Alberto De Conti

Calculation of Lightning-Induced Voltages on Overhead Distribution Lines Including Insulation Breakdown

This paper investigates the influence of considering actual insulation volt-time curves both in the calculation of lightning-induced voltages and in the estimation of the number of flashovers an overhead wire may experience per year due to nearby lightning strokes. The flashover mechanism is modeled according with the integration method, which is used as a reference for comparisons with the simplified 1.5 CFO flashover criterion traditionally used in the estimation of the lightning performance of overhead distribution lines. Sensitivity analysis show the dependence of flashovers on the shape and front time of the assumed channel-base current. The obtained results suggest that the simplified 1.5 CFO flashover criterion is likely to underestimate the number of flashovers an overhead line may experience per year due to nearby lightning strokes. This result is confirmed by statistical analyses considering a Monte Carlo-based approach. It is also shown that more realistic flashover rate estimates can be obtained in the statistical analysis of lightning-induced voltages provided a reduced threshold level (1.2 CFO in the particular case evaluated in this paper) is considered instead of the 1.5 CFO level traditionally used in this type of study.

Lightning Overvoltage Due to First Strokes Considering a Realistic Current Representation

Lightning overvoltages due to direct strikes over a 138-kV transmission line and voltages induced on a single-conductor overhead line by nearby strikes were simulated using a realistic representation for the first-stroke current waveform that includes a pronounced initial concavity followed by a sharp rise at the half peak and the double peaks, in addition to the median time parameters taken from traditional database of instrumented towers. The results were compared to those yielded by a single-peaked current given by two Heidler functions, which is commonly used in simulations. Significant differences were found on the amplitude of the resultant overvoltages. The elaborate waveform is responsible for higher direct-strike overvoltages and lower lightning-induced voltages in relation to those yielded by simplified single-peaked waveform.

Lightning-Induced Voltages Over Lossy Ground: The Effect of Frequency Dependence of Electrical Parameters of Soil

The impact of the frequency dependence of soil resistivity and permittivity on lightning overvoltages induced on overhead lines over lossy ground is investigated. The Visacro-Alipio expressions were implemented on the hybrid electromagnetic model to take this effect into account. Systematic simulations were performed considering representative current waveforms of first and subsequent strokes and different stroke locations near the simulated overhead line. In general, the results indicated that this effect is responsible for a reduction of the induced overvoltage. This reduction increases with increasing soil resistivity and becomes relevant for soil resistivity above 1000 Ω·m.

Lightning Performance of 138-kV Transmission Lines: The Relevance of Subsequent Strokes

The relevance of subsequent strokes on the lightning performance of 138-kV lines is assessed. An electromagnetic model was used to simulate lightning overvoltages experienced across insulator strings due to direct strikes to the towers of an existing line in order to determine the critical peak current required to flashover using the integration method. From peak current distributions, estimates of outage rate due to backflashover were developed considering the contribution of first and subsequent strokes. It was found that, depending on the value of tower-footing grounding resistance, the contribution of subsequent strokes can be relevant, notably for tall towers, with height above 30 m.

Voltages Induced in Single-Phase Overhead Lines by First and Subsequent Negative Lightning Strokes: Influence of the Periodically Grounded Neutral Conductor and the Ground Resistivity

The influence of a periodically grounded neutral conductor on voltages induced by first and subsequent negative strokes in single-phase overhead lines for different ground resistivity values is investigated by means of computational simulations using the hybrid electromagnetic model. Configurations of medium- and low-voltage networks were assumed. Currents of first and subsequent strokes with median parameters obtained from the measurements of Mount San Salvatore and Morro do Cachimbo stations were adopted. While the simulation of subsequent stroke considered the current departing from the ground, the first-stroke simulation considered 100-m-height attachment of the upward and downward leaders. For the conditions simulated herein, the results indicated the largest induced-voltage amplitudes for first-stroke events.

