Breakdown Modeling of Long Air Gaps in EHV Transmission Systems Under Positive Lightning Impulses

The current external insulation design for Extra High Voltage (EHV) transmission lines primarily relies on empirical formulas provided by standards. However, operational datas from Chinese power grids reveal an increasing rate of lightning-induced tripping events caused by air-gap flashovers, highlighting the need for fundamental studies on long air-gap breakdown models under lightning impulse conditions. This article investigates the breakdown characteristics of positive lightning impulses in 3–6 m rod-plane gaps, measuring the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> breakdown voltage, voltage-time characteristics, and high-speed optical discharge dynamics of the 5 m gap. Experimental results reveal a linear dependence of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> (kV) on gap length d (m), formulated as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> = 81.2 + 562.9d, whereas the average gap field strength <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> (kV/m) follows an inverse relationship: <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> = 545.1 + 77.4/<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i><sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.51</sup>. The measured voltage-time characteristics surpass CIGRE benchmarks, underscoring the necessity for refined breakdown models in insulation coordination. The streamer, preceding the leader, bridges the gap and governs the discharge dynamics. By synthesizing experimental data with Rizk, CIGRE, and Pigini breakdown models, this work refines the critical leader inception field strength and final jump height, proposing an enhanced leader progression model. Based on the proposed model, we quantify both the linear and exponential velocity coefficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> across gap lengths and voltages. Larger gaps stabilize <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> values, with the exponential model showing tighter convergence, better capturing leader progression’s nonlinear voltage dependence.