Ruijin Liao

A comparative study of physicochemical, dielectric and thermal properties of pressboard insulation impregnated with natural ester and mineral oil

Natural ester is considered to be a substitute of mineral oil in the future. To apply natural ester in large transformers safely, natural ester impregnated solid insulation should be proved to have comparable dielectric strength and thermal stability to mineral oil impregnated solid insulation. This paper mainly focuses on a comparative study of physicochemical, ac breakdown strength and thermal stability behavior of BIOTEMP natural ester/pressboard insulation and Karamay 25# naphthenic mineral oil/pressboard insulation after long term thermal ageing. The physicochemical and dielectric parameters including moisture, acids and the ac breakdown strength of these two oil/pressboard insulation systems at different ageing status were compared. The permittivity and ac breakdown strength of these two oil/pressboard insulation systems at different temperatures were also investigated. And a comparative result of the thermal stability behavior of these two oil/pressboard insulation systems with different ageing status was provided at last. Results show that though natural ester has higher absolute humidity and acidity during the long ageing period, the lower relative humidity of natural ester helps to keep its ac breakdown strength higher than mineral oil. The pressboard aged in natural ester also has higher ac breakdown strength than that aged in mineral oil. The lower relative permittivity ratio of natural ester impregnated paper to natural ester is beneficial to its dielectric strength. Using natural ester in transformer, the resistance to thermal decomposition of the oil/pressboard insulation system could be also effectively improved.

An Integrated Decision-Making Model for Condition Assessment of Power Transformers Using Fuzzy Approach and Evidential Reasoning

This paper presents an integrated model based upon the fuzzy approach and evidential reasoning decision-making approach to condition assessment of power transformers. An assessing index system, including the DGA data, electrical testing, and oil testing, is established to facilitate the assessing model. The model is composed of two levels, that is, the fuzzy model and evidential reasoning model. The fuzzy model is proposed for generating the original basic probability assignments for the second-level model. Afterwards, an evidential reasoning decision-making model is utilized to combine all of the evidence and give an overall evaluation conclusion. Based upon this integrated model, a decision-making procedure is put forward to serve as an effective tool for transformer condition assessments. The results show that the assessing model is capable of offering an overall evaluation of the observed transformer condition.

Charge trapping and detrapping in polymeric materials: Trapping parameters

Space charge formation in polymeric materials can cause some serious concern for design engineers as the electric field may severely be distorted, leading to part of the material being overstressed. This may result in material degradation and possibly premature failure at the worst. It is therefore important to understand charge generation, trapping, and detrapping processes in the material. Trap depths and density of trapping states in materials are important as they are potentially related to microstructure of the material. Changes in these parameters may reflect the aging taken place in the material. In the present paper, characteristics of charge trapping and detrapping in low density polyethylene (LDPE) under dc electric field have been investigated using the pulsed electroacoustic (PEA) technique. A simple trapping and detrapping model based on two trapping levels has been used to qualitatively explain the observation. Numerical simulation based on the above model has been carried out to extract par...

Thermal aging micro-scale analysis of power transformer pressboard

In order to analyze the thermal aging mechanism of the insulation paper inside the power transformer, a series of accelerated thermal aging tests were performed on pressboard. Subsequently, the atomic force microscope (AFM) together with scanning electron microscope (SEM) and X-ray diffraction (X-RD) were utilized to observe the micro surface of the thermal-aged pressboard. The experiments and analysis indicate that either the links among the D-glucopyranose units or the hexagonal mesh structures of the D-glucopyranose units were broken under thermal stress; the number of D-glucopyranose units after 6 weeks of aging was 0.8-1 per nm2, only about one third of un-aged value. The wall of a cellulose cell was deteriorated and thinned by thermal stress. At the same time, the cracks expanded gradually on the surface of the cellulose, which shortened the average width of cellulose fiber from about 40 mu of un-aged sample to about 25 mu after 6 weeks of aging. Meanwhile, the relative crystallinity and the size of the crystallite in the pressboard decreased nonlinearly with the thermal aging time.

Aging condition assessment of transformer oil-paper insulation model based on partial discharge analysis

This paper presents aging condition assessment of oil-paper transformer insulation based on partial discharge analysis in order to realize statistical parameters reduction. The extracted feature factors of this proposed model were used to identify oil-paper samples with different aging degrees. An accelerated aging test was implemented using artificial oil-paper samples with an internal flat air gap. During the aging test, partial discharge signal acquisition was conducted periodically. In the new model, conventional statistical parameters of phase resolved partial discharge (PRPD) patterns were analyzed using principal component and factor analysis (PCFA), and a group of new features constituted by the extracted factors was obtained. These factors were not only independent of one another, they had their own specific properties. To a great extent, these factors represent information on PRPD patterns through a limited number of variables. Through the use of the new features extracted from PCFA method, the clustering and discriminating results of the samples with different aging stages provided significantly referenced information on the condition assessment of oil-paper insulation.

