Gas-insulated Lines Research Articles

Surface charge inversion algorithm based on bilateral surface potential measurements of cone-type spacer

To study the surface charge distribution of high-voltage direct-current (HVDC) spacers for gas insulated lines (GIL), a surface charge inversion algorithm based on surface potential measurements is necessary. However, previous studies on inversion calculation of surface charge density only considered the surface potential of one side, neglecting the effect of residual surface charge on the other side of the spacer; this leads to an inaccurate inverted surface charge distribution. In this paper, the inversion algorithm methods used to determine surface charges over the past few decades were summarized, and their advantages and disadvantages were discussed. Based on the previous charge inversion algorithms, an improved surface charge inversion algorithm considering the surface potential of both sides of a cone-type model spacer was developed. The improved method was verified experimentally using a cone-type model spacer. Compared with that obtained by previous charge inversion algorithms, the charge distribution derived from the improved algorithm was closer to the situation determined from theoretical analysis. This work has practical relevance for the surface charge measurement of cone-type spacers and offers a new orientation in the surface charge analytical algorithm.

Influence of temperature on the characteristics of surface charge accumulation on PTFE model insulators

During the operation of gas insulated switchgear (GIS) and gas insulated line (GIL), the conducting bar generates joule heat, and a non-uniform distribution of temperature forms in the bulk of the insulators. As the load varies, the temperature distribution changes, and it has an influence on the bulk conductivity of the insulators. Moreover, the surface charge accumulation can be affected. In order to clarify this, a surface charge measurement system which could achieve temperature control was constructed, and a model insulator with truncated cone type was employed, as well as two types of high-voltage electrodes, i.e. plate and needle electrodes. It is found that both polarities of charges existed on the insulator surface when a dc high voltage was applied to the plate or needle electrode, and homo-charge density was much higher than that of hetero-one. As for the plate electrode, homo-charges resulted from micro-discharges, while generated by corona in the case with needle electrode. As temperature increased, homo-charge density decreased, kept unchanged and increased for three cases, respectively, i.e. when dc voltage was applied to the plate electrode, and when voltage of 3 and 20 kV was applied to the needle electrode. Moreover, a simulation model involving multi-physics was established, which included heat conduction in solid and transient field changing from the initial capacitive to stationary resistive field distribution. It is proved that the electric conduction of the insulator bulk contributed to the accumulation of hetero-charges, and temperature could enhance this. Besides, when temperature increased, the corona inception voltage reduced, and hence the homo-charge density increased. Due to the effect of neutralization and the differences in the sensitivity of corona with different intensity to temperature, the tendency of homo-charge density changing with temperature for the three cases showed distinct in the experiments.

Power losses in the three-phase gas-insulated line

Power losses in the three-phase gas-insulated line

Motion analysis of spherical metal particle in AC gas-insulated lines: random effects and resistance of the SF6/N2 mixture

This paper analyzes the electromechanics of the spherical metal particles in AC gasinsulated lines (GIL) with a pragmatic test rig comprising coaxial cylindrical electrodes. Analysis on fluid resistance during the particle moving process is presented based on fluid mechanics as well as the kinetic parameters of the SF6/N2 mixture. The interactive impacts between the particle and the conductor or the shell are also proposed based on collision theory of elastic mechanics considering random effects caused by the metal surface roughness. With the above aspects being included, a dynamic motion model for the spherical metal particles within AC GIL is established accordingly, and experimental studies are carried out to validate effectiveness and accuracy of the proposed model. Meanwhile, simulations are done regarding a real 220 kV AC GIL for further governance and verifications. The research observations indicate that, the applied voltage exerts an incremental effect on the maximum height of the moving particle, but won't affect the spread angle of the particle motion. Both the height and the spread angle of the particle’s motion show inflection points with increased particle radius. On the contrary, the maximum height of the moving particle decreases with the increase of SF6 ratio and the gas pressure.

