Oil Impregnated Paper Insulation Research Articles

Numerical Simulation of Trap Modulated Multi-Mobility Charge Transport Under AC Field

Based on the characteristics of charge distribution under AC electric field with a wide frequency range, this paper focuses on the applicability of the traditional bipolar charge transport model under AC field in oil-impregnated paper (OP) insulation by simulation. Three physical processes of charge injection, migration and trapping/detrapping are discussed. In reaction to the influence of amplitude variation of AC field on injection rate, the charge injection process is described by a concatenation of the Ohmic mechanism and the Schottky mechanism. On account of the correspondence between the frequency and carrier mobility, the single mobility mode is modified to the multi-mobility mode, which exhibits the effect of localized state differences in materials on charge migration. The charge hysteresis under AC field is to do with the trapped charges, so the multi-energy trap distribution model is adopted to reveal the matching relationship between the period of electric field and charge residence time in deep traps. By comparing the simulation results and experimental phenomena, the modified model achieves space charge accumulation under a wide frequency range, and the shape of charge distribution is similar to the experiments, which proves the validity of the model. Further, the simulation results demonstrate that the net charge accumulation decreases with the increase of the electric field frequency, while the total charge accumulation shows an inverse trend with frequency, which is related to the coexistence of positive and negative charges.

Performance Assessment of Cellulose Paper Impregnated in Nanofluid for Power Transformer Insulation Application: A Review

Insulation cellulose paper is a basic measure for a power transformer’s remaining useful life, and its advantageous low cost, electrical, and mechanical properties have made it an extensive insulation system when impregnated in a dielectric liquid. Cellulose paper deteriorates as a result of ageing due to some chemical reactions like pyrolysis (heat), hydrolysis (moisture), and oxidation (oxygen) that affects its degree of polymerization. The condition analysis of cellulose paper has been a major concern since the collection of paper samples from an operational power transformer is almost impossible. However, some chemicals generated during cellulose paper deterioration, which were dissolved in dielectric liquid, have been used alternatively for this purpose as they show a direct correlation with the paper’s degree of polymerization. Furthermore, online and non-destructive measurement of the degree of polymerization by optical sensors has been proposed recently but is yet to be available in the market and is yet generally acceptable. In mitigating the magnitude of paper deterioration, some ageing assessments have been proposed. Furthermore, researchers have successfully enhanced the insulating performance of oil-impregnated insulation paper by the addition of various types of nanoparticles. This study reviews the ageing assessment of oil-paper composite insulation and the effect of nanoparticles on tensile strength and electrical properties of oil-impregnated paper insulation. It includes not only significant tutorial elements but also some analyses, which open the door for further research on the topic.

The influence of aging time and moisture content on the tensile strength of transformer bushing insulating paper

The parameter of tensile strength for transformer bushing insulating paper occupies an important position in the structural design and life evaluation of transformer bushing. In order to precisely achieve the effect of aging time and moisture content on the tensile strength of transformer bushing paper, transformer insulating paper samples with a thickness of 0.13 mm were prepared for tensile strength and dielectric strength testing under different moisture contents and aging states. The results show that the tensile strength (MPa) of insulating paper decreases linearly in the range of 0–648 h under accelerated thermal aging at 130 °C. However, there is a more serious exponential decline in the tensile strength (MPa) of insulation paper with its moisture content is boosted. The moisture content in insulation paper is the most important factor for insulation paper deterioration in oil-impregnated paper insulation equipment, which could accelerate the end of insulation service life. The value of capacitance increment ΔC of the insulation paper samples displayed closely linear growth with a moisture content from 0.5% to 5.9%. Strictly controlling the moisture content by monitoring the ΔC value of transformer insulation paper is of significance for prolonging its service life.

