Identification of High-impedance Ground Faults Under Dynamic Variations of Voltage Unbalance in Distribution Networks

In distribution networks, single-phase high impedance ground faults and dynamically varying asymmetric phase voltages exhibit similar waveforms of neutral point displacement voltage, which can lead to misjudgment of grounding protection. To address this issue, this paper establishes an asymmetric admittance vector model to analyze the causes of dynamically varying asymmetric phase voltages and reveal the changing mechanism of neutral point displacement voltage under non-fault conditions. A full-response fault circuit model is also developed to depict the variation trajectory of neutral point displacement voltage with respect to transition resistance, examining the overlapping steady-state region and transient feature differences between high-impedance ground faults and asymmetric admittance dynamic changes. While demonstrating the overlapping steady-state features of the two, the influence of the initial fault phase angle on the DC decay component of the neutral point displacement voltage during transients is revealed. Empirical wavelet transform is employed to extract the DC decay component and high-frequency component of the neutral point displacement voltage, and the DC decay component is utilized for fault identification. For scenarios where the specific initial fault phase angle renders the DC component undetectable, a centroid frequency distribution feature identification method based on variance contribution ratio is proposed to recognize grounding faults. Simulation analysis indicates that this method provides distinct distinguishing features for high-impedance ground faults below 5 kΩ and asymmetric admittance dynamic changes, offering a new approach for accurate identification of single-phase ground faults.