Molecular relaxation and glass transition properties of epoxy resin at high temperature

Epoxy resin is widely used as a polymeric insulating material in power equipment, such as gas-insulated switchgear and gas-insulated lines. The motions of molecular chains or segmental chains in a polymeric insulating material can affect the material properties, such as dielectric relaxation, charge transport, breakdown, and glass transition temperature. Molecular or segmental chains may form dipoles, and their motions can contribute to dielectric relaxation properties. Molecular or segmental chains with different scales have different relaxation time constants. Their motions affect dielectric relaxation processes in different frequency ranges. The motions of molecular or segmental chains are also affected by temperature, since the magnitudes of motions are restricted by free volume in a polymeric insulating material. However, the effects of motions of molecular or segmental chains in epoxy resin on electrical properties have not been very clear to date. Therefore, it is important to investigate the relations between the motion of molecular or segmental chains and dielectric relaxation properties, the temperature and molecular scale dependence of the motions, and their effects on charge transport of epoxy resin. In this paper, the properties of dielectric relaxation and glass transition of epoxy resin are measured. Before the experimental tests, samples of pure epoxy resin are prepared by using epoxy raw materials supplied by Pinggao Group, and the curing temperature is 130 ℃. The glass transition temperature is around 105 ℃ measured by a differential scanning calorimetry (DSC). As for the dielectric relaxation measurement with Novocontrol broadband dielectric relaxation spectroscopy, the sample is processed into a disk with a diameter of 50 mm and a thickness of 1 mm. The measurement temperature and frequency are in ranges of 100-180 ℃ and 10-1-107 Hz, respectively. The results reveal that there are two relaxation processes at high temperature. In addition, above glass transition temperature, a relaxation peak occurs at high frequencies due to the motions of molecular chains or segmental chains, and a direct current (DC) conductivity resulting from the migration of charge carriers appears at low frequencies. Besides, molecular chains with different scales have different relaxation times. It is found that epoxy resin has a very broad distribution of relaxation times. The distributions of relaxation times at various temperatures are calculated. The results show that the temperature dependence of molecular relaxation and DC conductivity satisfy Vogel-Tammann-Fulcher equation. Through fitting the experimental results, the Vogel temperatures and strength parameters of molecular relaxation and DC conductivity are obtained. From the Vogel temperatures, the glass transition temperature is estimated to be 102 ℃, which is consistent with the DSC result. It means that free volume in epoxy resin increases with the increase of temperature, which facilitates the motions of molecular chains and the migration of charge carriers.

Lightning Protection of 500-kV DC Gas-Insulated Lines (GIL) With Integrated Surge Arresters

A 500-kV dc hybrid transmission line consisting of a gas-insulated line (GIL) with lengths up to 10 km connected at both ends to long overhead lines is applied for lightning overvoltage studies. For this system, the conventional application of external surge arresters results in high electrical stress on the insulation of GIL. Therefore and because of the modular structure of GIL, the application of internal surge arresters and two combinations of external and integrated surge arresters are investigated. The influence on the voltage buildup in GIL of variation in the position of integrated surge arresters is explained and the best positions are calculated by a sensitivity study. All variants of the application of integrated surge arresters result in a reduction of voltage stress within the GIL compared to the conventional application of external surge arresters.

Reduction of electric-field intensification inside GIS by controlling spacer material and design

Abstract This paper presents finite-element simulation to calculate electric-field distribution inside coaxial gas-insulated switchgear and lines (GIS and GIL) for different solid spacer configurations, namely, disc, conical and post spacers. Several troubles and systems’ outages have been reported worldwide due to insulation failures. Therefore, it is essential to minimize the electric-field intensification along their surfaces, especially at the triple point “electrode-spacer-gas”, and hence enhance their reliability and increase their lifetime. Finite-element-based OPERA-2D software was used to calculate electric-field distribution by studying the effects of solid spacer dimensions and relative permittivity, application of functionally graded material (FGM) and the use of spacers with metalized cavities and shields.

