Volt-time Curves Research Articles

Accurate Flashover assessment for coastal overhead lines based on volt-time criterion

This study investigates the impact of considering insulation volt-time curves on estimating the annual flashover rate of overhead lines subjected to nearby lightning strikes in the presence of ocean-land environments. A method based on the 3D Finite Element Method is employed to compute induced voltages, while a Monte Carlo simulation determines the annual flashover occurrence. To detect Flashover occurrence a volt-time method is employed and the results are compared with the conventional 1.5CFO criterion. The influence of line proximity to the coastline, pole spacing, and various striking distance calculation methods on flashover predictions is evaluated. Results indicate that the 1.5CFO criterion underestimates the annual flashover rate due to indirect lightning in mixed ocean-land terrains compared to the volt-time curve approach. A novel analytical formula is proposed to estimate annual flashovers considering volt-time curves in such environments.

Breakdown process experiments of cap‐and‐pin insulators and their EMT leader progression model implementation

Here, breakdown tests of cap-and-pin glass insulators with arcing horns, conducted at two different HV labs, are presented. Since only volt-time curves are measured, a curve-fitting approach to determine leader progression model parameters is taken, which involves modelling of the impulse generator in detail and inclusion of the Motoyama model to simulate the breakdown. Furthermore, a comparison of the breakdown simulation results with LPM parameters for a pure air gap and the measurements is accomplished. Finally, a curve-fitting procedure is developed to fit the resulting volt-time curves from simulations to the measurements.

Transformer insulation coordination using volt–time curve and limit–state surface formulation

This paper presents a novel method for the power transformer insulation coordination, based on the risk of failure analysis due to lightning surges, that uses its insulation strength volt–time curve and a limit–state surface formulation. The limit–state surface is derived in a novel way, from the optimal number of systematic numerical simulations of transformer terminal overvoltages—emanating from station impinging lightning surges—while accounting for the transformer insulation volt–time curve and surge arresters protective characteristics and disposition. The proposed method further employs a state-of-the-art transmission line (TL) and substation equipment models for lightning-surge transient analysis, constructed in the EMTP software package. It also uses the electrogeometric model of lightning attachment to TLs, in order to estimate the expected number of direct lightning strikes, along with a bivariate statistical distribution of lightning currents. The main aspects of the proposed method are demonstrated by means of the computational example featuring an air-insulated substation power transformer lightning insulation coordination. Simulation results exhibit many benefits of the proposed method. Sensitivity analysis further reveals different influences that the various model parameters have on the transformer insulation coordination design.

Volt–Time Curve Prediction of Distribution Insulators Under Standard and Typical Lightning Overvoltages Using the Disruptive Effect Method

Through the development of detection and measurement systems for lightning strokes, the possibility arose for the identification of the characteristic parameters of lightning current and the resulting voltage waveforms that stress the power system insulating equipment. The real lightning voltages that stress the insulation may vary over a wide type of waveforms and may also differ significantly from the standard 1.2/50 $$\upmu \hbox {s}$$ lightning impulse voltage. Therefore, the prediction of volt–time characteristics of any insulating material for such non-standard voltages is required for a proper insulation design. In this regard, this paper aims at evaluating the dielectric behaviour of medium voltage insulators subjected to voltage impulses with standard and non-standard waveforms, by applying the disruptive effect method. The non-standard lightning overvoltage waveforms considered in this study were based on lightning overvoltages produced by real return stroke current waveforms observed on conveyed measurement data. The investigation involves a comparison between measured and predicted voltage-time characteristics of insulators, considering single-peaked and double-peaked non-standard impulsive voltage waveforms as well as the standard lightning impulse voltage. Three different types of insulating arrangement involving 15- and 25-kV pin-type porcelain insulators were investigated. The estimation of the parameters required for this prediction method was performed using a simulation language available at ATP/ATPDraw software called MODELS.

Lightning performance of transmission lines based upon real return-stroke current waveforms and statistical variation of characteristic parameters

This paper presents investigative studies concerning the lightning performance of 500kV transmission lines considering the characteristic parameters of first stroke current observed in measurement data along with its statistical variation. Therefore, advanced electromagnetic models were applied during the development of computational models, thus allowing a realistic representation of the first stroke current waveform. Lightning performance studies were conducted applying two of the most commonly used return-stroke current models, besides the first stroke current model recommended by CIGRÉ. The aim therefore is to identify which of these present the most accurate results, when compared to the use of real return-stroke current waveforms. The volt–time characteristics produced by each of the return-stroke current waveforms were evaluated using both the leader progression model (LPM) and volt–time curve. The breakdown process modelled through LPM is able to represent accurately the physical process established along the perimeter of the insulator strings, during a backflashover event. Analysis of leader length and velocity can reveal the influence of particular characteristics of lightning overvoltage waveform across the insulators produced by different return-stroke current waveforms in the assessment of the lightning performance of a transmission line. The computational modelling developed herein, allows identifying the critical current, the amplitude of the overvoltage that produces the backflashover, and the instant of time that breakdown occurs. The proposed modelling also makes it possible to determine in which insulator string and tower the backflash happens.

