DC Gas Insulated Transmission Lines Research Articles

Charge interaction behaviors at interfacial domains in DC GIL insulators

Long-term operation of high voltage direct current at elevated temperatures can result in the accumulation of surface charges in DC gas-insulated transmission line (GIL) insulators. Such a phenomenon leads to localized electric field distortion, increasing the risk of surface discharge. The analysis of interaction behaviors between surface charge and space charge at interfacial domains of GIL insulators is a complex task, which requires a comprehensive understanding of physical mechanisms of the gas–solid interface charging. In this work, a two-dimensional bipolar charge transport and interaction (2D BCTI) model is established, with the consideration of both surface and space charge dynamics. Pulsed electroacoustic tests and surface potential measurements are conducted on DC GIL insulator materials under different electrical-thermal coupling conditions. Experimental results exhibit great consistency with the predictions from the 2D BCTI model. The local accumulation of space charge near interfaces has certain effects on surface potential distribution, which in turn influences charge injection behavior from electrodes. In comparison to traditional surface charge simulation models, the consideration of space charge–surface charge interaction behaviors proves to be essential for estimating the polarity and amplitude of surface potential distribution. This model holds promise for assessing charge characteristics in electrical equipment where direct measurement is challenging.

Commissioning and Service Experience With a ±550 kV DC GIL Conducted in Frame of a CIGRE Prototype Installation Test

A newly developed ±550 kV DC GIL (direct current gas-insulated transmission line) prototype was erected at a HVDC test facility at the Technical University of Darmstadt to collect first service experience with this new equipment. The 100 m long DC GIL prototype is periodically stressed with DC voltage and DC current as well as superimposed impulse voltage. The test equipment was procured for the project and partly newly developed. The overall procedure follows the prototype installation test recommendations from CIGRE TB 842. This paper presents experience collected during the commissioning and long-term testing of the DC GIL prototype. The commissioning process, events occurred, as well as resulting conclusions for type, factory acceptance and site acceptance tests of DC GIL systems are discussed. The long-term operation of the DC GIL is intended to run continuously. Interruptions occur due to external influences only. Partial discharge events and the overall performance of the DC GIL prototype during long-term stress are presented.

Temperature-Dependent Surface Charge Accumulation for Vertical and Horizontal HVDC GIL

In this paper, the heat transfer process of DC gas-insulated transmission line (GIL) is comprehensively considered, including thermal conduction, thermal convection and thermal radiation. An electric-thermal field coupling model is established to characterize the surface charge accumulation characteristics of insulators. The temperature and insulator surface charge density distribution of a 3-D GIL model under different GIL installation ways (i.e. vertical and horizontal GIL) are obtained by finite element software. The results show that: under the identical load current, a large temperature difference (up to 5.7 °C) is shown between both surfaces of vertical GIL insulator, and the temperature on the insulator surface is distributed symmetrically along the center of the insulator. However, the temperature on both surfaces of the horizontal GIL is basically the same, but the temperature distribution varies greatly along the different radial directions of the insulator (up to 5.3 °C). By comparing the surface charge density distribution of insulators under both GIL installation ways, it is found that the surface charge density of horizontal GIL is more than 20% higher than that of vertical GIL. In addition, the volume conductivity distribution of insulator varies greatly under different GIL installation ways. The volume conductivity is positively correlated with the temperature distribution, while the normal electric field on the insulator surface is negatively correlated with volume conductivity distribution. It is hoped that this study can serve as a reference for the design and safe operation of DC GIL.

Physical mechanism of electrode coatings to suppress wire particle’s firefly motion under DC stress and selecting criterion for in situ applications

