Ultra-high Voltage Gas-insulated Switchgear Research Articles

Surface modification of aramid fiber by cascade process with atmospheric plasma and TDI grafting

Aramid fiber (AF)/epoxy composites are often used as mechanical and insulating materials for ultra-high voltage gas-insulated switchgear (GIS) insulation pull rods. However, the product performance is believed to be restricted by the poor adhesion between AF and the resin matrix, affecting the operation reliability of GIS. In this work, a cascade process was applied to modify the AF surface to improve interfacial behavior. The AF was first treated with air dielectric barrier discharge at atmospheric pressure to obtain a rough surface and then reacted with toluene-2,4-diisocyanate (TDI) to introduce polar functional groups onto the surface. The plasma discharge power and treatment time were investigated as major parameters, and the aging effect was studied. The modified AF was characterized by scanning electron microscope, Fourier transform infrared spectrometer, x-ray photoelectron spectroscopy, atomic force microscope, and filament yarn tensile test. The interfacial behavior of the AF/epoxy composite was analyzed by interfacial shear strength (IFSS) test of micro-bond specimen and interlaminar shear strength (ILSS) test of Naval Ordnance Laboratory rings. Experimental results showed that polar functional groups were introduced onto the AF surface. The IFSS and ILSS of AF/epoxy composite were increased by 32.8% and 20.2% at most, respectively, and the tensile strength of the AF was mostly preserved. Moreover, with the increase in plasma discharge power, the interfacial strength of the modified AF/epoxy interface increased firstly and then decreased. The AF modified by TDI retained a relatively good modification effect after aging for some time. This study presents a cascade process for the AF surface modification with simplicity, effectiveness, and resource-saving, which is suitable for industrial applications.

Seismic vulnerability assessment on porcelain electrical equipment based on Kriging model

To improve the efficiency in assessing the seismic vulnerability of porcelain electrical equipment when considering different sorts of uncertainty, this paper proposes a seismic vulnerability analysis method based on Kriging model. The one-dimension and multi-dimension Kriging models were considered in the method. Afterward, case studies were conducted on an ultra-high voltage (UHV) gas insulated switchgear (GIS) porcelain bushing. The seismic vulnerability of the UHV GIS bushing obtained by incremental dynamic analysis (IDA) method and the Kriging method were compared. The validity and applicability of the method were verified. Moreover, the effects of the dimensions of the Kriging model were analyzed. The results indicated that the Kriging method reduced the calculation quantity by 40% compared with IDA method, and the analysis error was within 5%, which significantly improved the efficiency of seismic vulnerability analysis of porcelain electrical equipment. The multi-dimension Kriging model-based method is suitable for the seismic vulnerability analysis considering different sorts of uncertainty, while the one-dimensional Kriging model-based method can obtain more stable seismic vulnerability assessment results.

Experimental Evaluations on Seismic Performances of Porcelain and GFRP Composite UHV GIS Bushings.

To evaluate the seismic performances of the ultra-high voltage (UHV) gas-insulated switchgear (GIS) bushings made by porcelain and glass fiber reinforced polymer (GFRP) composite materials, shaking table tests were conducted on the two full-scale GIS bushings. The dynamic characteristics and seismic responses of the two UHV GIS bushings were obtained. The experimental results indicated that the two UHV GIS bushings meet the seismic requirements in the corresponding standards. The supporting frame and bus canister amplify the seismic responses of the UHV GIS bushings. Under earthquakes, the GFRP composite UHV GIS bushing is safer than the porcelain bushing. In the seismic design of the electrical substation, the large seismic displacement of the GFRP composite UHV GIS bushings should be considered.

Study on fluid model of disconnector arc applied in very fast transient overvoltage

The essence of the mathematical model of disconnectors' multiple reignition arc is the repeated breakdowns of SF6. In this study, the kinetic properties of microparticles are studies based on the streamer discharge theory, and a fluid arc mathematical model applied in centimetre clearance is established. Ultra-High Voltage (UHV) disconnector manufactured by Pingao Electric Co. Ltd of Henan Province is selected as the object and the resistance change process of contact clearance multiple reignition arc is simulated by finite difference method-finite element method-flux corrected transport self-programming in MATLAB based on the chamber structure and operating characteristics. UHV gas-insulated switchgear very fast transient overvoltage (VFTO) test platform is built to gain the characteristics of VFTO waveforms generated by the disconnector's operation. Compared with simulation results, the test data shows that the fluid arc mathematical model can scientifically explain the dynamic development process of microparticles in the contact clearance and the calculation results are consistent with the test data. In addition, the arc fluid mathematical model combines the structure and operating characteristics of the switch with the development process of microparticles, which can better guide the design of the switch, and provide a theoretical basis for the suppression of VFTO with the disconnector itself.

