Fernando H. Silveira

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.

Evaluation of Lightning-Induced Voltages Over a Lossy Ground by the Hybrid Electromagnetic Model

In this paper, the hybrid electromagnetic model is applied to calculate lightning-induced voltages over a lossy ground. Results provided by this model are compared with experimental data obtained from a reduced-scale model and with results simulated by the numerical electromagnetics code. Good agreement is achieved in all cases.

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.

Backflashovers of Transmission Lines Due to Subsequent Lightning Strokes

Lightning overvoltages across insulator strings of existing 138- and 69-kV transmission lines were simulated using the hybrid electromagnetic model. It was found that, though the reduction of tower-footing grounding resistance is very effective to decrease lightning overvoltages caused by first strokes, this reduction is saturated around 750-600 kV for typical currents of subsequent strokes. The lightning performance of these lines was estimated and subsequent strokes were found to be relevant cause of backflashovers in 69-kV lines.

The Use of Underbuilt Wires to Improve the Lightning Performance of Transmission Lines

An unconventional technique to improve the lightning performance of transmission lines, consisting of the addition of grounded wires properly positioned below the phase conductors, is assessed. Overvoltages developed across insulator strings of existing 230-kV lines in response to direct strikes to the towers were simulated using an electromagnetic model. Minimum reductions of these voltages in the ranges of 19%-32% and 26%-44% were achieved for 20- to 80-Ω tower-footing grounding resistances due to the addition of one and two underbuilt wires, respectively. This practice was shown to be more efficient than reducing the tower grounding resistance to its half value.

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.

The Impact of the Frequency Dependence of Soil Parameters on the Lightning Performance of Transmission Lines

This study assesses the lightning performance of 230- and 138-kV transmission lines under the assumptions of constant and frequency-dependent soil parameters. An electromagnetic model was used to simulate the response of tower-footing electrodes and overvoltages across insulators due to direct lightning strikes to the line, considering representative lightning current waveforms. Backflashover rates were determined using the Disruptive Effect Model. It was found that the decrease of soil resistivity and permittivity, resulting from the frequency-dependence effect, is responsible for significant decrease of the expected outage rate of the tested lines, in the range of 13-32% for usual distributions of soil resistivity along the lines.

Frequency dependence of soil parameters: The influence on the lightning performance of transmission lines

The impact of the frequency dependence of soil parameters on the lightning response of transmission lines was assessed by simulation, taking an existing 138-kV line as reference. This can be important for overvoltages experienced across insulators due to lightning strikes, in particular of first strokes and soil resistivity above 300 Ωm. The effect causes a percentage decrease of the backflashover rate expected under the assumption of constant soil parameters of around 8%, 20% and 25% for values of soil resistivity of 300, 1000 and 2000 Ωm.