Very Fast Transient Overvoltage Research Articles

Very fast transient overvoltage waveshapes in a 500-kV gas insulated switchgear setup

Very fast transient overvoltages (VFTOs) may be generated in SF6 gas insulated switchgear (GIS) due to a series of arc ignitions and extinctions, during the operation of disconnecting switches and circuit breakers. They may also occur during internal flashover in the GIS. Some recent reports of failures in power transformers suggest high internal overvoltages associated with arc restrikes as a possible cause; such internal overvoltages might be generated by resonant frequencies affecting the transformer coils [1]. In order to understand this phenomenon better, Itaipu Binacional, a 14,000-MW Hydropower Plant, in cooperation with LACTEC, a High Voltage Research Laboratory, assembled a 31-meter-long GIS test setup including a disconnecting switch in order to study VFTOs occurring during disconnecting switch operation. It was found that the time to voltage peak, the inner frequency bandwidth, and the voltage peak values around 2.0 to 2.8 times the nominal voltage [2] lay within the ranges suggested in IEC 60071-1 [3]. However, the measured frequencies of the envelope of the transitory overvoltage shape were at least 10 times the upper limit suggested in [2] and [3]. Detailed results are presented below.

Improved Cable Connection to Mitigate Transient Enclosure Voltages in 220-kV Gas-Insulated Substations

Very fast transient overvoltages (VFTOs) due to switching operations can appear between the gas-insulated substation (GIS) enclosures and the cable sheaths and between these grounding parts and the substation Earth structures. These VFTOs achieve hundreds of kilovolts, provoking hazardous disruptive discharges for human safety. This paper includes a sensitivity analysis of the different parameters in order to establish criteria to be adopted in the grounding system design of the GIS structures and the cable connection to mitigate transient overvoltages. The use of bypass surge voltage limiters (SVLs) between the GIS enclosure and the cable sheath is also needed to avoid uncontrolled discharges. The selection of these SVL according to insulation coordination rules is presented in this paper, ensuring electrical safety without affecting the current mode of exploitation of the 220-kV GIS or the cable.

Behavior Comparison of Metal Oxide Arrester Blocks When Excited by VFTO and Lightning

With the development of the ultrahigh voltage (UHV) and extra high voltage (EHV) electric power industry in China, gas insulated substation (GIS) is applied widely. Very fast transient overvoltage (VFTO) caused by disconnector operations in GIS may damage the insulation of equipment due to its high frequency and amplitude. As one of the main overvoltage protection equipments, the protection behavior of surge arresters has received extensive attention. In this paper, the residual voltage behavior, the impedance behavior, and the response behavior of five different representative arrester blocks when excited by VFTO and lightning are investigated, respectively. Experimental results present that when the current peak keeps constant, the residual voltage under VFTO is approximately 14.7%–18.2% higher than under the standard lightning impulse voltage (8/20 μs, LI). The arrester block presents different impedance behavior under VFTO and LI. It behaves dominantly inductively under LI and expresses an impedance converting behavior under VFTO. Furthermore, the diameter and the shape of arrester blocks have little impact on the ratio of $V_{0}$ to $V_{{\rm 1mA}}$ under VFTO. Therefore, the protective effect of arresters varies slightly under VFTO and LI. Investigating behavior of arrester blocks under VFTO in comparison with under LI could be applied not only in VFTO simulation and insulation coordination but also in the design and the operating of metal oxide surge arrester in GIS.

High-Frequency Model of Magnetic Rings for Simulation of VFTO Damping in Gas-Insulated Switchgear With Full-Scale Validation

Nanocrystalline magnetic rings have been experimentally proven for effective damping of very fast transient overvoltages (VFTOs) in gas-insulated switchgear (GIS). The application of any damping solution in a specific GIS setup (rated voltage, GIS arrangement) requires simulation-based design, which, in turn, requires reliable models with proven accuracy in demanding high-frequency and high-current VFTO conditions. This paper presents a new model of a magnetic ring, employing full frequency-dependent characteristics of the ring's complex impedance, as well as the dedicated approach on modeling the saturation effect of the material magnetization characteristics. The saturation effect is modeled by a bypassing branch, activated at a certain saturation current value calculated according to the specific magnetic material properties and for the VFTO main frequency component. The model was implemented in Electromagnetic Transients Program simulation software and validated experimentally in a full-scale 550-kV GIS test setup. The validation proved the applicability of the model for the assessment of VFTO damping effectiveness with the use of nanocrystalline magnetic rings and, thus, for the design of a particular magnetic-based damping solution in a specific GIS setting.

