Research On Current Compensation Control Strategy Of Self Synchronous Inverter In Non Ideal Power Grid

With the increasing demand for energy in the world,the environmental pollution caused by the exploitation and use of traditional fossil energy has become more and more serious,and the call for the development of clean energy is increasing.As a bridge between the sustainable development of clean energy and large power grid,grid connected inverter is very important.Among them,DC bus voltage and reactive power control type(DCVQ)grid connected inverter has attracted more and more attention.It can realize synchronous operation with large power grid without using PLL.At the same time,it can simulate the output characteristics of synchronous generator.This paper focuses on the design of DCVQ self synchronous grid connected inverter and the possible non ideal working conditions when it is connected to the power grid,and compensates the current fed into the power grid.Firstly,the average model of the main circuit of the inverter is established,and the operation characteristics of the synchronous generator are simulated.Taking the speed regulator and excitation regulator of the synchronous generator as reference,the DC bus voltage frequency control loop and reactive power voltage control loop of the DCVQ grid connected inverter are designed,and the specific implementation method of its stable operation is obtained.Then,the mode switching method of DCVQ grid connected inverter under different conditions of off grid and on grid is studied.In order to keep the advantage of DCVQ grid connected inverter that can operate in self synchronization without using PLL unit,the compensation control strategy based on virtual impedance and virtual power is adopted after the conditions of off grid switching are analyzed mathematically,By monitoring the virtual power information flowing on the virtual impedance as the condition of grid connection,the compensation strategy is simple and easy to implement,which can effectively reduce the generation of impulse current and a large number of exchange of active and reactive power.Next,the mathematical analysis of DCVQ grid connected inverter in the face of unbalanced grid voltage is carried out.The symmetrical component method and the mathematical method based on improved second-order generalized integrator are used to quickly and efficiently separate the positive and negative sequence of three-phase unbalanced voltage and current signals,PR controller’s current compensation loop can effectively suppress the three-phase negative sequence current,eliminate the output power,frequency and double frequency fluctuation on the DC side bus voltage caused by the three-phase negative sequence current,effectively reduce the three-phase imbalance of the current below the specified standard,and effectively improve the output power quality.Finally,aiming at the complex working conditions of the 5th and 7th power frequency harmonics and the 3rd background harmonics in the low-voltage distribution network caused by the use of a large number of power electronic devices and nonlinear loads,a power current coordinated control strategy of adding a current compensation loop to the control loop of DCVQ grid connected inverter is proposed,After the mathematical analysis of the traditional repetitive control(RC),the improved control strategy can effectively reduce the total harmonic distortion(THD)of the output current,and reduce the fluctuation of the output active and reactive power The output frequency fluctuation and DC bus voltage fluctuation have significant effect.