An Improved Grid Impedance Estimation Technique under Unbalanced Voltage Conditions

The coupling in synchronous reference frame (dq-axis) of grid-connected inverter reduces the estimation accuracy of grid impedance based on variations of inverter's output power. Therefore, in this paper, an improved grid impedance estimation technique (at fundamental frequency) based on small variations of the inverter's output power is proposed. Moreover, to cope with unbalanced grid voltage conditions, the estimation algorithm is embedded in a positive- and negative-sequence controller (PNSC) strategy. The technique relies on obtaining three operating points produced from a deliberate variations in the dq-current references of the positive-sequence control loop. Adopting three operating points to estimate the grid impedance, significantly reduces the effects of the coupling in the dq-axis during variation of the output power without the need to set either the active or the reactive output power of the inverter to zero during the estimation. Hence, accurate estimation of grid impedance is obtained in a short period of time. Simulation results, using MATLAB nSimulink and PLECS toolbox based on a discrete-time model, show the reliability of the proposed grid impedance technique.