Epoxy Resin-impregnated Paper Research Articles

Insulation Degradation of Epoxy Resin Impregnated Paper Bushing Under Discharge Arc Ablation

Insulation Degradation of Epoxy Resin Impregnated Paper Bushing Under Discharge Arc Ablation

A Promising De-Ionized Water Cooling Based ERIP Bushing-II: Cooling Mechanism and Effect of Parameters

Overheating has become a key factor that threatens the safe operation of the epoxy resin impregnated paper (ERIP) HVDC bushing and restricts its engineering application. The de-ionized water cooling based ERIP (DIWC-ERIP) bushing utilizes diffusion and convective heat transport to achieve low temperature distribution in the DIWC-ERIP bushing. In this manuscript, the temperature, heat flux density and the heat flux distribution of the DIWC-ERIP bushing are analyzed and the cooling mechanism is discussed. The heat flux density in the current carrying conductor (CCC) is mainly in axial direction. In the ERIP it is mainly in negative radial direction in oil tank side, and in positive radial direction in flange area and air box side of the bushing. The effect of the flow rate and temperature at inlet of de-ionized water on the temperature distribution of the bushing is studied by experiments and simulations, and the simulation results are very close to the measured results. The parameters of the thermal resistance per unit length of the thermal thin layer and the radii of water tubes have little effect on the temperature distribution of the bushing. The temperature distribution of the bushing can be controlled by the temperature of water at the inlet. Due to the strong heat dissipation capacity of de-ionized water, as the flow rate is 2 L/min, the maximum temperature of the ERIP doesn't exceed 120 ℃.

A Promising De-Ionized Water Cooling Based ERIP Bushing-I: Model Validation and High Cooling Efficiency of the Cooling Method

Overheating has become a key factor that threatens the safe operation of the epoxy resin impregnated paper (ERIP) high voltage direct current bushing and restricts its engineering application in high current field. In this paper, a new method based on cooling of de-ionized water is proposed to resolve this problem. A prototype of the de-ionized water cooling based ERIP (DIWC-ERIP) bushing of rated voltage ±400 kV is designed and built. The temperature distribution on the surface of the current carrying conductor (CCC) is measured under RMS AC currents of 4500 A and 6736 A. The simulation model of doubly coupled electro-thermal-fluid field is built, and the temperature distribution is simulated on the surface of the CCC for the prototype of the DIWC-ERIP bushing. Comparing the simulation results of temperature distribution with the experimental results, the simulation model is verified. Compared with other four cooling schemes of the conventional unmodified ERIP scheme, the material based BN modified ERIP scheme, the double-conductor cooling scheme and the oil cooling scheme, the hotspot temperatures are decreased at least 43.4 °C, 11.5 °C, 46.3 °C and 58.4 °C under the current of 4500 A, respectively. The results show that the cooling effect of de-ionized water is more obvious and the new DIWC-ERIP bushing has better heat distribution characteristics. The proposed bushing may resolve overheating issues in the ERIP at valve side of converter transformer in high current application.

SF6 decomposition behaviour under partial discharge of typical insulation defects in gas‐solid insulated valve‐side bushing of converter transformer

AbstractIn this paper we investigated SF6 decomposition behaviour of typical insulation defects in gas‐solid insulated converter transformer valve‐side bushing under partial discharge (PD). Additionally, we compared the results with that of gas insulated switchgear (GIS). The results indicated that SF6 decomposition behaviour varies with the insulation defects type. For metal protrusion defects, the proportion ranking of CO2 and SO2 can be related to whether the needle tip is wrapped with epoxy resin‐impregnated paper (ERIP). The proportion of carbon‐containing products is much higher under contamination defect than under other defects. Moreover, the proportion of carbon‐containing components is higher under flashover condition than under PD condition and is accompanied by the appearance of H2S and CS2. However, the yield of SO2F2 is always low. Compared to negative DC voltages, positive DC voltages have a higher proportion of carbon‐containing components, mainly in the form of more CO2 and less SOF2. Compared to the PD decomposition behaviour of typical insulation defects in GIS, when PD involves ERIP, the proportion of carbon‐containing products, especially CO2, is significant higher, while the proportion of sulphur containing products, especially SO2F2, is lower.

Effect of Aging Time on the Growth Characteristics of Electrical Treeing in Epoxy Resin-Impregnated Paper

The long-term operation of dry-type transformer bushings at high temperatures can impact the performance of their primary insulation material. This article presents an experimental study of the effect of thermal aging time on electrical treeing in the epoxy resin-impregnated paper. The experimental results show that the electrical treeing pattern along the middle path of the electric field and the interlayer creepage along the average direction of the electric field are closely related to the aging time. Under the effect of thermal aging at 150 °C, the tree-starting voltage of epoxy resin-impregnated paper tends to increase and decrease as the aging time increases. An analysis and discussion of the test results in conjunction with scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) tests suggest that the destruction of the cross-linked network of the epoxy resin-impregnated paper at 150 °C is a fundamental reason for the significant decrease in the material’s tree-starting voltage and the change in electrical treeing morphology and interlayer creepage. Long-term operation at high temperatures can seriously impact the primary insulation performance of dry-type transformer bushings and requires attention.

