Comprehensive evaluation of the transformer oil-paper insulation state based on RF-combination weighting and an improved TOPSIS method

The accurate identification of the oil-paper insulation state of a transformer is crucial for most maintenance strategies.This paper presents a multi-feature comprehensive evaluation model based on combination weighting and an improved technique for order of preference by similarity to ideal solution(TOPSIS)method to perform an objective and scientific evaluation of the transformer oil-paper insulation state.Firstly,multiple aging features are extracted from the recovery voltage polarization spectrum and the extended Debye equivalent circuit owing to the limitations of using a single feature for evaluation.A standard evaluation index system is then established by using the collected time-domain dielectric spectrum data.Secondly,this study implements the per-unit value concept to integrate the dimension of the index matrix and calculates the objective weight by using the random forest algorithm.Furthermore,it combines the weighting model to overcome the drawbacks of the single weighting method by using the indicators and considering the subjective experience of experts and the random forest algorithm.Lastly,the enhanced TOPSIS approach is used to determine the insulation quality of an oil-paper transformer.A verification example demonstrates that the evaluation model developed in this study can efficiently and accurately diagnose the insulation status of transformers.Essentially,this study presents a novel approach for the assessment of transformer oil-paper insulation.

Chaotic Characteristic of Time Series of Partial Discharge in Oil-Paper Insulation

The chaotic characteristics of time series of five partial discharge （PD） patterns in oil-paper insulation are studied. The results verify obvious chaotic characteristic of the time series of discharge signals and the fact that PD is a chaotic process. These time series have distinctive features, and the chaotic attractors obtained from time series differed greatly from each other by shapes in the phase space, so they could be used to qualitatively identify the PD patterns. The phase space parameters are selected, then the chaotic characteristic quantities can be extracted. These quantities could quantificationally characterize the PD patterns. The effects on pattern recognition of PRPD and CAPD are compared by using the neural network of radial basis function. The results show that both of the two recognition methods work well and have their respective advantages. Then, both the statistical operators under PRPD mode and the chaotic characteristic quantities under CAPD mode are selected comprehensively as the input vectors of neural network, and the PD pattern recognition accuracy is thereby greatly improved.

The Research of Connection between Degree of Polymerization and Frequency Domain Dielectric Spectroscopy Characteristics during Oil-Paper Insulation Aging

In order to study the linkage effects between degree of polymerization and frequency domain dielectric spectroscopy characteristics of oil-paper insulation, the frequency domain dielectric response test platform of oil-paper insulation is set up. Complex permittivity of oil-paper insulation respectively composed by new or aged oil and insulation paper with different DP are tested, and complex permittivity of oil-paper insulation respectively composed by insulation respectively composed by new oil and insulation paper with different DP and low or high moisture content are tested. The test results are analyzed, and the analysis results show that the degree of polymerization of insulation paper has an influence on complex permittivity of oil-paper insulation though influencing the distribution of moisture and acids between oil and paper.

Low Frequency Dispersion Mechanism of Dielectric Response for Oil-paper Insulation Diagnosis

Both the real part and imaginary part of complex permittivity approximately have a log-linear frequency dependency at low fre- quencies, especially at ultra-low frequencies under conditions of different moisture concentrations and temperatures, which is recognized as the low frequency dispersion (LFD). In order to explain this dispersion, a new mechanism of dielectric response of LFD of oil-paper insula- tion is proposed. A simplified one-dimensional mathematical model of concentration polarization carrier caused by slow migration is developed and solved, which indicates that ion mobility is closely related to the size of gap and the adsorption capacity of cellulose molecu- lar chains to ions. A stochastic statistical model of the carrier mobility induced LFD is also developed. Moreover, actual tests under 50 °C and 2% moisture content were put forward, as well as simulations with according current waveforms. The simulation results agreed well with the experimental data in that concentration polarization of carriers caused by slow migration is the probable cause of low frequency dispersion of dielectric response for oil-paper insulation diagnosis.

