An Adaptive Active Damper For Improving The Stability Of Grid-Connected Inverters Under Weak Grid

As the interface between the distributed generation unit and the power grid,the grid-connected inverter plays an important role in converting the generated dc power into high-quality ac power and injecting it into the grid.However,when connected to a weak grid through the point of common coupling(PCC),the grid-connected inverter may be destabilized due to the wide variation of the grid impedance,even though it is designed to be stable on its own.An active damper can be connected in parallel at the PCC to damp out the resonance and stabilize the system.This thesis focuses on the adaptive control method of the active damper.First,the impedance-based stability criterion is adopted to analyze the stability condition of the grid-connected system.It is shown that the system will be unstable if the amplitude-frequency curves of the grid-connected inverter output impedance and the grid impedance intersect with each other at a frequency where the phase of the inverter output impedance is below-90o.In order to stabilize the system,a parallel impedance can be connected to the PCC to shape the inverter output impedance,and it is pointed out that the simplest form of the parallel impedance is a pure resistor.To avoid introducing additional power loss,the resistor can be emulated by an active damper.Then,an adaptive tuning method of the virtual resistor is proposed,which can automatically regulate the virtual resistor to the critical value to stabilize the system by detecting the resonant components in the PCC voltage,and thus reduce the power loss of the active damper.Besides,considering that the primary objective of the active damper is to damp the resonant components in the PCC voltage,rather than suppressing the harmonic components introduced by the grid background harmonics,a method of increasing the port impedance of the active damper at the dominant low-order harmonic frequencies is proposed to further reduce the power loss.Moreover,a harmonic-currentreference compensation method is proposed,which allows the active damper to accurately emulate the virtual resistor in a wider frequency range,and thus better damp the resonance in the system.Finally,the parameters design method of the main circuit and the control loops of the active damper is presented.The prototypes of a 1-kVA active damper and a 6-kVA grid-connected inverter are built and tested in the lab.The exprerimental results verify the effectiveness of the proposed control scheme and parameters design method of the active damper.