Electric Field and Charge Characteristics at the Gas–Solid Interface of a Scaled HVDC Wall Bushing Model

A numerical model is established in the present research on the basis of a theoretical analysis, for describing and analyzing the electric field of High Voltage Direct Current (HVDC) wall bushing that demonstrates highly nonlinear characteristics. The relationship between the electric field intensity and the resistance of the insulators of the wall bushing is highly nonlinear and the wall bushing is subjected high voltage with nonlinear electric field. A numerical iteration technique is developed with the Parameter Design Language (PDL) of a Finite Element Analysis software package for carrying out the numerical calculations. The nonlinear characteristics of the HVDC wall bushing are investigated with the model and the iteration technique established. The methodology presented in the research provides a new approach for designing and manufacturing the HVDC wall bushing.

Facing the demand for key equipment of UHV DC transmission projects in China, and aiming at the technical difficulties of ±1000kV SF6 gas insulated through wall bushing, this project carries out the core technology research and device development of ±1000kV SF6 gas insulated through wall bushing. Through the research of this project, the problems in the design, manufacture and test during the development of ±1000kV DC through wall bushing, including the homogenization design of internal and external electric field, mechanical strength and current carrying capacity of bushing, put forward the design scheme of anti corona, anti pollution and external insulation of bushing, and master the performance of epoxy resin material for DC bushing and the electrical characteristics of epoxy resin insulator, formulate process flow, test scheme and technical standards. Finally, the prototype of ±1000kV DC SF6 gas insulated through wall bushing passing the type test is developed to meet the engineering application conditions and strive to realize the engineering application. The research of this project will solve one of the key difficulties restricting the development of ±1000kV UHVDC project in China, promote the manufacturing level of UHVDC key equipment in China to rank among international advanced ranks, and improve China's high-tech research and development capacity and the international competitiveness of the industry.

High voltage direct current (HVDC) based transmission of electric power, in comparison with high voltage alternating current (HVAC) transmission over a distance of over 800 km, has the advantages of higher transmission capability, lower line loss, more confined damage by fault, and that all AC lines connected to an HVDC system do not need to be synchronized. However, under DC conditions the electric field across the insulation space remains unidirectional and stable, which results in charge accumulation, leading to significant differences in the dielectric behavior and flashover characteristics of insulation materials in comparison with those under AC conditions. There exists limited amount of test data and research results on HVDC insulation behaviour. It is therefore necessary to carry out in-depth study of the discharge mechanisms and behavioral characteristics of the insulation materials required for HVDC transmission systems. The research in this thesis aims at the understanding of the charge transport and accumulation process inside and around insulators made of epoxy composite material. Firstly, the physical mechanisms are critically reviewed with appropriate experimental results selected for model verification. The model developed in the work encompasses all important mechanisms. Material properties obtained under different conditions are reviewed and values for the cases investigated in the present work determined. Charge transport is dominated by the drift of two types of oppositely charged particles in electric field with nonlinear generation source terms, giving rise to extreme difficulties for convergence of computation in strong electric field. As a result, special code is developed for a commercial software package (COMSOL based on finite element method) to implement the model and also the complicated boundary conditions. To gain confidence in the model and its implementation including the boundary conditions and material properties the model is verified in the work against experimental cases with different gas, geometry and applied voltage. An experimental study of the effect of surface charge accumulation on the potential distribution along the surface of an epoxy insulator under HVDC stress was carried out under non-ideal conditions.

An online Partial Discharge (PD) measurement performed on a High Voltage Direct Current (HVDC) wall bushing successfully identified the presence of internal discharges. The wall bushing is a sulfur hexafluoride gasinsulated bushing, rated for 500kVdc and terminated on a thyristor controlled HVDC converter bridge. The measurement of PD within the HVDC station environment is particularly challenging due to the high levels of electromagnetic noise caused by thyristor switching events, and external air-corona from the neighboring high-voltage equipment. An additional challenge is the “mixed“ voltage stress on the bushing insulation, which has both ac and dc high-voltage components. There are also fast transients during the firing of thyristors in the HVDC conversion process that cause added stress to the insulation. As a result, the analysis and interpretation of PD data for HVDC equipment is more complex; PD pulses may occur in response to the ac, dc, or switching transient voltage stresses. In this paper, an online PD measurement strategy for noise filtering and isolation of PD sources within the bushing are discussed. The PD measurement data is plotted on a phase-resolved diagram where the line cord voltage was used as a reference. The phase-resolved diagram appears to suggest that the fast transients caused during switching, trigger some PD events. The findings from the online PD measurements are verified with physical evidence, found after the bushing was removed from service.