Improvement on Condition Assessment Model for Oil-paper Insulation

Identification of the aging condition and the failure probability of oil-paper insulation in transformer is important for improving the reliability of electric power transmission system and applying life cycle cost (LCC) management to electrical equipment. Based on data obtained in a series of multi-accelerated-aging experiments, two approaches for calculating failure probability of oil-paper insulation were compared in aspects of degree of polymerization (DP) and condition ranking. In the experiments, mineral oil and cellulose paper are sub- jected to electrical and thermal stresses, and several parameters, including dissolved gases’ volume fraction, furfural content, moisture content, and degree of polymerization, are measured after the aging process. Results show that weight of carbon oxide, which has a close relationship with cellulose paper degradation, is much higher in DP model than in condition ranking model. Moreover, it is concluded that DP model is more practically accurate than condition ranking model, because aging of cellulose paper rather than mineral oil is the key and critical factor of oil-paper insulation aging.

Space Charge Characteristics of Oil-paper Insulation with New Paper and Aged Oil

Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performances. The insulation gradually degrades under combined thermal, electrical and chemical stresses during routine operations, mainly because of space charges inside. This work investigated the space charge characteristics in oil-paper insulation under oil aging circumstance. New trans- former oil samples are thermally aged to obtain different aging states, and their physical and chemical properties are analyzed. New Kraft papers are dried in vacuum and fully immersed in these different aged oil samples, and three kinds of oil-paper samples are obtained. We use the pulsed electro-acoustic (PEA) method to measure space charge under both DC voltage-on and voltage-off conditions at room temperature. The effect of oil aging state on characteristics of space charge injection, accumulation, and decay is analyzed and discussed. The results show that comparing with the DC voltage-off condition, more charges are injected into samples at the interface of electrode and dielectric when DC voltage is on. When the oil-aged state gets worse, more charges are induced at both cathode and anode, more space charges are accumulated in the bulk, the area of negative charges is larger, and local electric field is distorted more seriously. Moreover, for the voltage-off condition, aged oil is good for space charge decay, and trapped positive space charges decay faster than trapped negative charges.

A New Method for the Aging Evaluation of Oil-Paper Insulation Using η-Butanol and Methanol

To improve the accuracy and efficiency of the aging life prediction and assessment of transformer oil-paper insulation,and to make up for the deficiencies of traditional characterizers,such as 2-furfural,carbon monoxide,and carbon dioxide,a method for the simultaneous determination of methanol,ethanol,n−propanol,and η-butanol in oil with a single injection is established by headspace-gas chromatography-mass spectrometry.The measured results show that the determination limits of the four alcohol characterizers can be controlled to 10μg/kg level.Based on this method,the change patterns of the above four alcohols with thermal aging time and degree of polymerization are obtained through thermal aging experimental research.Ethanol,η-propanol,and η-butanol in oil indicate nearly linear correlations with thermal aging time and degree of polymerization,similar to that of methanol.By analyzing 52 sets of measured data of 500 kV EHV transformers in operation,η-butanol is found to have excellent performance,and a new method to evaluate the aging state of oil-paper insulation employing η-butanol and methanol is proposed along with the aging attention value model.The measured data of 500 kV EHV transformers in operation indicate that the combination of η-butanol and methanol as the preferred characterizers can effectively compensate for the shortcoming of traditional characterizers in the early stages of aging,and the feasibility of the method is verified.Two possible pathways for the generation of η-butanol by cellulose cleavage during the aging of oil-paper are proposed from the chemical structure of cellulose.

Dynamics of an impurity ion transport in oil-paper insulation under various electric fields

The ionic transport process is an important counterpart to charge transport in oil-paper insulation,and it significantly impacts oil flow electrification at the oil-paper interface.Despite this,the dynamics of this phenomenon and the underlying mechanisms remain unclear,particularly at the molecular level.To understand this fundamental aspect,we conduct Molecular Dynamics study on the transport behaviour of an impurity ion in different oil-paper insulation models under various external electric fields.Different influence factors,such as external electric fields,temperatures,and local structural characteristics,are investigated in relation to the corresponding ionic mobility in different polymer models.According to the simulations,ionic mobility and its response to electric fields are higher in weaker electrostatic models.As a result,mineral oil exhibits the highest ionic mobility and the most substantial enhancement of ionic mobility by external electric fields,followed by vegetable oil,oil-paper blends,and insulating paper.This significant deviation in ionic mobility between oil and paper leads to the formation of an electric double layer near the oil-paper interface.The underlying physical mechanisms of different ionic mobility and its response to the electric field in different polymer models could be explained by the different polymer structural influences in terms of interaction energy and coordination numbers in a static manner,as well as by the interactions between the impurity ion and its surrounding atoms in terms of lifetime correlation functions and velocity autocorrelation functions in a dynamic manner.In addition,the influence of temperature on ionic mobility in mineral and vegetable oil is examined,and their activation energies are calculated.Advancing in the fundamental understanding of the dynamics of the ion transport process in oil-paper insulation is vital to improving their insulating properties for oil-impregnated power transformers.

