Research On Suppressed Technology Of The Resonant Problem Of Grid-Connected Inverter

As an interface between distributed generation and a power grid,a grid-connected inverter plays an important role in injecting high-quality power into the grid.A low-pass passive filter(typical LCL-filter or High-order filter)is often inserted between a grid-connected inverter and the grid to attenuate the PWM carrier and sideband harmonics which was injected into the point of common coupling.However,the inherent resonance characteristics of the filter brings challenges to stabilize the system.In order to achieve high performance of the system,this dissertation studies the three-phase three-wire grid-connected inverter from four aspects:selection of resonant damping strategy,optimal design of the damping parameters,grid impedance estimation and harmonic controller scheduling method.An accurate mathematical model of the three-phase three-wire grid-connected inverter is established in the s-domain under the premise of the digital delay.Based on this model,the performance of the structures of different passive damping methods are analyzed from the perspective of the damping losses and the attenuation rate in the high frequency,and the optimal damping structure was finally obtained.After considering factors such as resonant frequency and position of the current sensor etc.,an analytical method is presented to study the sphere of application of different active damping methods.This dissertation presents a damping selection strategy which is applicable for grid-connected inverters with different configuration,it solves the problem that the damping performance cannot be maximized due to the inappropriate selection of the damping methods.The topological structure of the high-order filter is given,and an equivalent circuit analysis method is presented which is used to simplify the mathematical model of the high-order filter.The computational method of damping losses and the analysis of the effects of the damping capacitor on damping losses and damping performance are given based on the equivalent model.A systematic step-by-step design method which based on the pole position optimization is presented to maximize the damping performance of the hybrid damping,and this method is also applicable for passive damping and active damping.The influence of the parasitic parameters on the damping of the system is analyzed to make the analysis of the later experimental results more accurate.This dissertation analyzes the influence of the variation of the grid impedance on the harmonic controller and the resonant frequency.A quasi-passive grid impedance estimation method based on the Goertzel algorithm is presented to calculate the system crossover frequency,and the harmonic controller scheduling strategy is presented on this basis.Finally,a series of operation rules for the inverter are presented to ensure that as little as possible to activate the grid impedance estimation.Through the analysis and study of this dissertation,the grid-connected inverter can achieve the goal of safe and stable operation under various complex grid condition,and it pave the way to the research of the miniaturization and high performance of the grid-connected inverter.