Oil-immersed Paper Research Articles

Model moisture transport in oil‐paper insulation of transformer: Theory and experiment

AbstractThe insulation performance of oil paper insulation is significantly affected by moisture such that monitoring moisture content is important. However, it is difficult to obtain the moisture content accurately due to its dynamic change under multi physical fields. Studying the moisture transportation in oil paper insulation under multi physical fields becomes a vital method in solving the problem. A multi physical model describing moisture migration was proposed, which considered the effects of temperature on moisture in different existing states in oil‐immersed paper (OIP). The temperature distribution formed a vapour pressure gradient to drive moisture migration and affected migration speed. Then, experiments and simulations of moisture migration were performed, which showed that the experiments were in good agreement with simulation. The results revealed that the temperature gradient caused uneven moisture distribution and the increased temperature reduced OIP moisture content. The established model could fully characterise moisture migration under temperature gradient, which provided a theoretical reference for predicting the risk of partial dampness and evaluating insulation performance.

A comprehensive evaluation method for moisture and aging state of oil-immersed paper bushings based on dielectric modulus and K-nearest neighbor algorithm

The insulation performance of oil-immersed paper bushings is prone to deteriorate, primarily due to moisture intrusion and thermal aging. The frequency domain spectroscopy (FDS) method is commonly employed to assess the insulation condition of the bushing. However, identifying and extracting relaxation polarization information from the low-frequency region of the FDS curve can be challenging, and there is little research about the condition evaluation under the combined effects of aging states and moisture content. To address this issue, this article uses the Taylor formula mathematical model to extract characteristic parameters from the dielectric modulus curve of OIP bushings and uses the KNN algorithm to achieve the evaluation of aging and moisture status. Then, the effectiveness and accuracy of the proposed method are validated on three field OIP bushings. The results demonstrate that the evaluation accuracy of the proposed method exceeds 83%, which has significant advantages compared to other classification algorithms. The innovation of this article lies in extracting new feature parameters and combining them with intelligent classification algorithms to evaluate the moisture and aging state of the bushing.

Pengembangan Uji Eksperimental Penuaan Dipercepat Sistem Isolasi Kertas Terendam Minyak Transformator

A transformer is a static electric device that is used to change the alternating voltage to a higher or lower level and is used to transfer energy from one electric circuit to another without changing the frequency. When the transformer is working, there is a temperature rise in the transformer which affects the degradation of the insulating paper and transformer oil. This accelerated aging tester applies the drying and vacuuming process as well as the arrangement of paper insulation racks to achieve maximum test results. The research method refers to IEC 60216-3 2006 for aging time, IEC TS 62332-1 2014 and IEC TS 62332-1 2011 as a reference for the Development of Accelerated Aging Experimental Test Cells for Transformer Oil-immersed Paper Insulation Systems. After achieving the existing goals, the addition of the drying process resulted in quite good Breakdown Voltage test results, not only that after the addition of vacuum it produced a good oil color according to ASTM D1500 as well as after the presence of paper placement racks the results of good tensile strength testing were proven by a graph which relatively decreased from sample 1 to sample 3.

Accurate Assessment of Moisture Content and Degree of Polymerization in Power Transformers via Dielectric Response Sensing.

Power transformers are essential apparatuses used to transfer electrical energy from one voltage-level circuit to another. For reliable systems, preventive maintenance of the transformers is required to ensure good services of all mechanical, electrical, and insulation parts. Oil-immersed paper is most often used for transformer insulation. To ensure such good insulation performance and for assessing insulation conditions, advanced transformer sensing, monitoring, and effective assessment techniques are required. This paper introduces an effective technique for assessing the insulation conditions in power transformers, which are crucial for ensuring reliable energy transfer. The method utilizes advanced transformer sensing and monitoring, focusing on oil-immersed paper insulation commonly used in transformers. The technique employs dielectric response sensing, obtained from frequency-domain spectroscopy tests, to estimate degrees of polymerization (DP) and percentages of moisture content (PMCs) in the oil-immersed paper insulation. These parameters are well-known indicators of insulation performance. The approach is based on the weighted k-nearest neighbor regression, using a database of dielectric loss factors at low frequency and oil conductivities. To overcome limited data availability, linear interpolation and extrapolation techniques are applied to enlarge the database. Experimental verification and comparison with a previously developed method demonstrate the proposed technique's superiority in accuracy and complexity. The maximum deviations of DP and PMC in the validation cases are 6.2% and 18.7%, respectively. In addition, to evaluate the validity of our proposed method in the case of a real power transformer, a comparative analysis of the DP and PMC values determined by the proposed method with those obtained through a previously developed and complicated approach was performed. The predicted results indicate that the DP and PMC values of the oil-immersed insulation fall within the ranges of 800 to 1000 and 1.5 to 2.0, respectively, which agree with the results determined by the complicated approach and closely align with real conditions. By offering a reliable and advanced means of assessing insulation conditions, this technique contributes to the preventive maintenance and overall efficiency of power transformers.

