Pressboard Insulation Research Articles

Evolution laws of charge mobility of transformer oil and pressboard material with temperature and electric field strength

Abstract Charge mobility, a pivotal parameter in the space charge analysis model for oil-pressboard insulation of converter transformer under DC electric fields, predominantly relies on theoretical or empirical data. The prevalent omission of empirical, measurement-based evaluations, combined with the neglect of temperature and electric field intensity effects, introduces uncertainties in the calculation of electric field and charge dynamics. Grounded on the principles of the electroacoustic pulse technique for space charge measurements, our investigation established a temperature-controlled platform to determine the charge-charge mobility in oil and pressboard insulation mediums. Utilizing diverse oil-pressboard insulation systems, this study identified the temporal patterns of both positive and negative charge mobilities and unravelled the mechanisms modulated by temperature and electric field intensity: 1) With increasing temperatures (from 20°C to 80°C) and intensifying electric field strengths (from 5kV/mm to 25kV/mm), the charge mobility within transformer oil and pressboard exhibits a rising trend. Notably, temperature variations exert the most significant influence, yielding amplifications up to 24.8 times. 2) The intrinsic properties of different transformer oils and oil-impregnated pressboards result in pronounced differences in their charge transport behaviours. Specifically, naphthenic-based oil demonstrates enhanced charge mobility relative to its paraffin-based counterpart. In contrast, transformer oils with higher kinematic viscosities exhibit diminished charge mobility compared to those with lower viscosities. 3) A linear relationship is observed between the oil absorption rate and electrical conductivity, aligning closely with the insulating pressboard's charge mobility. The evaluation of electric field and interface charge dynamics in oil-pressboard insulation under both DC and polarity-reversed conditions revealed that the electric field decay in naphthenic-based oil is more rapid than in paraffin-based oils. Moreover, both the magnitude and rate of interface charge accumulation in naphthenic-based oils exceed those in paraffin-based variants, further validating the accuracy of our charge mobility measurements and ensuing analysis.

Effect of Temperature and Electric Field Strength on Carrier Mobility of Oil-Impregnated Pressboard Under DC Voltage

The influence of carrier mobility on the space charge transport behavior inside the oil-impregnated pressboard insulation of converter transformers cannot be neglected. However, at present, current knowledge is usually derived from empirical or theoretical values, lacks experimental studies, and often ignores the effects of temperature and field strength under actual operating conditions. In this paper, based on the variable-temperature surface potential decay (SPD) method, a carrier mobility measurement platform for oil-impregnated pressboard is established, and the carrier mobility values for different combinations of oil and oil-impregnated pressboard are obtained experimentally to analyze and reveal the influence mechanisms of temperature and field strength on the carrier mobility. The results indicate the following: (1) The positive and negative carrier mobilities of oil-impregnated pressboard are in the range of 10−12–10−13 m2·V−1·s−1, and the negative carrier mobility is always higher than the positive carrier mobility. (2) The carrier mobility is positively correlated with the changes of temperature and field strength, and when the temperature increases from 20 °C to 80 °C, the positive and negative carrier mobilities increase by 4.01 times and 4.72 times, respectively; when the field strength increases from 1 kV/mm to 7 kV/mm, the positive and negative carrier mobility increases by 2.53 and 2.72 times, respectively. (3) The carrier mobility of the pressboard with a higher oil absorption rate changes more significantly with temperature; when the field strength is 7 kV/mm and the temperature increases from 20 °C to 80 °C, the positive polarity carrier mobility increases from 3.96 × 10−13 m2·V−1·s−1 to 2.64 × 10−11 m2·V−1·s−1, an increase of 66.67 times, while the increase in the carrier mobility of the pressboard with a lower oil absorption rate is only 1.59 times. (4) The carrier mobility of the naphthenic transformer oil-impregnated pressboard is higher than that of the paraffin-based transformer oil-impregnated pressboard, and the carrier mobility of two kinds of naphthenic transformer oil-impregnated pressboard is 3.16 times and 2.47 times higher than that of the paraffin-based transformer oil-impregnated pressboard, respectively, under the conditions of 60 °C and 7 kV/mm. (5) Utilizing the Darcy model and microscopic scanning results of the pressboard morphology, it was revealed that permeability and fiber structure are key factors influencing the variation in carrier mobility. The research results of this paper can provide theoretical basis for the calibration and optimization of the oil-pressboard insulation structure of converter transformers.

