Abstract: To address the lack of quantitative analysis regarding line resonance and directional protection misoperation caused by sudden impedance changes of low-voltage grid-connected photovoltaic(PV) inverters under voltage asymmetry resulting from phase-to-phase short-circuit faults in medium-voltage distribution lines, an analytical method for inverter positive and negative sequence impedance and quantitative error evaluation considering control loop effects is proposed. Based on the harmonic linearization principle, the analytical models of positive and negative sequence impedance of the inverter, incorporating the phase-locked loop(PLL) and current control loop, are derived, which theoretically clarifies the impedance frequency-domain evolution and multi-frequency coupling mechanisms under asymmetric faults. Hardware-in-the-loop(HIL) simulation experiments based on StarSim are performed to investigate the impedance distortion characteristics under various control parameters and asymmetric voltage amplitudes. The experimental results show that in the dominant frequency band of 100 Hz to 800 Hz, the mean absolute percentage errors(MAPE) of the positive and negative sequence impedance magnitudes between the established model and the measured data are within 5%, and the maximum absolute error(MAE) of the phase does not exceed 5°. These results demonstrate that the proposed analytical model possesses high theoretical calculation accuracy and can accurately reveal the internal physical mechanisms of high-order harmonic back-injection and the polarization angle deviation of protection elements.

Key words: distribution line fault, distributed photovoltaic, grid-connected inverter, positive and negative sequence impedance, protection misoperation