Epoxy Spacer Research Articles

Surface Flashover Characteristics of the Interface Between PPLP and Epoxy According to Tensile Stress Under Cryogenic Environments

Stop joint boxes (SJB) serve to maintain constant cooling efficiencies by separating the cooling sections on superconducting DC cable systems. SJBs are composed of polypropylene laminated paper (PPLP) and an epoxy spacer. PPLP is wound against a superconductor and epoxy spacer to improve surface flashover performance. The interface between the insulating paper and epoxy is vulnerable to surface flashover due to the extreme distortion of electric fields at the interface. In addition, PPLP is subject to constant tensile stress that affects the dielectric characteristics of insulating paper. The breakdown characteristics of insulating paper with diverse tensile stress in cryogenic environments has been previously reported. However, research with regard to the surface flashover characteristics of composite insulation structures under cryogenic environments depending on tensile stress has not yet been reported. Therefore, in this paper the DC surface flashover characteristics of the interface between PPLP and epoxy was investigated in liquid nitrogen in accordance with the tensile stress of PPLP. TM systems which are able to convert rotational motion into linear motion by torque were devised for changing and subjecting the tensile stress to PPLP. From the experiments, it was confirmed that the DC surface flashover strength of PPLP decreased over 40% due to tensile stress. In the case of the interface, tensile stress had no effect on the DC surface flashover strength. Consequently, tensile stress was an important design factor in a single insulation structure with PPLP but was not essentially considered in the composite insulation structure with PPLP and epoxy.

Temperature Dependence of Surface Charge Accumulation on DC-GIS Insulating Spacer

Surface charge accumulation on an insulating spacer is a critical issue that causes electric field distortion and flashover voltage reduction in DC-GIS. In the operating environment, high-voltage electrode is heated by Joule heat, and the surface charge accumulation is strongly affected by the temperature. This study reports the temperature dependence of surface charge accumulation on DC-GIS insulating spacer and its impact on electric field distortion. Surface charge density distribution is measured on a downsized epoxy spacer under isothermal conditions at room temperature, 40 °C, and 70 °C. Special care is taken to realize a low-moisture and low-dust environment. We obtain the saturation tendency of homo-charge accumulation with less random charges and its temperature dependence. Homo-charge accumulation develops more rapidly and more extensively on the spacer at the higher temperatures. Our results indicate that the surface charge accumulation cannot be explained by a conventional resistive field calculation, and that an explanation based on charge trapping is reasonable. They also show that the homo-charge accumulation significantly contributes to the electric field enhancement after the polarity reversal in our experimental system, especially at high temperatures.

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.

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.

Electric Field Optimization of 1MV DC-GIL Spacer With Surface Conductivity Gradient for ITER NBI

The flashover along epoxy spacer installed in 1 MV DC GIL caused by the excessive concentration of electric field threatens the safe operation of the ITER NBI (neutral beam injector). A spacer with surface conductivity gradient has been proposed to optimize the electric field distribution. In this paper, a simplified model of transmission pipeline with spacer is established. It is found that the electric field is mainly concentrated on the high voltage triple junction and the turning position of spacer. The surface conductivity gradient distribution on the spacer is designed by an iterative method. Results indicate that surface conductivity gradient can weaken the electric field distortion on both sides of spacer, especially the electric field strength of turning position of the concave surface is decreased obviously compared with that of the conventional spacer.

Electric Field Stress in a Three Phase Common Enclosure Gas Insulated Busduct with FGM Post Type Spacer and Reduction with Metal Inserts

The establishment of supporting insulators plays a vital role in the continuous operation of the three-phase Gas Insulated Busduct (GIB), which primarily depends on the distribution of electric field stress on the spacer surface. Shape control is a technique used along the spacer surface to achieve uniform field intensity, but this technique can also lead to very irregular shapes. The Triple Junction (TJ) is the region where the field stress has to be reduced. Over a period, high field stresses along the spacer surface will result in a surface flashover and thus de-energize the complete GIB, leading to enormous losses economically. In this paper, a Functionally Graded Material (FGM) spacer for a three-phase GIB has been designed with more number of gradings and further field stress at TJ is minimized by inserting and reshaping the metal inserts (MI) at the end of the enclosure. By doping them with different permittivity values, functionally graded materials are spatially distributed with multiple filler materials to achieve uniform stress on the electric field. Simulation is carried out with the FGM and epoxy (post type) Spacers for different voltages and FGM gradings. With MI, the stress is minimized and an enhanced uniform field allocation along the surface is obtained.

