Research On PV Grid-connected With LCL And Harmonic Suppression Technology

By using power electronic devices, Photovoltaic grid-connected power generationmay inject harmonic into power system. LCL filter could eliminate high order harmonicof Photovoltaic grid-connected effectively, it can improve the quality of electricity. Asthe photovoltaic grid consistent with the basic structure of the active power filter, so itcan achieve unified control of photovoltaic grid and active filtering. In the absence oflight or low light, the unified control system in turn effectively filter out harmonics ofgrid side. From now, when unified control system compensate harmonics of grid side,the harmonic detection method are basing on the three-phase grid balance voltage, butin the actual working conditions, the three-phase grid balance voltage usually goes intoimbalance, at this condition, the detection error of voltage phase angle is larger, thetracking accuracy of harmonic become lower, even failure. The response of harmonicdetection become more slowly, also it reduce the compensation speed. The effect ofharmonic compensation is not good. In addition, the elimination of LCL filter resonanceproblem in harmonic current closed-loop control to be studied.Do the following researches on the precision and speed of harmonic compensationfor three-phase unbalanced grid voltage control system:Firstly, as the core part of PV grid connected, inverter has similarities in structurewith active power filter. This paper adopts unified control mode to suppress harmonic ofsupply side and improving power quality. In the absence of light, photovoltaic systemcan eliminate harmonic of grid side.Secondly, in the three-phase imbalance voltage condition, when voltage phase shiftand amplitude variation occurs, taking positive sequence of voltage Park transformationby using average current module to get fundamental voltage phase angle, so it canaccurately extract the positive sequence voltage to ensure the accuracy of harmonicdetection, to keep the effect of dynamic compensation of harmonic wave. At this time,the harmonic detection delay will be greater. Instead of LPF, integral module caneffectively shorten the detection delay time.Thirdly, by improving the calculation method of THD, instead of calculating theRMS of harmonic, the system reduce the calculation amount.Fourthly, Harmonic detection of harmonic instruction values are three phasevolume, and is usually the control current loop in D, Q coordinates. So it will getharmonic reference current D, Q coordinate value by making difference, realize thecurrent closed-loop control and active damping technology, then make inversetransform, it can get three-phase harmonic current command value.Fifthly, Based on the simulation environment of MATLAB/Simulink, establishingthe model of unified control system. In order to realize the function of unified controlsystem, design a unified control system’s hardware and software basing on DSP.The simulation results verify the feasibility of theory unified control system, andrealized the quick and accurate detection of harmonic. Multifunctional grid connectedphotovoltaic inverter control theory is applied to deal with special situations.