DQ Impedance-Based Analysis of An APF -Type Active Damper to Stabilize the Grid- Tied Inverter System

An active power filter (APF)-type active damper is proposed to mitigate the high-frequency oscillations in the grid-tied inverter system. This damper follows the rules of APF by sampling the harmonic current of the original system, which will introduce a coupling effect between the damper and the original inverter. This is different from the traditional active damper which only extracts the voltage signal of the point of common coupling (PCC), thus the original inverter and the damper are independent. When extracting the harmonic current, the coupling effect poses a great challenge to the impedance modeling and stability analysis. This paper first establishes the dq-impedance model of the active damper with the coupling effect included. Then the harmonic current mitigation control is equivalent into an additional damping impedance that is decided by the original inverter and the capacitor. Later, the stability effect of the active damper is analyzed and verified via experimental results. It is finally concluded that the APF -type active damper can provide a well damping effect to improve the system stability by increasing the phase margin at resonant frequency area.