Quantitative analysis ageing status of natural ester-paper insulation and mineral oil-paper insulation by polarization/depolarization current

Polarization and Depolarization Current (PDC) technique is an effective tool to assess the condition of oil-paper insulation system in power transformers. So far the PDC behaviors of mineral oil-paper insulation have been widely investigated. However, with the increasing number of transformer choosing natural ester as its insulation oil, it is important to investigate the PDC characteristics of natural ester-paper insulation to see whether the PDC technique can also be used to assess the condition of new insulation system using natural ester in transformers accurately. In this research, natural esterpaper insulation sample and mineral oil-paper insulation sample were subjected to thermally accelerated ageing experiment at 110°C for 120 days. The PDC characteristics of natural ester-paper insulation sample and mineral oil-paper insulation sample were compared over the ageing process. A new method for assessing the ageing condition of the oil-paper insulation in terms of the depolarization charge quantity was proposed. Results show that the polarization/depolarization current of natural ester-paper insulation sample is higher than that of mineral oil-paper insulation sample with the same ageing intervals. The depolarization charge quantity of both kinds of oil-paper insulation sample is very sensitive to their ageing conditions. There is an exponential relation between the stable depolarization charge quantity of both kinds of oil-paper insulation sample and the degree of polymerization (DP) of paper. The depolarization charge quantity can be used to predict the ageing condition of oil-paper insulation providing the measurement temperature is kept the same.

Influence of moisture on space charge dynamics in multilayer oil-paper insulation

The main insulation system in HVDC transformer consists of oil-paper insulation. The formation of space charge in oil/paper insulation system can not only affect the dielectric strength, but also relate to the stability and security of HVDC transformer. Moisture has detrimental effect on the electrical breakdown strength and insulation life of oil-paper insulation. Thus a better understanding the influence of moisture on the charge distribution in multi-layer oil-paper insulation system becomes increasingly practical importance. In the present study, the space charge dynamics in three layers oil-paper insulation system with three different moisture concentrations (0.28, 1.32 and 4.96%) were investigated using the PEA technique under 6 kV. The influence of moisture on the charge distribution in multi-layer oil-paper insulation system was provided. Results show that the higher the moisture content of the oil-paper sample, the higher the mobility of the trapped charges, and the more obvious the positive charge injection and negative charge injection were observed. The negative charges can inject across the paper-paper interface near to the cathode when the moisture content increases to 4.96%. The higher moisture content of the oil-paper sample, the faster the dissipation rate of the charges trapped in the sample. Moreover, the higher mobility of the trapped charges leads to less slow moving charges trapped in the sample with higher moisture content.

Oil-paper aging evaluation by fuzzy clustering and factor analysis to statistical parameters of partial discharges

A thermal aging experiment was conducted for oil-paper insulation to evaluate the insulations response under thermal stress. Oil-papers at different aging stages were used to create oil-paper bound gas cavity specimens, which were used to collect data on degrees of polymerization (DP) and partial discharge (PD). Evaluation of oil-paper aging was based on statistical operators of PD and factor analysis was used to extract the principle parameters of PD. Three types of fuzzy clustering approaches were used to classify PD of aged oil-paper: the fuzzy c-means, the kernel fuzzy c-means, and the possibilistic fuzzy c-means. The clustering results showed that the possibilistic fuzzy c-means clustering was capable of classifying PD of oil-paper bound gas cavity specimens. The factor analysis method was also verified to be helpful in fuzzy clustering of PD data samples by reducing number of parameters.

Optimal features selected by NSGA-II for partial discharge pulses separation based on time-frequency representation and matrix decomposition

This paper presents a feature extraction algorithm for partial discharge (PD) pulses separation using S transform (ST)-based time-frequency representation. Firstly, the algorithm acquires a series of base vectors in the frequency domain and location vectors in the time domain obtained by applying a non-negative matrix factorization (NMF)-based matrix decomposition technique to compress ST amplitude (STA) matrices of PD pulses. Then, a new group of features including sharpness, sum of derivatives, sparsity, entropy, mean value and standard deviation is extracted from the base and location vectors, which is further separated by a fuzzy C-means (FCM) clustering algorithm. Finally, non-dominated sorting genetic algorithm II (NSGA-II) is introduced as a feature selection tool to improve the FCM clustering performance and acquire the corresponding selected feature subsets. The 600 PD pulses sampled from four typical defect models are adopted for testing. It is shown that a minimum clustering error of 7.67% with 4 dimensional optimal feature subset selected by NSGA-II is achieved when NMF parameter r = 1. In addition, NSGA-II can not only reduce the feature dimension but also dramatically improve the FCM clustering performance compared with the original extracted features. The selected four features are also examined by the data of two PD sources simultaneous active. The results demonstrate that it is feasible to apply the proposed algorithm to PD pulses separation.

Comparison of ageing results for transformer oil-paper insulation subjected to thermal ageing in mineral oil and ageing in retardant oil

The properties of insulation paper immersed in ageing-retardant oil and mineral oil were studied through a 264 days ageing experiment. Measurements of the color of the oil, the furfural concentration in the oil, water concentration, the degree of polymerization, and the acid concentration in the paper were performed. The results show that the ageing-retardant oil cannot only resist the oxidation of the oil itself, but also the ageing of the paper immersed within. The relevant inhibition mechanisms were studied. This paper demonstrates the ageing-retardant oil is capable of bounding water molecules. In addition, the esterification, which can resist the hydrolysis of the insulation paper, was studied through infrared spectrum analysis. Furthermore, this paper proposes that the combination of esterification and the hydrogen bonds will contribute to the anti-ageing ability.