The Wire metal particles in gas-insulated equipment such as GIS (Gas-Insulated Substation) and GIL (Gas-Insulated transmission lines) under DC stress present a special firefly motion behaviour, namely a long-term levitation on the surface of conductors. However, the electrode coating, as a key device available to suppress the movement of particles, lacks of a theoretical basis as well as an effective selecting criterion to suppress wire particle’s firefly. In this paper, a test platform is established to obtain the charge dynamics and ionic wind characteristics of the wire particles on the coated electrodes. Experimental results illustrate that, the polarity alternation of wire particle charge due to the space charge accumulation near the wire particles as well as the ionic wind generated by the wire particles, is the key impact factor upon firefly motion. The coated electrodes will reduce the surface conduction current and thereby suppress the particle corona discharge, consequently suppressing the firefly motion by limiting the charge acquisition and ionic wind speed of the wire particles. Based on the above research, a selecting criterion of electrode coating material is further proposed. Electrode coatings is primarily expected to increase the corona onset voltage of the wire particles on the coating surface, so as to lower the ionic wind at the end of the particles less than 1 m s−1. Regarding the high voltage conductor, the coating material with relatively large conductivity is recommended, while for the grounded enclosure, the coating material with smaller conductivity is preferable as to constrain the particles adhere to the enclosure, thereby preventing the firefly motion of wire particle. Both the clarified physical mechanism and the proposed selecting criterion of electrode coating presents theoretical and quantitative guidance for DC GIL design from a specific point of view in suppressing the firefly motion.

Particle behavior and trap design for ±320 kV gas-insulated power transmission line (GIL)

The movement of metal particles in the electric field of the DC gas-insulated transmission line (GIL) may cause local electric field distortion on the surface of the spacer, which seriously affects the operation stability of the GIL. In this paper, based on a ±320 kV GIL platform, the movement characteristics of metal particles (aluminum blocks, aluminum wires, aluminum balls) inside the GIL are studied. The suppression effect of particle activity for particle trapping and surface coating are experimentally studied in DC electric field. The relationship between particle trap porosity and particle suppression effect are discussed and verified with experiments. The research results show that under negative voltage, the minimum take-off voltage of metal particles in ±320 kV GIL is −190 kV, which is far lower than the steady-state operating voltage of GIL. Once the spherical and blocky particles take off, they will continue to reciprocate rapidly between the conductors. When the block particles are close to the spacer, they might be attracted and adsorbed on the surface of the spacer. Metal wires after taking off tend to show ‘firefly’ movement near the high-voltage conductor. The coating has a significant effect on increasing the take-off electric field of metal particles. It is verified that the traditional AC GIL particle trap is not effective in inhibiting particles in DC voltage. The suppression of the DC GIL particle is positively related to the porosity of the particle trap to a certain extent. The conclusions of this paper can be reference for the development of future stable and reliable DC gas-insulated equipment.

Introducing Chlorine Into Epoxy Resin to Modulate Charge Trap Depth in the Material

The epoxy insulators in dc gas-insulated transmission lines (GIL) tend to accumulate surface charges, which causes insulation flashover. Increasing the surface conductivity of epoxy resin, which can restrain the accumulation of surface charges on the epoxy insulator, is a potential method to improve the insulation performance of dc GIL insulators. The conductivity of polymer dielectric is strongly influenced by the charge trap characteristics of the polymer. In this work, we introduce chlorine with strong electronegativity and a larger atomic radius into the epoxy group segment of epoxy resin to improve the distributed energy level structure, which in turn reduces the electron trap depth, to increase the surface conductivity of epoxy insulating material without affecting its intrinsic dielectric strength. Based on the results of quantum chemistry calculation, the modulation laws of introduced chlorine (including the chlorine in the form of a hydrolyzable chlorine atom and a nonhydrolyzable chlorine atom) on distributed energy levels of epoxy resin molecule are anticipated. These laws are explained from the microscopic perspectives of electron energy structure and electron cloud offset. Both the inductive effect of the chlorine atom and the conjugation effect of the 2p electron orbital of the oxygen atom in the epoxy group impact the distributed energy levels by changing the spatial distribution of electron cloud density between and on valence bonds.

Surface Charging Affecting Metal Particle Lifting Behaviors Around Epoxy Spacer of HVDC GIL/GIS

In this article, the mechanisms of surface charging and surface modification of epoxy spacer affecting the lifting voltage and the motion trajectory of spherical metal particles in a downsized dc gas insulated transmission line (GIL) are investigated by simulations and experiments. Results show that the lifting voltage can be predicted with much accuracy by calculating the electric field distribution under the capacitive condition. Prestressing leads to a 27% decrease of lifting voltage, which is traceable to that surface charging making a major contribution to the enhancement of electric field force. Direct fluorination is proven as an effective modification method to improve the insulation performance of epoxy spacer again. The lifting inclination is significantly inhibited after treatment because of less surface charge accumulation and more rationalized electric field distribution. The trajectory simulation results indicate that more serious surface charging can activate more metal particles to lift up and drive them to more collisions. The lifting probability and velocity of metal particles around the epoxy spacer are markedly decreased by fluorination, which is a promising treatment for dc GIL/gas insulated switchgear (GIS) application.