Degradation mechanism of oil–pressboard insulation by multiphysical field aging characteristics and cellulose crytallinity

Degradation mechanism of oil–pressboard insulation by multiphysical field aging characteristics and cellulose crytallinity

Analysis of converter transformer pressboard insulation degradation under surge using mathematical morphology

Nowadays, with the significant expansion of industrial growth, the bulk power requirement can only be satisfied through high-voltage direct current HVDC transmission. The converter transformer is the utmost vital part of the HVDC transmission. Pressboard insulation is most commonly used as inter-disc insulation in converter transformers. During working conditions due to elevated temperature and different operational stresses, insulation material gets deteriorated. It may cause a risk to the life of the converter transformer. The effects of elevated temperatures as well as frequency on pressboard insulation of the converter transformer are examined in this study. The condition evaluation and morphological changes in pressboard insulation at elevated temperatures can evaluate with the help of frequency domain spectroscopy (FDS) and atomic force microscopy (AFM) techniques. The impact of elevated temperatures on insulation material can be analyzed based on surface roughness and dielectric parameters. In MATLAB Simulink environment, a dual winding single-phase converter transformers valve side star winding 60 discs model is constructed for impulse test. Based upon arrival time and velocity of traveling wave, insulation degradation location can be identified by using mathematical morphology. The simulation results demonstrate that the suggested method can notably located degradation across disc winding.

Photodetector array for measuring the spatial–temporal distribution characteristics of electric field in oil–pressboard insulation based on the Kerr electro‐optic effect

AbstractAccurate measurement and effective analysis of the space electric field and charge characteristics within the oil–pressboard insulation structure of converter transformer are essential for achieving precise insulation structure design. Presently, most electric field measurement methods relying on the Kerr electro‐optical effect are limited to single‐point measurements. Given the complexity of converter transformer insulation structures, including their large scale and the use of various materials, existing technologies and findings struggle to provide comprehensive electric field observations within the oil–pressboard region. After leveraging the Kerr electro‐optic effect, a high‐precision 32‐unit photodetector array has been developed to achieve regional measurements of spatial‐temporal electric field in oil–pressboard insulation. The array boasts a measurement spatial resolution of 1.4 mm2 and a sensitivity of 0.15 kV/mm, ensuring measurement accuracy exceeding 96.50%. Through practical measurements of the spatial electric field distribution within the oil–pressboard insulation under direct current voltage, the non‐uniform distribution of the electric field is effectively captured in oil. Notably, the maximum deviation in field strength at different positions within the transformer oil can reach 18.5%. Moreover, as the applied voltage increases, the unevenness coefficient of the field strength gradually rises, peaking at 1.19, which signifies a progressive expansion in the area affected by electric field distortion. By conducting tests to assess the dispersion of volume resistivity in samples from various positions within large sheets of pressboard, the variation in volume resistivity of materials is posited as one of the contributing factors to the non‐uniform distribution of electric field within the oil. The detailed accomplishments aim to provide essential technical and theoretical support for optimising insulation structure design of converter transformers.

