Resonance Mitigation Techniques For LCL-Type Grid-Connected Inverters

The undergoing energy transition is leading to a paradigm shift in the electrical grid to a renewable-energy dominated power system,which features the “double high”characteristics,i.e.,high proportion of renewable energy and high penetration of power electronics,as well as the characteristic of weak grid.As the key interface between the renewable-energy based power generation system and the power grid,the grid-connected inverter converts the generated dc power into ac power and feeds it into the power grid,which plays a crucial role in the safe,stable,and high-quality operation of the power system.This dissertation is dedicated to the resonance mitigation for the LCL-type grid-connected inverter,aiming to achieve the high robustness under weak grid and ensure the safety and reliability of the power system.The typical resonance mitigation techniques include the pole-zero cancellation and the state-variable-feedback active damping.In essence,the pole-zero cancellation technique eliminates the resonant poles in the control loop from the current reference to the controlled current by configuring zeros.Thereby,the stability of the controlled current which can be either inverter current,grid current,or the weighted average current,is ensured.A general mathematical model is developed to describe the three kinds of current control,and the stability analysis is conducted in this dissertation.One finding is drawn that,when controlling the inverter current or weighted average current,the resonant poles still exists in the path from the current reference to the grid current;when controlling the grid current,the resonant poles can still arise in the path from the grid voltage disturbance to the grid current.Therefore,oscillations can be incurred in the grid current.In order to clarify its mechanism intuitively,the equivalent circuit models are further established.From the physical insights,it illustrates that the pole-zero cancellation does not essentially eliminate the LCL resonance but merely “hide” it in the control system,which can still be triggered,resulting in oscillations of the grid current.A 6-k W prototype of a single-phase LCL-type grid-connected inverter was built to verify the correctness of the above findings.Different from the pole-zero cancellation technique,the state-variable-feedback active damping technique shifts the resonant poles on the imaginary axis onto the left half plane by inserting a virtual resistance in the filter.However,the time delay in the digital control has a negative effect on the damping performance.In this dissertation,the damping performances of the proportional,integral,and derivative capacitor-current feedback are investigated.It points out that the time delay can cause their equivalent resistances to be negative,resulting in ineffective damping.In light of this,the capacitor-current proportional-integral positive feedback active damping is proposed and a dedicated design method of the feedback coefficients and current regulator parameters is given,which can ensure the total equivalent resistance positive almost within the entire Nyquist frequency range.Accordingly,the high robustness of the inverter against the grid impedance variation and filter parameter fluctuation is achieved.Experimental results from the 6-k W inverter prototype confirm the effectiveness of the proposed active damping scheme and the controller parameter design method.Moreover,it is known that the point of common coupling(PCC)voltage proportional feedforward is usually applied to improve the grid current quality.In this dissertation,its potential damping ability is revealed,which is actually equivalent to the capacitor-current integral positive-feedback active damping.Note that its equivalent resistance is dependent on the grid impedance.The larger the grid impedance is,the stronger the damping ability will be.It is found that no matter how much the grid impedance is,the equivalent resistance can be positive only within the frequency range below one-third of the sampling frequency.In order to extend the range to the Nyquist frequency,the hybrid active damping method is proposed,combining the capacitor-current proportional feedback and the PCC voltage proportional feedforward,and the parameter design procedure is presented to harvest their optimal cooperation.Furthermore,a family of splitting-current based implementation schemes are proposed which can realize the hybrid active damping without sensing the capacitor current.Finally,the improved grid current quality,the hybrid active damping ability,and the effectiveness of the splitting-current based implementation schemes are verified by experimental results.