Research On Droop Control Technology For Parallel Inverters In Photovoltaic Micro-grid

The combination of photovoltaic power generation and micro-grid technology promotes the development of photovoltaic micro-grid.On the issue of power shortage in remote areas,the off-grid photovoltaic micro-grid has a good development prospect.Considering the expansion of power source and the increment of load,the power supply scheme with parallel inverters is a good choice.In the parallel system,each inverter ofen uses the droop control strategy without communication line.Firstly,the development situations of photovoltaic power generation and photovoltaic micro-grid are summarized in this paper.Meanwhile,the droop control technology of inverter and its research status are introduced.What’s more,control strategies of the devices on DC bus side are selected and the simulation verification is made in MATLAB.Secondly,this paper analyzes the problems existing in the inductive droop control of inverter and the series virtual impedance method is also introduced.The small signal model of inverter’s control system with the series virtual impedance method is established.Based on the control target and the analysis method of small signal stability,LCL type filter of the inverter is designed.Meanwhile,the relevant simulation model is built in MATLAB.Simulation results show that when inverter uses the inductive droop control method which is based on series virtual inductance,the power sharing precision is improved by using the LCL type filter which is designed by this paper.Then,this paper analyzes the problems existing in the series virtual impedance method.Based on the constraint of equal currents,a parallel virtual resistance method is proposed in this paper by improving the voltage tracking link of inverter.At the same time,the relative control parameters are selected in this paper.Simulation results show that the parallel virtual resistance method proposed in this paper is superior to the traditional series virtual impedance method in terms of the inverter’s integrated control effect.Finally,problems of the method with constant droop coefficient are analyzed.Through the improvement of inverter’s power droop link,a new method with adaptive droop coefficient is proposed in this paper.Meanwhile,simulation verification is made and the simulation results show that the method with adaptive droop coefficient can balance the voltage quality,frequency quality and power sharing precision of the system well.