Gas Insulated Research Articles

Theoretical Investigations on the Oxidation of Heptafluoro-iso-butyronitrile by Atomic Oxygen in Dielectric Breakdown.

Heptafluoro-iso-butyronitrile [(CF3)2CFCN, in abbr., C4] with CO2 gas mixture is a promising dielectric candidate to replace sulfur hexafluoride (SF6) for the sake of environmental concern. Detailed mechanisms for the reactions of C4 with atomic oxygen in both triplet and singlet due to dissociation of CO2 were proposed using high-level ab initio methods including density functional theory, quadratic complete basis set, multireference Rayleigh-Schrodinger perturbation theory, and state-averaged multiconfiguration self-consistent field. The reaction of C4 with O(3P) proceeds via the stepwise C-O and N-O addition/elimination and direct displacement mechanisms. The reaction paths are bifurated into cis and trans conformations. The predominant product channel is the C-O association with the barriers 6.1-6.8 kcal/mol to form the C3F7C(O)N intermediates and followed by the C-C bond cleavage to produce C3F7 and NCO radicals. On the singlet surface, the three-center intramolecular conversion to form the C3F7OCN isomer becomes competitive. The singlet-triplet intersystem crossing is favorable due to the significant spin-orbital coupling. Master equation calculations were carried out to obtain the temperature- and pressure-dependent rate coefficients and yields of the O(3P) + C4 reaction and were compared with the analogous reactions of nitriles. It was found that only the NCO + C3F7 product channel is significant. The overall rate coefficients are pressure-independent in the range 1-10 atm and can be expressed by k∞(T/K)=(6.41 ± 0.22) × 10-13(T/298)1.30±0.01e-(3241.5±24.5)/T cm3 molecule-1 s-1. The present theoretical work provides useful insights into the breakdown diagnostic of the C4/CO2 insulation gas.

Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application

This study presents the surface discharge characteristics of insulating gases, including sulfur hexafluoride (SF6), dry air, and N2, under a non-uniform field. Surface discharge experiments were conducted, with the gas pressure ranging from 0.1 to 0.6 MPa, on samples of epoxy dielectrics under an AC voltage. The experimental results showed that the surface insulation performance significantly improved in insulating gases possessing electronegative gases, such as SF6 and dry air. Surface flashover voltages of SF6 were saturated with an increasing pressure, compared to dry air and N2. The surface discharge mechanism is proposed to explain the improvement and saturation of dielectric characteristics of the electronegative gas in complex dielectric insulations, as well as its influence on the surface flashover voltage. As an application, an insulation design method is discussed with regards to replacing SF6 gas in high-voltage power equipment based on the knowledge of the physics behind gas discharge.

Fault Diagnosis of Traction Transformer Based on Bayesian Network

As the core equipment of a traction power supply system, the traction transformer is very important to ensure the safe and reliable operation of the system. At present, the three-ratio method is mainly used to distinguish transformer faults, whereas such a method has some defects, such as insufficient coding and over-general fault classification. At the same time, on-site maintenance personnel make an empirical judgment based on various test data, which is subjective and uncertain to a certain extent. For cases with multiple abnormal data and relatively complex conditions, on-site personnel often need to discuss and even dismantle the transformer to identify the fault, which is time-consuming and costly. In order to improve the effect of fault diagnosis for traction transformer, this paper uses Bayesian network to correlate the cause and effect of various tests and faults. By combining the results of field tests, the fault is diagnosed by the causal probability of the Bayesian network, rather than relying on the exception that occurred in a single experiment to judge its fault. The diagnosis results are more accurate and objective by using the Bayesian network. In this paper, the frequent test anomalies of the traction transformer are taken into account in the network, so that the network can more comprehensively analyze the operation situation of the traction transformer and judge the type of fault. According to field situations, based on the existing set of symptoms of the Bayesian network fault diagnosis, this paper further considers the insulation resistance, dielectric loss tangent value, oil and gas, power frequency voltage, and leakage current. By combining the association rules algorithm and the experience of the field operators, the cause–effect relationship of test data and the conditional probability parameters of the network are obtained. Then, the Bayesian network is constructed and used for traction transformer fault diagnosis. The case study shows that the four types of fault diagnosed using the Bayesian network model proposed in this paper are consistent with the fault types inspected by on-site operators, which shows promising engineering application prospects.