Polishing Orientation Affecting Surface Charging and Flashover Characteristics of GIL/GIS Epoxy Spacer

In this article, the influence of polishing orientation on surface charge accumulation and flashover characteristics of the basin spacer in a downsized dc-gas insulated transmission line (GIL) are explored by experiments. Results show that the surface charge distribution of the polished spacers is different from the untreated one. Heteropolar charges can be observed in the polished area and outward expansion area under prestressing condition. The flashover voltages of all polished spacers are decreased. The radial polishing (RP) has the greatest impact on the flashover voltage, and the flashover voltage of the radially polished spacer is decreased by 11% under positive prestressing condition. The metal dust adsorptions of the spacers are also measured, which shows that the radially polished spacer is easier to absorb metal dust and the metal dust diffuses in the radial direction. SEM and energy spectrum scanning of different spacers show that there are more exposed alumina particles on the surface of the polished area. The microdischarge induced by alumina particles is the source of heteropolar charges. When the polished area of the spacers is repaired with epoxy paint, heteropolar charges disappear on the surface of the repaired spacer. It is confirmed that charge accumulation and flashover along the spacer surface are mainly affected by the change of surface morphology on the polished area.

Study on Movement and Distribution Characteristics of Metal Particle Dust in DC GIL

Gas-insulated transmission lines (GILs) have a wide application prospect in special occasions such as cross-terrain power transmission. Metal particle contamination is an important reason for low insulation performance of GIL. To better suppress metal particle contamination, it is necessary to master the motion characteristics of metal particles. Based on that, this article took the metal particle dust (MPD), which is common in engineering, as the research object. The boundary element method (BEM) was introduced, and the boundary integral equations (BIEs) of multimedia domain Poisson equation in 3-D were given. The electric field distribution considering charged particles was obtained. Then the motion model of MPD was established. Combined with experiments, the movement and distribution characteristics of MPD in direct-current (dc) GIL were studied. The results show that the randomness of MPD movement in dc GIL is strong, and near the ground, MPD presents a “sandstorm” movement. Second, due to the influence of charged metal particles on electric field distribution, the collision between metal particles, and the randomness of the collision between metal particles and wall, the range of MPD’s motion in the axial direction of GIL chamber is wide, which increases the probability of MPD adhering to the insulator. The adhesion of MPD is more obvious on the convex surface of the insulator near the high-voltage electrode. The above research results can be used to guide the suppression of metal particle contamination in dc GIL.

Surface Charge Accumulation and Electric Field Distribution of Cone-Type Spacer in DC GIL

The surface charge accumulation on the spacer and the distortion of the surface electric field caused by it are the main reasons for the reduction of flashover voltage of DC GIL (Gas Insulated Transmission Line) spacers. A reasonable spacer structure helps to improve the surface electric field distribution. This paper considers the generation, migration, diffusion and recombination of free ions in GIL gas, and the nonlinear relationship between gas current density and electric field strength. A numerical model for the charge transport and accumulation in DC GIL is established. Based on this model, the surface charge accumulation and electric field distortion characteristics of cone-type spacer with different inclination angles and curvatures of the surface are investigated. The results show that the polarity of the accumulated charge on the two sides of the cone-type spacer is different from that on the inside and outside (concave and convex surface). The negative charge density in the concave surface is 1.6 times that of the positive charge density in the convex. The normal component of the electric field is the main reason for the formation of charge accumulation; The surface charge density of the cone-type spacer increases as its inclination angle increases. and there is a key position for the electric field along the both surfaces. The high-voltage side electric field decreases with the increase of the angle, and the electric field near the ground side increases with the increase of the angle; The smaller the curvature along the spacer, the higher the charge density. The research results can provide reference for structural optimization of cone-type spacer in DC GIL.

