Polarity Reversal and Over Voltage Affecting Discharge Inception of Tri-Post Insulator in ±800 kV GIL

Surface charge and electric field distributions of a tri-post insulator in +/- 800-kV gas-insulated transmission line (GIL) are calculated under the superimposed dc-impulse voltage and the polarity reversal voltage. The theoretical discharge inception voltage of the tri-post insulator under the abovementioned conditions is estimated by the volume-time theory. The accumulated homocharges on the insulator surface significantly relax the electric field at the dc steady state, thus improving the theoretical discharge inception voltage under dc superimposed homopolar impulse voltage but reducing that under dc superimposed heteropolar impulse voltage. When the voltage polarity is reversed in two minutes, the intensified electric field on the insulator surface shifts from the grounded (GND) electrode toward the vicinity of the high voltage (HV) electrode, in which the maximum electric field grows by 21.4% and increases as the reversal time shortens. The micro-discharge around the particle trap may induce the accumulation of heterocharges on the insulator surface between the lower two posts, aggravating the electric field distortion in the surrounding region and degrading its insulation performance.