Breakdown characteristics of oil-paper insulation under lightning impulse waveforms with oscillations

The non-standard lightning impulse waveforms (N-SLIWs) including oscillating lightning impulses, very fast transient voltages (VFTOs) and steep waveforms in the power system gravely endanger the inner insulation of power transformer. To prevent insulation failure and provide guidance for robust transformer design, in this paper, two typical insulation models of oil-immersed transformer were established and their breakdown characteristics under N-SLIWs were investigated. The research mainly focused on the influence of front time and oscillation frequency of N-SLIWs on the breakdown characteristics. The experimental results showed that the breakdown voltage increased with the decrease of front time. For low frequency-oscillating lightning impulse waveforms (LF-OLIWs), breakdown voltage increased with the increase of oscillation frequency and was up to 30% higher than that under standard lightning impulse waveform (SLIW). For VFTOs, with the increase of frequency, breakdown voltages presented a law of inverted U type. Insulation model could withstand the highest value of VFTO when the oscillation frequency was around 2.4 MHz. But when the frequency was as high as 9 MHz, breakdown voltage was close to that under SLIW. A very high frequency oscillation was the main reason for the reduction of breakdown voltage as its steepness dV/dt assisted the effective transmission of the sphere’s potential to the head of the streamer channel.

New Concept for VFTO Attenuation in GIS with Modified Disconnector Contact System

This paper investigates on a new concept for very-fast transient overvoltages (VFTOs) attenuation in gas-insulated switchgear (GIS). The concept, first introduced by the authors in this paper, involves modification of the GIS disconnector contact system in order to dissipate the VFTO energy in damping elements placed inside the GIS busbar conductor. The concept is an alternative to the state-of-the-art method where magnetic rings are placed in the space between the GIS conductor and the enclosure. The proposed arrangement has no impact on the damping element on the dielectric design of the GIS busbar, and it highly reduces the impact on the GIS thermal design. This paper presents analyses which show feasibility of the concept, through 1) principles which govern voltage conditions inside the GIS busbar and 2) full-Maxwell finite-element method electromagnetic simulations of VFTO attenuated with the use of the proposed concept.

Discharge characteristics of long SF6gas gap with and without insulator in GIS under VFTO and LI

Gas insulator switchgears (GISs), widely used in electric power systems for decades, have many advantages due to their compactness, minimal environmental impact, and long maintenance cycles. However, very fast transient overvoltage (VFTO) increases caused by a rise in voltage levels can lead to GIS insulation failures. In this paper, a generating system of VFTO and standard lightning impulse (LI) is established. The insulation characteristics of SF 6 gas with and without insulators under VFTO and standard LI are investigated. Experimental results show that the 50% breakdown voltages of the inhomogeneous electric field rod-plane gap under positive VFTO and standard LI are higher than that under negative VFTO and standard LI. The research shows that the 50% breakdown voltage under VFTO could be lower than that under standard LI at 0.5 MPa for the negative polarity. Moreover, the polarity effect of the insulator without defect is different from that with defect. Similarly, the breakdown voltage of the defective insulator under VFTO could be lower than that under standard LI by 8%. The flashover channel under VFTO is seen as more than that under standard LI. Based on the analysis of discharge images and experimental results, it is concluded that the polarity effect is related to the distortion effect of ion clusters formed by SF 6 on the electric field. Additionally, the steepness and front time of impulse plays an important role in the initiation and further development of discharge on insulator surface. Finally, the research shows that different discharge characteristics between VFTO and standard LI may be caused by different wave fronts and oscillation on the tails of the impulses.