Detection of SOF2 and SO2F2 through aluminium nitride nanosheets: A DFT study

Detection of sulfuryl fluoride (SO2F2) and thionyl fluoride (SOF2) can be used to find out the degradation of sulfur hexafluoride (SF6), indicating gas insulation switchgear equipment failures. In the present work, we study through first-principle DFT calculations the SOF2 and SO2F2 adsorption on aluminium nitride nanosheets (AlNNS), with the aim of to evaluate the performance of this nanomaterial as gas sensor. First, we performed an energetic, geometric and bonding analysis, obtaining considerable adsorption energy values for SO2F2 and SOF2 (−1.63 and −1.93 eV, respectively), which suggests these gases are chemisorbed on the AlN surface. Furthermore, certain SF bonds were broken during adsorption, while new AlF and SN overlaps were developed. Simultaneously, an important weakening was observed for AlN bonds close to the adsorption site. The adsorption selectivity of AlNNS toward SO2F2 and SOF2 was also studied, noting that other molecules normally present in air show a lower reactivity.Band structure and electronic density of states were realized in order to evaluate the sensing capacity of AlNNS surface. The obtained plots show a notorious narrowing of the band gap after gas adsorption, particularly in the case of SOF2, so an important change in the conductivity of AlNNS could be expected during gas exposition. Charge density analysis reveals an important charge transfer from AlNNS to both adsorbed gases, which suggest that this nanomaterial could behave as an n-type semiconductor. These results seem to indicate that AlNNS could act with good sensibility and selectivity for SOF2 and SO2F2 detection, showing high potential as gas sensing material.

Study of partial discharge characteristics in HFO-1234ze(E)/N2 mixtures

HFO-1234ze(E)(trans-1,3,3,3-tetrafluoropropene, chemical formula: C3H2F4) is an extremely environmentally friendly SF6 alternative gas with high electrical strength. In this paper, the partial discharge (PD) characteristics of HFO-1234ze(E)/N2 mixtures were studied using the gas insulation test platform. The PD inception voltage of insulating gas under positive and negative half cycles of power frequency was tested. Using SF6/N2 mixtures as a control group, the effects of electrode spacing, mixing ratio and pressure on the insulation performance of HFO-1234ze(E)/N2 mixtures were explored. The test results show that the PD inception voltage of the negative half-cycle of pure HFO-1234ze(E) under short electrode spacing can reach 0.96–1.04 times of pure SF6 under different pressures; the PD inception voltage of 40%HFO-1234ze(E)/60%N2 mixtures at 0.3 MPa is 0.67–0.89 times that of SF6/N2 mixtures under the same conditions, which has great application prospect.

Study of electric field stress on the surface contour and at the triple junction in three phase GIS with FGM spacer under the depression defect

Abstract Now days, the establishment of spacers is in wide usage in three-phase Gas Insulated Busduct (GIB) for providing mechanical support and better insulation to the conductors. The region of the intersection of SF6 gas, enclosure end and the spacer is one of the weakest links in GIB, so the major concentration is done on minimization of electric field stress at this junction by using Functionally Graded Material (FGM) technique. The other incidents of insulation failures are due to several defects like depression, delamination etc. reduces the dielectric strength of the spacers. In this paper, an FGM post type spacer has been designed for a three-phase GIB under depression and further electric field stress at Triple Junction (TJ) is reduced by introducing a metal insert (MI) nearer to the TJ. Several filler materials are used as doping materials for obtaining different permittivity values using FGM technique to achieve uniform electric field stress. Simulation is carried out for the designed spacer at various operating voltages with different types of FGM gradings. The effect of depression with different dimensions and positions is analyzed before and after inserting MI to the FGM post type spacer in three-phase GIB.