Lightning Impulse Breakdown Improvement Properties of Cellulose Insulation Pressboard Impregnated With Three-Element Mixed Insulation Oil

Lightning Impulse Breakdown Improvement Properties of Cellulose Insulation Pressboard Impregnated With Three-Element Mixed Insulation Oil

Health assessment of converter transformer pressboard insulation based on FDS and digital image processing

The stable operation of the converter transformer is an essential task for power system operation reliability and security. Nowadays, Frequency domain spectroscopy (FDS) technology is prominently utilized for assessing oil-impregnated pressboard insulation. The present study examines the effect of the pressboard insulation material as a function of frequency and elevated temperature. The experimental analysis on oil-impregnated pressboard insulation is carried out at temperatures from 30 °C to 130 °C with an incremental rise of 20 °C intervals with frequency variation from 1 Hz to 10 MHz. The frequency-dependent permittivity, conductivity and loss tangent angle studies also confirm the deterioration of oil-impregnated pressboard insulation. The surface morphological changes inside the pressboard insulation are recorded with the help of scanning electron microscopy (SEM). The synergistic effect generated on pressboard insulation is examined by fiber width changeand image processing approach by randomly selecting the average of three local areas of SEM image. A canny operator is selected to extract the exact boundaries of images and more change is recorded in the edge detection count after 90 °C.The porosity and pore size distribution can be increased with elevated temperatures. A single-phase, 315 MVA valve side star winding with 60 discs of single-phase converter transformers model is developed in MATLAB Simulink. An impulse of 100 kV, 1.2/50μsec is applied across the star winding to identify the pressboard insulation degradation derived from FDS data with the help of mathematical morphology and wavelet transform technique. The energy of the wavelet coefficient on the neutral current capture during the impulse test adds a significant contribution to analyzing pressboard insulation degradation. The results presented are in good agreement with the published work.

Investigation of the dielectric properties and mechanical stability of lignocellulosic biomass based electrical insulation material for high voltage applications

The laminated insulation pressboards are used in electrical transformers and they are made from pure wood cellulose fibre. It is used in high-voltage transformers for electrical insulation due to its superior electrical and mechanical properties. Many researchers are searching for alternatives to wood raw materials to reduce deforestation. The banana is one of the lignocellulose-based raw materials; the banana fibre contains 48% cellulose. In this work, different frequencies and temperatures are used to investigate the electrical characteristics of epoxy resin composites reinforced with banana leaf fibres, including their dielectric constant, dissipation factor, and loss factor. At frequencies ranging from 1 to 10 kHz and temperatures ranging from 30 to 150 °C, the dielectric properties of several composites containing banana leaf fibre are studied. The composite’s dielectric properties gradually increased with temperature and decreased with frequency. Correspondingly, the mechanical tests involving a banana leaf fibre epoxy composite are conducted for tensile, bending, impact, and water absorption. The banana leaf fibre has been treated with a 5% sodium hydroxide (NaOH) solution to increase its dielectric and mechanical strength. According to this study, the mechanical strength of biocomposites containing up to 60% treated banana leaf fibre epoxy (TBLFE) composites is greater than that of pure epoxy. A thermogravimetric investigation of composites reinforced with banana fibres has revealed remarkable thermal stability up to 220 °C. In a composite made from chemically treated banana fibre pressboard, there is a good bond between the fibres and the matrix. The FESEM surface analysis shows that treated banana leaf fibril insulation boards have a better texture than composites that have not been treated. The testing results depict that banana leaf fibre is one of the good alternatives to wood cellulose for electrical insulation on pressboard for high-voltage applications.

Natural Ester Replacement Effect on Thermal Degradation of Cellulose Insulation From Macroscopic Behavior to Atomic-Scale Mechanism

The accelerated thermal ageing test was conducted with samples composed of mineral oil (MO), FR3 natural ester, kraft insulation pressboard (KIP) and thermally upgraded paper (TUP) at 120 °C. In the middle ageing period, aged MO was replaced with new FR3. Molecular dynamics was used to simulate the mechanism of delaying cellulose aging by replacing natural esters. The experiment results show that, compared to the controls, KIP and TUP impregnated in FR3 had higher degrees of polymerization (DP) values and lower ageing rates. After the FR3 replacement, the relative chain scission rate of KIP and TUP was significantly slowed. FR3 has better aging retarding effect on TUP than KIP. Atomic scale mechanism analysis shows that FR3 natural ester could inhibit the diffusion of water and acid in the oil-paper insulation system before and after oil change, which has a significant ability to delay the acid hydrolysis of cellulose. These is the main reason why KIP and TUP exhibited a higher DP in natural ester. This paper provides theoretical support for transformer replacement of natural ester.