Space Charge Behavior in Oil-Impregnated Paper Insulation Under Sinusoidal Electric Field With Wide Frequency Range

This article focuses on the space charge transport under the sinusoidal electric field with a wide frequency range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> –10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> Hz. Three questions are studied: whether there is obvious charge accumulation in oil-impregnated paper (OP) insulation under the ac field, the effect of charge accumulation on electric field, and the physical processes affected by the periodic electric field. Based on the pulsed electro-acoustic method, the original signal is obtained by a detection strategy that combines the pulse cluster method for low frequency and automatic equal phase shift (AEPS) principle for high frequency. It shows that charge hysteresis is a characteristic phenomenon of charge distribution under the ac field. Charge accumulation under a low frequency field is more than that under a dc field due to the higher interfacial field. Furthermore, numerical simulation is carried out to investigate the charge dynamics under ac stress. Two specific physical processes of effective charge injection and charge migration are discussed, and a charge conduction mechanism with a multi-mobility mode is proposed for charge transport under the ac field. It shows that the maximum recombination rate increases with the increase in frequency. Charge hysteresis essentially creates conditions for the charge recombination process.

Paper Insulation Aging Estimation Using Swept-Source Optical Coherence Tomography

In this article, an algorithm that uses subsurface structural images, obtained by swept-source optical coherence tomography (OCT), is developed to estimate the aging of insulation paper. Swept-source OCT, a novel and noninvasive diagnostic technique, is used to study the subsurface morphological changes in thermally stressed paper insulation. Obtaining subsurface structural images of the insulation paper enables an accurate estimation of the age of thermally deteriorated insulation paper. Accelerated thermal aging is employed to generate samples of oil-impregnated insulation paper with different levels of deterioration. Textural features, based on a spatial grayscale-level dependence matrix (SGLDM) obtained from the OCT images, are used to estimate the aging time of insulation paper based on changes in the subsurface morphology of the paper. Principal component analysis (PCA) is applied to the feature sets to reduce its dimension. The PCA output is then used to construct the age estimation model, and the leave-one-out-cross-validation (LOOCV) method is used to assess the estimation performance of the fitted model. The results of this work demonstrate that the development of OCT-equipped scanning probes to assess the condition of paper insulation is promising.

Analysis of FDS Characteristics of Oil-impregnated Paper Insulation under High Electric Field Strength Based on the Motion of Charge Carriers

Frequency Domain Spectroscopy (FDS) is a powerful tool to study the dielectric properties of oil-paper insulation, which is widely used under high electric field strength. The motion of charge carriers under high electric field strength will cause new dielectric characteristics in FDS curves, from which more information could be extracted for insulation state assessment. In this paper, FD spectra are measured under high electric field strength. The electric field dependence of the loss peak in middle frequency and the characteristics of dielectric loss in low frequency are studied. With the influence of trapping on the motion of charge carriers under electric field being considered, charge carriers hopping polarization model and charge carriers motion dependent conductance model are proposed. The trap energy level of oil-paper insulation decreases when the moisture content increases, thus, the motion characteristics of charge carriers change, leading to decreasing loss peak in middle frequency and inverse dielectric loss characteristics in low frequency. These two models introduce the influence of traps on carrier motion into the dielectric response and enable one to gain a better understanding of the dielectric properties of oil-paper insulation under high electric field strength.

Aging Characteristics of Transformer Oil-Impregnated Insulation Paper Based on Trap Parameters.

Oil-impregnated insulation paper is an important part of transformers; its performance seriously affects the life of power equipment. It is of significance to study the aging characteristics and mechanism of oil-impregnated insulation paper under thermal stress for transformer status detection and evaluation. In the work, the accelerated thermal aging was carried out at 120 °C, and DP1490, DP787, and DP311 samples were selected to represent the new, mid-aging, and late-aging status of the transformer, respectively. The space charge distribution within the specimens was measured by the pulsed electro-acoustic (PEA) method and the trap parameters were extracted based on the measurement curves. Further, the aging mechanism was studied by molecular simulation technology. A typical molecular chain defect model was constructed to study the motion of cellulose molecules under thermal stress. The experimental results show that the corresponding trap energy levels are 0.54 eV, 0.73 eV, and 0.92 eV for the new specimen, the mid-aging specimen, and the late aging specimen, respectively. The simulation results show that the trapped energy at the beginning of aging is mainly determined by the loss of H atoms. The changes in trap energy in the middle stage of aging are mainly caused by the absence of some C atoms, and the trap energy level at the end of aging is mainly caused by the breakage of chemical bonds. This study is of great significance to reveal the aging mechanism of oil-impregnated insulation paper and the modification of insulation paper.

