The energy absorption capability of metal-oxide varistors, decidedly influenced by the electrothermal processes occurring within their microstructures, is a vital parameter for the effective surge protection. This paper proposes a numerical model for the simulation of the electrothermal phenomena occurring in metal-oxide varistors. The model is based on a 3D representation of the varistor's polycrystalline microstructure and the finite element method analysis. With the aid of the proposed model, the interdependence of the varistor's energy absorption capability, nonuniform current conduction, and the thermal behavior is investigated. Results are in good alignment with experimental data and discussed in the context of electrothermal properties of the varistor's microstructure. An unambiguous dependency of the varistor's energy handling capability on the applied overvoltage is shown to exist. A quantitative evaluation of the effective volume decrease due to the severe current localization at temporary overvoltages is demonstrated. The presented work analyses varistor's current conduction under temporary overvoltages and provides the means for extension of the varistors I-V characteristics into the time domain so as to accurately predict their dynamic electrothermal behavior. The introduced dynamic (I-V-t) model may have significant implications in energy absorption requirements and design principles for surge protective devices.

The maximum shielding failure current of overhead transmission lines is an important parameter in evaluating the shielding performance of the lines and in insulation coordination of substations. General expressions for the estimation of the maximum shielding failure current of transmission lines, derived by employing several lightning attachment models in shielding analysis, are presented. An application to typical 110 kV up to 1150 kV overhead transmission lines shows that there is a great variability in maximum shielding failure current among lightning attachment models. The importance of maximum shielding failure current in insulation coordination of substations is demonstrated with the aid of alternative transients program-electromagnetic transients program (ATP-EMTP) simulations. The computed overvoltages impinging on 150 and 400 kV gas insulated system (GIS) substations because of shielding failure of the incoming overhead transmission lines, being dependent upon shielding failure current, vary with the lightning attachment model employed in shielding analysis of the lines. Implementation of the electrogeometric model adopted by IEEE Std 1243:1997 in shielding analysis imposes high requirements on protection of the substations against incoming shielding failure surges.

General expressions for the estimation of lightning incidence to overhead transmission lines on the basis of electrogeometric and generic models are introduced, which consider, besides transmission-line geometry, lightning crest current distribution and, based on the recently proposed statistical model, interception probability distribution. An application to typical 115 kV up to 765 kV and large scale 500 kV and UHV overhead transmission lines is performed and the computed results, varying significantly among lightning attachment models, are validated through comparisons with field data from literature; the IEEE Std 1243 overestimates significantly lightning incidence to shield wires of large scale transmission lines. The expected annual number of lightning strikes to shield wires of transmission lines depends on lightning crest current distribution; this dependence, easily quantified with the aid of the introduced general expressions, is not considered by the relevant IEEE standard. Lightning incidence results are discussed in the context of the backflashover rate of overhead transmission lines.

Lightning is a major cause of distribution line outages affecting reliability of power supply thus, consequently, resulting in economic losses. The lightning performance of overhead distribution lines is associated with line insulation flashover due to direct and nearby lightning strokes. The present paper introduces a statistical method for the estimation of the flashover rate of overhead distribution lines. The proposed method, implementing a recently introduced statistical lightning attachment model and the simplified coupling model suggested by the IEEE Std. 1410:2011, yields a range for the expected flashover rate of distribution lines associated with lightning interception probability distribution of the line conductors. It also provides the means to easily quantify the effects of line parameters, soil resistivity, shielding of nearby objects and lightning crest current distribution on lightning performance of distribution lines. Results of the statistical method, discussed and compared with that obtained by the IEEE Std. 1410:2011, are in close consistency with field data.