Study on Micro Topography of Thermal Aging of Insulation Pressboard Based on Image Processing

The insulation pressboard is very important in the insulation of oil-immersed transformers, which determines the useful life of the transformer. In order to analyze the change in the micro topography of the pressboard under the condition of thermal aging in detail, the insulation pressboard samples of different thermal aging periods were made. Image enhancement, denoising, segmentation, optimization, edge detection, expansion, and corrosion were used to process SEM images of the pressboard after thermal aging, so as to extract the fiber width, hole size, and surface roughness. By comparing the relationship between the micro topography and the degree of polymerization of the insulation pressboard in different thermal aging periods, it can be concluded that when the fiber width was 91% of the unaged pressboard fiber width, and the corresponding roughness was 0.162, the insulation life was only approximately half of its original value, and when the fiber width was 79% of that of the unaged pressboard, the corresponding roughness was 0.225, and the insulation life was nearing the end of its life. This shows that the image processing method used in this paper can observe the micro topography changes during thermal aging and can estimate the thermal aging degree of the insulation pressboard.

Effect of vacuum drying the insulation pressboard on partial discharge characteristics under ripple voltage conditions

The object of the research is partial discharges arising in a sample of the insulation pressboard. In most modern high-voltage direct current transmission schemes based on voltage-source converters six-pulse or twelve-pulse converters are used. The signal waveform on the direct current voltage side of a high-voltage direct current transmission is not pure direct current voltage and it has alternating current component. Since the partial discharge activity under distorted voltage is much different than that under pure direct current voltage, voltage ripples and voltage harmonics are the subject of various studies. This article examines the effect of vacuum drying the insulation pressboard on partial discharge characteristics at direct voltage ripples. The insulation pressboard was dried in a vacuum chamber with a residual pressure of 1 mm Hg. Among all the characteristics of a partial discharge, the main focus has been on the apparent charge of a partial discharge. The greater is the apparent charge value, the stronger is the destructive effect on high-voltage insulation. It was shown that drying the insulation in a vacuum chamber has the greatest effect on reducing the apparent charge of a partial discharge at direct current voltage than at alternating current voltage. After drying the insulation in a vacuum chamber, the amplitude of the partial discharge pulses was decreased by 99.3 % at direct current voltage in comparison with the moistened sample. After vacuum drying, at DC voltage, rare and very low magnitude partial discharges were recorded. The conducted research contributes to the development of methods for partial discharge detection under non-standard voltage conditions.

Aging assessment method of oil–paper composite insulation based on repeated polarization current characteristics

To explore the aging assessment method of the oil–paper insulation in power transformers, the polarization and depolarization current (PDC) characteristics of oil–paper composite insulation samples are investigated under high applied voltage. Based on the asymmetry of polarization absorption current and depolarization current and the polarity effect of insulation pressboard, the repeated polarization current test method is proposed. A novel aging characteristic parameter, which is the ratio of the normalized charge for the first polarization and the normalized charge for the second polarization, is extracted. The difference between the decay rates of the first polarization current and the second polarization current increases at first and then decreases with the polarization time, and decreases with increasing aging degree of the pressboard. The more serious the insulation aging is, the smaller the variation range of aging characteristic parameter with polarization time is, and the closer the value is to 1. With decrease in the DP, the maximum of aging characteristic parameter changes in a decreasing trend. The aging condition of the oil–paper composite insulation can be assessed by the repeated polarization current method and the relationship between the aging characteristic parameter and the polarization time.