The compatibility between environmentally friendly insulation gas C4F7N and α-Al2O3 (0001) surface: Theoretical and experimental insights

In this paper, the compatibility between C4F7N and α-Al2O3 (0001) surface has been evaluated based on the density functional theory (DFT). The adsorption energy, total charge transfer, states analysis, and electron density difference were obtained to analyze the interaction behavior of C4F7N on α-Al2O3 (0001) surface. The results demonstrated a chemisorption between C4F7N and α-Al2O3 (0001) surface. Moreover, the pathway of C4F7N dissociated on α-Al2O3 (0001) surface after the adsorption was determined. However, the activation energy required for the dissociation is too large to achieve, which means that the dissociation of C4F7N on α-Al2O3 surface is difficult to occur. Additionally, the thermal compatibility between α-Al2O3 and C4F7N were verified by experiments, which shows a certain compatibility between C4F7N and α-Al2O3 (0001) under up to 170 ℃. The results demonstrate that α-Al2O3-based materials can be used as insulators in C4F7N-filled electrical equipment.

Research and Development of NDIR Sensor and Its Application In On-Line Detection of CF4 Gas

Due to the excellent insulation and arc extinguishing properties of SF6 gas, it is widely used in the production and use of high voltage GIS. However, due to the breakdown of discharge and overheat, SF6 gas decomposes and produces a variety of low fluorine sulfides. Therefore, the concentration of CF4 gas in the high-voltage switch state can be effectively determined by detecting high-voltage GIS. This paper puts forward a kind of semiconductor laser based gas detection (NDIR) technology, the flange fixed installation in high voltage switch, optical fiber sensing structure can realize remote, real-time online monitoring of CF4 gas content. The experiment shows that the flange fiber optic sensor on-line monitoring device can realize the effective measurement of CF4 gas. The research results show that maximum field intensity on the surface of basin insulator is 10.788kV/mm, which is lower than the control field strength of 20kV/mm under the SF6 gas insulation condition, therefore the E-field near the basin insulator does not affect the operation of the optical fiber sensor. In the case of 1m light path, the detection limit of CF4 gas near 1312nm is less than 0.5ppm. The results of this study can provide an entirely new scheme for on-line monitoring of high-pressure open-light SF6 gas decomposition products, which can realize real-time on-line monitoring of the internal gas of the high voltage GIS equipment.

Improving the Geiger Muller Counter Characteristics by Optimizing the Anode and Cathode Radius Dimensions

This article considers the influence of the anode (r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) and cathode (r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> ) radius dimensions in the Geiger Muller counter chamber on its characteristics such as efficiency, measurement uncertainty, dead time, slope, plateau length, and aging. The article is of a theoretical and experimental nature. In the theoretical part of the article, the expression for the dependence of the electrical discharge parameters of a cylindrical electrode system is derived from various radius dimensions of the anode and cathode. For this purpose, the enlargement law of the probability was applied. The experimental part of the work was performed on the models of the Geiger Muller's chamber of different radius dimensions of the anode and cathode. The chamber models (and insulation gas) were made to maintain the geometrical similarity of the commercially applicable chamber. In the first experimental part of this article, the effect of radioactive radiation was simulated with a fast pulse voltage, while real radioactive elements were used in the second experimental part. The testing has shown that the synergy of the stochastic volume effect of the breakdown voltage of the Ruffer-Kuilling mechanism and the corona of the plasma can significantly improve all tested characteristics.