Degradation characteristics of insulation near aluminium foil edges inside dry‐type bushing cores under electrothermal compound stress

Insulation breakdown of dry-type valve-side bushing cores under electrothermal compound stress has become a key factor limiting the safe and stable operation of the converter transformer. Based on slices of the real bushing core, it is observed for the first time that fold, peeling off, and fracture defects exist near the aluminium (Al) foil edge, accompanied by many metal spikes with a curvature radius <4 μm. Coupled with a capacitor screen thickness of ∼2 mm, an electrical tree model is designed to simulate insulation defects near aluminium foil edges inside dry-type bushing cores, whose insulation degradation characteristics under electrothermal compound stress are investigated accordingly. The results show that high temperature significantly increases the dielectric loss heat energy density and Joule heat energy density, accelerating the insulation degradation near Al foil edges. The parallel crepe paper interface significantly accelerates the growth of electrical trees under AC-superimposed positive DC voltage, while this effect is not significant under AC voltage. As the crepe paper consists of wooden fibres of 10 μm diameter, the electrical trees tend to grow along interfaces between fibres and epoxy resin to form “feathery” electrical trees in epoxy resin-impregnated paper samples.

Space Charge Characteristics in Epoxy Resin Impregnated Paper under AC and AC-DC Superimposed Electric Fields

In this paper, charge distributions in three-layered epoxy resin and epoxy resin impregnated paper samples under AC and AC-DC superimposed electric fields are measured by the improved high-speed pulsed electro-acoustic system. To exclude the interferences caused by the discontinuity of relative permittivity, a method to distinguish between net dipolar charges and free charges is proposed. Through analysis, the influences of frequency, AC component and DC component on the charge transport and trapping characteristics in multi-layered epoxy composites are obtained, which could provide important reference to the structural design and optimization of converter transformer bushings.

The Development of Electrical Tree Discharge in Epoxy Resin Impregnated Paper Insulation

An electrical tree discharge model of multilayer insulation paper was prepared, and a voltage of 20 kV was used for electrical tree cultivation and initiation tests. Based on the microscopic image information and partial discharge (PD) information during the aging of the electrical tree, the development of electrical tree discharges between layers of paper comprising epoxy resin-impregnated paper insulation was studied. The results show that, by analysing PD information such as the discharge volume phase spectrum and the discharge repetition rate phase spectrum, the development of the electrical tree branch progresses in four stages: initial, growth, stagnation, and bursting. During the growth and stagnation stages, the main discharge paths of the electrical tree are mainly formed, and there are many filamentous fibres on the path edge. During the burst stage, more branches will be formed on the electrical tree, which will cause the greatest damage to the insulation. With the extension of the electrical tree path, the discharge magnitude and discharge times also increase. There are specific discharge patterns in different growth stages of the electrical tree, and the number of discharge events in each positive half-cycle is significantly greater than that in a negative half-cycle. Combining the microscopic image information and PD information during the aging of such an electrical tree, a theoretical basis for evaluating insulation state by PD monitoring is realised. The long-chain molecules of cellulose and epoxy resin break bonds and oxidise, which decompose into a large number of free radicals, monosaccharides, and other low-molecular-weight products.

The Influence of Temperature and Aging on the Characteristic Parameters of Dielectric Spectroscopy of Epoxy Resin Impregnated Paper Insulation

This study aimed to investigate the effects of temperature and aging on the characteristic parameters of dielectric spectra of epoxy resin impregnated paper insulation. A broadband dielectric spectra test platform was established and an equivalent physical model of the valve side dry-type bushing of the converter transformer was developed. Different degrees of thermal aging were performed at 130 °C for 0, 4, 8, 16, and 32 days. In addition, variation of the complex permittivity and dielectric loss factors of epoxy resin impregnated paper at different temperatures and different degrees of aging was revealed. The dielectric spectra test results were fitted using the Havriliak-Negami (HN) relaxation model. The characteristic parameters were extracted, and the influences of temperature and aging on the characteristic parameters of the HN model were analyzed. The results show that in the low frequency range, temperature and aging exert greater influences on the complex permittivity. In the high frequency range, temperature and aging have smaller effect. The relaxation time and temperature satisfy the relationship of the Arrhenius equation, and the relaxation strength increases exponentially with increasing temperature. The relaxation time τ, the relaxation intensity Δe, the shape parameter α, and the shape parameter β can characterize the aging degree of paper insulation. A linear relationship between relaxation and aging times, and an exponential relationship between relaxation intensity and aging time were revealed.