Research On Harmonic Control And Optimization Of Photovoltaic Inverter Based On Virtual Impedance

The trend of power electronization in distribution network is accelerating while the power electronics technology is developing rapidly.The power quality problems caused by the large number of non-linear loads are becoming worse and worse,among which the harmonic problem of low-voltage distribution network is serious that has been widely concerned.In view of the wide application of distributed PV power in low-voltage distribution network and the limited working time of PV inverters,most of DG is in standby state.So it is necessary to consider the effective utilization of DG.Since the topology of PV inverters is similar to that of active power filters,it is feasible to design PV inverters with both filtering functions and power generation.Therefore,the main purpose of this paper is to research a harmonic suppression strategy and optimization of photovoltaic inverters based on virtual impedance.The system controller is improved.The harmonic extraction method are improved through detecting the harmonic voltage of PCC point.And the value of virtual impedance is optimized by PSO to achieve the goal of suppressing the effect.Firstly,the LCL PV inverter is studied.The mathematical model of LCL PV inverter in synchronous rotating coordinate system is established,and the influence of coordinate transformation on LCL resonance is analyzed.On this basis,aiming at the defect of poor tracking effect or large amount of calculation of existing controllers for harmonic signals,an improved PI + repetitive controller suitable for this topic is selected.The performance of the controller is compared by simulation analysis which shows that the control accuracy and dynamic response of harmonic signals are improved without increasing the amount of calculation.Secondly,the forward channel of resonant frequency and the input signal of SOGI are improved.A frequency-adaptive and cross-feedback harmonic extraction method is designed,which makes the proposed harmonic detection method not only can extract harmonics accurately in unbalanced and distorted power network,but also ensures voltage phase-locked.Then,in order to solve the problem that the traditional harmonic compensation method needs to install current transformer in line reconstruction,a harmonic suppression strategy based on virtual impedance for photovoltaic inverters is proposed.It only needs to detect the voltage harmonic of grid-connected point,so that the PV inverters can suppress the harmonic of grid-connected point while generating active power.In addition,the existence of the virtual impedance which makes the harmonic suppression effect the best is pointed out,so the optimal virtual impedance is searched and optimized.Furthermore,a PSO algorithm is proposed to solve the uncertainty of the optimal virtual impedance value so as to achieve the best harmonic suppression effect.Then,the feasibility of the above algorithm is verified by simulation.Finally,an experimental platform of PV inverter prototype with harmonic suppression function is built.The experiment proves that the proposed control theory can detect harmonic signals accurately and reduce the harmonic voltage of PCC effectively.