Comparative study on direct fluorination and surface properties of alumina-filled and unfilled epoxy insulators

In view of the significant performance improvement of unfilled epoxy insulators by direct fluorination, and the fact that all spacers in SF 6 gas insulated systems are alumina-filled epoxy composites, to promote the application of the method for improving the performance of epoxy spacers, in this paper, alumina-filled epoxy (composite) and unfilled epoxy (pure) samples were prepared with the same formulation and process as those for epoxy spacers. The pure and composite epoxy samples were fluorinated under the same conditions, and comparative studies were performed on their surface physicochemical characteristics and properties. IR analysis and SEM imaging show no obvious influence of alumina particles on the fluorination of the epoxy matrix under the used fluorination conditions, while many nanoparticles and agglomerates appear on the surface and large microsized particles in the SEM imaging depth become less visible after the composite epoxy sample was fluorinated. Contact angle and electrical property measurements indicate that like the pure epoxy sample, surface hydrophilicity and surface conduction of the composite epoxy sample are also increased by the fluorination, while the increase in surface hydrophilicity or surface conduction is more significant or less significant, compared to the pure epoxy sample. All these are associated with the fluorination of the alumina particles and the interface change or reconstruction between the particles and the matrix.

Effects of metal particle material on surface flashover performance of alumina-filled epoxy resin spacers in SF6/N2 mixtures under DC voltage

When metal particle is attached on spacer surface, it will distort surface electric field and may reduce surface insulation level. Therefore, it is important to research the relationship between metal particle attachment and surface flashover property. In this study, the effect of metal particle materials on surface flashover performance of alumina-filled epoxy resin spacers in SF6/N2 mixtures is investigated. Three kinds of metal particle materials including copper, iron and aluminum are considered. The results reveal that, at low gas pressure or low surface electric field, the material of metal particle has no obvious effect on flashover voltage. At high gas pressure and high surface electric field, the flashover voltage is almost the same when the iron or aluminum particle is attached, which is lower than that with copper particle attached. It is revealed that at high pressure and high electric field, metal powders and vapor of micron size can be generated from the attached metal particle surface by electric field forces and Joule Heating Effect, which may be responsible for the difference in flashover voltage for different metal particle materials.

Surface functional graded spacer for compact HVDC gaseous insulated system

Electrical field distortion induced flashover is a bottleneck for the stable operation of HVDC gas insulated system (GIS). Functional graded materials (FGM) have attempted to homogenize the electrical field distribution along the gas-solid interface; however, the complicated fabrication process and huge dielectric loss makes it difficult into industrial application. This paper proposed a novel concept as that a spacer with functional surface was fabricated by magnetron sputtering ZnO onto the epoxy resin forming a surface conductivity gradient, namely surface functional graded materials (SFGM). The feasibility was first demonstrated by simulating the electrical field distribution. Two kinds of disc-type spacers with SFGM were designed as linear and inverse proportional annular gradients. Results show that the more uniform electrical field distribution contributes to the improved surface flashover voltage of the SFGM spacer. Therefore, the SFGM epoxy spacer has great potential in HVDC GIS insulation.

Effects of surface roughness on surface charge accumulation characteristics and surface flashover performance of alumina-filled epoxy resin spacers

The accumulated charges on the insulator surface become a key factor to incur surface flashover. The charge accumulation process is closely related to the surface condition. This research investigates the effects of surface roughness on surface charge accumulation behavior and surface flashover performance of alumina-filled epoxy resin spacers in SF6/N2 mixtures under DC voltage. The insulator is prepared with only half surface subjected to rough treatment. The accumulated charges are distributed in two regions. Considerable homo-charges are located near the high voltage (HV) electrode and few hetero-charges are near the grounded (GND) electrode (region I). These charges are injected by electrodes. The bipolar charges between HV and GND electrodes (region II) originate from gas ionization. Surface rough treatment can suppress surface charge accumulation and improve surface insulation strength. When increasing surface roughness, surface charge declines first slowly (stage I) and then rapidly (stage II), which coincides with the two stages of surface flashover voltage increment. Surface charge declination at stage I is mainly due to the increase in surface conductivity, while at stage II, the introduced deep traps also play a role. Surface flashover voltage increment at stage I is mainly due to the extended creepage distance, while at stage II, the declination of surface charge also plays a role. Besides, the block of electron avalanche development by the roughed surface is also responsible for the increase in surface flashover voltage at these two stages.