Research on the performance of green and environmentally friendly epoxy resin impregnated paper 110 kV current transformer

As an important high-voltage electrical equipment in the substation, the performance stability of high-voltage current transformers is directly related to the safe operation of the power grid. The insulating medium of the traditional oil-immersed current transformer is an oil-immersed paper structure. When the oil-immersed current transformer malfunctions, it is easy to cause fire and explosion, resulting in very serious consequences. If the transformer with the epoxy resin impregnated paper insulation medium is used, there is no oil and gas inside; thus, it has high reliability, which greatly reduces the risk of fire and explosion. Based on this, considering the application research on the high-voltage current transformers epoxy resin impregnated paper technology, this paper mainly analyzes some precautions for the application of epoxy resin impregnated paper technology to the high-voltage current transformers from three aspects: the research on the insulation performance of the epoxy resin impregnated paper transformers, the research on the mechanical properties, and the research on the relevant experimental methods of the epoxy resin impregnated paper technology application to the high-voltage current transformers.

Evaluasi Kinerja Sistem Isolasi Transformator Kertas Kraft Terendam Minyak Mineral Pada Sel Uji Penuaan Termal Dipercepat

The transformer insulation system in the form of oil-immersed paper, is the most important part of a transformer. This isolation system needs to be ensured that it has the appropriate characteristics, and is resistant to stress due to transformer operation. The most commonly used method for evaluating insulation life is the accelerated thermal aging test. Insulating oil-immersed paper is sealed in a vessel with a ratio in the Accelerated Thermal Aging Chamber with a volume capacity of 3600 ml. It takes about 3000 ml of oil to be poured into a tube with a ratio of insulating oil to insulating paper rolled on copper which is 10:1 and then heated in a thermal oven to accelerate aging. This study aims to develop an Accelerated Aging Chamber for evaluating the performance of transformer isolation systems. The developed test cell is able to reach a set-point of 150°C in 16 minutes with on/off control while PID control in 37 minutes with a Human Machine Interface for monitoring and collecting temperature data in real time, and maintaining that temperature until the end of the experimental period. Theresults of the sample will later be tested for the characteristics of the transformer insulation, namely tensile strength, color scale, andbreakdown voltage due to thermal stress.

Acquisition of FDS for Oil-Immersed Insulation at Transformer Hotspot Region Based on Multiconstraint NSGA Model

The evaluation of the state of transformer oil-immersed insulation based on the traditional methods becomes unreliable due to the nonuniform aging effect. To address this issue, a novel model for accessing the aging information of transformer oil-immersed insulation at the hotspot is proposed. In this article, frequency-domain spectroscopy (FDS) is selected as the carrier for characterizing the aging states of oil-immersed insulation. The multiobjective function for reversing the FDS of the hotspot is constructed based on the dielectric response equivalent circuit. Then, the nondominated sorting genetic algorithm with the multiconstraint scheme is proposed to solve the multiobjective function, by which the FDS data of the transformer hotspot region are acquired. The verification results indicate that the average standard deviation between the computed FDS of the hotspot region and the measured data is less than 0.26. The contribution of this article is in the exploration of the proposed model as a potential tool to extract the FDS data of the hotspot region. This will promote a more reliable aging evaluation of the transformer oil-immersed paper insulation at the hotspot.