Decomposition Gases of Insulating Oil in Oil-impregnated Paper Insulation System with an Oil Gap

Decomposition Gases of Insulating Oil in Oil-impregnated Paper Insulation System with an Oil Gap

Effects of AC Electrical Stress, Oil Pressure and Insulation Construction on Time Transition of Partial Discharge Characteristics on Oil-impregnated Paper Insulation System for Oil-filled Cables

Effects of AC Electrical Stress, Oil Pressure and Insulation Construction on Time Transition of Partial Discharge Characteristics on Oil-impregnated Paper Insulation System for Oil-filled Cables

A Circuital Model-Based Analysis of Moisture Content in Oil-Impregnated-Paper Insulation Using Frequency Domain Spectroscopy

The explicitly numerical or well-defined correlations between the dielectrics responses (DRs) and dielectric essences of oil-impregnated paper (OIP) insulation have not been fully understood yet. As a result, it is rather difficult to quantitatively diagnose the critical insulation condition, like determining moisture content in insulation paper (MCP) of power equipment only using electrical-based techniques such as frequency domain spectroscopy (FDS). To obtain MCP value, from a new perspective of parametric study on the circuital DR equivalence of OIP insulation-extended Debye model (EDM), the present contribution introduces a novel approach by exploring the pattern when EDM parameters vary with MCP and temperatures T. Further, mathematical correlations are developed between sensitive R-C values and MCP-T values. On the above analysis, small-scale physical models of real-life transformer bushings were prepared as test samples. Their OIP condenser bodies were artificially absorbed different controlled quantities of moisture and conditioned at different temperatures to record corresponding FDS results. Then a hybrid genetic algorithm combined with the Levenberg-Marquardt algorithm was proposed to estimate the EDM parameters by fitting the measured FDS data. Sensitive parameters therein were identified using subspectrum decomposition and formulated with MCP-T values so that a reliable moisture content estimation can be achieved once the testing temperature and FDS recordings of an insulation body are known.

Semi-simultaneous Observation of Partial Discharges Signal, Light Emission and Bubbles, and Detection of Dissolved Gasses in Oil Gap on Oil Impregnated Paper Insulation System

Semi-simultaneous Observation of Partial Discharges Signal, Light Emission and Bubbles, and Detection of Dissolved Gasses in Oil Gap on Oil Impregnated Paper Insulation System

Accelerating Moisture Content Sensing of Oil-Impregnated Paper Insulation Using Frequency Modulated Square Wave Excitations

Assessment of insulation condition by sensing moisture content of oil-impregnated paper at periodic intervals is necessary for reliable operation of transformers. The conventional method of moisture sensing requires application of two cycles of sinusoidal excitation voltage at discrete measurement frequency points over a wide range starting from 0.1 mHz to 1 kHz, which is inadvertently time consuming and practically problematic. Besides, the insulation system in real-life operation is often stressed by voltage waveforms which deviate significantly from sinusoid. Therefore, modification of the existing method is necessary for a reliable insulation diagnostic. Considering the above-mentioned fact, in this article, a new measurement technique for sensing moisture content of oil-paper insulation is proposed using frequency modulated square wave excitations. The proposed technique was experimentally validated on several real-life transformers tested in the laboratory. Investigations revealed that the use of frequency modulated square wave excitations is capable of sensing moisture content in oil-paper insulation almost accurately in comparison with the conventional method but with a significantly reduced measurement time without compromising on measurement accuracy.

Influence of Switching Impulse Voltage Repetitive Superimposition and Internal Conditions of the Oil Gap Defect on Partial Discharge Characteristics of Oil Impregnated Paper Insulation System

Influence of Switching Impulse Voltage Repetitive Superimposition and Internal Conditions of the Oil Gap Defect on Partial Discharge Characteristics of Oil Impregnated Paper Insulation System

Polarization and Trap Characteristics Modification of Oil-Impregnated Paper Insulation by TiO₂ Nanoparticles.

