Gas-insulated Systems Research Articles

Computational evaluation of resistance of fracture capacity for SUS304L of liquefied natural gas insulation system under cryogenic temperatures using ABAQUS user-defined material subroutine

A computational method for evaluating the resistance of fracture capacity of austenitic stainless steel (SUS304L) under ambient and cryogenic temperatures was proposed in the present study. To do this, a unified elasto-viscoplastic-damage model which can take into consideration the material nonlinearity under cryogenic temperature was adopted and implicitly formulated. In addition, the developed model is implemented into ABAQUS as a type of user-defined material subroutine UMAT. In order to verify the proposed method, the simulation results of resistance of fracture capacity are compared to a series of crack tests such as double-edge-cracked and center-cracked tension tests of SUS304L plates under ambient and cryogenic temperatures. Namely, the relationships between force and displacement as well as allowable crack length and allowable stress regarding to room and cryogenic temperatures are evaluate/predicted using the proposed method.

Monte Carlo simulation of positive corona discharge in SF6

Gas-insulated systems are now a major component of power transmission and distribution systems all over the world. There has been considerable interest in non-thermal discharges over the past decade due to the increased number of industrial applications. Diverse applications demand a solid physical and chemical understanding of the operational principals of such discharges. This paper focuses on the most important and widely used variety of non-thermal discharges, the corona discharge. Positive corona in SF6 is studied for a point to plane electrode gap using Monte Carlo simulation technique. The initiation and development of successive avalanches are traced as function of time. The simulation provides a detailed structure of avalanches from which essential properties of corona discharge with regard to total field distribution, propagation of successive avalanches and ion distribution can be discerned.

ANALYSIS OF TRANSIENT ENCLOSURE VOLTAGES IN GIS (EMTP SIMULATION STUDIES)

Abstract Transient Enclosure voltage is special case of very fast transient overvoltages which occurs due to disconnect switch operation or earth ground. Transient Enclosure Voltage appears on external of earthed enclosure of Gas Insulated Systems. Despite of proper grounding, this phenomenon indicates presence of high potentials on Gas Insulted System enclosures so the grounding system impedance is thoroughly examined and designed. In this study EMTP Software is used for analysis. Simulation was done by varying the different parameters. Variations of waveforms of the Transient Enclosure Voltage with various parameters have been studied. Index Terms: Transient Enclosure Voltage (TEV), Gas Insulated System (GIS), Very Fast Transient Overvoltages (VFTO) Transient Ground Potential Rise (TGPR), Disconnector Switch ----------------------------------------------------------------------***------------------------------------------------------------------------ 1. INTRODUCTION

Calculation of impulse characteristics for gas-insulated systems with homogenous electric field

The possibility of generating a statistical sample of the pulse breakdown voltage random variable numerically is examined for arbitrary shaped pulses. Impulse characteristics are then determined on the basis of the generated statistical sample. Numerically generated statistical samples of the pulse breakdown voltage random variable are compared to the corresponding experimentally obtained statistical samples. Impulse characteristics obtained from the numerically generated statistical samples are compared to the corresponding impulse characteristics derived from the semi-empirical Area Law and the Time Enlargement Law. The set of impulse characteristics obtained in this way is compared to the results obtained experimentally for different shapes of the pulse voltage load. Gases used in the experimental and numerical models include SF6, N2 and Ar. Gas pressures range from 1 × 102 Pa to 6 × 102 Pa, and inter-electrode gaps from 0.1 to 10 mm. A homogenous electric field is considered.