Comparison of different procedures to predict the volt-time curves of 15 kV insulators

This paper presents the results of the application of a novel method to estimate the parameters of the Disruptive Effect Model for the prediction of the volt–time curves of 15kV insulators. Besides the standard lightning impulse voltage waveshape, four non-standard impulse waveshapes (namely 1.2/4μs, 1.2/10μs, 3/10μs, and 7.5/30μs), of both polarities, were adopted in the tests. The measured volt–time characteristics of a 15kV class pin-type insulator were compared with those predicted by three different procedures related to the Disruptive Effect Model: Darveniza and Vlastos, Hileman, and the one recently proposed by the authors. The results show that for some impulse waveshapes and polarities, the methods by Darveniza and Vlastos and by Hileman do not predict insulator breakdown for the lower voltage levels. On the other hand, a relative good agreement is found between theoretical and experimental results for the calculations performed using the method proposed by the authors.

Impulse withstand voltage of single-phase compact distribution line structures considering bare and XLPE-covered cables

This paper is dedicated to investigate the impulse withstand voltage of single-phase compact distribution line structures of 15kV class. The structures are tested in laboratory with the application of standard lightning impulse waveforms of positive polarity. Three situations are considered: (i) XLPE-covered cables with brand new insulation; (ii) XLPE-covered cables with punctured insulation; (iii) bare cables, aiming at assessing the critical case in which the insulating layer of the cable is completely deteriorated. The results indicate that the presence of an XLPE layer covering the cable leads to a significant increase in the impulse withstand voltage of the investigated structures compared to the use of bare cables. In the cases considering brand new XLPE-covered cables, insulation breakdown leads to the formation of a pinhole in the insulating layer at points up to about 200cm from the tested structure. The presence of pinholes in the tested cable specimens has no significant effect in the breakdown voltage of the tested structures, except when the pinhole is sufficiently close to the pole. Finally, to provide elements for investigating the performance of the structures when subjected to nonstandard impulse voltages, parameters for volt-time curves and for the disruptive effect (DE) method are provided for the critical case of structures with bare cables.

Flashover Probability Distribution and Volt-Time Curves of Medium Voltage Overhead Line Insulation Under Combined AC and Lightning Impulse Voltages

Lightning overvoltages are considered as one of the major causes of insulation failure in medium voltage overhead lines. Therefore, the evaluation of the dielectric strength of overhead line insulation is of great importance for insulation coordination studies. This paper presents the flashover probability distributions and the volt-time characteristics of different types of insulator gaps subjected to both positive and negative polarity standard impulse (SI) and short tail lightning impulse (STLI) voltages. In practice, the transient overvoltages generated by a lightning stroke are superimposed over the power frequency AC voltage. Accordingly, the flashover characteristics of the insulator gaps under combined AC and lightning-induced overvoltages are also presented in this paper. The results indicate that superimposition of the impulse voltage on the power frequency AC voltage changes the flashover characteristics of the insulation gaps. Therefore, it is recommended that the effect of power frequency AC voltage should be considered in the risk assessment of insulation for the insulation coordination studies.

Modeling of flashover arcs in medium voltage networks due to direct lightning strikes

Abstract Distribution line faults in many cases are caused by direct lightning strokes to overhead lines resulting in a disruption between phase and ground in the form of flashover arc. Therefore, it is important to accurately simulate the lightning behaviour for the protection studies of distribution networks. This paper investigates the influence of considering the dynamic arc model and the actual volt–time characteristics of line insulators in the evaluation of lightning overvoltages. A full-scale experimental set-up was installed in the high voltage laboratory to study the dynamic interaction between fault arc and power system with grounded and ungrounded cross arm configurations. The least square method has been used to extract the arc parameters from the experimental results for modeling purpose. The model of experimental set-up has been implemented in Alternative Transients Program–Electromagnetic Transients Program (ATP–EMTP) software and the experimental results have been reproduced by computer simulations with reasonable accuracy. Based on the comparison of experimental and simulation results, it is concluded that the actual volt–time curves and the arc model that characterize the behaviour of line insulators should be considered during the simulation of the lightning performance of overhead distribution networks.

Lightning Flashover Rates of Overhead Distribution Lines Applying EMTP and IEEE Std.1410

This paper calculates the lightning flashover rates of 22 kV overhead distribution line. In order to calculate the backflashover rate (BFOR), the performance of line which related to the insulator backflashover has been modeled and analyzed using Electromagnetic Transient Program (EMTP). The flashover models, volt time curve (VT), disruptive effect model (DE) and leader progression model (LPM) have been compared. The shielding failure flashover rate (SFFOR) and induced voltage flashover rate (IVFOR) have been calculated follow equations given in IEEE std 1410-2010. Finally the total numbers of flashovers of line have been determined. The lightning flashovers rates of BFOR, SFFOR and IVFOR have also been compared with the data taken from the line outages recorder. From the system being studied shown that, the total numbers of flashovers from simulation and calculation is closed to the data from the line outage recorder.

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.

