Online Grid Impedance Measurement Based on Virtual Reference Axis

This paper proposes an online grid impedance measurement method using the virtual reference axis (VRA). The method is based on producing a small perturbation that the inverter transmits different power to the grid, and measures the grid impedance by observing the steady-state response of the grid. In the practical applications, the voltage of the point of common coupling (PCC) is sensed and the Phase Locked Loop (PLL) generates a synchronous rotating reference axis. However, the PCC voltage will change when the grid-tied inverter operates in different power delivery scenarios. The voltage change at PCC will affect the rotating reference axis which produces errors for the grid impedance measurement. This article proposes an online grid impedance measurement method based on VRA to improve the problem caused by the PLL. The impedance measurement theory and derivation of important mathematical equations will be presented in this paper. The computer simulations and the hardware experimental results will be provided to verify the performance of the proposed VRA method.