Frequency-Domain Spectroscopy and Charge Trap Characteristics of Cellulose Based Pressboard Modified by Nano-TiO2 Particles

Dielectric response and charge decay test are widely used for evaluating the electric properties of the insulation pressboards in the converter transformer. In this paper, Frequency Domain Spectroscopy (FDS) test, breakdown test, and charge decay characteristics experiments are carried out on the cellulose pressboards modified by TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> with different mass fractions and diameters. The FDS of the modified cellulose pressboards at ambient temperature is measured from 0.0001 to 1000 Hz by IDAX 300. The process is conducted at the laboratory to study the characteristics parameters of the dielectric response behaviors, such as complex relative permittivity. The results show that the values of the relative permittivity for the insulation pressboards modified by TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (5 wt%, 10 nm) are reduced. To verify the accurate results, the microstructures of the pressboards modified by 5 wt% with 10 nm diameter are observed by a Scanning Electron Microscope (SEM). The breakdown strength tests are carried on the samples modified by 1 wt%, 3 wt%, 5 wt%, 7 wt% with 10 nm TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> under the intensity electric and high-temperature field, respectively. The values of the breakdown strength are increased by 38% and 11.5% for single layer samples and oil/pressboard composite insulation models modified by 5 wt% nano-TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> filler. In addition, the charge decay characteristics are tested on the samples. The proper proportion additions can reduce the trap density.

Moisture‐dependent interface and bulk behaviour of space charge in DC‐stressed oil‐impregnated pressboard insulation

Moisture‐dependent interface and bulk behaviour of space charge in DC‐stressed oil‐impregnated pressboard insulation

Experimental and simulation analysis of the influence of moisture on dielectric and breakdown properties of insulation pressboard used in power transformer

The influence of water molecules on cellulose has attracted much attention. The influence of water molecules on the dielectric property and breakdown property of cellulose requires in-depth research. In this paper, firstly, the effect of water on the dielectric spectrum in the frequency domain dielectric spectrum and the breakdown voltage of cellulose were studied. Then the parameters such as free volume, hydrogen bond, energy level and state density of cellulose under different moisture content were analysed by molecular simulation. The changes of dielectric properties and breakdown properties of cellulose under different water content were studied and summarized. The variation mechanism of dielectric property and breakdown property of different moisture content was explained by molecular simulation.

Experimental Investigations of Mechanical and Chemical Properties of PVP Precompressed Pressboard Insulation for HV Power Transformers

As the rating of power transformers increases, oil–paper insulation requirements become extremely difficult as insulation system should have more withstanding capabilities of high voltages and high reliability. Thus, it is important to find pressboards and precompressed pressboards using new insulating polymers with acceptable mechanical, chemical and electrical insulation properties that can used as an insulation barrier for high voltage–power transformers. The cellulose material-based pressboards are in great demand for high voltage–power transformers. Many researchers have investigated pressboards prepared from kraft pulp and found suitable in solid insulation of power transformers since decades. Pressboards are usually compressed with water soluble and insoluble polymer resins to develop precompressed pressboards. Water soluble insulating polymer resigns precompressed pressboards are of great importance, since they are recyclable. This paper mainly focuses on preparation of precompressed pressboards using a closed ring water soluble insulating polymer polyvinylpyrrolidone (PVP) and in order to ensure a correct functioning of this insulation in transformers, it is very important to characterize the mechanical and chemical properties of precompressed pressboards. These properties have been studied, and the results are compared with precompressed pressboard prepared from polyvinyl alcohol (PVA). It is observed that mechanical and chemical properties of PVP precompressed pressboards have shown substantial improvement compared to PVA precompressed pressboards.

Analysis of deterioration characteristics in oil-immersed insulation pressboard with different durations of aging based on an image-processing method

Insulation pressboard samples were obtained by thermal aging (according to Montsinger’s formula, at 130 °C, the pressboard is heated for 0 to 32 days) and discharge experiments. SEM images of samples were analysed. Image segmentation was applied to calculate the fibre width, cross-sectional porosity, and carbon-trace area. Inter-layer fibre models were established to observe fibre morphology using 3-D reconstruction. The initial discharge voltage decreased with age, and the discharge amounts increased. After 16 days of aging, the fibre width had decreased to between 68.1% and 81.8% of unaged pressboard. As the aging increased, cellulose hydrogen bonds were broken, which affected the expansion of interlayer pores, increasing the porosity of the pressboard. After 32 days of aging, the porosity increased to 2.38 times that of a new pressboard. In addition, the longer the aging, the larger the area of carbon marks caused by the discharge breakdown. With the aggravation of thermal aging, the insulating property of pressboard decreased due to the decrease of fibre width and increase of porosity that further accelerated the damage to the fibre structure. It was concluded that the fibre width and porosity could be used as criteria to judge the degradation of pressboard.

