Design Of The Injected Grid Current Regulator And Capacitor-current-feedback Active Damping For Grid-connected Inverters With LCL Filters

Nowadays, due to environmental pollution and resource shortage, distributed powergeneration system (DPGS) based on renewable energy has been drawing more and moreattention. As the interface between the DPGS and power grid, LCL-type grid-connectedinverter plays an important role in injecting high-quality power into the grid. To avoidpossible instability, the active damping based on capacitor current feedback is an effectiveway to damp the resonance caused by the LCL filters in grid-connected inverters. Thispaper focuses on the characteristics analysis and design method of the feedback coefficientof capacitor current and injected grid current regulator.This paper investigates the frequency responses of the injected grid current regulatorand capacitor-current-feedback active damping in analog control. By carefully dealing withthe interaction between the current regulator and active damping, the complete satisfactoryregions of the controller parameters for meeting the specifications of the systemperformance, including the steady-state error, phase margin and gain margin, can beobtained with the proposed design method, and then the controller parameters can be easilypicked out. Based on this design method, the trial-and-error procedures are effectivelyreduced, and it is more convenient and explicit to analyze and optimize the systemperformance.When the LCL-type grid-connected inverter adopts digital control, the control delaysare introduced. It is proved in this paper that the active-damping method is no longerequivalent to a resistor but a frequency-dependent impedance in parallel with the capacitorin digital control. Thus, although the resonance peak can be damped, the frequency of theresonance peak is also changed, and two right-half hand poles exist in the loop gain if theequivalent impedance contains negative resistor component at the frequency of theresonance peak. This equivalent impedance changes the system characteristics and makesthe controller parameters hard to design. This paper proposes that the system stabilitymargin in this case can be described as the phase margin and gain margins at the resonancefrequency of LCL filter and one-sixth of the sampling frequency. Thus by carefully investigating the system stability and steady-state error, the proposed controller parametersdesign method is extended to the situation where digital control is adopted.It is also proved in this paper that, if the resonance frequency of the LCL filter is largerthan one-sixth of the sampling frequency, the LCL-type grid-connected inverter can workwell when only the injected grid current regulator is used, and the phase margin of thecompensated system is larger but the crossover frequency is lower.A6-kW single-phase prototype with analog and digital control circuits has beenconstructed in the lab. The experimental results verify the effectiveness of the theoreticalanalysis and the proposed controller design method. The proposed controller parameterdesign method also can be extended to the cases when the inverter-side inductor current iscontrolled, or in three-phase and passive-damping systems.