Cryogenic Fatigue Strength Assessment for MARK-III Insulation System of LNG Carriers

The objective of this study is to investigate the typical failure mode and to obtain the stress range versus number of cycles to failure (S-N) data of MARK-III type liquefied natural gas (LNG) insulation system under the fatigue loading at actual cryogenic environment. A systematic experimental research is carried out for the assessment of the fatigue strength of MARK-III insulation system at cryogenic temperature. Three different types of test specimens are tested for the evaluation of fatigue performance of MARK-III insulation system. Test specimens are determined considering the fatigue vulnerable locations such as mastic area, slit area, and top bridge pad area inside the actual LNG cargo tanks. All test specimens are fabricated as close as possible to the actual yard practice. A series of fatigue test results is represented as S-N curves. Cyclic fatigue loadings were carefully considered similar to the actual sloshing loads. The effect of sloshing impacts is considered by selecting the stress ratio (R=−10). The load levels have been determined based on the ultimate strength of reinforced polyurethane foam as 12.2 bars. Different cryogenic temperatures are employed according to the test locations in consideration of temperature gradient within the insulation system. All test results including fatigue life, as well as failure locations of MARK-III insulation system at cryogenic temperatures, are reported and compared with those at room temperature. Consistent S-N curves of MARK-III insulation system at both room and cryogenic temperatures are obtained and compared. The slopes of S-N curves from both fatigue test results are observed to be almost identical, and the fatigue strengths are found to exhibit similar trend. The results from this research can be used for the fatigue assessment of the LNGC insulation system, as well as a design guideline of LNG CCS at cryogenic temperature.

Effect of Metallic Particles on E-field Enhancement in Extra High Voltage Gas-insulated Transmission Lines

Gas-insulated transmission lines (GITL) are valued as technological solutions in hydropower stations due to their enormous power handling capabilities. The performance of GITL is a function of the size of metallic particles inside the gas-insulated chamber. Electrostatic field (E-field) enhancement is a common phenomenon in gas-insulated lines due to these metallic particles. In this study, the E-field enhancement factor is calculated by considering metallic particles at various locations in the gas-insulated line/bus section, such as high-tension (HT) conductor, high-voltage shields, support insulator, and inner surface of grounded enclosure. For this purpose, a two-dimensional model based on finite element (FE) method is developed. The length of the metallic particle is in the range of 1 to 10 ㎜ while the diameter is between 1 to 3 ㎜. E-field enhancement is also computed for various particle configurations of the gas-insulated system, with focus on dielectric coating made of epoxy on HT conductor and inner surface of grounded enclosure.

Lightning attachment models and maximum shielding failure current of overhead transmission lines: implications in insulation coordination of substations

The maximum shielding failure current of overhead transmission lines is an important parameter in evaluating the shielding performance of the lines and in insulation coordination of substations. General expressions for the estimation of the maximum shielding failure current of transmission lines, derived by employing several lightning attachment models in shielding analysis, are presented. An application to typical 110 kV up to 1150 kV overhead transmission lines shows that there is a great variability in maximum shielding failure current among lightning attachment models. The importance of maximum shielding failure current in insulation coordination of substations is demonstrated with the aid of alternative transients program-electromagnetic transients program (ATP-EMTP) simulations. The computed overvoltages impinging on 150 and 400 kV gas insulated system (GIS) substations because of shielding failure of the incoming overhead transmission lines, being dependent upon shielding failure current, vary with the lightning attachment model employed in shielding analysis of the lines. Implementation of the electrogeometric model adopted by IEEE Std 1243:1997 in shielding analysis imposes high requirements on protection of the substations against incoming shielding failure surges.

Understanding the partial discharge activity of conducting particles in GIS under DC voltages using the UHF technique

AbstractThe major cause of failure of DC‐GIS is due to presence of foreign particles causing partial discharges in the insulation structure. The particle movement in gas insulated system (GIS) radiates electromagnetic waves and the bandwidth of the signal lies in the range 1–2 GHz. Increase in applied DC voltage/pressure has not altered the frequency content of the ultra high frequency (UHF) signal generated due to partial discharge formed by particle movement. The UHF sensor could recognize the breakdown of sulfur‐hexa‐fluoride (SF6) gas under DC and Lightning impulse voltages and the frequency content of the signal captured by the UHF sensor lies up to 500 MHz. Mounting UHF sensor in GIS could allow one to classify internal partial discharges from breakdown, at the time of testing/during operation. Copyright © 2009 John Wiley & Sons, Ltd.

