Effect of Waveform Parameters on Creeping Discharge of Oil-Paper Insulation Under Bipolar Oscillation Impulse Voltage

When lightning impulse voltages invade substation equipment via transmission lines, the oil-paper insulation is subjected to bipolar oscillation impulse voltage with variable waveform parameters. As one of the most critical failure modes, the creeping discharge characteristic under bipolar oscillation impulse voltage remains unclear. This paper establishes a platform for generating bipolar oscillation impulse voltage with adjustable waveform parameters and investigates the effects of these parameters on creeping discharge characteristic. The results indicate that, under a single application of bipolar oscillation impulse voltage, the breakdown voltage increases with the damping factor following a power-law relationship and increases linearly with the oscillation frequency. The cumulative effect was analyzed through the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-N</i> characteristic, and it is proposed to use the negative reciprocal of the exponential term in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-N</i> equation as the cumulative coefficient to quantify the strength of the cumulative effect. Within the experimental conditions, as the damping factor and oscillation frequency increased, the cumulative coefficient rose by 40.5 % and 6.7 %, respectively, suggesting that the cumulative effect of creeping discharge weakened. The growth of white marks and changes in characteristic peaks in spectra of Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) on the pressboard surface indicate that impulse voltages trigger the cracking of cellulose chains and the formation of gas channels. Under the cumulative application of the impulse voltage, each creeping discharge progresses along gas channels formed by previous discharges, expanding the damage and degrading the insulation step by step. This study provides insights for optimizing transformer insulation design and mitigating the risk of insulation failure.