Statistical analysis of induced magnetic fields on oil-impregnated insulation pressboards

Electrical insulation materials are highly exposed to electrical network-based electric and magnetic fields in power systems. In electrical fields, electrical insulation materials are prone to breakdown and cause failure. A transformer failure which is related to pressboard insulation may lead to total breakdown and hence system malfunction in a total manner. In this study, a test setup is used to conduct discharge tests for pressboards in different thicknesses where main interest is originated magnetic fields on the pressboards. These tests are fulfilled with spherical and rod electrodes in transformer oil where magnetic field sensors are used to acquire discharge-based magnetic field data. By investigating high-voltage stresses with different levels, possible breakdown voltage of a pressboard is predicted and statistically analyzed. In addition to magnetic field measurements, discharge current measurements are taken; however, contrary to conventional studies, this study assesses magnetic field data which are dependent on the thickness of pressboard insulation. For different voltage levels (13 kV and 22 kV for different stress levels), magnetic field measurements and current waveforms are obtained by using magnetic field sensors and high-speed oscilloscope. Magnetic field time series signals are subjected to wavelet analysis, and wavelet coefficients are obtained. Rather than utilizing time series current signals or time series magnetic field signals, wavelet coefficients of magnetic field signals are taken into consideration as a novel approach. These coefficients are processed by multifractal analysis, and finally, the integrity of the pressboard is determined as in proper mode (no failure) or in breakdown mode.

Research on Partial Discharge Characteristic of Nano MMT Modified Insulation Pressboard

In order to restrain partial discharge phenomenon of oil-paper insulation in convertor transformer, the nano montmorillonite (MMT) modified pressboards were developed by nano-doping method. The influence of nano modification on dielectric characteristics of modified pressboards were investigated, and their partial discharge characteristics in pin-plane electrode system were also studied. Meanwhile, the inhibition mechanism to partial discharge of nano-modification was analyzed by software simulation, and its impact to breakdown characteristics was verified as well. The measuring results showed that the conductivity rose with the increase of nano-doping content, while the relative permittivity fell firstly and then rose, which reaching the minimum value at 1%. Also, the breakdown strength of modified pressboard at nano doping ratio of 1% is higher than other ratios under DC and AC voltage. Further, the partial discharge inception voltage increases and maximum PD quantity decreases under same voltage level in oil-paper insulation by using nano modified pressboard at this ratio. Moreover, it helps to homogenize the electric field distribution of insulation structure in PD experiment, and finally contributes to the improvement of breakdown characteristics.

Failure modes of insulating pressboard subjected to high electric fields and durations: Evidence of partial discharge and changes at the microscopic scale

The deterioration of insulation pressboard under needle-plate discharge was tested, and the degradation stage was divided according to observed experimental phenomena. Based on the improved Top-hat watershed image segmentation method, the white marks area of the pressboard were tested. Fibres with different discharge states were extracted from SEM images. The fibre width was calculated, and the porosity of the surface of the insulation pressboard at different degradation stages was calculated. Simultaneously, the 3D reconstruction technique was used to observe the 3D morphology of fibres at different discharge stages. The study found that with the deepening of the discharge process, the discharge of the pressboard increased, and the white marks of the pressboard continued to expand from point to surface. In addition, the diameter of the fibre of the insulating pressboard decreased obviously with the increase of the pressing time, and the fibre diameter was 89.6% after the breakdown. Moreover, the electrical stress had a great effect on the expansion of the interlayer pores of the pressboard; the cross-section porosity of the insulating pressboard gradually increased with the deepening of the discharge process, and after breakdown, the interlayer porosity reached 12.5%.