A study on the influence of corona on currents and electromagnetic fields predicted by a nonlinear lightning return‐stroke model

This paper investigates the influence of corona on currents and electromagnetic fields predicted by a return-stroke model that represents the lightning channel as a nonuniform transmission line with time-varying (nonlinear) resistance. The corona model used in this paper allows the calculation of corona currents as a function of the radial electric field in the vicinity of the channel. A parametric study is presented to investigate the influence of corona parameters, such as the breakdown electric field and the critical electric field for the stable propagation of streamers, on predicted currents and electromagnetic fields. The results show that, regardless of the assumed corona parameters, the incorporation of corona into the nonuniform and nonlinear transmission line model under investigation modifies the model predictions so that they consistently reproduce most of the typical features of experimentally observed lightning electromagnetic fields and return-stroke speed profiles. In particular, it is shown that the proposed model leads to close vertical electric fields presenting waveforms, amplitudes, and decay with distance in good agreement with dart leader electric field changes measured in triggered lightning experiments. A comparison with popular engineering return-stroke models further confirms the models ability to predict consistent electric field waveforms in the close vicinity of the channel. Some differences observed in the field amplitudes calculated with the different models can be related to the fact that current distortion, while present in the proposed model, is ultimately neglected in the considered engineering return-stroke models.

Lightning strikes to tall objects: A study of wave interactions at the return-stroke front using a nonlinear transmission line model

A theoretical study is presented to investigate lightning strikes to towers, with focus on wave interactions occurring at the return-stroke front due to the arrival of current pulses that propagate upward on the channel after being transmitted from the tower. The lightning channel is represented as a transmission line including corona and nonlinear losses. Analyses for the hypothetical case of a lossless channel considering matched tower and channel impedances show that the arrival of current pulses at the upward moving return-stroke front leads to an increase in corona currents leaving the channel. This transient process generates current pulses whose arrival at the tower top can be interpreted as the effect of a current reflection at the upward moving front, even though no impedance discontinuity exists at that point if return-stroke and leader channel properties are assumed the same. In the more realistic case of a lossy channel considering unmatched channel and tower impedances, the nonlinear channel resistance modifies the current pulses that propagate along the channel so that they merge smoothly with the return-stroke front. In this case, the interaction of the current pulses transmitted from the tower to the channel with the upward moving return-stroke front does not lead to features that can be clearly interpreted as the result of current reflections at that point in the evaluated conditions. Finally, it is argued that the current reflection coefficient at the tower top should be viewed as a current dependent parameter as opposed to a constant, linear value.

Two-port wideband models of a single-phase distribution transformer with center-tapped secondary

This paper proposes a pair of two-port wideband models for a 10 kVA 7.967 kV/240-120 V single-phase transformer with center-tapped secondary used in distribution lines in Brazil. The models are valid from 10 Hz to few MHz and are derived from measurements of the admittance matrix and the voltage ratio between the high-voltage and low-voltage transformer terminals. Two different conditions are assumed for deriving the models for the transformer. One assumes that one of the low-voltage terminals is short-circuited, while the other assumes that the same terminal is left open-circuited. Both models are validated by means of comparisons with experimental data. It is shown that voltages transferred from the high-voltage side to the low-voltage side of the transformer reach higher amplitudes for the condition where one of the low-voltage terminals is left open-circuited.

Influence of XLPE-covered cables on the impulse withstand voltage of a single-phase structure used in compact distribution lines

This paper investigates the influence of XLPE-covered cables on the impulse withstand voltage of a typical structure used in single-phase compact distribution lines in Brazil. For comparison purposes, two different procedures are adopted for estimating the impulse withstand voltage of this structure under standard positive lightning impulses. One considers a bare cable and the other assumes a XPLE-covered cable. For the impulse withstand analysis considering the bare cable, the up-and-down method is used for determining the critical flashover overvoltage (CFO) of the structure. The corresponding Vxt curve is also determined using the standard procedures. For the impulse withstand analysis considering the XPLE-covered cable, the breakdown voltage was determined in two different laboratory tests, the first one considering a brand new cable and the second one considering a punctured cable. Both tests were performed for two different cable manufacturers.

Preliminary analysis of the impulse breakdown characteristics of XLPE-covered cables used in compact distribution lines

This paper presents preliminary results related to the impulse breakdown characteristics of XLPE-covered cables. Tests with standard lightning impulse voltages (1.2/50 μs) applied to a typical single-phase structure employed in compact distribution lines used in Brazil were performed. Results in terms of breakdown voltage, time to breakdown and pinhole location are presented.