Electrical field distribution along SG6/N2 filled DC-GIS/GIL epoxy spacer

The flashover faults due to the accumulation of surface charge have restricted the further development of spacers in HVDC GIS/GIL transmission systems. Many researchers have proposed a variety of mathematical models to simulate the surface charge accumulation process; however, few of them have taken into consideration the gas collision ionization and charge trapping-detrapping processes. This paper combines plasma hydrodynamics and charge transport equations to build a modified model. For the basin-type spacer, the model shows that the surface charge has the same polarity as the applied voltage on the lower surface but the opposite polarity on the upper surface of the spacer. A decreased charge density and even a polarity reversal is seen near the conductor. For the disc-type spacer, the surface charge has the same polarity as the applied voltage near the shell but opposite polarity near the conductor under negative voltage. But, under positive voltage, negative charge exists almost on the whole surface. The most serious distortion of the electric field occurs at the triple junction of the epoxy spacer. Under load condition, joule heating of the conductor increases the temperature which has a great influence on the electric field distribution. As the conductor temperature increases, the field strength near the conductor is reduced owing to the temperature-dependent volume conductivity of the epoxy spacer. The application of shielding electrodes grades the electric field at the triple junction, and is referred to as the DC GIS/GIL design.

Charge transport and accumulation around a spancer insulator for application in HVDC wall bushing

A charge transport model based on ion drift in gases of strong electron affinity has been studied in details and validated against experimental results in air and SF 6 at different pressure and voltage levels. The size of the insulators also differs significantly. The choice of the model parameters, i.e. the solid and gas material properties, is carefully examined with information from existing literature. Results show that using the selected parameters, satisfactory agreement between predicted and measured potential distribution and surface charge density along the insulator surface can be obtained. Computational results for an 1100 kV epoxy spacer for use in HVDC wall bushing show that the concept of electrical conductivity as a material property is no longer valid in strong electric field for the gas surrounding the insulator. The polarity and density of the accumulated surface charge depend on the relative largeness of the electrical conductivity of the insulator material and the effective conductivity of the gas. The nonlinear effect due to ion generation and recombination becomes significant at very high voltage. The metallic cap providing shielding to the triple junction at the high voltage end of the insulator can induce much stronger local electric field near the insulator surface, by a factor of 1.48, if its shape is not optimized against the insulator geometry.

Simulation and experimental study on mechanical performance of UHV GIS spacer in hydrostatic test

Gas insulated switchgear (GIS) spacer is required to be of good electrical and mechanical properties to ensure high safety and reliability. The requirements are stricter for ultra high voltage (UHV) GIS spacer under higher voltage and pressure. In previous studies, the insulation characteristics of GIS epoxy spacer were investigated. But researches on the mechanical properties of the GIS spacer were rarely reported. In this study, UHV GIS spacer in hydrostatic test is simulated with the finite-element method. Strain and stress distributions along spacer surface are investigated. Results indicate that the maximum stress appears on the spacer surface near flanging ring, where cracks usually occur. Based on the calculation results, fiber Bragg grating (FBG) sensors are placed on UHV GIS spacer surface to monitor the strain variation in hydrostatic test. Measured results present a strain distribution similar as calculation results. The calculated failure pressure is consistent with those of tested spacers. The crack initiates in the proximity of sealing groove as predicted in calculation.

Shape Optimization of the Spacer of 220 kV DC GIL

Both experimental and numerical studies demonstrate that electric field formation mechanisms and insulation behavior at SF6/epoxy interface are substantially different for ac and dc voltage excitations. In this paper, based on fixed dc GIL dimensions and spacer-electrode joints, three types of alumina filled epoxy resin spacers with different shapes were designed with FEA software in order to find a way to provide a reliable spacer performance in HVDC gas insulated systems. The generalized design criteria for dc spacers were put forward. Results show that shape design of spacer in dc GIL is distinct from that in ac GIL. Moreover, distinctions between dc spacer and ac one in shape design are proposed, which could be beneficial for practical use of dc GIL.