Application of X-Ray Inspection for Ultra High Voltage Gas-Insulated Switchgear

Owing to the strong penetration ability of X-rays and the imaging visibility, X-ray inspection has been gradually applied to power grids. However, various problems occur in the promotion and application of X-ray inspection in ultra high voltage gas-insulated switchgear (UHV GIS) owing to its large size. These problems relate to parameter selection, photography, protection area, random defect recognition, and risk assessment based on the X-ray image. In this paper, an X-ray digital imaging system is set up for UHV GIS inspection. Parameter selection for UHV GIS is discussed, and we demonstrate that UHV GIS can be photographed under a source-to-detector distance (SDD) of 2.8 m, a tube voltage of 300 kV, and an exposure of >240 mA·s. A photographic method for UHV GIS, entailing adjustment of the position of the imaging plate with a fixed position of the X-ray generator, is proposed to reduce the difficulty in image stitching. The influence of the parameters on imaging quality is discussed, and we show that the brightness and contrast increase with increasing exposure or decreasing SDD. The interactions between parameters are qualitatively introduced. Moreover, the sensitivity of X-ray inspection in UHV GIS is studied under selected parameters, and the sensitivity of X-ray inspection to copper particles and aluminum particles in UHV GIS are shown to be <0.7 mm and <1.1 mm, respectively. Radiation protection for operators is also discussed. In addition, a pseudo-color scheme for identifying random tiny defects is proposed to reduce human error. Finally, the risk level of defects with X-ray inspection is defined by size feature.

Conical voltage sensor for measuring very fast transient overvoltage up to 3 MV in ultra‐high‐voltage class gas‐insulated switchgear

Very fast transient overvoltage (VFTO) engenders great threat to gas-insulated switchgear (GIS) in ultra-high-voltage (UHV) power system. A novel conical voltage sensor was developed based on combination of the conical voltage divider and coaxial integral device to measure VFTO in UHV GIS. The results indicate that the conical structure of the voltage sensor can make the wave impedance translate smoothly from the high-voltage arm to the coaxial cable, which significantly reduces refraction and reflection of VFTO. The optimum value of the integral capacitance is between 100 and 200 pF, which is determined by the integral resistance, capacitance of the sensor, and the length of the transmission line. To avoid refraction and reflection of VFTO in the process of transmission, the length of the transmission cable should not exceed 20 cm. Calibration results show that the response time of the conical voltage sensor is <;4.8 ns, the voltage attenuation ratio of the sensor can reach up to 43,402 and the frequency band is between 7.8 Hz and 73 MHz. The conical voltage sensor was tested on a steep impulse voltage equipment, and the result showed the sensor could measure VFTO up to 3 MV.

Multi-spark modeling of very fast transient overvoltages for the purposes of developing HV and UHV gas-insulated switchgear and of conducting insulation co-ordination studies

AbstractVery fast transient overvoltages (VFTO) originate from steep voltage breakdowns in SF6gas that are inherent to operation of any switching device of the gas-insulated switchgear (GIS) type. For power stations with voltage ratings exceeding 500 kV, the ratio between equipment rated- and withstand-voltage levels becomes relatively low, which causes the VFTO peak values to reach the component’s insulation withstand-voltage levels, thus becoming a design factor for high- and ultra-high voltage GIS. While well-established approach to VFTO analyses involves only single VFTO events (the so-called single-spark approach), there is often the need to analyze the entire VFTO generation process, for which the multi-spark approach to VFTO modeling is to be employed. The multi-spark approach allows one to evaluate the VFTO impact on the GIS disconnector design along with the impact of the VFTO on selection and dimensioning of the VFTO damping solutions. As the multi-spark approach to VFTO modeling is now being increasingly used in UHV GIS developments as well as for the insulation co-ordination studies of power stations, the present paper is motivated by the need to report on the VFTO multi-spark modeling approach and to lay a common ground for development works that are supported extensively with VFTO simulations. The paper presents physical assumptions and modeling concepts that are in use in such modeling works. Development of the multi-spark GIS disconnector model for VFTO simulations is presented, followed by an overview of examples of the model application for the GIS development works and for insulation co-ordination studies.

Simulation and experimental study on mechanical performance of UHV GIS spacer in hydrostatic test

Gas insulated switchgear (GIS) spacer is required to be of good electrical and mechanical properties to ensure high safety and reliability. The requirements are stricter for ultra high voltage (UHV) GIS spacer under higher voltage and pressure. In previous studies, the insulation characteristics of GIS epoxy spacer were investigated. But researches on the mechanical properties of the GIS spacer were rarely reported. In this study, UHV GIS spacer in hydrostatic test is simulated with the finite-element method. Strain and stress distributions along spacer surface are investigated. Results indicate that the maximum stress appears on the spacer surface near flanging ring, where cracks usually occur. Based on the calculation results, fiber Bragg grating (FBG) sensors are placed on UHV GIS spacer surface to monitor the strain variation in hydrostatic test. Measured results present a strain distribution similar as calculation results. The calculated failure pressure is consistent with those of tested spacers. The crack initiates in the proximity of sealing groove as predicted in calculation.