Determination of Pulse Tolerable Voltage in Gas-Insulated Systems

In this paper we expound on the procedure of determining the pulse tolerable voltage characteristic in the voltage-versus-time frame, by applying the time enlargement law to the breakdown time random variable, and using a single statistical sample for this variable, obtained through experiments with a predefined shape of the voltage load. The suggested algorithm has been experimentally tested for Ar, N2, and SF6 gases, in the pd product (pressure × interelectrode gap) range from 10-4 to 300 bar mm. The testing was performed by comparing the pulse tolerable voltage characteristic of a two-electrode configuration obtained by applying a particular shape of the pulse voltage load with the values corresponding to other pulse voltage shapes, covering a wide range of frequencies. Satisfactory results have been obtained concerning the applicability of the procedure, with certain minor limitations, which are pointed out.

Partial discharges and breakdown at protrusions in uniform background fields in SF6

The breakdown mechanism of compressed SF6 in gas insulation is known to be controlled by stepped leader propagation. This process is reasonably well understood for strongly non-uniform insulation gaps (‘point-to-plane’) and in the absence of pre-breakdown discharge activity (‘corona stabilization’). Open questions still remain for weakly non-uniform insulation gaps with small electrode protrusions (particles, surface roughness), in which pre-breakdown partial discharge (PD) activity is present. This paper presents a first attempt to derive a consistent picture under these conditions, which are characteristic for practical gas insulation systems. Experiments were carried out in a uniform field gap with a short protrusion on one electrode. This configuration was studied at various pressures from 0.1 to 0.5 MPa and both polarities using electrical and optical diagnostics. The results are interpreted using a quantitative model and order-of-magnitude estimates. The emerging picture allows prediction of most of the technically relevant aspects of the discharge processes and their main parameter dependences. It comprises statistical time lags, formative time lags including pre-breakdown PD activity and breakdown fields as a function of gas pressure, protrusion length and polarity.

Partial discharges at a triple junction metal/solid insulator/gas and simulation of inception voltage

In gas-insulated systems, the presence of solid insulators used to hold conductors at high voltage induces the presence of triple interfaces where the metal, solid insulator and gas are in contact. These regions, usually named “triple junction”, constitute weak points for the insulation, mainly due to the mismatch between permittivities of solid and gas, which induces an enhancement of the electric field. If the design of these regions is not carried out with great care, partial discharges (PDs) may appear and accelerate the ageing of the insulator. This paper deals with an experimental study of PDs in a “triple junction” electrode configuration, carried out with sensitive electrical and optical PD measurements. Experiments are done in two pure gases, nitrogen and SF6. Two discharge regimes can be observed: very small discharges (<1 pC) occurring at low voltage, probably initiated by defects, and more intense and homogeneous discharges spreading around the triple junction. A model based on the calculation of the electric field close to the triple interface and on the application of the Townsend breakdown criterion is then presented and compared to experiments.

Insulation characteristics of CO2 gas for nonstandard lightning impulse oscillations: Evaluation method of nonstandard lightning impulse waveform for CO2 gas insulation

AbstractSF6 gas is widely used in electric power apparatus such as gas‐insulated switchgears (GIS), because of its superior dielectric properties; however, it has been identified as a greenhouse gas at COP3 in 1997, and alternative insulation gases to SF6 have recently been investigated. One of the candidates is CO2 gas, which has lower global warming potential (GWP). However, CO2 gas has a lower withstand voltage level than SF6 gas; therefore, it is necessary to rationalize the equipment insulation level and reexamine the insulating test voltage for electric power apparatus as low as possible. From our previous investigation, in SF6 gas insulation system, we obtained that the insulation requirements of the real surges (called nonstandard lightning impulse waveform) are not as severe as those of the standard lightning impulse waveform. This paper describes the evaluation method for real surges, based on insulation characteristics of CO2 gas gaps. Furthermore, the method was applied to typical field overvoltage waveform in the lightning surge time region for 500‐kV systems and it is obtained that the equivalent peak value of the standard lightning impulse waveform is possibly reduced by 10 to 15%. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(3): 1– 9, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20560