Study on the Influence of Disconnector Characteristics on Very Fast Transient Overvoltages in 1100-kV Gas-Insulated Switchgear

The very fast transient overvoltage (VFTO) generated during switching of the disconnector (DS) in gas-insulated switchgear (GIS) may threaten the insulation of electrical equipment. DS characteristics are important influence factors for VFTO. To study the influence of DS characteristics on VFTO in GIS, an experimental study was conducted in two established full-scale 1100-kV GIS test circuits. More than 1800 tests of switching of DS in test circuits were carried out. In one circuit, the effect of operating speed on VFTO amplitude and trapped charge voltage were studied by changing the operating speed of DS. The results show that the amplitude of the trapped charge voltage and VFTO decreases with a decrease of operating speed. Operating speed was proposed to be 0.5 m/s $ \sim $ 0.8 m/s to suppress VFTO. In the other circuit, VFTO characteristics were studied by tests under two connection ways of DS. One is that the moving contact is connected with the ac power source, and the other is that the fixed contact is connected with the ac power source. The results show that the connection way will affect the breakdown characteristics of the DS, and then affect the polarity of trapped charge voltage. However, the connection way has little influence on the amplitude of VFTO. The achieved results provide a reference for the design and optimization of DS in GIS.

V-t Characteristics using extended disruptive effect model for impulses of varying front times

Overvoltages due to internal and external sources stress the insulation of the power system equipment. Usually the insulation design of any power equipment is tested with standard lightning impulse voltage. In practical cases, the insulations are stressed with overvoltages of non-standard waveshapes like steep fronted, Very Fast Transient Overvoltages (VFTO) and oscillatory impulses also. This demands the study on behavior of insulating materials under non-standard impulses. The prediction of v-t characteristics of any insulating material for such non-standard overvoltages is required for proper insulation design. In this regard, an attempt is made to characterize air insulation using existing Disruptive Effect model for standard lightning impulse (1.2/50 μs) and is found successful only for time to breakdown (tb) greater than 2 μs. Hence, an extended Disruptive Effect model is proposed to cater to tb less than 2μs and the model is verified up to VFTO. The model is also verified for impulses of varying front times between 90 ns to 1.2 μs with oscillations.

Introduction of Insulation Coordination for UHV AC Systems

This paper introduces the insulation coordination study results for UHV AC systems which was performed by CIGRE working group. The study results will be discussed within IEC technical committee in order to amend the IEC Insulation Coordination 60071-1 Ed. 8 2010 and 60071-2 Ed. 3, 1996. This paper includes the insulation coordination of transformer, gas insulated switchgear, metal oxide arresters and clearances of transmission towers. This article also illustrates the overvoltage specific to UHV systems such as TOV(temporary overvoltage), switching overvoltage, lightning overvoltage and VFTO(very fast transient overvoltage).

Experimental Research and Simulation on VFTO Mitigation by Ferrite Rings in UHV GIS

Experiments were performed on two different ultra-high voltage gas-insulated switchgear test circuits to evaluate the mitigation performance of ferrite R2KB rings with very fast transient overvoltages (VFTOs). Significant decreases in the overshoot factor and the VFTO level were observed, although the steepness was not much improved owing to saturation. It was found that ferrite rings are more effective for VFTO mitigation at the source side than at the load side. Simulations were carried out with a new circuit model of the ferrite ring, and the simulation results were consistent with the experimental results. The method of drawing the contour plot of VFTO and very fast transient current in the time-space domain was proposed to facilitate the analysis. With this method, the principle of determining the optimal installation location of ferrite rings for the best mitigation performance was clarified.

3-MV compact very fast transient overvoltage generator for testing ultra-high-voltage gas-insulated switchgear

Gas-insulated switchgear (GIS) is widely used today in electrical power systems because of its reliability, compaction, long maintenance cycle, and small impact on the environment [1]. In its use the normal operation of the disconnector generates very fast transient overvoltages (VFTO) with frequencies ranging from 1 to 100 MHz [2] and amplitudes up to 2.5 p.u. [3]. For systems above 330 kV, the insulation strength of the GIS under VFTO is of concern as insulation failures caused by VFTO have exceeded those caused by lightning impulse (LI) [4]. This characteristic has attracted the attention of researchers worldwide and currently is a hot topic in the field of ultra-high-voltage (UHV) GIS insulation.