As a critical part in High Voltage Direct Current (HVDC) transmission, wall bushing undertakes the task of electrical insulation and mechanical support for other equipment. However, the dirt on the surface of wall bushing may influence the electrical field distribution. In this paper, the calculation model of ±400kV wall bushing was established using the FEM (finite element method) software ANSYS. Effect of several different types of filth locating on the surface of the wall bushing was separately studied to get the diverse features, including that contamination was uniformly distributed on the surface of the wall bushing as well as the unevenly damp filth. Also, the intermediate shielding electrode was brought in to optimize the structure. The electric field was obtained in all circumstances above and the distribution feature was concluded.

UHV DC wall bushing is the only channel connecting the valve hall and the DC electric field in the converter station. For the epoxy - SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas composite insulation wall bushing, the main insulation material is the epoxy/crepe paper core and SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas. The transient process of establishing the DC electric field for the wall bushing is impacted by the temperature gradient. In this paper the 3-D temperature field of the wall bushing has been simulated considering the effect of heat conduction, convection and radiation. The simulation results have been confirmed using a temperature rise experiment. The transient process of the wall bushing under the actual operation condition has been simulated using the gas saturation current density method. The temperature impact on space charge density in the core, the surface charge on the interface, and the electric field distribution on the bushing is investigated. In this way a theoretical basis is developed for the optimal design of the bushing that is conductive to the safe operation of the bushing.

The charge accumulation phenomenon at the gas–solid interface under the co-action of DC electric field and temperature gradient is of great significance, but its long-time evolution characteristics are still unclear. In this work, the accumulation and dissipation characteristics of the surface charge on the epoxy-SF6 interface within 216 h were obtained, indicating that, (a) the normal surface electric field is increased by the temperature gradient along the gas–solid interface, which results in a prominent increase in the surface charge density; (b) there are different evolution processes of surface charge in high and low temperature regions under the co-action of DC-temperature gradient, and the increase of the charge trap density caused by the degradation of solid material is considered to be an important reason for the charge evolution; (c) the total charge dissipation ratio within 600 s decreases with the increase of charging time in DC electric field, and the surface charge dissipates mainly to the gas side of the interface; (d) the large amount of charge accumulated at the interface distorts the surface electric field at the triple junction points. The total electric field strength at the low-temperature tri-junction point increases by 35.5%, while the direction of the tangential field at the high-temperature tri-junction point reverses during the long-time co-action of the DC-temperature gradient. The results of this work may be helpful to understand the long-time charge evolution characteristics of the gas–solid interface under the DC-temperature gradient and to reveal the failure mechanism of the gas–solid interface.

In high-voltage direct current transmission systems, charges accumulate at the gas—solid interface, distorting the local field strength, causing a reduction in the flashover voltage, and threatening the safe and reliable operation of the power system. The latest research has found that doping metal nanoparticles into an epoxy resin effectively suppresses the surface charge accumulation on insulators and improves their flashover voltage. This paper further analyzes the microscopic mechanism of this phenomenon, establishes a single-electron tunneling mode, and draws two conclusions: when there is no agglomeration of the doped nanoparticles, a higher doping concentration can be achieved, which provides a better insulative performance. The optimal metal nanoparticle radius is several to tens of nanometers. This work provides theoretical guidance for the future improvement of insulating materials through metal nanoparticle doping and has good prospects in engineering applications.

DC wall bushing is one of the key equipment in high-voltage direct current (HVDC) transmission project. At present, post insulators based on epoxy composites are widely used for supporting insulation in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> -insulated apparatus. However, the flashover faults caused by surface charge accumulation can be a severe threat to the long-term operation of the post insulators under dc field. Therefore, the issue of charge accumulation on epoxy post insulators in HVDC wall bushing and its effect on flashover has been widely concerned. Aiming at this problem, the flashover characteristics of epoxy samples in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas under polarity reversal conditions with dc voltage prestressed for 10–600 min were measured in this article. The flashover process is analyzed through surface charge distribution measurement and electric field computation. The results show that with the increase of prestressed time, the accumulated negative charge density could reach <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-14.8\,\,\mu \text{C}/\text{m}^{{2}}$ </tex-math></inline-formula> , which led to a 12% increase in maximum field strength. As a result, the polarity reversal flashover voltage could decrease by 12% and the flashover time delay could decrease by 84.6%. This article can offer as a reference for the effect of surface charge accumulation on flashover in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas-insulated equipment.

Chapter 30 - HVDC transmission

The cable terminal composed of rubber and epoxy resin is a critical component connecting high voltage direct current (HVDC) cable and gas insulated substations (GIS). The interface is the most vulnerable part of cable terminal insulation because of the interface charge accumulation issues under DC voltage, which causes electric field distortion and even interface discharge or breakdown. In this paper, dynamic interface charge behaviors in HVDC cable terminal are simulated, and the interface charge characteristics during polarization and depolarization process under single polarity voltage and polarity reversal voltage are analyzed. Results indicate that the charges mainly accumulate at the solid-gas interface after polarization, which leads to a serious electric field concentration in SF6. The charge accumulation between the different solid dielectrics first increases and then decreases with time during polarization and depolarization process. The electric field intensity in the solid domain is strengthened obviously under the polarity reversal voltage. The interface charge simulation provides an accurate reference for DC cable terminal insulation design.

