The dielectric strength of cellulose–liquid composites is always about several times higher than that of the cellulose paper and insulating liquids. However, this experimental phenomenon has not ye...

Substantial Improvement of the Dielectric Strength of Cellulose-Liquid Composites: Effects of Traps at the Nanoscale Interface.

The dielectric properties of oil-paper insulation degrade due to moisture ingression and ageing. This degradation significantly impacts the space charge accumulation and charge trapping behaviour in the insulation, which are vital parameters for insulation health under the high-voltage direct current environment. In this work, an improved model based on the isothermal relaxation current (IRC) has been developed to study the charge trapping behaviour of an oil pressboard under the influence of moisture and ageing. The conventional IRC model considers the total relaxation current because of charge de-trapping only. However, in the case of a composite dielectric like oil pressboard, dipolar relaxation also affects the relaxation current. In this work, a methodology has been proposed to investigate charge de-trapping behaviour of oil-pressboard insulation considering the dipole relaxation process from IRC measurements. For this purpose, frequency domain spectroscopy measurements and IRC measurements have been performed on oil-impregnated pressboard specimens carefully prepared in the laboratory having different ageing conditions and moisture contents. Results presented in this work depict that charge trapping parameters, i.e. the trap depth and trapped charge distribution are highly affected both by ageing and moisture. It was observed that ageing leads to the generation of deeper traps, while moisture mainly enhances the density of shallow traps.

Studies on the effects of moisture and ageing on charge de-trapping properties of oil-impregnated pressboard based on IRC measurement

The life of transformer oil-paper insulation is determined by the insulation property of transformer cellulosic insulation. Existing studies have shown that sulfur corrosion poses a serious threat to the properties of transformer oil-immersed cellulosic insulation due to the reaction of corrosive sulfur dissolved in oil with copper winding. This paper investigated the effect of sulfur corrosion on the characteristics of transformer cellulosic insulation, cellulosic insulation winding samples with different degrees of sulfur corrosion were obtained through an accelerated thermal aging experiment. Then, short-/long-term withstand voltage tests for the cellulosic insulation winding were conducted. Experimental results showed that the contamination of sulfur corrosion had a minimal effect on the power–frequency breakdown voltage of the cellulosic insulation winding but considerably decreased its long-term electrical life. Combined with the analysis of the electrical/physical and chemical parameters of oil-immersed cellulosic insulation, the failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion was synthetically discussed and analyzed.

Failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion

Investigating the relationship between tack and degree of conversion in DGEBA-based epoxy resin cured with dicyandiamide and diuron

PMIA (Poly-m-phenylene isophthalamide) is an ideal substitute for traditional cellulose paper in oil-immersed paper insulating scenarios. This paper studies the partial discharge characteristics of oil-immersed laminated PMIA. According to the $\mathrm{q}{-}\varphi$ scatter diagram, discharge quantity and discharge phenomenon, the partial discharge process of PMIA is divided into 3 stages: creeping discharge, co-existence discharge and pre-breakdown. The micro-structure and chemical components of damaged PMIA are presented. The carbon trace, functional groups, carrier trap characteristics and residual insulation strength in different stages of partial discharge are studied. This paper provides a theoretical basis for the application of PMIA in electrical insulation.

Experimental Study on Damage Mechanism of Partial Discharge of Oil-Immersed Laminated PMIA Paper

To study the ageing characteristics of epoxy resin composite insulation in dry-type transformers in high-temperature environments, glass fibre-reinforced epoxy resin samples were placed in a hot ai...

Thermal-ageing characteristics of dry-type transformer epoxy composite insulation:

The insulation of oil-immersed paper bushing in a transformer bushing is mainly made of multi-layer paper insulation. This work investigates the whole process from discharge initiation to breakdown of an oil-paper insulation model. On the basis of establishing a laminated insulation test sample of a capacitive bushing immersed in oil paper as found in an analogue transformer, a constant voltage method was used to pressure-test to breakdown of the insulation paper. After the analysis of carbon marks on the surface, internal fibre structure, surface functional groups, and surface element content, the results show that: during each stage of partial discharge, the area of carbon marking on the surface of the insulation paper decreases gradually from high the voltage electrode to the ground electrode on a layer-by-layer basis, the extent of damage to the fibre surface and molecular chain is reduced, and the tendency to carbonisation is weakened. With increasing partial discharge time, the area of the carbon mark on the surface of the insulation paper increased layer-by-layer, the damage to the fibre surface and molecular chain increased, and the tendency to carbonisation increased. These results provide a guidance to those interested in the partial discharge mechanism of multi-layer insulation paper.

