliability of the system and the power quality of the grid-connected current.In addition,with the continuous increase of the proportion of new energy power generation in the system and the wide distribution of the location,the power grid is more and more showing the characteristics of weak power grid.However,the weak grid contains rich background harmonics and large grid impedance,which will put forward higher requirements for grid-connected inverters.Therefore,the control strategy used in grid-connected inverter system has become one of the hot topics.In this paper,the grid-connected inverter system of LCLCL type filter is taken as the research object.The parameter constraints of LCLCL type filter,the current double closed-loop control with additional grid voltage feed forward control,the influence of grid impedance on system stability under weak current network and the method to reduce the influence of grid impedance on system are studied.Firstly,according to the topology structure of grid-connected inverter system,the mathematical model of LCLCL type filter in the αβ coordinate system is established,and the parameters of LCLCL type filter are designed according to the parameter constraints of the filter.Secondly,in order to improve the power quality of the grid-connected current in the system,the double closed-loop current control strategy with proportional feedforward control of the additional grid voltage is adopted.The proportional control of capacitance current internal loop can solve the problem of resonance spike produced by the filter.The grid-connected current outer ring is controlled by QPR(quasi-proportional resonance)and PI(proportional integral),which can approximately realize the accurate tracking of the reference current by grid-connected current,and it can also restrain the DC component of grid-connected current effectively.The proportional feedforward control of grid voltage can reduce the influence of grid voltage disturbance on grid-connected current.Finally,the Norton equivalent circuit of two parallel inverters connected to the grid in weak power grid is established.The coupling effect of the grid impedance between the parallel inverters and the inverter and the grid is analyzed.Under the condition of weak grid,the grid impedance will reduce the phase margin of the system,and which will affect the stable operation of the system.In view of this phenomenon,the PCC voltage resonance feedforward control method is proposed.This method can reduce the adverse effect of the grid impedance on the system and achieve the purpose of system stability and power quality improvement of grid-connected current.In addition,the control strategy described in this paper is simulated and verified in Matlab/Simulink.