Analysis of a Direct Lightning Impact on a Substation and its Protection Equipment

This paper examines electromagnetic field effects inside a substation when lightning hits a transmission tower located just outside a substation. An integral formulation of Maxwell's equations is used to model the whole structure including power line, towers, grounding network, control cables, shielding system and lightning channel. Computations are carried out using the Method of Moments (MoM) for surface-wire integral equation in a stratified medium. This paper focuses exclusively on transient voltage and current stresses that appear mainly because of significant ground potential difference (GPD) between conductors and cables and surrounding grounded structures. The currents flow along cable wires and shields and voltages appear across a wire of an unshielded or shielded cable and a nearby metallic grounded panel or cable shield. Such currents and voltages are a critical reference or indicator of the vulnerability of control cables to transient surges. The voltage and current stresses along a subset of the substation control cables that may result in possible damages because of a nearby lightning strike are computed accurately based on a realistic complete model of the substation and the surrounding electric network. Various scenarios are considered based on the same cable topology and the effectiveness of each scenario in reducing current and voltage stresses are discussed. This exact computation analysis can help develop more appropriate standards and mitigation measures to prevent most severe, lightning strikes in a well-designed substation, resulting in significant economical savings in outages avoidance and equipment damages.