Resistance of surface fluorinated epoxy resin to corona discharge in SF<inf>6</inf> gas

Our previous results have shown that direct fluorination can modulate surface electrical properties and suppress surface charge accumulation of epoxy insulators, and thus has potential applications for improving the flashover performance of epoxy spacers in gas-insulated equipment. In this study, to investigate the resistance of the fluorinated epoxy surface layer to partial discharge in SF 6 gas, surface fluorinated epoxy samples together unfluorinated (virgin) ones were corona treated using a multi-needle-to-plate electrode system. ATR-IR analyses and SEM surface and cross section images show that the corona discharge caused the chemical modification and chain scission at some weak points in the fluorinated layer and the crosslinking reaction, and thus led to the formation of nodules on the corona treated fluorinated surface. However, the corona treatment did not change the fluorinated layer thickness. In contrast with this, severe degradation, most likely in the form of etching, occurred for the virgin surface layer during the same treatment. Measurements of surface potential decay and water contact angle indicate that the corona discharge obviously reduced surface conduction of the fluorinated sample, and rendered the surface conduction insensitive to humidity although the corona treatment almost did not change surface wettability of the fluorinated sample. On the contrary, the measurements show that surface conduction of the virgin sample increased and became sensitive to humidity after the corona treatment.

Insulation Design of a Stop Joint Box of 80-kV DC HTS Cables Based on DC Electric Field Analysis

Among the accessories of the dc high-temperature superconducting (HTS) power cable, a stop joint box (SJB) is an essential component for the long-distance transmission cable connections and the separation of the cooling section. The main insulating material of the SJB is a polypropylene laminated paper (PPLP), which is already generally used for dc HTS power cable. In order to separate the cooling section, an epoxy spacer is located at the center of the SJB. Due to the weakness of surface breakdown characteristics compared with that of punctual breakdown and different nature of dc electric field distribution determined by the ratio of conductivities of dielectric materials, the epoxy spacer configuration of the SJB should be carefully designed based on dc electric field analysis. In this paper, the optimum configuration of the epoxy spacer of the SJB was investigated based on dc electric field analysis. On the basis of the initial SJB model, which is already designed for the joint box of the conventional ac oil-filled power cable, a variety of improved SJB models with different epoxy spacer configurations were considered. In dc electric field simulations, the main objective was to reduce the tangential dc electric field intensity at the interface between PPLP and epoxy spacer. To find the optimum epoxy spacer configuration, a parametric sweep technique was applied, and an appropriate SJB model could be identified. However, its tangential dc electric field intensity remained still higher than that of the design criteria for the dc electric field from the experimental results. Therefore, as a novel method, a thin Kraft layer between the PPLP and the epoxy spacer was added to reduce electric field intensification. The simulation result of the newly designed insulation structure revealed the possibility of lowering the tangential electric field. Furthermore, different electrical conductivity values were applied to the additional layer instead of the original Kraft value in order to meet the critical electric field intensity. Finally, 1E-11 S/m, among those values, showed the best simulation result, which was lower than the design criteria for the dc electric field. To verify the effect of the additional layer for dc electric field mitigation, miniature SJB specimens were fabricated, and dc breakdown tests were performed using a step-by-step test method. Based on the experimental results, SJB specimens with an additional layer showed the highest breakdown strength compared with that of SJB specimens without an additional layer. Thus, the optimum configuration of SJB determined by dc electric field analysis was approved by experimental works. Consequently, these insulation design skills based on dc electric field analysis could be usefully adopted in developing the SJB of 80-kV dc HTS power cables.

Insulation characteristics of GIS insulators under lightning impulse with DC voltage superimposed

To study the dc insulation design of gas insulated switchgear (GIS), the insulation characteristics under lightning impulse (LI) voltage with a superimposed dc voltage (superimposed voltage) must be clarified. The paper experimentally examined the GIS breakdown characteristics under this superimposed voltage. The test models simulating an insulator creepage surface were used for which consideration of the influence of dc voltage among various other GIS insulating elements is particularly important. To be specific, a cylindrical model made of epoxy resin or fiber-reinforced plastic (FRP) as the material and a conical epoxy spacer model were tested. For the cylindrical model, a cap-shaped electrode was placed on the insulator and a small gap was established between the end of the electrode and the insulator. When the dc breakdown voltages were measured using these samples, they were higher for the applied voltage of positive polarity than that of negative polarity for all samples. The post-test observation of the electrification condition revealed greater electrification on the insulator surface for the applied voltage of positive polarity. The electrification charges are considered to have relaxed the electric field and increased the breakdown voltage. Subsequently, the breakdown test was conducted using a superimposed voltage, whereby a foregoing dc voltage was applied to samples for a certain period, whereupon a LI voltage was applied with the dc voltage continually applied. The breakdown voltage when the LI voltage and dc voltage had equivalent polarity was approximately same to the LI alone breakdown voltage. Conversely, when they were opposite in polarity, the breakdown voltage under the superimposed voltage obviously tended to decrease from the LI alone breakdown voltage. It is considered attributable to the fact that the insulator surface was electrified by the foregoing dc voltage and applying LI voltage opposite in polarity to this electrification intensified the electric field where the breakdown started to occur. Accordingly, it emerged that the GIS breakdown characteristics changed significantly depending on the polarity combinations of LI and dc voltages. The influence of these polarities must be taken into consideration when studying the GIS dc insulation design.