Effect of Spacer's Defects and Conducting Particles on the Electric Field Distribution along Their Surfaces in GIS

Researches in the area of gas insulated systems (GIS) reliability are still attracting a considerable attention from the electric utilities and the scientific community in many countries. Solid insulating spacers in GIS represent the weakest points in these systems, and several troubles and systems' outages have been reported all over the world due to their failure. So it is essential to determine the electric field distribution along their surfaces and hence evaluate the degree of their reliability. This paper discusses the electric stress distribution at the solid-gas interface with spacer's defects and contaminating spherical conducting particles on the surface. The effects of the defects size, type and its location and the particles size and its location on the electric stress distribution at solid-gas interface are presented and discussed.

Characterization of Partial Discharges in a Gas Insulated System Using an Acoustic Emission Technique

The major cause of failure of GIS is the presence of foreign particles causing partial discharges/corona in the insulation structure. In the present work, partial discharge activity in GIS under AC, DC, and the composite voltage formed by AC and DC voltages, was studied using the acoustic emission technique. It is observed that the magnitude of the acoustic emission (AE) signal produced due to partial discharges under positive DC is high compared to negative DC and AC voltage. Under the composite voltages, it is observed that superimposing AC voltage over positive DC enhances the partial discharge activity, and superimposing the AC voltage over the negative DC voltage could limit the partial discharge activity. The rate of rise and the duration of the AE signal indicates that partial discharge activity under positive DC voltage is severe compared to AC and negative DC voltages. The measured counts and energy content of the AE signal produced due to partial discharges under positive DC is high compared to the negative and AC voltages. It is also realized, based on a partial power analysis, that the maximum energy content of the AE signal lies in the range 125–250 kHz, for the signals generated under AC/DC/composite voltage formed by AC and DC voltage.

Partial Discharge Activity in Gas-Insulated System Under Composite Voltage

In the present study, the partial-discharge activity under nonstandard voltage formed by ac and dc voltages was studied. It is identified that the ac superposed with positive dc enhances the discharge activity in the medium and if the ac voltage is superposed with the negative dc, it can mitigate the discharge activity. The acoustic emission signal generated due to the partial discharges in gas-insulated systems under the composite voltage formed by ac superposed with dc voltage/ac/dc voltages, has a dominant frequency content at around 250 kHz.

Development of 72-kV high-pressure air-insulated GIS with vacuum circuit breaker

SF6 gas has excellent dielectric strength and interruption performance. For these reasons, it has been widely used for gas-insulated switchgear (GIS). However, use of SF6 gas has become regulated under agreements set at the 1997 COP3. Thus, investigation and development for GIS with a lower amount of SF6 gas are being carried out worldwide. Presently, SF6-free GIS has been commercialized for the 24-kV class. Air or N2 gas is used as the insulation gas for this GIS. On the other hand, SF6-free GIS has not been commercialized for the 72-kV-class GIS. The dielectric strengths of air and N2 gas are approximately one-third that of SF6 gas. To enhance the insulation performance of air and N2 we have investigated a hybrid gas insulation system which has the combined features of providing an insulation coating and suitable insulation gas. We have developed the world's first 72-kV SF6-free GIS. This paper deals with key technologies for SF6-free GIS, such as the hybrid insulation structure, a bellows for the high-pressure vacuum circuit breaker, a newly designed disconnector and spacer, and prevention of particle levitation. Test results of the 72-kV high-pressure air-insulated GIS with the vacuum circuit breaker are described. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 157(4): 13–23, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20451