SF6 breakdown in GIS

SF/sub 6/ gas breakdown and spacer surface flashover volt-time characteristic curves were established for a GIS of D/d=55/25 (15 mm gap) and D/d=45/25 (10 mm gap) for 1.2/50, 0.5/50, 0.2/50 and osc/50 (1.64 MHz) /spl mu/s impulse and disconnector induced VFT voltages. VFT volt-time curves established using actual breakdown voltages against actual time to breakdown were found to give a better representation of the dielectric performance, and thus are recommended for future applications. Highly inhomogeneous field geometries, such as particle contaminated gaps, caused downward turning of the v-t curves during VFT breakdowns at short times to breakdown.

Lightning overvoltage protection of the Paddock 362-145 kV gas-insulated substation

Backflashovers close to the Paddock 362-145 kV gas-insulated substation (GIS) have been analyzed with the electromagnetic transient program (EMPT) using a frequency-dependent multiconductor system. The severities of the lightning stroke currents were based on recent recordings in the eastern United States. The effects of corona attenuation and distortion were accounted for using a shunt linear model approach. Turn-up effects of both line insulator flashover voltages and surge arrester protective characteristics were based on manufacturer's volt-time curves. Wave-shaping effects of substation capacitances were modeling details in determining the overvoltages inside the GIS due to close backflashovers, which are caused by lightning strokes with varying intensity. These results are aimed at better evaluation of lighting protection requirements for GIS protected by metal-oxide surge arresters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance of Large Air Gaps under Lightning Overvoltages: Experimental Study and Analysis of Accuracy of Predetermination Methods

The paper summarizes the results of the research recently carried out at CESI, with lightning overvoltages. Tests were performed on various gap configurations and with several impulse shapes of both polarities to get information about macroscopic parameters and basic processes associated with discharge development. The basic aim was to have a consistent amount of data to carry out a validation and comparison of the existing models, while providing the required information to support the refinement of those based on physical approach. Breakdown voltages versus time to breakdown characteristics, (Volt-time curves), were determined. The curves give an indication of the importance of the impulse-shape and configuration, thus underlying the necessity of calculations methods. Discharge parameters have been presented and discussed to put into evidence the relative importance of the impulseshape characteristics. Data about the discharge process have been compared with other available data and with evaluations according to methods suggested in the literature. A refinement of the available formulas with reference to leader velocity has also been proposed, as in the following: v1 = 170 * d * e(1.5* 10-3* U/d) * (U/x-Eo) where ?1 is the velocity (m/s), U is the applied voltage (kV), d the gap clearance (m), x the part of the gap unbridged by the leader (m) and Eo the average gradient in the gap of the configuration examined at 50% breakdown voltage with standard lightning impulse (kV/m). The equation is of general type, applicable for all the configurations and for impulses of different shape and polarity.

The generalized integration method for predicting impulse volt-time characteristics for non-standard wave shapes-a theoretical basis

A theoretical basis for the integration method of predicting the strength of insulation subjected to impulses of nonstandard wave shapes is presented. Two theoretical approaches are developed for a general physical model of impulse breakdown applicable to gaseous, liquid, and solid insulation, and from an energy-balance model applied to an incipient breakdown channel in a general dielectric. It is therefore not surprising to find that the integration method is reasonably successful as a tool for predicting nonstandard wave-shape insulation strengths, using as input the data derived from tests with standard-wave-shape impulses. Numerical results are presented that provide an understanding of the shape of volt-time curves. An energy balance model has also been used to derive the resistance of the channel after breakdown has occurred.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Volt-Time Curves for a Coaxial Cylindrical Gap in SF6-N2 Mixtures

The impulse breakdown voltage-time characteristics of a coaxial cylindrical gap in 10% SF6-90% N2 mixtures are presented. The theoretical models for the determination of the envelopes of the v-t characteristics are examined. A method has been suggested for the determination of the characteristic parameters used in the theoretical models.

Switching Surge Response of Transformer Insulation Designed on the Basis of Power Frequency PD Inception Volt-Time Curves

Switching Surge Response of Transformer Insulation Designed on the Basis of Power Frequency PD Inception Volt-Time Curves

Switching Surge Response of Transformer Insulation Designed on the Basis of Power Frequency PD Inception Volt-Time Curves

The paper presents the results of power-frequency and switching-surge tests performed on transformer insulation models.

Sparkover Characteristics of High Voltage Protective Gaps

Published data on sparkover characteristics of high-voltage protective gaps with 50-200 cm spacings have been reviewed and statistically analyzed. The results are presented, including: volt-time curves for times to sparkover up to 8 Ms, average critical sparkover gradient for various types of gaps for lightning impulse and for switching impulse, and average sparkover gradient for power-frequency voltage. Wherever justifiable, 95 percent confidence intervals are also given.

Impulse Characteristics of Aerial Spacer Cable

Although the overall performance record of aerial spacer cable on the Public Service Electric and Gas Company system has been excellent, the number of failures caused by lightning appears to be disproportionately high. To determine the optimum design criteria of this type of distribution system, a program of laboratory impulse tests was established. This paper presents the results of the laboratory tests, with volt-time curves for various components of the spacer cable system. This study is one of several performed by the Public Service Electric and Gas Company.