Research On The Method Of Current Harmonic Suppression In Single-phase Grid-connected Inverters

The grid-connected inverter is an important part of distributed energy generationsystems such as PV and wind turbine. It is widely used as the energy interface of the DCsystem and the AC grid, and has a major impact on the stability and efficiency of the wholesystem. The inverter is supposed to inject energy into the grid in the form of low THD andhigh power factor current. However low-order current harmonics can be observed whichare caused by the harmonics existed in the grid voltage and the dead time effect in thePWM modulation. It is impossible to eliminate them by an L or LCL filter. So theharmonics must be suppressed by a proper-designed current controller.PR controller and repetitive based controller have the similar frequency response withinfinite gain at particular frequencies. So both of them can achieve zero steady state errorwhen the reference is sinusoidal, or cancel the effect of disturb signal in those frequencies.In this paper, multiple resonant controller and repetitive based controller are used tosuppress the low order harmonics in the current. The stability analysis and the parametersdesign of both types of controllers are discussed. Simulations and experiments are carriedout to validate the theoretical analysis.The main content of each chapter is as follows: In the second chapter, an accuratemodel of the grid-connected inverter is established. The causes of current harmonics areexplained. In the third chapter, current controller based on multiple resonant controller isdiscussed. Due to the sudden change of the frequency response at the resonant frequency,the stability analysis is complicated when performed by Bode diagrams and thephase-margin criterion.But the Nyquist curve and the Nyquist stability criterion are used toanalysis the stability condition of the control system and it is shown that in this way theprocess is easier and the result is more intuitive and reliable. The next chapter is on thedesign method of two types of repetitive based current controller: the traditional one andthe odd harmonic repetitive controller.