Controlled Voltage Breakdown in Disconnector Contact System for VFTO Mitigation in Gas-Insulated Switchgear (GIS)

Several methods have been proposed and investigated so far on mitigation of very fast transient overvoltages (VFTO) in gas-insulated switchgear (GIS). The state-of-the-art methods are primarily based on dissipation of the energy associated with electromagnetic waves that the VFTO originate from and are composed of. Present paper reports on an alternative concept of VFTO mitigation based on the principle of controlling voltage conditions preceding voltage breakdown in SF6 gas that leads to VFTO generation. The paper introduces different control algorithms and shows how the algorithms can limit VFTO maximum value and total number of voltage breakdowns during operation of the GIS disconnector. The concept is applied for mitigation of VFTO in ultra-high voltage GIS. As the study case, an 1100 kV test set-up is used as recently reported for Wuhan (China) GIS station, with the disconnector characteristics obtained from 1100 kV development tests.

Repeated Strike Process During Disconnector Operation in Ultra-High Voltage Gas-Insulated Switchgear**supported in part by National Natural Science Foundation of China (No. 51277106) and in part by the National Basic Research Program of China (973 Program) (No. 2011CB209405)

Very fast transient over-voltage (VFTO), induced by disconnector operations in gas-insulated switchgears, has become the limiting dielectric stress at ultra-high voltage levels. Much work has been done to investigate single-strike waveforms of VFTO. However, little study has been carried out investigating the repeated strike process, which would influence VFTO significantly. In this paper, we carried out 450 effective experiments in an ultra-high voltage test circuit, and conducted calculations through the Monte Carlo simulation method, to investigate the repeated strike process. Firstly, the mechanism of the repeated strike process is proposed, based on the experimental results. Afterwards, statistical breakdown characteristics of disconnectors are obtained and analyzed. Finally, simulations of the repeated strike process are conducted, which indicate that the dielectric strength recovery speed and polarity effect factor have a joint effect on VFTO. This study enhances the understanding of the nature of VFTO, and may help to optimize the disconnector designed to minimize VFTO.

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.

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.

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.

Development of Full Frequency Bandwidth Measurement of VFTO in UHV GIS

In order to study the characteristics of very fast transient overvoltage (VFTO) in ultra-high voltage (UHV) gas-insulated switchgear (GIS), the authors have developed a full frequency bandwidth measurement system based on a porthole-type capacitive voltage divider. Three kinds of measurement circuits for the measurement system are studied and the influence of cable length on measurement results is analyzed in this paper. The calibration of the measurement system using step response was conducted. The results show that the measurement system of the capacitive voltage divider integrated with an impedance converter has a bandwidth ranging from 0.003 Hz to 300 MHz at least. It can be used to measure the full VFTO during DS operation perfectly. A full-scale UHV DS (disconnect switchgear) is used to establish the test circuit. The measurement system with good anti-electromagnetic interference ability was constructed. Four different measurement locations are chosen on which to install the measurement system. An approximate total of 3000 closing and opening operation tests were conducted. Due to the wide frequency band of the measurement system, it is possible to obtain the full VFTO waveform accurately and conveniently. According to the waveforms, the key parameters of VFTO, such as trapped charge voltage, peak value of VFTO, breakdown characteristic of DS, frequency spectrum of transient waveform, and the overvoltage coefficient could be obtained.

Experimental Research on Very-Fast Transient Overvoltage in 1100-kV Gas-Insulated Switchgear

Very-fast transient overvoltage (VFTO) generated during switching of the disconnector in gas-insulated switchgear (GIS) may threaten the safety operation of electrical equipment. As the rated voltage of GIS increases, the difference between the level of VFTO and its lightning impulse withstand voltage gradually decreases. Hence, VFTO becomes the main dielectric stress to switchgear optimization and insulation coordination for ultra-high voltage (UHV) systems. To obtain the characteristics of VFTO in UHV GIS, two full-scale 1100-kV GIS test circuits were established in China. Two types of capacitive sensors were developed to measure VFTO generated in two test circuits. The measurement systems based on both types of the sensors were calibrated by a special calibration device in laboratories and onsite. The measurable frequency range of the systems is 0.1 Hz ~ 100 MHz and their division ratios are stable. More than 2000 tests by switching the disconnector in GIS had been carried out in the test circuits, and VFTO waveforms were captured by the measurement systems. From the test results, the waveform properties, the striking number, the predominant frequency, and distribution of the trapped charge voltage for full-voltage waveforms, the typical characteristics and oscillation coefficient of single-strike waveform, the distribution of VFTO magnitude was analyzed. The mitigation of VFTO by a damping resistor in the disconnector was illustrated. The maximum magnitude of VFTO was 2.27 p.u., and it was mitigated to 1.33 p.u. by a damping resistor of 500 Ω. The achieved characteristics of VFTO in the UHV GIS have become the basis of VFTO simulation and insulation coordination for engineering design.