State of the Art in Ultrahigh-Voltage Transmission

This paper discusses ultrahigh-voltage (UHV) DC as an efficient solution for bulk power transmission especially of renewable energy. The European policy and legal framework is used to illustrate how this new technology, international electricity market integration, and the distribution of wind and solar natural resources lead to the need for continent-wide internationally coordinated network planning with a view on long-term greenhouse gas reduction. First, the technical and economic aspects of UHV transmission are described, including, e.g., converter configurations and internal and external insulation. Then, UHV transmission is put into the context of other broad current questions of power system development: AC and DC, microgrids, smart grids, power electronics, overhead lines, cables, and gas-insulated lines. The final part discusses the European legal framework for climate protection and for transmission system operator (TSO) cooperation in the new, legally mandated European transmission system operator association ENTSO-E, in particular for joint European grid planning and for R&D. This example shows how UHV technology development can contribute to market integration and economic and environmental goals for continent-wide and internationally coordinated transmission development as one option in international long-term system studies.

가스절연송전선로(GIL) PQ시험 방안에 관한 연구

As the expension of metropolis and increasing of housing site, GIL(Gas Insulated Line) comes to the fore as an alternative to overhead transmission line avoided by local resident. The certification and experimental test for the GIL is now in progress in under ground test lab, Go-chang. For the test, we propose a method of PQ(pre-qualification) test which is essential for development and field application in this paper. The test following this method take about 200 days or 300 days under the circumstances.

The Transmission Line for the SPIDER experiment

The 100 keV Ion Source Test facility – Source for the Production of Ions of Deuterium Extracted from RF plasma (SPIDER) – is aimed to test the full scale prototype of the Ion Source for the ITER 1 MeV Neutral Beam Injector (NBI). The SPIDER facility requires the construction of a High Voltage Deck (HVD) and of a High Voltage Transmission Line (TL) respectively to host the Ion Source Power Supplies system polarized at 100 kV and to carry the power and signal conductors to the beam accelerator. In already existing NBI systems with beam energy above 100 keV, the TL is realized with the SF 6 Gas Insulated Line technology. In the SPIDER TL case, the presence of a large inner conductor (half meter diameter), would make the pressurized TL a complex and costly component; therefore a free air insulated solution has been proposed. The paper focuses on the design of this TL, which has to host inside the complex high potential (100 kV) inner electrode a number of power and measuring conductors and has to minimize the Electro Magnetic Interferences (EMI) produced by the frequent grids breakdowns. Finite Element (FE) analyses have been performed to verify the configuration from the electrostatic point of view, to evaluate EMI screening effectiveness and to assess the impact of the relatively high thermal dissipation of power conductors located inside the high potential electrode. Moreover, an experimental test campaign has been carried out on a TL mockup to validate the TL electrostatic configuration under DC voltage. Finally, the paper reports on the status of procurement activities for the Transmission Line.

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.

Thermal Behavior of EHV Gas-Insulated Lines in Brenner Pass Pilot Tunnel

The paper deals with the thermal behavior of a double-circuit gas-insulated line inside the pilot tunnel of the future high-speed railway galleries between Italy and Austria (Brenner Basis Tunnel (BBT)). Since the active power losses generated by the power transmission line may likely be about 7 MW, the thermal behavior of the BBT pilot tunnel and its ventilation requirements have been evaluated by resorting to a suitable computer model. This model is based on a “control volume” approach: a discretization of the space and time domains leads to a network of thermal capacities and resistances to be solved time after time by means of a linear algebraic system.

Thermal and magnetic analyses of gas-insulated lines

This paper presents the factors affecting current rating “ampacity” of buried three-phase, naturally cooled isolated-phase gas-insulated lines (GIL) with flat formation and finite-element simulation of double-circuit, three-phase GIL inside a tunnel to calculate the magnetic-flux and current densities. In the former analysis, the depth of GIL burial, interspacing of phases and soil thermal resistivity are investigated to show their impacts on the GIL ampacity. In the latter analysis, the effects of electrical and physical arrangements of GIL phases on the magnetic-flux density inside and outside the tunnel, and the current density in few installed liquefied natural gas (LNG) pipelines, which run in parallel to the GIL inside the tunnel, are also studied. In any GIL configuration, the magnetic field impact is always extremely low. Localized hot spots along the periphery of the GIL enclosure are found and corrosion can be accelerated due to temperature gradient. Moreover, there is a possibility of galvanic corrosion of the enclosures when they are bonded together and with the gallery steel reinforcement at very short length intervals. The simulation results also give the best location of the LNG pipelines or any cables to have the lowest electromagnetic induction. The configuration that has the best “magnetic behavior” is distant-placed arrangement (DPA) with (abc–cba) phase arrangement. The induced eddy-current density in the LNG pipelines is very low and hence its thermal effects can be neglected.