Protocol for characterizing the molar mass distribution and oxidized functionality profiles of aged transformer papers by gel permeation chromatography (GPC)

Oil-immersed paper insulation and paper pressboards for structural support are widely used in electrical power transformers. Cellulose thus fulfills an essential task for the smooth power supply of our societies. However, the prevailing temperatures in such equipment, combined with a targeted service life of several decades, pose a serious challenge to the long-term integrity of cellulosic paper insulation. Therefore, numerous studies have been conducted to obtain kinetic data on the degradation processes that contribute to the thermally induced decomposition of cellulose. These studies usually rely on the assessment of the average degree of polymerization by viscosity measurements. In this work, we applied and optimized more advanced methods for the characterization of cellulosic materials based on gel permeation chromatography for the special case of thermally stressed unbleached Kraft paper samples. This allowed studying the molar mass distributions of paper polymers upon exposure to heat, as well as the investigation of changes in their conformation in solution and the observation of thermally induced cross-linking. In combination with group-selective fluorescence labeling, it was possible to track over time the changes in molar mass-dependent profiles of carbonyl and carboxyl groups of authentic Kraft insulator paper samples under thermal stress. In addition, changes of the hemicellulose composition were quantified. We hope that this analytical approach to the in-depth characterization of thermally stressed insulator paper will prove useful for future studies of this important cellulose product, and that our findings will contribute to a better understanding of the thermal decomposition of paper in general.Graphical abstract

The Study of VFTO Distribution in the Insulation System of IOCT Used in Traction Network

The FDS (Frequency-domain Dielectric Spectroscopy) of oil-immersed insulation paper, and semi-conductive paper with different moisture content, has been measured. The data measured are fitted as a function of frequency and moisture content using the amendatory Cole–Cole model utilizing the least square technique. Then, the broadband MTL model of the insulation system of IOCT (Inverted-type Oil-immersed Current Transformer) is established considering the capacitive electrodes thin layer, and the distribution parameters consider the moisture and frequency dependence. A new method for VFTO (Very Fast Transient Overvoltage) distribution calculation of insulation systems is proposed.

Extracting Frequency Spectroscopy of Oil-Immersed Paper Based on Havriliak–Negami Model Without Known Insulation Structure of Transformer

When using the Frequency Dielectric Spectroscopy (FDS) method to evaluate the insulation condition of a transformer, it is essential to calculate the frequency spectroscopy of the oil-immersed paper from the major insulation of transformer with the structural parameters. However, in on-site measurement, the structure of the transformer is often unknown, which greatly limits the application of the FDS method. This paper proposed a method for extracting the frequency spectroscopy of oil-immersed paper from the oil-paper insulation without the known insulation structure. Firstly, the frequency spectroscopy of the oil-immersed paper with different moisture content, aging states, and temperatures was measured and analyzed. The results showed that the Havriliak-Negami (H-N) model with hopping conductivity is the most suitable frequency spectroscopy model for the oil-immersed paper in different conditions. Secondly, the transformer insulation model was constructed, and the frequency spectroscopy of the corresponding oil-immersed paper was calculated. The calculation showed that the error of the structural parameters influences the calculated frequency spectroscopy of oil-immersed paper, especially in the low moisture condition, which leads to the increase of the fitting residual of the H-N model. According to the smallest fitting residual, the structural parameters can be analyzed, and the frequency spectroscopy of oil-immersed paper can be extracted accurately from the oil-paper insulation without the known insulation structure.

Moisture Assessment of Oil-Immersed Paper Based on Dynamic Characteristics of Frequency Domain Spectroscopy

In on-site measurement, as the transformer’s structure is often unknown, the application of frequency domain spectroscopy (FDS) is greatly limited in moisture assessment of transformer. This paper proposes a moisture assessment method for oil-immersed paper based on the dynamic characteristic frequency even for an unknown transformer structure. FDS tests are performed on the oil-paper insulation, corresponding oil-immersed paper, and mineral oil under different moisture conditions. The moisture criterion of oil-immersed paper and the influence of the structural parameters on the calculated frequency spectroscopy at different frequencies are discussed. It is concluded that the moisture criterion’s effectiveness, which indicates the sensitivity of assessed moisture content to the error of the tan <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> , changes at different frequencies. Moreover, there are some frequencies at which tan <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> is not influenced by structural parameters and is close to the measured value, which are regarded as “characteristic frequencies.” Based on the characteristic frequency, the moisture content of oil-immersed paper is assessed according to the frequency spectroscopy of oil-paper insulation without the structural parameters. The result shows that the assessing frequency changes under different moisture conditions, and the error of the assessed moisture content is less than 0.1%.