Polarization and traps determine the electrical property of oil-paper insulation, but most attention has been paid to the modification of insulating oil with nanoparticles, so there are is little research about oil-impregnated paper, and the origin for performance variation is not understood yet. In this paper, spherical nanoscale titanium dioxide was prepared by the hydrolysis method and nanofluid-impregnated paper (NP) was fabricated through oil-impregnation. The frequency domain spectrum was measured for polarization analysis, and both thermally stimulated depolarization current (TSDC) and isothermal surface potential decay (ISPD) methods were used to reveal trap parameters. Results show that NP’s low frequency permittivity is much larger, and another peak appears in the spectrum even though the content of nanoparticles is very low. With the addition of TiO2 nanoparticles, TSDC’s amplitude and peak temperature increase, and the trap energy becomes shallower. TiO2 nanoparticles’ strong polarization and high activation energy contribute to NP’s larger interface polarization intensity and activation energy. Furthermore, because of oxygen vacancies, TiO2 nanoparticles offer a transfer site for holes and electrons to escape from deep traps; thus, the trap energy is greatly reduced.

Examination of State of the Cable Insulation by the Return Voltage

Abstract The relevance of the study is conditioned by the need to determine the state and residual life duration of high-voltage cable lines to identify faulty and maintainable cables. The aim of the article is to determine a reliable scientifically grounded criterion for assessment of insulation characteristics of the cables in use and to perform a comparative analysis of the results obtained by the traditional method of diagnosing insulation with the results of a new method of assessment by the return voltage. In this regard, the article deals with the issues related to the testing of cables having oil-impregnated paper insulation, as well as with the issue of switching from planned replacement of cables to assessment of their actual state and period of residual life. The authors propose to use the method of examining the cables by the return voltage using the device for testing electrical insulation “UDEI-1” developed at the department of Electrification and Automation of the Nizhny Novgorod State University of Engineering and Economics. The article presents the results of measuring the return voltage of three cables that operated under different conditions. The cables had different technical state. The analysis of the estimation of the residual life of cables by the return voltage was carried out using such criteria as the PIRV polarization index, the LIRV electrical conductivity index, and the P-factor. The P-factor is the physical criterion demonstrating the aging of paper-oil insulation by the shape of the return voltage curve. It represents such characteristics of insulation aging as moistening. To compare the results of testing the cables by the return voltage with the conventional methods of diagnostics and to determine the actual technical condition of power cables, the authors applied the method of spatiotemporal reflectometry and the method of measuring insulation resistance with the determination of such indicators of state as insulation resistance normalized per one kilometer, absorption coefficient, and polarization index. The results of this article confirm that the return voltage gives a qualitative assessment of the state and degree of aging of cables with impregnated paper insulation. The authors proposed a new system for evaluation of cable condition by weighting coefficients. In this approach, the determination of residual life of cables with impregnated paper insulation is based on the values of the return voltage. Application of the new system gives opportunity to improve reliability of the power lines. Recommendations for the further operation of the studied cables are given. The materials of the article are of practical value for carrying out complex assessment of the technical condition of power cables by the return voltage and can be useful for drawing up a schedule for replacement or repair of cable lines depending on their actual state.

Ageing characterization of oil impregnated paper insulation based on dispersion staining colors

The life expectancy of a transformer is closely related to the ageing state of its paper insulation. This paper presents novel markers for ageing characterization of paper insulation in oil-filled transformers based on the dispersion staining colors of cellulose fibres. Accelerated thermal ageing experiments were performed for oil-paper specimens at 120 and 130 °C to obtain aged oil-paper samples of different ageing degrees. The average degree of polymerization (DP), crystallinity index, and refractive index of insulation paper were measured; the experimental results show that an increase in refractive index of insulation paper was observed with the increase of ageing degree. Meanwhile, the dispersion staining colors of cellulose fibres in oil were observed using dispersion staining method; the experimental results show that the refractive index of cellulose fibres detached from insulation paper increases with the increment of ageing degree of insulation paper. Finally, two ageing markers—that is, the primary color ratio-based ratio of blue to red and the primary color ratio-based Mahalanobis distance were presented. Their relationship with DP and ageing time of insulation paper were obtained, and were compared with the corresponding relationship with 2-furaldehyde (2-FAL) and carbon oxides. The proposed ageing markers demonstrate an advantage over 2-FAL and carbon oxides and show potential in ageing state evaluation and residual life prediction of paper insulation in transformers.

