Oil-impregnated Insulation Research Articles

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.

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.

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.

Evaluation of Abnormal Conditions of Oil-impregnated Insulation Papers in Aged Oil-filled Cable Joints and Terminations Removed After Service and Proposal of Classification Method for Ranking of their Abnormal Conditions

Degradation rate of electrical insulation performance of oil-filled (OF) cables has been considered small since void-less conditions are realized due to their oil-impregnated insulation systems under insulation oil pressure that exceeds atmospheric pressure. However, recently, insulation breakdowns have occurred in OF cable joints at extra-high voltage, and they were not predicted by dissolved gas analysis (DGA) of insulation oil. Therefore, it is necessary to improve the prediction accuracy of DGA for detection of partial discharges (PDs), which are known as a pre-breakdown phenomenon. Recently, it has been reported that traces of black color in oil-impregnated insulation papers are sometimes compounds of oxide sludge or copper sulfide, which were all regarded as carbonization due to PDs. Moreover, it has been reported that PD inception fields decrease with an increase in the rate of the total thickness of insulation papers with discoloration to the insulation thickness. To improve the prediction accuracy of DGA, such new findings should be taken into account when ranking abnormal conditions of insulation systems of cable joints and terminations, which gives the basic data for the prediction by DGA. In this study, abnormal conditions of oil-impregnated insulation papers were redefined and classified into trace of occurrences of PDs and two types of discoloration, and those of 264 aged OF cable joints and terminations removed after service were investigated. In addition, the above-mentioned rate was quantitatively evaluated for cable insulation systems, and locations of the abnormal conditions were investigated in detail for reinforced insulation systems. Taking these results into account, a new classification method for the ranking of abnormal conditions of oil-impregnated insulation systems was proposed.

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

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

The influence of polarity reversal time on interface charges in oil-impregnated pressboard insulation under non-uniform electric field

The polarity reversal test is one of the routine tests for the evaluation of electric insulation in converter transformers. The polarity reversal time (PRT) has significant effect on the accumulation of interface charges in oil-impregnated pressboard insulation. IEC standard recommends that polarity reversal should be performed within 2 min so as to match with the dispersion rate of interface charges in uniform filed, but not necessarily in the non-uniform field. In order to investigate the influence of PRT on the dissipation rate of interface charges in non-uniform electric field, the density of interface charges was measured by an electrostatic capacitance probe in the needle-plate non-uniform field at the PRT of 2 ms, 4 ms and 2 min respectively. The results showed that the polarity of interface charges did not change with the applied voltage jump at the PRT of 2 ms and 4 ms, while the polarity of interface charges almost synchronously changed with the polarity reversal of applied voltage at the PRT of 2 min. For the PRT of 2 ms and 4 ms, the electric field nearby the needle electrode would be enhanced with applied electric field of the same direction, whilst the total electric field of insulation system would be weakened at the PRT of 2 min. It was inferred that polarity of interface charge might have not enough time to change in polarity reversal test within the PRT of 4 ms under non-uniform electric field, which was more close to the severe condition of covert transformer. The paper suggest that the value of PRT be reconsidered when the polarity reversal test is performed in non-uniform electric field.

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.

Influence of temperature and moisture joint action on oil-paper insulation compound electric field distribution

In the paper, the electric field change in transformer oil and oil-impregnated insulation paperboard within wide temperature range (20°C~100°C) was studied. The influence law of temperature on oil-paper insulation electric fields was summarized. Oil-paper insulation electric field distribution under the action of different wave-form voltages was simulated for analysis. The electric field distribution condition during moisture balance steady state and transient state was obtained. The reasons of electric field distribution change were analyzed.

Frequency domain dielectric response of oil gap in time-varying temperature conditions

The frequency domain dielectric response method has been one of the most effective approaches for the condition assessment of transformers. Prior to conducting the test, the transformer should be taken offline and given adequate time to reach internal temperature equilibrium in order to diminish any temperature-related effects. This can greatly increase the maintenance time; moreover, there is not enough time for some special transformers to reach this internal temperature equilibrium; for example the traction transformers used in the railway industry. The oil-paper insulation in transformers was composed of oil gap, oil-impregnated insulation paper (board). The study related to the dielectric response of the oil gap under time-varying temperature conditions is the basis for studies regarding oil-paper insulation. In this work, the frequency domain dielectric spectrums of oil gap were tested in three cases with different temperature change rates, and the currents in the three cases were tested when the oil gap was applied with low frequency excitations. A current model of the oil gap has been presented here, which is applicable to the time-varying temperature conditions. The current model was verified by comparing the simulated and measured currents. During the verification process, the parameters used in the current model were obtained on the basis of experimental tests. When combined with the established model, a method was developed to obtain the modified results in time-varying temperature conditions; this method was verified by comparing the results calculated from the proposed model with experimentally measured values. In the modification process, the parameters used in the current model were obtained by directly analyzing the tested frequency domain dielectric spectrum of oil gap, rather than through experimental tests, which is more suitable for the maintenance of field transformers. Lastly, the frequency domain dielectric spectrum of oil gap tested in time-varying conditions can be modified based on the proposed model and method.