A Novel Arc Model for Very Fast Transient Overvoltage Simulation in a 252-kV Gas-Insulated Switchgear

One of the important issues of concerns for gas-insulated switchgear (GIS) is the generation of very fast transient overvoltage (VFTO) during switching operations, especially at operational voltage levels of 330 kV and beyond. It becomes extremely important to build an arc model for VFTO simulation, since the arc determines VFTO characteristics to a great extent. However, existing arc models for VFTO simulation are too simple to provide simulation results that agree well with the experimental ones. As the arc resistance is often taken as a function of the very fast transient current (VFTC), it is necessary to get the experimental data on the VFTC. Therefore, a 252-kV GIS platform has been built as well as the VFTO and VFTC measuring system. On the basis of the experimental investigation, a novel arc model for VFTO simulation was proposed, in which the time dependent characteristics of the arc resistance is revealed. The model may be used for VFTO simulation for extra high voltage or ultrahigh voltage GIS.

New Methods of Damping Very Fast Transient Overvoltages in Gas-Insulated Switchgear

Very fast transient overvoltages (VFTO) are a well-known phenomenon in gas-insulated switchgear. They are mainly generated due to several pre- or restrikes during disconnector switching operations. Since VFTOs may cause complications in substations, a reliable method for damping these transient voltages is of great interest, especially in ultra-high voltage applications. Two new approaches are presented in this paper and compared with a matched radio-frequency resonator, which is an already existing theory. The new methods are an inductive arrangement with surge arrestors and rings of nanocrystalline tape-wound cores. All methods were investigated inside a high-voltage experimental test setup. Therefore, VFTOs with amplitudes of 600 kV and more were generated. The first VFTO amplitude was damped by up to 20%. Even higher damping is possible depending on the damping method and the test conditions. In addition to high-voltage tests and their results, this paper illustrates the theoretical background of all damping methods.

Simulation of VFTO by ATP/EMTP with Various Arc Models

Several arc models representing a disconnector’s closing operation in a 252kV gas insulated switchgear (GIS) are implemented with a computer program ATP/EMTP to simulate very fast transient overvoltage (VFTO). Arc models are used as black box models in connection with a test circuit. Theoretical bases, formulas and realizations of these models are described. Arc models’ parameters are mainly pinpointed by fitting their simulation results with VFTO waveforms measured in a real 252kV GIS test setup. Comparisons between arc models’ simulation results and measurement indicate that dynamic arc models based on Cassie and Mayr theories give more realistic results with particular regard to VFTO’s peak value and frequency spectrum.

Evaluation of Gas Insulated Disconnector Switch for Bus Charging and Bus Transfer Currents

Abstract Gas insulated disconnector switches (DSs) are employed in a gas insulated substation (GIS) to interrupt and to make capacitive currents, in principle. During switching of capacitive currents very fast transient over-voltages (VFTOs) are common to DS. Intense VFTOs may result in peak voltage close to basic insulation level (BIL) of the connected equipment, leading to progressive deterioration and secondary breakdowns in very high voltage gas-insulated systems. At the same time, arcing during switching of bus transfer currents, by the DS, is considered as a source of substantial amount of decomposed products. This contamination may result in significant reduction in insulation strength of the medium. In order to ensure reliability of the design, it is essential to establish the DS for bus charging and bus transfer current duties under most onerous service conditions through suitable tests. The optimal design of DS to meet these test duties is established with suitable analytical techniques. The tests, which have been discussed and analyzed as part of this study, were performed on an indigenously developed 145 kV gas insulated DS as per IEC 62271-102. A detailed arrangement of the DS (main and auxiliary) and guidelines for conducting the above tests are also explained. Different performance parameters like arcing time, pre-arcing time, secondary discharges, trapped voltages, and level of contamination are analyzed. The behavior of DS during these evaluations is also reported in the present paper. Finally, the effect of different design parameters on performance of the DS has been analyzed.