Effects of TiO2 Nanoparticles on Streamer Propagation at the Surface of Oil-Impregnated Insulation Paper

Recent studies have found that the surface flashover strength of oil-impregnated insulation paper can be improved by the modification of nanoparticles. To reveal the working mechanism, this paper presents an experimental study on the effects of TiO2 nanoparticles on streamer propagation process at the surface of oil-based nanofluid-impregnated insulation paper under lightning impulse voltage. The results show that the surface flashover voltage of nanofluid-impregnated insulation paper is significantly increased by 34.6%. Streamer characteristics including shape and length at the surface of oil-impregnated insulation paper were investigated using the schlieren technique. It is indicated that streamer characteristics are dramatically changed by the addition of TiO2 nanoparticles. For nanofluid-impregnated insulation paper, streamers with more but much shorter branches tend to develop into adjacent nanofluid rather than clinging to the paper compared to that for pure oil-impregnated paper. The reason responsible for the shape dissimilarity lies in the change of electrical force due to the permittivity difference between oil and paper. Furthermore, more shallow traps introduced by nanoparticles contribute to the faster surface charge dissipation of nanofluid-impregnated paper, reducing the charge accumulation at the streamer head and suppressing the electric field distortion. Thus, it is hard for streamers to develop at the surface of nanofluid-impregnated paper, resulting in an increase of surface flashover strength.

Space Charge Modulated Electrical Breakdown of Oil Impregnated Paper Insulation Subjected to AC-DC Combined Voltages

Based on the existing acknowledgment that space charge modulates AC and DC breakdown of insulating materials, this investigation promotes the related investigation into the situations of more complex electrical stress, i.e., AC-DC combined voltages. Experimentally, the AC-DC breakdown characteristics of oil impregnated paper insulation were systematically investigated. The effects of pre-applied voltage waveform, AC component ratio, and sample thickness on AC-DC breakdown characteristics were analyzed. After that, based on an improved bipolar charge transport model, the space charge profiles and the space charge induced electric field distortion during AC-DC breakdown were numerically simulated to explain the differences in breakdown characteristics between the pre-applied AC and pre-applied DC methods under AC-DC combined voltages. It is concluded that large amounts of homo-charges are accumulated during AC-DC breakdown, which results in significantly distorted inner electric field, leading to variations of breakdown characteristics of oil impregnated paper insulation. Therefore, space charges under AC-DC combined voltages must be considered in the design of converter transformers. In addition, this investigation could provide supporting breakdown data for insulation design of converter transformers and could promote better understanding on the breakdown mechanism of insulating materials subjected to AC-DC combined voltages.

Impact of Low Molecular Weight Acids on Oil Impregnated Paper Insulation Degradation

Aging of a power transformer’s insulation system produces carboxylic acids. These acids—acetic, formic and levulinic—are absorbed by the paper insulating material, thus accelerating the degradation of the whole insulation system. In this contribution, the effect of these acids on the aging of oil-impregnated paper insulation used in power transformer is reported. A laboratory aging experiment considering different concentrations of these three acids was performed to assess their effect on the insulation system’s degradation. Each acid was individually mixed with virgin oil, and a mixture of acids was also blended with oil. The paper’s degradation was assessed by the degree of polymerization (DPv). It was found that the DPv of paper aged with formic acid decreased much faster in comparison to the other acids.

New Method for Shallow and Deep Trap Distribution Analysis in Oil Impregnated Insulation Paper Based on the Space Charge Detrapping

Space charge has close relation with the trap distribution in the insulation material. The phenomenon of charges trapping and detrapping has attracted significant attention in recent years. Space charge and trap parameters are effective parameters for assessing the ageing condition of the insulation material qualitatively. In this paper, a new method for calculating trap distribution based on the double exponential fitting analysis of charge decay process and its application on characterizing the trap distribution of oil impregnated insulation paper was investigated. When compared with the common first order exponential fitting analysis method, the improved dual-level trap method could obtain the energy level range and density of both shallow traps and deep traps, simultaneously. Space charge decay process analysis of the insulation paper immersed with new oil and aged oil shows that the improved trap distribution calculation method can distinguish the physical defects and chemical defects. The trap density shows an increasing trend with the oil ageing, especially for the deep traps mainly related to chemical defects. The greater the energy could be filled by the traps, the larger amount of charges could be trapped, especially under higher electric field strength. The deep trap energy level and trap density could be used to characterize ageing. When one evaluates the ageing condition of oil-paper insulation using trap distribution parameters, the influence of oil performance should not be ignored.