Analysis of VFTO during the failure of a 550-kV gas-insulated substation

This paper presents an analysis of a very fast transient overvoltage (VFTO) experienced by a 550-kV gas-insulated substation (GIS) that caused the failure of a disconnector switch (DS) and circuit breaker (CB), after manual closure of the DS. Simulations were performed using an electromagnetic transient program with GIS component modeling for high-frequency analysis and multi-spark electric-arc modeling. The simulation results were verified by comparison with field test measurements and showed an acceptably accurate waveform and amplitude prediction for VFTO on the 550-kV GIS under study, allowing for improved failure analysis. The simulation results using equivalent electrical circuit models indicated that the failures occurred at a voltage below the lightning impulse withstand voltage (LIWV) with a high quantity of sparks. After the analysis, it was possible to conclude that the manual closure of the DS generated excessive sparks, causing significant damage to the fixed and mobile contacts and abnormal stress in the insulation of SF6 gas in the inhomogeneous electric field region of the CB, impacting on the reduction of the dielectric strength of SF6 gas, thereby producing a high-intensity arc between the shield and the enclosure.

Time-Frequency Analysis of Partial Discharge Current Pulses in Different Gas Environment under Lightning Impulse

Abstract This paper deals with a time-frequency analysis of the measured partial discharge (PD) currents in different insulation gases. These gas environments consist of a pure SF6 (sulphur-hexafluoride) and sulphur-hexafluoride and nitrogen (1 % SF6 + 99 % N2) mixture, under both positive and negative lightning impulse (LI) voltage stresses. In this study, the short time Fourier transform was used to extract the time-frequency information of PDs for different gases at different pressures, and these results were compared to each other. Thus, the relationship between the time, amplitude, and frequency of PD currents was studied. Moreover, some statistical formulas, such as mean, standard deviation, kurtosis, and skewness were applied to the time-dependent PD current data. As a result, a correlation between obtained statistical results and PD frequencies was examined. In most cases, the frequency of partial discharge decreased when the pressure increased. The amplitude of the partial discharges for negative polarity was more than that for positive polarity gas insulations. The partial discharge amplitudes of the pressure of 2 bar were mostly high compared to other pressures. This case demonstrated that SF6 had a maximum minimum character in terms of breakdown. The partial discharge frequency of a 1 % SF6 mixture was higher than that of pure SF6. It is thought that SF6 suppresses the discharge frequency, and statistical evaluations support the experimental results.

Research of GIS partial discharge type evaluation based on convolutional neural network

This paper presents a GIS (Gas Insulated Switchgear) partial discharge type evaluation based on the convolutional neural network. It is significant to study GIS partial discharge and effectively classify the types of GIS partial discharge. In order to effectively and efficiently distinguish the types of GIS partial discharge, phase resolved partial discharge is researched and evaluated based on the convolutional neural network. First, the hardware of the evaluation system is introduced in detail. Second, the partial discharge signal is analyzed. Third, the partial discharge signal evaluation based on the convolutional neural network is studied. Finally, the experiment of the evaluation is presented to verify that the evaluation system can effectively and accurately perform GIS partial discharge monitoring and improve the reliability of power grid operation.

Theoretical and experimental investigation on decomposition mechanism of eco-friendly insulation gas HFO1234zeE

Decomposition experiments under corona discharge and theoretical calculations using the density functional theory (DFT) method were accomplished to clarify the dissociation behavior and decomposition mechanism of HFO1234zeE (trans-1,3,3,3-tetrafluoropropene), an eco-efficient SF6 alternative gas. The discharge decomposition products of HFO are mainly fluorocarbon, unsaturated hydrocarbon and saturated hydrocarbons, which are containing no more than three carbons. Free radicals, CF3·, F· and H·, generated via bond-cleavage reaction are important structures to promote the decomposition, and HFO is more likely to dissociate with them by abstraction reaction to form CF4 and CF3H. Long-chain radicals, such as CF3CHCF·, CF3CHCH· and CF3CCHF·, will be decomposed into CF3CCH and CF3CCF, and small intermediates would be easier to combine to form HF, C2F6, C2F4, C2HF5, CF3HCF3H and C3F8. It is also likely to be converted to Z-isomer. Due to the high discharge intensity and ion bombardment, solid by-products appeared on the electrode surface may contain carbon dust and metal compounds. The solid attached to the surface has little effect on the electric field distribution, and most gas decomposition products still maintain the insulation strength, so the air-gap breakdown voltage only dropped by about 6.2% after long-term corona discharge. The obtained results not only reveal the decomposition mechanism in a comprehensive way, but also present useful reference for exploring the application potentials of HFO1234zeE.