Conductor Surface Roughness-dependent Gas Conduction Process for HVDC GIL—Part I: Simulation

Charges due to ionization at conductors at low electric field serves as the main source of charge carriers that is responsible for surface charge enhancement, especially for hetero-polar charges on the convex surface side of spacers inside DC gas-insulated transmission lines (GILs). In this paper, simulation results of charge generation processes from gas side, including natural ionization, micro-discharge and field emission, are presented. The results show that for the case where the electric field strength is -6.548 kV/mm and the gas pressure is 0.4 Mpa, when conductor roughness Ra is lower than 24.16 μm, natural radiation ionization is the main cause of the dark current. When Ra is between 24.16 μm and 188.59 μm, the dark current is dominated by micro-discharge. The Fowler-Nordheim (F-N) field emission occurs when Ra exceeds 188.59 μm. Considering the roughness of the conductor in the actual GIL, the increase in the dark current due to local roughness is mainly due to micro-discharge rather than F-N field emission. The results in this paper provide an important part for further determining of conductor surface conditions in high voltage direct current (HVDC) GILs and serve as an important reference for completion of the surface charging model of spacers under DC.

Particle trajectories and its charging behaviors: effect of gas type and electric field

Conductive particles in the gas insulated transmission lines (GIL) induce the breakdown between electrodes or the flashover along insulators. To solve the problem of particle moving and realize the particle-moving regulation, particle trajectories should firstly be determined in air and SF6. This paper presents the results of particle trajectories and its charging behaviors at vacuum and SF6, respectively. Metal particles with different materials and sizes were introduced and the charge quantity was calculated. The results showed that the particle lift-off electric field in SF6 was higher than that in air under the same gas pressure, that is, the charge on particle in SF6 was about 0.789 times lower than that in air under the same condition. Besides, the lift-off electric field of particle increased with the pressure increase of SF6. The charge on particle was affected by the concentration of electric field near particle and the electrical negative features of SF6. The work provided the data support for development of DC GIL in particle defect suppression.

A Review on Real-Size Epoxy Cast Resin Insulators for Compact High Voltage Direct Current Gas Insulated Switchgears (GIS) and Gas Insulated Transmission Lines (GIL)—Current Achievements and Envisaged Research and Development

Due to the ever-increasing demand for electricity in the one hand and the environmental constraints to use clean energy on the other hand, the global production of energy from remote renewable sources, particularly from large hydropower plants and offshore wind farms and their connection to the grid are expected to grow significantly in the future. Consequently, the demand to carry this electric power by high voltage direct current (HVDC) technology will increase too. The most suitable HVDC power transmission technology to deliver large amounts of power, exceeding a capacity of 5 GW per bipolar system over long distances with lower losses is by using compact HVDC gas insulated transmission lines (DC GIL) and gas insulated switchgears (DC GIS) with rated voltage (maximum continuous operating voltage) of ±550 kV and 5000 A which are presently under development worldwide. Among the critical challenges for the development of these HVDC gas insulated systems, there are the epoxy cast resin insulators that are used to separate gas compartments also called spacers. Indeed, thorough research studies have been and still being carried out to well understand and clarify the electrical insulation characteristics of HVDC spacers using mainly cylindrical samples and small insulator models, where useful results have been obtained and proposed for implementation in real compact gas insulated systems. However, few practical investigations have been undertaken on real size spacers (product scale) to verify such research outcomes and validate the reliability of the spacers to collect experiences or for commercial use. This paper reviews the current achievements of real size HVDC spacers development. It describes the basic electric field calculation and spacers design, the verification of the insulation performance and validation testing. It gives today’s commercially available compact HVDC GIS/GIL and finally it presents the envisaged future research and development.

A novel nonlinear conductive ZnO micro-varistor/epoxy resin composite film for metallic particle deactivation in DC GIL

Enclosure coating is a promising technique of metallic particle contamination deactivation in DC gas-insulated transmission lines (DC GIL). However, the conventional insulation film cannot inhibit partial discharge (PD) at triple junction around the contact between the particle and the film, which has adverse effects on metallic particle deactivation. In the present work, we therefore fabricated electrically nonlinear conductive film by doping ZnO micro-varistors in epoxy resin matrix. And we measured the conductivity and dielectric characteristics of film with various doping concentrations. We experimentally and theoretically investigated the effects of film properties and particle radius on suppression of PD and particle lift-off statistics. The developed nonlinear conductive film was proved to effectively suppress PD and better increase the particle lifting voltage than conventional insulation film, which is of significance in the long-term reliable operation of DC GIL.

