Improved Dielectric Strength of GIL Insulator by Multi-Dimensional Functionally Graded Materials Under Polarity Reversal Voltage

In direct current gas-insulated transmission lines (DC-GILs), insulators often operate under temperature gradients and may experience polarity reversal voltages. To reduce flashover faults of DC-GIL insulators, this paper introduces a novel multi-dimensional functionally graded material (MFGM) insulator, which integrates a surface coating made of nonlinear conductivity material (SNCM) with a bulk constructed from permittivity functionally graded material (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ϵ</i>-FGM). Simulation and experimental studies were conducted under polarity reversal conditions and across different temperature gradients. Results show that the SNCM insulators can uniform the electric field (E-field) distributions before polarity reversal under various temperature gradients, but enhance the E-field distortion after polarity reversal at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HV</sub> of 25 and 50 °C. The MFGM insulator can effectively regulate the E-field distributions across all temperature gradients both before and after polarity reversal, leading to flashover voltage improvements of 6.5%, 9.8%, and 21.9% at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HV</sub> of 25, 50, and 70 °C, respectively. Moreover, flashover voltages of all insulators under the negative-to-positive condition are notably higher than those under the positive-to-negative condition, exhibiting a pronounced polarity effect. Results suggest that the MFGM insulator holds promise for applications in DC-GIL projects.