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

The 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.

Microscopic mechanisms analysis of various dielectric response processes in oil-paper insulation with different insulating states

Various dielectric response processes in the oil-paper insulation are sensitively affected by the insulating states(ageing degrees and moisture contents).However,the existing research is still incomplete in revealing the microscopic mechanisms of various dielectric response processes in oil-paper insulation with different insulating states.Given this issue,the genetic algorithm is first adopted to extract the Dissado–Hill(D–H)model parameters by simulating the frequency domain spectroscopy(FDS)of oil-paper insu-lation.Then,the change laws of the extracted D–H model parameters are adopted to reveal the microscopic mechanisms of various dielectric response processes.Microscopic mechanisms of four dielectric response processes are studied,which are quasi-dc relax-ation,loss peak relaxation,optical frequency relaxation,and DC conductance.Meanwhile,due to the dielectric response processes being dominated by various polar particles(methanol,ethanol,furfural,and water molecule),the contents of various ageing by-products dissolved in the insulating oil are measured to support the above analysis.In this respect,a dielectric theoretical reference for the FDS technique to research the insulating states of oil-paper insulation is provided.

Research on the variation of dielectric properties of oil-paper insulation for power equipment over a wide temperature range

The main part of oil-immersed power equipment is oil-paper insulation.At lower ambient temperatures,transformer oil will become viscous.As a result,compared to those at room temperature or operating temperature,the frequency domain dielectric properties of oil-paper insulation at low temperatures are drastically different.For evaluating the insulation condition of electrical equipment,it is essential to accurately determine their dielectric properties.This paper described the FDS test,the DC conductivity test,and the transformer oil viscosity test,which was carried out in a laboratory environment for different ageing of oil-immersed pressboard and transformer oil at 233–373 K.The ef-fects of temperature on the dielectric properties at the competing mechanisms of polarisation and conductivity loss were clarified based on variations of the FDS curves.Considering the viscosity change of ageing transformer oil at low temperatures,the Arrhenius and VFT viscosity equations were used to achieve a fitting calculation in the different temperature zones.Based on the molecular/ion transition model in the force field and electric field,the characterisation relationships between the dynamic viscosity,DC conductivity,and test temperature of transformer oil were established.The limita-tions of conventional transformer oil equivalent dielectric relaxation models over a wide temperature range were finally clarified by combining the test findings of the DC con-ductivity over a wide temperature range with observed and simulated calculations.This paper provides an essential theoretical reference when using the FDS test to diagnose the insulation performance of oil-immersed power equipment in extremely cold regions.

Research on the feature extraction of DC space charge behavior of oil-paper insulation

As the ultra high DC voltage grid comes into an important stage in the ‘Twelfth Five-year Plan’ of China, the requirement on the operation safety of power equipment needs to be improved. Oil-paper insulation plays a significant role as the main insulation material of many key power equipments. The space charge behaviors of oil-paper insulation not only affect the dielectric strength, but also relate to the stability and security of power transmission facilities. Until now, the research on the space charge behaviors of oil-paper insulation remains on the stage of objective description and phenomenon explanation, thus it is very urgent to make some feature extractions on the test results in order to further investigate the DC space charge behaviors of oil-paper insulation. In this paper, based on the large numbers of test data in the first stage, data mining and feature extractions were performed on the DC space charge behavior of oil-paper insulation. The variation laws of some key characteristic parameters obtained in different experiment conditions such as the total charge injection amount, apparent charge mobility, and trap energy distribution were analyzed. In addition, the distribution of trapping level inside oil-paper was calculated by using the PEA test results based on the mechanism of the isothermal decay current, which realized the approximate calculation of trap depth by charge decay characteristics. What’s more, the exponential injection and decay equations of DC space charge changing with time were acquired for the first time, and the physical meanings of related parameters were defined. This has laid a foundation for the prediction and simulation of space charge in the oil-paper insulation.

