A Noninvasive Feeder Impedance Estimation Method for Parallel Inverters in Microgrid Based on Load Harmonic Current

As a key parameter in microgrids, feeder impedance is closely related to power distribution, power quality, and system stability. This letter proposes a novel noninvasive feeder impedance estimation method, which uses the microgrid central controller (MGCC) and inherent harmonic signals introduced by a nonlinear or unbalanced load. First, the sequence impedance model for a droop-controlled inverter is built to demonstrate that the inverter output impedance is negligible at low-order harmonic frequencies. Then, the bus voltage and output current at a specific dominant harmonic frequency are extracted in MGCC, based on which feeder impedance can be obtained. Finally, the estimated feeder impedance is used to generate proper virtual impedance to enhance power sharing and voltage quality as an example, based on a hierarchical control structure. Experimental results are provided to validate the proposed method.