A VHF-Voltage Double Time Integral Method for Apparent-Charge Estimation of Partial Discharge in Gas-Insulated Switchgear

Gas-insulated switchgear (GIS) is widely used in modern power systems, but transportation, installation, and manufacturing defects can trigger partial discharge (PD). Apparent charge is a key indicator of PD intensity and insulation deterioration, yet conventional pulse current measurements perform poorly in field environments because of interference, pulse superposition, non-flat sensor responses, and impedance mismatch. Very-high-frequency (VHF) sensing is sensitive to on-site PD but lacks a direct quantitative mapping to apparent charge. This study derives and optimizes a double time integral of voltage (DTIV) method for capacitively coupled VHF sensors. Simulation and experimental platforms were established, and through-origin linear regression was used to relate DTIV output to reference apparent charge. The first and second voltage zero-crossings (ZC1 and ZC2) were evaluated as integration upper limits, and the preferred DTIV_ZC2 strategy was further assessed using thresholds of 1% and 5% of the signal peak. For the simulated library, using the second zero-crossing increased the coefficient of determination from 0.8957 to 0.9585 and reduced the average relative error to 4.5%. A 5% threshold further improved these values to 0.9641 and 3.9%, respectively. The results support the feasibility of calibrated DTIV estimation for the tested sensor and ideal laboratory configuration.