Multiconductor analysis of underground power transmission systems: EHV AC cables

It is widely ascertained that the multiconductor analysis is a powerful tool which can solve any structurally complex circuit (e.g., high-speed railway supply system, gas insulated lines, etc.). Moreover, this method can be also used as an in-depth analysis of the electric networks after the power-flow studies when they introduce simplifying hypotheses especially in the presence of asymmetry. In this paper, the multiconductor cell analysis has been applied to AC underground cable lines (UGC). This multiconductor procedure based on the use of admittance matrices, which account for the line cells (with earth return currents), different types of sheath bonding, possible multiple circuits, allows predicting the steady-state (and faulty) regime of any cable system. The method calculates the proportion and behavior of the phase currents carried by each parallel conductor, the circulating current in the sheath of each cable and the stray current in the earth. Moreover, some comparisons have been made with traditional programs showing the great accuracy of multiconductor cell model.

High-Capability Applications of Long Gas-Insulated Lines in Structures

The paper deals with the possibility of installing gas insulated transmission lines (with acronym GIL) in dedicated or shared structures. The applications involve railway galleries (both in the proper galleries and in service or pilot tunnels), highway networks, gas lines. The high GIL power capability together with very low power losses configures these applications as high transmission efficiency corridors. Some issues of this application is shown in this paper (steady-state regime of the line, voltage drops, power losses, voltage unbalance factor, ampacity, electro-magnetic field impact and limit lengths): moreover, the paper discusses the necessary studies to be carried out and the problems that have to be faced in order to develop this kind of application towards a feasible technical solution

Operating capability of long AC EHV transmission cables

The operating constraints involved in power transmission of long AC cable links are presented and here throughout developed. The possibility of transmitting high power rating is examined and the transmission length limits are evaluated. The procedure, which uses the great computation and graphical facilities of modern computers, is essentially based on the classical transmission equations and their pertaining diagrams. It has been particularly applied to XLPE cables and gas-insulated lines but it is worth applying to any distributed-parameter transmission line (including overhead lines). The capability chart proposed in the paper is a useful tool to highlight the operating characteristics of a cable link and a guide to evaluate the results of different degrees of reactive compensation. The authors hope that the results can offer the transmission system operators very effective means in order to evaluate the existing and future underground cable links.

Effect of the percentage of SF6 (100%–10%–5%) on the decomposition of SF6–N2 mixtures under negative dc coronas in the presence of water vapour or oxygen

Low SF6 content SF6–N2 mixtures have recently been proposed as a replacement for pure SF6 in the insulation of gas insulated lines (GIL). Among the areas of investigation of such gas mixtures, their electrical decomposition under corona discharges must be studied considering the possible occurrence of such stress in GIL.This paper presents data concerning the decomposition of high-pressure SF6–N2 (5 : 95) mixtures (400 kPa) submitted to negative dc coronas in the absence or presence of 0.3% H2O or 0.3% O2. The chemical stability of these mixtures is compared with that of SF6–N2 (10 : 90) mixtures or undiluted SF6 investigated in the same conditions in a previous paper.The corona discharges were generated with a point-to-plane set-up and the gaseous by-products were assayed by gas chromatography at the end of each run carried out over a range of transported charge covering 0–13 C. The following by-products were detected and assayed: SOF4, SO2F2, (SF4 + SOF2), SO2, S2F10, S2O2F10, S2O3F6, (SF5)2NF, NF3 and N2O.Whatever the type or the concentration of impurity added to the SF6–N2 mixtures the major compound groups, (SOF4 + SO2F2) and (SF4 + SOF2 + SO2), were formed with 5% SF6 in quantities very close to those observed with 10% SF6. However, the lesser production rates of S2F10, (SF5)2NF and NF3 measured with the most dilute SF6–N2 mixtures makes the use of SF6–N2 (5 : 95) more advantageous than that of SF6–N2 (10 : 90) in all cases.Considering the overall quantity of by-products formed in the presence of water or without any impurity added, it appears that SF6–N2 mixtures are, from this point of view, preferable to pure SF6.