Surface Breakdown Characteristics of Composite Insulating Papers in LN<sub>2</sub> for a Stop Joint Box of DC HTS Power Cable

One of the key components for dc HTS cable for long-distance line is a stop joint box (SJB). Up to now, the SJB has not been commercialized yet due to some difficulties of dc electrical insulation matters in LN 2 . Basically, the structure of SJB would be similar to the SJB of oil-filled cable because they adopt liquid insulation and sectionalized cooling separations. The insulation structure of conventional SJB based on ac electric field was no more suitable for dc; hence, there is a need to design suitable insulation structure, which can be applied for dc. In this work, in order to find a solution to reduce dc electric field concentration on the polypropylene laminated paper (PPLP) above epoxy spacer, double layer structure composed of PPLP and Kraft were suggested. To verify the effectiveness of this structure for relieving dc electric field, simulation and experiments were performed. From analytical works, the Kraft layer between PPLP and epoxy has shown noticeable electric field mitigation. Furthermore, surface breakdown tests considering PPLP/Kraft combined layers were conducted to verify the insulation properties of double layer structure. Finally, it was deduced that Kraft layer was effective to enhance surface breakdown voltage. Surface breakdown characteristics of PPLP and Kraft were relevant to the surface roughness of insulating materials.

The masquelet induced membrane technique with BMP and a synthetic scaffold can heal a rat femoral critical size defect.

Long bone defects can be managed by the induced membrane technique together with autologous bone graft. However, graft harvest is associated with donor site morbidity. This study investigates if a tricalcium phosphate hydroxyapatite scaffold can be used alone or in combination with bone active drugs to improve healing. Sprague Dawley rats (n = 40) were randomized into four groups. (A) scaffold, (B) BMP-7, (C) BMP-7 + scaffold, and (D) BMP-7 + scaffold + systemic bisphosphonate at 2 weeks. Locked femoral nailing was followed by 6 mm segment removal and implantation of an epoxy spacer. At 4 weeks, the spacers were removed and the defects grafted. Eleven weeks later, the bones were explanted for evaluation with radiography, manual assessment, micro-CT, histology, and Fourier Transform Infrared spectroscopy (FTIR). Isolated scaffolds (A) did not heal any defects, whereas the other treatments led to healing in 7/10 (B), 10/10 (C), and 9/10 (D) rats. Group D had greater volume of highly mineralized bone (p < 0.01) and higher bone volume fraction (p < 0.01) compared to all other groups. A synthetic scaffold + BMP-7 combined with a bisphosphonate improved the callus properties in a rat femoral critical size defect, compared to both BMP-7 and scaffold alone or the two combined.

Insulation characteristics of GIS epoxy insulators with non-uniform surface resistance under DC voltage

Now that gas-insulated switchgear (GIS) for ac systems are becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. For GIS insulating epoxy spacer, the present paper experimentally and analytically studied the influence of spacer surface condition on dc withstand voltage performance. In specific terms, initially, an insulating spacer model with an area of non-uniform resistivity present in the surface layer was created and the dc voltage breakdown characteristics were obtained. As a result, it was clarified that the time delay to breakdown increased with lowering the applied voltage, or in other words, even if a low voltage was applied, breakdown may occur after an extended period. Subsequently, using the same non-uniform resistivity model, the change in the electric field distribution over time under dc voltage was investigated through transient electric field analysis. Consequently, it was found that the electric field distribution varied from a capacitive to a resistive field and the maximum electric field was generated in the boundary between the high- and low-resistivity areas. It was further clarified, based on these breakdown characteristics and electric field analytical results, that the presence of an area of nonuniform resistivity on the insulating spacer creepage surface caused electric field concentration over time and breakdown occurred if the electric field in the creepage surface direction exceeded 40 kV/mm or so. Moreover, the breakdown characteristics were investigated where a lightning impulse voltage was superimposed over a dc voltage. Consequently, the creepage breakdown electric field where a dc voltage was applied alone was almost identical to that where the superimposed voltage was applied. It means that the application of the superimposed voltage may potentially allow the insulating spacer withstand voltage performance to be evaluated by a shorter time test for a dc voltage alone or the dc voltage with a superimposed VFTO.