SF6 Alternatifi Yalıtkan Gazların Çevresel ve Fizyokimyasal Özellikleri

Research on alternative dielectric gases to eliminate the disadvantages of SF6, which is widely used in GIS and switching systems in the power system engineering, has been an important study topic in the literature for nearly 40 years. Because of environmental priorities defined by international agreements such as the Kyoto Protocol and Doha Amendment, the restrictions on the use of SF6 make these studies an obligation. Although the number of alternative dielectric gases studied for this purpose is quite high, these gases can be classified under the titles of non-synthetics, hydrocarbons (HCs), fluorocarbons (FCs), hydrofluorocarbons (HFCs), fluoronitriles (FNs), fluoroketones (FKs) and other synthetic gases. In this study, the gases classified under these titles are compared using the dielectric constant relative to SF6, Global Warming Potential (GWP), atmospheric lifetime, boiling point and toxicity parameters used in the comparison of dielectric gases. When compared with these parameters, non-synthetic air, CO2, and N2, C3F7CN from FNs, and C5F10O and C6F12O from FKs stand out among alternative gases to SF6. These alternatives are used in some innovative power system industry applications and have a widespread use potential in the insulating gas industry instead of SF6.

Field-Time Breakdown Characteristics of Air, N2, CO2, and SF6

The dielectric performance of gases in insulation systems used in high voltage power and pulsed power applications is a subject of intensive theoretical and experimental investigations. Transient breakdown processes in gases stressed with short, high-field impulses, have been studied for many decades. However, there are still significant gaps in the understanding of the main breakdown processes and mechanisms associated with fast transient breakdown processes in gases. This knowledge is important for the optimization of gaseous insulating systems and for the coordination of gaseous insulation in power and pulsed power apparatuses. This information is also required for the development of gas-filled components such as circuit breakers and plasma closing switches. This article is aimed at the analysis of the field-time breakdown characteristics of air, N2, CO2, and SF6, using kinetic and drift-diffusion approaches. The kinetic approach is based upon the avalanche-to-streamer transition criterion, while the fluid drift-diffusion model requires self-consistent numerical solution of the continuity equations for charged species, and the Poisson equation for the electric field. The time to breakdown as a function of the applied field was obtained for all investigated gases. The obtained analytical results agree well with the experimental data reported in the literature, which suggests that both approaches can be used for insulation coordination, and for the development of gas-insulated power and pulsed power systems and components.

A four-way coupled CFD-DEM modeling framework for charged particles under electrical field with applications to gas insulated switchgears

Particulate flows with charged particles under electric flow field exist in many practical applications, including metal particles in Gas Insulated Switchgear (GIS) or manipulation of powders in fluidized bed. The motions of these particles are affected by the electrical field, ambient flow and collisions. This work further develops the four-way coupled CFD-DEM framework under electric field, which not only computes the coupling of flow field, electric field with particle motion and particle charge, the particle-occupied solid fractions on CFD cell, particle-particle interactions and particle-wall interactions, but also considers the dielectrophoretic force, particle-particle Coulomb force, virtual mass force and Magnus force. Particle electrification on its contact with electrode is computed as well. Besides, the multi-sphere model is also introduced into the present framework in order to simulate particles of irregular shapes. The model is validated with theoretic solutions and experiments, and is applied to GIS and fluidized bed. Based on the framework, statistic information from the particulate flow in GIS, e.g., time-averaged solid fraction, time-averaged electric field or particles' velocity and charge distribution are analyzed. A dimensionless number is proposed to predict the particle motion in GIS with Alternating Current (AC), which is further conducive to reducing the parameter window in relevant simulations and studies.