Intrinsic hetero-polar surface charge phenomenon in environmental friendly C3F7CN/CO2 gas mixture

The hetero-polar charges on the spacer surface due to the gas ionization has been verified as one of the hazard charge sources threatening the safe operation of HVDC gas-insulated transmission line (GIL). This letter reports the differences in surface charging properties of spacers in SF6 and C3F7CN/CO2 gas mixtures, and a rigorous re-examination of environmental firendly gas assessment for DC GIL is introduced. The results show that when the maximum value of electrical field strength at the spacer surface is in 0.45 kV mm−1, homo-polar surface potential in different insulating gases indicates a dominant charge originating from bulk injection. As the maximum value of electrical field strength at the spacer surface reaches 0.68 kV mm−1, local hetero-polar surface potential areas are detected and the area ratio of these regions is higher in CO2 gas, followed by 4% C3F7CN/96% CO2 gas mixtures. While in the 10% C3F7CN/90% CO2 gas mixtures, homo-polar potential regions are still dominant with very low area ratio of hetero-polar potential regions, which is close to that measured in SF6. An indicating factor is put forward to further clarify the key role of hetero-polar charges plays in potentially triggering surface flashover. This letter provides an important basis for determining contents and components of insulating gases, which serves as one of the key issues in developing HVDC GIL.

±100-kV HVDC SF6/N2 Gas-Insulated Transmission Line

This paper introduces an HVDC SF6/N2 gas-insulated transmission line (GIL) prototype that operates under 100-kV DC application. In the first part, the development history of GIL products over the past 50 years is reviewed. After that, the design concept of the ±100-kV HVDC SF6/N2 GIL is introduced. A GIL prototype was prepared and the DC withstanding voltage test and DC superimposed lightning impulse voltage test were performed. The results show that this GIL prototype passed all the required tests, including hydrostatic test, gas tightness test, thermal cycling test, DC withstanding test, and DC superimposed impulse test, etc. The successful preparation of this product promoted a major step in researches of the DC GIL and the prototype laid a solid foundation for the development of subsequent industrial products.

Charge transport and accumulation around the cone‐type insulator in SF 6 ‐filled 550 kV DC GIL

The gas-insulated transmission line (GIL) has the advantages of large transmission capacity, high operation reliability and environmental friendliness. It is suitable for the high voltage direct current (HVDC) power transmission of hydropower stations or nuclear power stations. However, due to the serious surface charge accumulation problem of the cone-type insulator under DC, its flashover performance is reduced, which restricts the engineering application of HVDC GIL. In this study, a charge transport model of SF6 gas and epoxy is established for a 550 kV cone-type insulator and the charge accumulation characteristics of the concave and convex surfaces of the cone-type insulator are obtained. Computational results show that the gas conductivity around the cone-type insulator is not uniform due to the charged particles drifting under a constant electric field. Compared with the AC, the difference of the electric field distribution caused by charge accumulation under DC stress is analysed. Based on the established model, the influence of the gas ion-pair generation rate and epoxy volume conductivity on the charge accumulation on the surface of 550 kV cone-type insulator was studied. This study is helpful to further optimise the geometry and material properties of the cone-type insulator and reduce the surface charge accumulation and critical area electric field to improve HVDC GIL insulation design.

