A novel P-Q variations method using a decoupled injection of reference currents for a precise estimation of grid impedance

In a weak grid condition, the precise grid impedance estimation is essential to guaranty the high performance current control and power transfer for a grid-connected inverter. This paper proposes a precise estimation method for grid impedance by P-Q variations employing a decoupled injection of reference currents. The operation principle of grid impedance estimation is fully presented, and the negative impacts of the Phase Locked Loop are analyzed. The estimation error by a synchronization angle in the park's transformation using the Phase Locked Loop is derived and as a result, the variation method of reference currents for accurate estimation is introduced. The validation of the proposed method is verified by mathematical expressions and several simulation results.