Entropy generation in concentric annuli of 400 kV gas-insulated transmission line

The object of this study is entropy generation analysis in the concentric annuli of a gas insulation transmission line enclosure (GIL) loaded with air and SF6-N2 mixture gas. The inner and outer cylinders of the cavity are hot and cold, respectively. Because of symmetric conditions the ½ cylinder is considered for modeling. The standard k–ɛ turbulence model, the equation of entropy generation, and the conservation equations are transformed from dimensional form to non-dimensional form utilizing the definition of dimensionless parameters, stream function, and vorticity. Then dimensionless governing equations are solved by a finite volume method (FVM) using an innovative ANSYS Fluent non-dimensionalization scheme. Inflated layers applied to the fine grids to improve the simulation capability for turbulent modeling. The effects of the Rayleigh number at Ra = 2.5 × 106, 1.7 × 109, 2.1 × 109, 4.2 × 109 and 5.7 × 109 are investigated. The results demonstrate that the Nusselt number grows as the Rayleigh number grows and entropy generation number and the Bejan number decreases as the Rayleigh number increases. The GILs have a proper morphology in geometry that reducing the high irreversibility in higher Rayleigh numbers conditions.

Modularization of High Voltage Gas Insulated Substations

Many existing petrochemical facilities are replacing outmoded high voltage (HV) air insulated switchyards (AISs) with new gas insulated switchgear (GIS) substations. Modularization or prefabrication has strategic advantages over site-built substations. While medium voltage (MV) and low voltage (LV) substations have been modularized for many years, the HV GIS substation modularization has not caught up with the construction industry. The modularized HV GIS substations are significantly different compared to other electrical MV or LV substations. This article addresses the modern philosophy and approach to modularization and how the end-user can benefit, in terms of design flexibility, cost-effectiveness, reduced lead times, premium quality, and accelerated site execution in a new or an operating facility. It describes the technological developments, design concepts, quality assurance, and environmental impact in delivering complete solutions based on site conditions.

Feasibility of C 3 F 7 CN/CO 2 gas mixtures in high‐voltage DC GIL: a review on recent advances

With the rise of renewable energy resources, especially offshore wind power plant, the DC transmission concept has attracted more and more attention. The selection of eco-friendly insulating gas, as most critical insulating part of gas-insulated equipment is top priority for developing highly reliable system. Moreover, it is acknowledged that eco-friendly C3F7CN/CO2 gas mixtures have become a potential alternative to SF6 gas due to its excellent performance. This study reviews the basic physical properties of C3F7CN gas/gas mixtures and insulation properties including the gas gap breakdown and surface flashover performance at DC, as well as lightning impulse voltage. Investigation about the gas stability is presented, including the compatibility of gas with solid materials in gas-insulated transmission line (GIL), and the decomposition characteristics under long-term electrical and thermal stresses. The important progress on the charge accumulation characteristics of gas–solid interface in the novel gas environment under DC voltage stress is analysed. Finally, the key issues that need to be paid attention to and further followed through regarding the application of this novel gas in GIL are summarised and put forward. This study hopefully can provide a complete reference for the development of eco-friendly DC gas insulation equipment with C3F7CN/CO2 gas mixtures.

Study on the combined characteristics of UHF and optical signals induced by partial discharge at spacer surface in GIS

In this study, to improve the sensitivity and accuracy of the detection of surface partial discharge (PD) in gas insulation switchgears (GISs), a combination of ultra-high frequency (UHF) and optical methods is proposed based on an integrated sensor. The PD triggered by a fixed metal needle defect attached to the spacer is detected from PD inception to flashover in a real GIS. The characteristics of UHF and optical signals under different applied voltages are compared and analysed in terms of PD waveform, Phase Resolved Partial Discharge (PRPD) pattern, PD count and amplitude. The results show that the surface discharge process can be precisely divided into three stages according to the combined characteristics of the UHF and optical signals. The comparison of the two detection methods shows that the optical method can detect the weak avalanche PD under low voltage while UHF cannot, so it can detect more PDs than UHF method in an AC cycle. In addition, with the deterioration of solid insulation material, the amplitude of the optical signal increases significantly while the amplitude of the UHF signal only increases slightly.