CO&lt;sub&gt;2&lt;/sub&gt;ガスの非標準雷インパルス振動波形に対する絶縁特性（その1）

Since SF6 gas was identified as a greenhouse gas at COP3 in 1997, alternative insulation gases to SF6 have been investigated. The CO2 gas insulation is one of the candidates, which is a natural gas and has lower global warming potential (GWP). However, CO2 gas has a lower withstand voltage level than SF6; therefore, it is important to rationalize the equipment insulation level and reduce the test voltage of electric power equipment as low as possible. The actual lightning surge waveform (called non-standard lightning impulse waveform) in actual substations is different from the standard lightning impulse waveform (1.2/50μs). The actual lightning surge waveform generally rises steeply, however, the decay of the overvoltage is large, and the insulation requirements are not as severe as those of the standard lightning impulse waveform.This paper describes the insulation characteristics of CO2 gas for single-frequency oscillatory waveforms with various frequencies from 1.3 to 4.0MHz and damping ratios. From experimental investigation, similarly to SF6, it might be possible to reduce the test voltage of CO2 gas insulation system by evaluating actual lightning surge waveform in terms of the equivalent standard lightning impulse waveform.

Particle-triggered pre-breakdown phenomena in atmospheric air gap under ac voltage

Corona discharge mechanism and breakdown voltage characteristics of air gaps in the presence of metallic particles were investigated including the relationship between the corona discharge mechanism and the phase resolved partial discharge patterns. These results are of interest for detection of foreign particles in gas insulated systems (GIS). The results with different shaped particles show that the corona mechanism depends on the particle shape and applied voltage. Moreover the corona discharges on both sides of the particle interfere with each other by inflow of charge carriers into the particle. The breakdown voltage is a minimum when the particle is in the vicinity of the electrode due to active interference of coronas. The phase-charge characteristics as the phase resolved partial discharge patterns vary with the particle shape. This suggests that the corona discharge mechanisms for all types of particles should be considered in the particle-detection method for GIS based on the phase resolved partial discharge patterns.

A Novel Approach Based on Simulated Annealing Coupled to Artificial Neural Network for 3-D Electric-Field Optimization

This paper deals with the optimization of stress distribution on and around electrode-spacer arrangements for ensuring economical and higher reliability of gas-insulated systems. The optimization technique adopted in this paper is the artificial neural network-aided simulated-annealing (SA) algorithm. By coupling a trained neural net with the annealing algorithm, the execution speed of the latter is greatly enhanced to evaluate the optimum values for the design parameters of the electrode-spacer arrangements compared to calculating the cost function via the entire process for field calculation at every move of the optimization algorithm. The convergence of the optimization algorithm has also been compared statistically with the genetic algorithm. The results presented in this paper show that optimized electrode-spacer contours have been obtained with an acceptable degree of accuracy using the neural-network-aided SA algorithm. The statistical analysis shows a promising result for the proposed method.

真空遮断器搭載72kV高圧空気絶縁GISの開発

SF6 gas has excellent dielectric strength and interruption performance. For these reasons, it has been widely used for gas insulated switchgear (GIS). However, use of SF6 gas has become regulated under agreements set at the 1997 COP3. So investigation and development for GIS with a lower amount of SF6 gas are being carried out worldwide. Presently, SF6 gas-free GIS has been commercialized for the 24kV class. Air or N2 gas is used as insulation gas for this GIS. On the other hand, SF6 gas-free GIS has not been commercialized for 72kV class GIS. Dielectric strengths of air and N2 gas are approximately 1/3 that of SF6 gas. So to enhance insulation performance of air and N2, we have investigated a hybrid gas insulation system which has the combined features of providing an insulation coating and suitable insulation gas. We have developed the world's first 72kV SF6 gas-free GIS.This paper deals with key technologies for SF6 gas-free GIS such as the hybrid insulation structure, bellows for the high pressure vacuum circuit breaker, a newly designed disconnector and spacer and prevention of particle levitation. Test results of 72kV high pressure air-insulated GIS with the vacuum circuit breaker are described.