The insulation structure research and design optimization of &plusmn;1100kV DC SF6 gas insulated through wall bushing is an important basis and technical key for the core technology research and device development of &plusmn; 1000kV DC SF6 gas insulated through wall bushing. It is also an important basis for the formation of bushing structure design and process technology, which is of great significance to improve the electrical performance of bushing. The &plusmn;1100kV DC SF6 gas insulated through wall bushing monomer bears the full voltage and current of the system, and the distribution of internal insulation electric field is complex. At the same time, it is affected by many factors such as electric field shielding, insulator surface charge accumulation, material non-linearity and so on. The improvement of voltage level puts forward higher requirements for bushing structure type, material performance and electric field uniformity regulation. Based on the actual engineering and scientific needs, this topic carries out the insulation structure research and design optimization of &plusmn;1100kV DC SF6 gas insulated through wall bushing. The design and selection of the bushing insulation structure, high current carrying structure and mechanical support structure based on multi physical field analysis. Through electrical, thermal and mechanical analysis, design and select reasonable structural type for casing under the action of multiple factors. Design optimization of typical insulation structure of bushing. The electric field simulation and intelligent algorithm are used to design the configuration parameters of shielding electrode and epoxy cast insulator, optimize the electric field intensity on the electrode surface, inhibit the charge accumulation on the insulator surface and improve the flash-over voltage of insulator.

High voltage direct current (HVDC) cable accessory is the key equipment of the HVDC transmission system. HVDC cable accessory insulation suffers from charge accumulation caused by the non-uniform electric field distribution. Temperature intensifies charge injection and accumulation. This paper prepares silicone rubber (SiR) composites with nonlinear conductivity and studies the space charge transmission under thermal-electric field. The nonlinear conductivity of SiC/SiR composites is obtained at different temperatures. It is found that as the SiC content increases, the conductivity-temperature coefficient changes from positive to negative. This is mainly because the lattice vibration of SiC. The space charge properties of SiC/SiR composites under DC field at different temperatures are obtained. It is found that space charge accumulation is suppressed in SiC/SiR composites. The effect of charge accumulation suppression and fast dissipation increases with the SiC content. Besides, high temperature can also increase the conductivity and further suppress the space charge accumulation. Moreover, the conductivity-temperature coefficient of SiC/SiR composites is smaller than that of SiR sample, which leads to a smaller decrease in space charge accumulation at high temperature.

Surface flashover on an insulator caused by the charge accumulation at the gas–solid interface is one of the main reasons limiting the development of gas insulated equipment in high voltage direct current (HVDC) condition. Therefore, it is helpful to clarify the mechanism of surface flashover and surface charge behavior by measuring charge distribution with high accuracy. In this article, a surface charge inversion algorithm based on constrained least square filter (CLSF) is proposed for surface charge measurement in a shift-invariant system. By taking the image restoration technology as an analogy, the algorithm transfers matrix equation into frequency domain by 2-D Fourier transform, which significantly simplifies the computational process. Then the CLSF is used to suppress the noise signal and actualize the conduction from surface potential distribution to surface charge density distribution. The calculation process and accuracy of the algorithm are discussed in detail with the simulated examples, and the inversion performance of CLSF is compared with Wiener filter. At last, the effectiveness of the algorithm is verified by performing surface charge measurement and dust figure. This methodology gives an innovative idea of potential-charge transformation process with higher accuracy and stability, which benefits the development of surface charge measurement technology.

UHVDC wall bushing is one of the key equipments of UHV DC power transmission. The ±800kV SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas-insulated DC wall bushing has been applied in many China's UHVDC projects. However, ±800kV SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas-insulated DC wall bushings exposed some problems during operation. In some bushings, flashover discharges occur on the legs of the three-support insulators used to support the conductive rods, and there is evidence of overheating in the butt joint part. Besides, a large amount of aluminum fluoride powder is accumulated on the surface of the conductive rods. In this paper, multiphysics simulation calculations are performed for two typical design structures of ±800kV SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas insulated wall bushings. The comparison of the electric field strength under four working conditions and stress distribution under the conditions of self weight, wind load, and icing load is given. The results show that the point of maximum electric field strength occurs at the maximum curvature of the shield of the epoxy insulator. The maximum field strength point of the shield cover surface at the flange appears at the R angle. When the structure is changed from three-support to double-support, the maximum field strength of the insulator shield is increased, the maximum surface field strength of the inserts is reduced. The maximum stress of the bushing occurs at the joint between the through-wall cylinder and the hollow composite insulator, and the maximum displacement occurs at the small shield ring at the outdoor end. This paper provides guidance for the structural optimization and operation and maintenance strategies of bushing.