Lightning protection of substations using EMT modelling

IEEE Std. 998-2012 is an industry standard guideline for designing lightning protection systems for high voltage (HV) substations. It describes the use of an Electrogeometric model (EGM) for design and provides an equation for the approximation of the allowable stroke current. The stroke current is used to determine a strike distance for the substation lightning protection system. The allowable stroke current equation is a function of the surge impedance of the busbar and the Basic Insulation Level (BIL) of the equipment. The equation does not take into account the substation's surge arresters which will limit the overvoltages within the substation. This report discusses how an Electromagnetic Transient (EMT) model can be used to estimate an allowable stroke current through consideration of the substation plant and arrangement. An example is provided to show how an EGM and an EMT model can be combined to calculate a striking distance that meets designed Mean Time Between Failures (MTBF).