Effect of BTA on Interface Charge Barrier and Charge Transportation in Oil-paper Insulation

It is known that 1,2,3 benzotriazole (BTA) can effectively limit the oil flow electrification in oil&#x2013;paper insulation used in HVAC transformers, but it is still unclear whether BTA is applicable to converter transformers. Thus, the effects of BTA on interface charge behaviors and charge migration in oil&#x2013;paper system under dc stress are investigated in this article. The experimental results show that BTA reduces the interface barrier between oil and oil-immersed paper by altering the charge distribution of the electric double layer (EDL) at the oil&#x2013;paper interface, which is beneficial to the operation stability of converter transformers in the condition of polarity inversion. However, it is also found that the BTA additive promotes the charge accumulation at oil&#x2013;paper interface and enhances the local electric field by increasing the carrier mobility of oil&#x2013;paper insulation. The experimental results in this article provide references for the selection of insulating oil additives for converter transformers.

Effective Oil-Immersed Paper Insulation Condition Assessment of Power Transformers

Effective Oil-Immersed Paper Insulation Condition Assessment of Power Transformers

Comparative implementation of graphene sheet insulation heat effect in dry-type transformers

Since the distribution transformers are widely used in industry, many variables, including materials, methods, and manufacturing processes directly affect its lifetime. The dry-type transformers are usually used in inner applications while heat dissipation directly influences the lifetime of a transformer, a significant improvement in this area, will result in further design like using the dry-type transformer in a closed box for outdoor applications. The reduction of the internal temperature of DTT also will conclude the reduction of its size. According to insulation properties of graphene sheet, the contribution of insulation of the winding and core is examined. Since the dry-type transformer uses standard oil-immersed paper insulation, the heat dissipation performance could be improved by addition of graphene sheet material. The results of this study showed that the graphene sheet provided a better heat transfer performance than the standard insulation, with decreases of 13–27% in the winding temperature and 24–37% in the core temperature.

Study on partial discharge characteristics of oil-immersed paper in wrinkled state under alternating current and direct current compound voltage

The insulation state of the converter transformer is closely related to the properties of the oil-paper insulation material. The study of the partial discharge characteristics of the wrinkled of the insulation paper has practical engineering significance for the fault diagnosis of the converter transformer. According to the actual operating environment inside the converter transformer, a set of oil-paper-insulated partial discharge measurement platform under the action of AC and DC composite voltage was built. The test results show that insulation paper wrinkles under the action of AC/DC composite voltage will cause insulation damage due to higher surface electric field and partial discharge; the discharge pulse repetition rate and discharge volume increase in the initial stage, development stage and final dangerous stage of discharge. The partial discharge caused by paper wrinkled significantly increases the CH4 content in the dissolved gas in the oil. This conclusion provides a certain basis for the realization of converter transformer fault diagnosis and condition monitoring.

Effect of nano-SiO2 particles modified by 3-aminopropyltriethyloxy silane on mechanical properties and thermal stability of meta-aramid insulation paper

ABSTRACT To further enhance the performance of nano-silica modified M-phenylene isophthalamide (MPIA) insulation paper, nano-SiO2 particles are modified by 3-aminopropyltriethyloxy silane (APTS) surface into meta-aramid insulation paper to form grafting nano-silica particles (GNP). In order to find the relative best performance of the nanoparticle doping ratio, molecular dynamics simulations were used to study the effect of different GNP volume fractions on the mechanical parameters of the MPIA amorphous region, and the highest mechanical performance was obtained with a volume fraction of 9.4 vol. %. Under this doping condition, the interaction between GNP and MPIA fibers was enhanced by the formation of a large number of hydrogen bonds. The addition of GNP decreased the diffusion of water, acid and gas in MPIA fibers and the influence of these substances on the thermal stability of aramid fibers. All these indicate that the addition of GNP further improves the performance of aramid insulation paper, improves the thermal stability of aramid insulation paper and delays the aging process of oil-immersed aramid insulation paper. This article provides a reference and basis for the preparation of meta-aramid insulation paper with relatively good thermal stability.

