Research On Control Strategy Of Inverter In Microgrid Supplies Nonlinear Loads

The environmental pollution problems caused by traditional fossil energy power generation are becoming increasingly serious.With the rise of new energy power generation,the development of microgrid has received growing number of attention.Microgrid,as a small-scale power generation system that can operate both in parallel and isolated-island,has a great randomness and diversity in the type of load.The problems of power quality caused by nonlinear load in microgrid have also become one of the concerned issues to be solved,which put forward higher requirements on the control strategy performance of the inverter.This paper mainly analyzes and studies the control strategy of microgrid inverter under nonlinear load.First of all,this paper introduce several traditional control strategies in different operating modes of microgrid inverters.And the main characteristics and dififerent types of nonlinear load are analyzed.The reason that the rectified load brings specific harmonics to the inverter is studied and the equivalent model is established.The voltage quality problems caused by rectified load are known in essence.Secondly,the dynamic mathematical model of the inverter is established under three-phase stationary coordinate system and rotating coordinate system.The double closed-loop control strategy of inductor current as inner loop is analyzed.Meanwhile,the parameters of the controllers are set by using the pole placement method.Through the analysis of Matlab/Simulink,the problems existing in the traditional double loop control strategy are pointed out and the causes are analyzed.Based on the above,a compound control strategy of voltage loop based on resonant controller is proposed.The feasibility of this compound control strategy is reflect by simulation results,which can effectively make up the shortcomings of double loop control strategy.Then,the performance of discrete controllers with several different numerical integration methods are compared.An optimization method is proposed to solve the high frequency offset problem exciting,in Tustin transformation method.The simulation results test and verify the effectiveness of the proposed method.Finally,aiming at the problem that the virtual impedance is easily influenccd by harmonic components and the control performance is not good under nonlinear load,the influenpe of harmonic components is pointed out,and several virtual impedance control methods are compared and analyzed.In order to solve the problem of harmonic amplification when deriving output current,the method based on the second order generalized integrator(SOGI)is adopted in this paper to improve the control performance.It makes the virtual impedance act only on the fundamental component of the output current,without being affected by the harmonic components,and effectively overcomes the problem of harmonic amplification.In addition,for the problem that the distance between inverter output and common connection point(PCC)is far from the actual situation so that PCC harmonic voltage cannot be detected directly.This paper analyzes the improvement of the voltage quality of PCC by detecting the harmonic components of the output current of the inverter and adding the harmonic virtual impedance.The effectiveness of the method was verified by simulation.