Study on Very Fast Transient Overvoltage of Closing No-load China UHV Transmission Demonstration Project Jindongnan-Nanyang-Jingmen 1000 kV Transmission Lines

When the ultra high voltage (UHV) substation used the gas insulated switchgear (GIS) system to switch lines with capacitive current, the pre-breakdown and multiple reignition will occur between switching contacts. Because of discharge characteristics of SF6 and special structures of GIS system, the variations of the electromagnetic transient process and very fast transient overvoltage (VFTO) are very fast and extremely complex. The development process and characteristics of VFTO remain to be dealt with to seek effective suppression measures. The overvoltage of closing no-load long lines on GIS system is analyzed in detail. The simulation model of the very fast transient process of closing no-load long lines on UHV GIS system is established. The VFTO of closing no-load China UHV transmission demonstration project Jindongnan-Nanyang-Jingmen 1000 kV transmission lines is analyzed using electromagnetic transient simulation software ATP-EMTP. The VFTO of closing no-load long lines on UHV GIS system is simulated and calculated, when the system has no additional closing resistors and lightning arresters, additional single stage closing resistors, additional multistage closing resistors, lightning arresters in two terminals of lines, lightning arresters in two terminals and the middle of lines, and additional multistage closing resistors and lightning arresters in two terminals and the middle of lines. The simulation results show that the additional multistage closing resistors and lightning arresters in two terminals and the middle of lines can effectively restrain the VFTO of closing no-load long lines on UHV GIS system.

New Techniques for Disconnector Switching VFT Mitigation in GIS

Switching operations in a Gas Insulated Substations (GIS) generate very fast transient over voltages (VFTO) which are dangerous for the transformer and the system insulation because of their short rise time. Under special circumstances the overvoltages can arise close to the transformer Basic Insulation Level (BIL). The reduction of VFTO amplitudes is considered main challenges. Therefore, VFTO in the 220 kV Wadi-Hoff GIS is analyzed and the worst case for disconnector switching is predicted using EMTP/ATP in this paper. VFTO mitigation techniques are studied in this work. Furthermore, proposed techniques are presented for mitigating the VFTO. The proposed techniques can be used by the maintenance engineers, transformer designers, and GIS insulation manufactures. The results show that the proposed techniques highly reduce the VFTO in a simple manner. DOI: http://dx.doi.org/10.11591/ijece.v4i2.5186

Fractional Lumped Parameter Modeling of Transformer Windings under VFTO

Very fast transient overvoltage (VFTO) is very dangerous for the transformer which is directly connected to gas insulated substations (GIS). At the same time, due to the continuous increasing of operating frequency in the power system, under the high frequencies of the transmission line calculation and simulation process, it is necessary to consider the frequency-dependent properties of the parameter. In order to study the voltage distribution in transformer windings under VFTO, a new Lumped parameter model which takes into account the fractional characteristics is proposed, named fractional lumped parameter model (FLP). The model divides each turn of transformer windings into proper sections, whose parameters are all lumped parameters. Finally it shows the correctness and validity of the model through the comparation of the experiment datas and simulation results.

A novel algorithm for frequency dependent modeling of ferrite ring and its optimum design to study VFTO behaviors within GIS

One of the fundamental issues in gas insulated substations (GIS) which has destructive effects on GIS equipment is the very fast transient over-voltages (VFTOs). This paper models a 400/230 kV substation in order to study the effects of VFTO extensively implemented on EMTP-RV. In addition, the application of ferrite rings for suppressing VFTOs is assessed thoroughly. The main advantage of this paper is its new proposed algorithm according to the ferrite ring frequency dependent modeling that is validated with experimental results. This paper examines the effects of three compositions of the ferrite ring on VFTO suppression. Moreover, it estimates the dimension of the ferrite ring based on the SF6 gas insulation withstand and the maximum effect of ferrite rings on VFTO suppression constraint with the COMSOL multiphysics software. Furthermore, it gains VFTO attenuated percentages due to the installation of the ferrite ring in different GIS nodes. Finally, it analyzes the offered VFTO amendment technique in various GIS switching scenarios.