Research progress on environmental-friendly insulating gases for HVDC gas-insulated transmission lines

Environment-friendly gas insulating mediums adapted to a DC gas-insulated transmission line (GIL) electric field condition is the key to the next generation of Environment-friendly HVDC GILs. In this paper, we review the literature on sulfur hexafluoride (SFo) alternatives including the scientific understanding, control, and implementation of gas-solid systems in this type of power transmission. First, the structure-activity relationship between the molecular structure and physico-chemical properties of Environment-friendly insulating gases is presented. Then, the search and prediction of important physicochemical properties of gases are summarized. Subsequently, in view of the potential of environmental friendly insulating gases, the swarm parameters of gas discharge and breakdown properties in a quasi-uniform field, inhomogeneous field, and at the gas-solid interface, that need to be taken into account with industrialized DC GILs are discussed. The latest research progress on insulation characteristics, especially the polarity effect in DC gas-solid insulation systems, the sensitivity to the electrode surface state, and the non-uniformity of the electric field, and the influence of metal particles and their variation with air pressure, is highlighted. In addition, the heat transfer characteristics of insulating gases, related to DC GIL transmission with a large current-carrying capacity and the influence of alternative gases on the heat transfer characteristics are described. Finally, aiming at solving the contradiction of low environmental impact, high dielectric strength and low liquefaction temperatures in the selection of alternative gases, an coordinated regulation model for Environment-friendly gases in DC GILs is established. Considerations for future work on this topic are also presented.

Flashover behavior of cone-type spacers with inhomogeneous temperature distribution in SF6/N2-filled DC-GIL under lightning impulse with DC voltage superimposed

Before designing DC gas insulated transmission lines (GIL), the insulation characteristics of spacers under a lightning impulse (LI) voltage with a superimposed DC voltage must be clarified. Many experiments have been performed to guide the development of the DC spacer. However, the impacts of thermal stresses on their insulation performance under combined voltage are always neglected, and little attention has been given to the insulation characteristics of spacers in an eco-friendly gas filled DC-GIL. To solve these problems, an experiment setup with the abilities to load the thermal field and the DC voltage with LI voltage superimposed on the cone-type spacer are built. After that, the insulation characteristics of spacers under the combined voltage of DC with lightning impulse at 0.1 MPa SF 6 /N 2 gas mixtures are investigated, under different thermal stress conditions. The experiment's results indicate that: 1) The flashover voltage of spacers under the same polarity superimposed voltages is slightly higher than that under LI alone. Conversely, the flashover voltage under reverse polarity superimposed voltages is lower than that under LI alone. 2) The longer the duration of the foregoing DC voltage, the differences between the flashover voltage under superposed voltage and that under LI alone are larger. 3) The combined flashover voltages are reduced under thermal stress. To ensure the insulation performance of newly designed DC spacers, the insulation test is suggested to be applied under thermal stress and combined voltage of the DC with lightning impulse, especially for superimposed reverse polarity.

Insulation characteristics of fluoronitriles / CO2 gas mixture under DC electric field

Environmentally friendly DC gas insulated transmission line (GIL) is currently one of the hot topics in advanced electric power equipment. Recently, fluoronitriles have been widely studied and considered to replace SF6 in GILs. In this paper, the breakdown voltage, insulator flashover voltage, and the partial discharge inception voltage of fluoronitriles/CO 2 gas mixtures are reported in a DC electric field. The mole ratio of the mixed gases are 4 : 96 % and 8 : 92 %, and the gas pressure is in the range 304.7 to 717.83 kPa. Based on the results, the breakdown field strength of fluoronitriles/CO 2 gas mixture shows a growth tendency with increasing gas pressure in a uniform electric field. At 717.83 kPa, the positive breakdown field strength is higher than the negative and reached a maximum. In a non-uniform electric field, the negative breakdown voltage of fluoronitriles/CO 2 gas mixture increases with increasing gas pressure. In the contrast, the positive breakdown voltage decreases with the rise of gas pressure, which exhibits a significant polarity effect. The flashover voltage of fluoronitriles/CO 2 gas mixture increases with increasing gas pressure. At the same gas pressure, the negative flashover voltage is slightly higher than the positive. The negative flashover voltage of fluoronitriles/CO 2 gas mixture at 717.83 kPa is equal to 96.06 % of SF6 gas at 0.5 MPa abs. The negative partial discharge inception voltage (PDIV) of the fluoronitriles/CO 2 gas mixture is greater than the positive, and both 4 : 96 % and 8 : 92 % gas mixtures showed higher PDIV than the SF6. Considering the insulation characteristics, the fluoronitriles/CO 2 mixture has the potential to replace SF6 in DC gas insulated transmission lines.