Partial discharge damage mechanisms in laminated oil-paper insulation

To analyze the changes in microscopic morphology of the pressboard under surface partial discharge conditions, some methods such as enhancement, denoising, segmentation, optimization, edge detection, expansion, and corrosion were used to process scanning electron microscope images of the pressboard, so as to extract the fiber width, hole size, and surface roughness. Experiments show that after different thermal aging times, the trends to change in the microscopic morphology of insulation pressboard under partial discharge are different; the microscopic morphology of the pressboard is changed by the breaking of the chemical bonds in the internal structure of the fiber. At a degree of polymerization of 500, the insulation life is only about half of its original value: at this time, the corresponding fiber width is 91.1% of the unaged pressboard fiber width, and the corresponding roughness is 0.162. At a degree of polymerization of 250, the insulation pressboard is nearing the end of its life: at this time, the corresponding fiber width is 79.2% of that of the unaged pressboard, and the corresponding roughness is 0.225.

The microscopic morphology of insulation pressboard: an image processing perspective

To investigate the effect of aging on surface discharge of the pressboard on the shielding electrode at the end of bushing, a thermal aging preparation methods of different aging degrees of pressboard samples, surface defect model, and the associated hemisphere are designed and the step-up method is used for subsequent experimental work. Finally, the partial discharges (PDs) under different degrees of aging are compared and combined with Fourier Transform Infrared spectra and electric field simulation. Results show that the pre-discharge occurs mainly in the oil gap between the hemispherical electrode edge and the pressboard. The aging of the pressboard exerts no obvious effect on the discharge characteristics in the initial stage of PD; with the development of the discharge, the development of aging of pressboard leads to a higher electric field intensity acting over a greater area, the more intense the discharge, the faster the relative discharge increases and the larger its amplitude. This discharge will cause decomposition of the oil in the pores of the aging pressboard thus generating gas bubbles which will move as discharge develops. When the bubbles reach the surface of the pressboard, the local electric field intensity will increase and the discharge will become more intense.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-smt.2018.5138

Effect of the oil-paper insulation aging on partial discharge characteristics in a hemispherical surface model

Pressboard samples with different aging degrees were prepared to study the material and discharge characteristics of pressboards with a shielding electrode. The step-up method was used for the subsequent experiment. The partial discharges of the pressboards with different aging degrees were compared and combined using Fourier transform infrared spectra, as well as electric field simulations and analysis. The microstructure of the aged pressboards was observed with scanning electron microscopy. The results showed that pre-discharge occurred mainly in the oil gap between the hemispherical electrode edge and pressboard. Aging the pressboard had no obvious effect on the discharge characteristics at the initial partial discharge stage. As the partial discharge developed, aging the pressboard resulted in a higher electric field intensity acting over a greater area, which led to a more intense discharge. Microscopic observation showed that aging and discharge destroyed the fibre, reduced the fibre width, and caused holes and fractures, which promoted further development of the discharge. The spectroscopic analysis showed that aging destroyed O-H functional groups and reduced the intermolecular forces and mechanical properties of the pressboard.

https://ojs.cnr.ncsu.edu/index.php/BioRes/article/download/BioRes_14_1_1303_Wang_Aging_Material_Properties_Discharge/6583

Yongqiang Wang

Papers

Thermal-ageing characteristics of dry-type transformer epoxy composite insulation:

Partial discharge damage mechanisms in laminated oil-paper insulation

The microscopic morphology of insulation pressboard: an image processing perspective

Effect of the oil-paper insulation aging on partial discharge characteristics in a hemispherical surface model

Effect of Aging on Material Properties and Partial Discharge Characteristics of Insulating Pressboard