Space Charge Characteristics of Oil-paper Insulation in the Electro-thermal Aging Process

High electric field and temperatures can result in electrical property degradation of oil-paper insulation and accelerate its aging process.This paper describes the regulation of space charge in the process of electro-thermal aging.Electric field distortion and differences in characteristic parameters during electro-thermal aging are analyzed,along with the effects of electro-thermal aging on space charge from the perspective of trap energy.High temperature and an electric field are used to accelerate the aging of oil-paper,and space charge characteristics are measured using the pulsed electro-acoustic(PEA)method throughout the electro-thermal aging.Based on results obtained,it can be concluded that homo-charge injection occurs at the anode and the type of the charge injection at the cathode varies throughout the electro-thermal aging;the decrease of permittivity during aging allows for space charge injection to take place;growth of trap depth makes the space charge accumulate in the middle of the sample;and space charge accumulation after electro-thermal aging results in inner electric field distortion,which leads to large current and decreased breakdown in voltages.

Flexible and Robust Functionalized Boron Nitride/Poly(p‑Phenylene Benzobisoxazole)Nanocomposite Paper with High Thermal Conductivity and Outstanding Electrical Insulation

With the rapid development of 5G information technology,thermal conductivity/dissipation problems of highly integrated electronic devices and electrical equipment are becoming prominent.In this work,“high-temperature solid-phase&diazonium salt decomposition”method is carried out to prepare benzidine-functionalized boron nitride(m-BN).Subsequently,m-BN/poly(pphenylene benzobisoxazole)nanofiber(PNF)nanocomposite paper with nacremimetic layered structures is prepared via sol–gel film transformation approach.The obtained m-BN/PNF nanocomposite paper with 50 wt%m-BN presents excellent thermal conductivity,incredible electrical insulation,outstanding mechanical properties and thermal stability,due to the construction of extensive hydrogen bonds andπ–πinteractions between m-BN and PNF,and stable nacre-mimetic layered structures.Itsλ∥andλ_(⊥)are 9.68 and 0.84 W m^(-1)K^(-1),and the volume resistivity and breakdown strength are as high as 2.3×10^(15)Ωcm and 324.2 kV mm^(-1),respectively.Besides,it also presents extremely high tensile strength of 193.6 MPa and thermal decomposition temperature of 640°C,showing a broad application prospect in high-end thermal management fields such as electronic devices and electrical equipment.

Hindered phenolic antioxidant grafting on tailoring the DC electrical characteristics of polypropylene cable insulation

The authors focus on the impact of melt-free radical grafting with hindered phenolic antioxidants(AO3052)on the electrical properties of polypropylene(PP)for DC cable insulation.The DC conductivity,space charge distribution and breakdown characteristic tests of grafting-modified PP are performed by comparing unmodified PP.The results demonstrate that the grafting of antioxidants can effectively suppress space charge injection,owing to the deeper trap sites at the grafting molecule.The breakdown strength of the grafted PP is significantly enhanced from 30°C to 90°C and especially achieves a 5.3%-6.7%increase after the same DC-prestressed time at 90°C.The surface electrostatic potential and molecular orbitals of the grafted PP are calculated.Simulation shows that the antioxidant introduces multi-level local state traps that can effectively trap the injected space charge,thus decreasing the destruction of molecular chains by electrons and increasing the breakdown strength level.In conclusion,antioxidant grafting modification can improve the breakdown characteristics with or without DC prestress,and thus it appears to be promising in the application of PP-insulated cables.