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

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 q-φ 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.

Comparison of AC breakdown characteristics on insulation paper (pressboard) immersed by three‐element mixed insulation oil and mineral oil

Insulation oil is an important dielectric in power devices, and many studies on mixed insulation oil have been conducted in recent years to improve the performance of insulation oils. To replace mineral oil directly, the authors previously developed a novel three-element mixed insulation oil successfully; the main parameters of which satisfy the IEC 60296-2012 standard for mineral oil. In the present study, the AC breakdown properties of insulation paper (pressboard) immersed by the new mixed oil and naphthenic mineral oil were compared. For both insulation oils, the increase in temperature cannot significantly reduce the breakdown strength of oil-immersed insulation paper (pressboard) at a low moisture content, and the increment of moisture content cannot reduce the breakdown voltage at a low temperature (25°C). The breakdown voltage decreases only when the two factors increase simultaneously. For the mixed and mineral oils, the AC breakdown voltage of oil-immersed paper (pressboard) with different thicknesses has significant difference. The mineral oil has a high breakdown voltage for the thin insulation paper, whereas the mixed oil has a high breakdown voltage when the thickness of the insulation paper (pressboard) exceeds 0.2 mm. This phenomenon is mainly caused by the breakdown field strength that varies with the increase of dielectric thickness and the different change trends of paper (pressboard) immersed with the mixed and the mineral oils. Moreover, the stack of thin multilayer insulation paper enables the mixed oil-immersed paper to have a higher breakdown strength than the mineral oil-immersed paper. For the AC breakdown voltage of oil-immersed pressboard with an oil-gap structure, the mixed oil is comprehensively superior to the mineral oil due to its larger relative permittivity.

Parameters Identification and Application of Equivalent Circuit at Low Frequency of Oil-Paper Insulation in Transformer

The low frequency section of frequency domain spectroscopy (FDS) of oil-paper insu- lation system can effectively reflect the ageing state and moisture of the oil-immersed paper. Meanwhile, steady-state insulation resistance is the one of indexes for the condition of transformer. In actual measurement, these two both are time-consuming and poor accessibility, thereby restraining their field applications. Aim at this problem, this paper analyses the polarization process of the oil-paper insulation in transformer, and adopts a low frequency equivalent circuit model for the oil-paper insulation to characterize the polarization process at low frequency. The simulated annealing particle swarm optimization (SAPSO) algorithm is proposed to identify the parameters in equivalent circuit, which are affected by change of insulation state. Compared with the experimental results, the errors of the steady-state insulation resistance and the tan $\delta $ below 10−3 Hz, which are calculated by the identified parameters, are less than 2.5%. This method can avoid the influence from external environment and reduce the measurement time. The time and frequency domain parameters calculated can provide reference for the ageing assessment of oil-immersed paper.

Improved Inversion Method for Multi-Regional Oil-Immersed Paper Resistivity in Transformer

Oil-paper insulation ageing of transformer has characteristics of spatial locality due to uneven temperature distribution. The transformer is sealed, it is difficult to obtain the local ageing state of oil-paper insulation by sampling directly. Currently, there are no effective detection method to detect the insulation state of local regions. The oil-immersed paper resistivity inversion method can nondestructively assess the local state of oil-paper insulation. However, the current inversion methods still have shortcomings and need to be improved. In this paper, combining the back propagation neural network (BPNN) and Newton downhill method (NDM), an improved inversion method is proposed to optimize the current inversion method, so as to reduce the effect of initial value on it and make calculated results converge better. Finally, a 3D 10 kV transformer model is established to verify the effectiveness of the proposed method. The resistivity calculated by the improved method show that the method can provide initial resistivity values for technicians and improve the convergence and accuracy of current inversion method, which is more suitable for extending to the engineering practice.