An Electromagnetic Model for the Calculation of Tower Surge Impedance Based on Thin Wire Approximation

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Abstract:
When a lightning strikes the top of a transmission line tower or shield wires, electromagnetic waves propagate through the tower back and forth, increasing the voltage across insulator strings. ‡is can eventually lead to a back-flƒashover (BF), which may cause damage to equipment or costly power outages. To calculate the over-voltages and predict the probability of a BF, an accurate model of the tower and its grounding system is needed in electromagnetic transient (EMT) type simulators. ‡There are a number of theoretical models for the equivalent circuit of a transmission tower. However, they either are not accurate enough or they are derived for a certain type of transmission tower, which limits their applicability. Numerical electromagnetic analyses have less simplifications compared to the theoretical solutions and are by far less expensive than field measurements. They also have the flexibility to analyze any type of tower. In this paper, the direct method for the measurement of tower impedance is implemented by NEC4 and applied to a 400-kV double circuit tower with all its details. Th‡e process of obtaining the wire network of the tower used in this paper is completely automated and it can be applied to any other type of transmission tower. Th‡e results of the numerical simulations are compared to those obtained with existing tower models. Th‡e developed model in this paper is capable of considering all the details of the tower and including the €finite resistance of the ground and grounding electrodes.