Second generation gas-insulated line

Environmental requirements make it necessary to build underground transmission systems in special areas with special requirements. Limiting values for the use of overhead lines could be magnetic field restrictions, or just visibility of the overhead line, or limited space. The gas-insulated line (GIL) is one technical solution to allow the transmission of electricity under ground at high transmission ratings. The cost reduction of the second-generation GIL compared to the first generation, and the increased productivity in laying of GIL directly buried or in a tunnel, makes the GIL a high power rating underground transmission solution for the future.

Proximity effect and magnetic field calculation in GIL and in isolated phase bus ducts

This paper deals with an integral numerical method suitable to predict the current density distribution in typical multiconductor systems represented by gas-insulated lines (GIL) or by isolated phase MV bus ducts, highlighting their proximity effect. Besides the prediction of power density distribution due to Joule losses, the integral approach makes the evaluation of the external flux density easy. The simplicity of the approach, the conciseness of the matrix algebra applied, and the close-to-reality procedure make this method extremely attractive for self-made software implementation on any commercially available math packages such as Matlab. For the sake of comparison, a finite-element method (FEM) model of the system has been implemented. In this model, a coupling between electric circuit and electromagnetic model has been used in order to take into account the real power supply conditions.

Decomposition of high-pressure (400 kPa) SF6 and SF6/N2(10:90) mixtures submitted to negative or 50 Hz ac corona discharges in thepresence of water vapour and/or oxygen

From the point of view of environmental protection, as SF6 has astrong greenhouse effect, high-pressure SF6/N2 (10:90) mixtures could bea good alternative to pure SF6 for use in gas-insulated lines (GIL); thus,the decomposition of these gases should be compared following stress such ascorona discharges, susceptible to occur in such electrical equipment.So, the present study concerns the decomposition of sulphur hexafluoride(SF6) and the mixture SF6/N2 (10:90) at high pressure (400 kPa) underthe effect of negative dc or 50 Hz ac corona discharges in the presence orabsence of added impurities (H2O and/or O2). The chemical stability ofSF6/N2 was compared to that of pure SF6 over a range of transportedcharge covering 0-12 C in the presence and absence of various proportionsof impurities added to the gas.The discharges, generated with a point-to-plane set-up, led to the formationof the following compounds: SOF4, SO2F2, (SF4 + SOF2), SO2, S2F10, S2OF10,S2O2F10, S2O3F6, NF3, SF5NF2, (SF5)2NF, N2O and CO2, which were assayed by gaschromatography at the end of each run.In the absence of impurities added to the gas phase, the overall quantities ofby-products formed in SF6 and SF6/N2 (10:90) were very close. In thepresence of 0.3% added oxygen, greater amounts of degradation product werefound in SF6/N2 (10:90) than in pure SF6. On addition of largequantities of water (⩾0.3%) the opposite was observed, even when 0.3% oxygenwas added at the same time as the water. So, the use of SF6/N2 (10:90)instead of pure SF6 was beneficial in the dampest conditions studied. Thiscan be accounted for by the inhibitory action of water, especially in diluteSF6, on the formation of by-products that require the SF5 radical such asSOF4 and S2F10, to mention only the most important. It follows that in theseconditions (damp, dilute SF6) the pair of degradation products (SF4 + SOF2)becomes preponderant over (SOF4 + SO2F2).However, the quantity of gaseous by-products formed in SF6/N2 (10:90)was much greater than suggested at first sight by the proportion of SF6 inthe mixture. This is mainly due to the fact that the dilution of the SF6fragments in the N2 at the same time inhibits the SF6 regenerationreactions and enhances the hydrolysis/oxidation of the SFx fragments whichlead to the formation of sulphur oxyfluorides. This dilution effect is alsoresponsible for the very low levels of S2F10 formed in the mixture.