Full-wave black-box transmission line tower model for the assessment of lightning backflashover

Abstract

In order to calculate the overvoltages across the insulator strings of overhead lines, electromagnetic transient (EMT)-type simulators require a model of the tower and its grounding system. In this paper, an electromagnetic field simulation model based on the solution of the full-wave frequency-domain Maxwell’s equations is established to obtain an EMT-compatible black-box representation of the transmission line tower for time-domain simulations. Details of the system, such as the grounding system consisting of counterpoise electrodes, shield wires, phase conductors, and tower cross arms and slant elements are considered in the simulation model. The frequency dependence of soil electrical parameters is included as well. The input data to the frequency-domain simulation model consists of the topology of the tower structure, grounding system, and the parameters of the surrounding medium. As a case study, the developed simulation model is applied to obtain the black-box representation (macromodel) of a 400kV double circuit tower, where the lightning current injected at the tower top is the input, and the output is the induced voltage across the insulator strings. The critical current leading to the flashover of the insulator strings is calculated according to various existing flashover models and two different first stroke lightning current waveforms. Finally, a probabilistic analysis is performed to obtain the percentage of downward negative first strokes leading to a backflashover.