Thermal runaway and induced electrical failure of epoxy resin in high-frequency transformers:Insulation design reference

Solid-state transformers(SSTs)have applications in medium-voltage direct current(MVDC)grids and compact power systems.High-frequency transformer(HFT)is the core component of SSTs.High levels of high frequency high dv/dt voltage stresses challenged the integrity of the galvanic insulation of HFTs.However,dielectric thermal runaway and resultant electrical failure mechanisms in epoxy resin(EP)cast insulation remain unclear.Dielectric heating of EP across varying voltages,frequencies,rising edges,duty cycles and DC biases were measured and corroborated by simulation.The thermal runaway threshold mainly depends on the tangency point of the loss generation and heat dissipation curves below the glass transition temperature.Observations reveal that thermal runaway does not directly cause breakdown;instead,thermal decomposition above 200°C triggers discharge and eventual failure.Simulations demonstrate that temperature rise mainly depends on the average field within the electrode region and intersegment and inter-layer distances within the HFT winding definitively impact insulation thermal runaway.By applying different criteria for MV and high-voltage(HV)transformers,the reference electric fields for insulation design with unfilled and filled EP were obtained.For instance,limiting dielectric heating below 5 K at 50 kHz necessitates an RMS average field less than 0.44 V/mm,which is much lower than dry-type transformer conventions.The authors prove the necessity of re-evaluating the permissible field strength in HFT insulation design.

Nano-silica modified lightweight and high-toughness carbon fiber/phenolic ablator with excellent thermal insulation and ablation performance

Lightweight and high-toughness carbon fiber/phenolic ablator(CFPA)is required as the Thermal Protection System(TPS)material of aerospace vehicles for next-generation space missions.To improve the ablative properties,silica sol with good particle size distribution prepared using tetramethoxysilane(TMOS)was blended with natural rubber latex and deposited onto carbon fiber felt,which was then integrated with phenolic aerogel matrix,introducing nano-silica into the framework of CFPA.The modified CFPA with a low density of 0.28—0.31 g/cm3exhibits strain-in-fracture as high as 31.2%and thermal conductivity as low as 0.054 W/(m·K).Furthermore,a trace amount of nano-silica could effectively protect CFPA from erosion of oxidizing atmosphere in different high-temperature environments.The oxyacetylene ablation test of 3000°C for 20 s shows a mass ablation rate of 0.0225 g/s,a linear ablation rate of 0.209 mm/s for the modified CFPA,which are 9.64%and 24.82%lower than the unmodified one.Besides,the long-time butane ablation test of 1200°C for 200 s shows an insignificant recession with mass and linear ablation rate of 0.079 g/s and 0.039 mm/s,16.84%and 13.33%lower than the unmodified one.Meanwhile,the fixed thermocouple in the test also demonstrates a good thermal insulation performance with a low peak back-face temperature of 207.7°C,12.25%lower than the unmodified one.Therefore,the nano-silica modified CFPA with excellent overall performance presents promising prospects in high-temperature aerospace applications.

Intelligent identification method of insulation pull rod defects based on intactness-aware Mosaic data augmentation and fusion of YOLOv5s

The authors introduce the intactness-aware Mosaic data augmentation strategy,designed to tackle challenges such as low accuracy in detecting defects in insulation pull rods,limited timeliness in intelligent analysis,and the absence of a comprehensive database for information on insulation pull rod defects.The proposed strategy incorporates the YOLOv5s algorithm for detecting defects in insulation pull rods.Initially,the YOLOv5s network was constructed,and a dataset containing photos of insulation pull rods with white spots,fractures,impurities,and bubble flaws was compiled to capture images of defects.The research presented a data enhancement approach to improve the images and establish a dataset for insulation pull rod defects.The YOLOv5s algorithm was applied for both training and testing purposes.A comparative analysis was conducted to assess the detection performance of YOLOv5s against a conventional target detector for identifying defects in insulation pull rods.Furthermore,the utility of Mosaic's data augmentation technique,which incorporates intactness awareness,was evaluated to enhance the accuracy of identifying insulation pull rod defects.The research findings indicate that the YOLOv5s algorithm is employed for intelligent detection and precise localisation of flaws.The intactnessaware Mosaic data augmentation strategy significantly improves the accuracy of detecting faults in insulation pull rods.The YOLOv5s model used achieves a performance index mAP@0.5:0.95 of 0.563 on the test set,distinct from the training set data.With a threshold of 0.5,the mAP@0.5 score is 0.904,indicating a substantial improvement in both detection efficiency and accuracy compared to conventional target detection methods.Innovative approaches for identifying defects in insulation pull rods are introduced.