In-situ three-dimensional surface charge mapping device for full-scale 500 kV GIL tri-post insulator

Abstract Gas-insulated transmission lines (GIL) offer high transmission capacity and compact layout, but surface charge accumulation on epoxy insulators can distort the local electric field and reduce the surface flashover strength. This work develops and evaluates a measurement system for in-situ three-dimensional surface charge mapping of a full-scale 500 kV GIL tri-post insulator. Finite-element analysis is first used to determine the electric field distribution and identify field-critical regions on the post, transitional arc and abdominal surfaces. Guided by these results, four charge probes and corresponding measurement paths are defined, and a three-dimensional motion-control mechanism is designed to scan the insulator surface inside the GIL enclosure, achieving full 360° coverage of the post region and an effective coverage of approximately 88% in the abdominal region. Based on the passive electrostatic capacitive-probe method, the system is calibrated, yielding a charge resolution of 0.0588 μC (m 2 ·mV) −1 and a spatial resolution of 3.7 mm 2 , with the average charge density decreasing by less than 8% over 60 min, indicating good stability. AC voltage tests are then performed to investigate the effects of voltage amplitude (200–300 kV) and charging duration (15–60 min) on surface charge accumulation. The results show that negative charges dominate and concentrate in the abdominal region, where the surface charge density is up to about five times that in the post region. A nearly linear correlation is observed between the measured surface charge densities and the simulated normal electric field, demonstrating the effectiveness of the proposed system for quantitative, field-guided surface charge analysis of GIL tri-post insulators.