An analysis of AC conductivity in moist oil-impregnated insulation pressboard

The paper presents research results of testing AC conductivity in a moist oilimpregnated electric insulation pressboard, depending on the moisture content level and temperature of the samples. A new method has been elaborated for converting experimentally obtained dependences from frequency-domain spectroscopy (FDS) to the reference temperature of 293 K (20 /dpl deg/C). Activation energy values have been determined for the conduction and relaxation time. It has been found the conductivity is realized by electron tunnelling between potential well formed by water molecules contained in the structure of moist cellulose.

Space Charge Behavior in Oil-Impregnated Insulation Paper Reinforced with Nano-TiO2

Oil-impregnated insulation paper is widely used in power transformers, and the insulation properties of oil-impregnated insulation paper play an important role in the reliability of power equipment. The formation and dynamics of space charge can affect the performance of insulation material. However, methods to improve the space charge distribution in oil-impregnated insulation paper are rarely reported. In this paper, space charge behavior in oil-impregnated insulation paper has been investigated using the pulsed electro-acoustic (PEA) technique. A series of measurements was applied when the oil-impregnated insulation paper reinforced with different nano-TiO2 contents was subjected to various electric field strengths. The accumulation and decay of space charge are discussed, and the internal electric field strength distribution of oil-impregnated insulation paper is analyzed. The test results show that the space charge distribution is improved and the distortion rate of the internal electric field strength is reduced by adding nano-TiO2 to the oil-impregnated insulation paper. The results show that the proposed method offers a new way to improve the properties of oil-impregnated insulation paper.

Measurements and analysis of surface damage on oil-impregnated insulation paper caused by partial discharge

Surface topography, surface roughness, and surface conductivity of oil-impregnated insulation paper were studied during the damage process caused by partial discharge within the cavity of the paper. Products generated on the surface during this process were studied, as well. According to phase-resolved partial discharge patterns, the damage process can be divided into five stages. At each of the stages, surface conditions of insulation were analyzed by optical microscopy, scanning electron microscopy, atomic force microscopy and high-resistance meter. ‘Ablating’, ‘peeling’, ‘cracking in silk’, ‘pitting’ and ‘electrical treeing’ appear on insulation surfaces one after another during the five stages of the damage process, along with sequential generation of droplets and crystalline solids. Surface roughness initially decreases, then increases. Finally, surface conductivity exhibits a general increasing trend, before it eventually stabilizes. However, its growth rate varies in different stages of damage.

Product analysis of partial discharge damage to oil-impregnated insulation paper

Surface products of oil-impregnated insulation paper during the damage process caused by partial discharge (PD), as well as gas within the cavity, were studied. An optical microscope and a scanning electron microscope (SEM) were used to investigate surface morphology, while an infrared spectroscopy (IR) and an X-ray photoelectron spectroscopy (XPS) were used to study surface products and their components. The volume variation in cavity gas was also analyzed. Furthermore, gas constituents and their relevant contents were studied using a gas chromatography–mass spectrometer (GC–MS). The study results reveal the following: during the PD damage process, the total gas volume and the content of electronegative gasses alternately decline and increase, while discharge types alternate between pulse type and pseudo-glow type (or glow type); “surface droplets” and “crystalline solids” appear on the insulation surface one after another; surface droplets mainly consist of (CO)-group-containing compounds, whereas crystalline solids are mainly carboxylic acids, with carboxyl groups also found in cellulose chains; and the discharge type related to the oxidization of decomposition products is the main factor that determines the state (liquid or solid) of the surface products.

Investigation on dielectric response characteristics of thermally aged insulating pressboard in vacuum and oil-impregnated ambient

Dielectric response measurements are non-invasive and promising diagnostic methods that are being gradually used for assessing the insulation and aging condition of transformer oil-paper insulation system. It is known that moisture content in oil-impregnated insulation has significant effect on its dielectric response phenomena, and whereas at present there is few research about the characteristics of aged cellulose without the impact of water. In this work, a series of experiments are designed and carried out in well-controlled laboratory. Accelerated thermal aging is performed on oil-immersed pressboard samples with different time intervals (0, 120, 250 and 400 h). Then these pressboard samples with different aging states are put into petroleum ether to remove the residual oil inside, and dried in a vacuum oven to remove the petroleum ether and moisture. The degree of polymerization (DP) and scanning electron microscope (SEM) are implemented to characterize the aging status of pressboard samples. Two kinds of dielectric response tests, i.e., the polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS), are performed on each oil/water-removed pressboard sample in vacuum and new-oil-impregnated ambient, respectively. Samples with different aging states show quite different dielectric response characteristics in both ambients. With the aging status more serious, the pressboard sample exhibits higher values of polarization/depolarization currents and complex capacitances. It is confirmed that, besides the influence of water induced by aging, the deterioration of cellulose itself also causes the increasing of pressboard conductivity, and hence greatly affects the PDC and FDS test results. Dielectric response diagnostic techniques have a potential to reflect the aging information of solid insulation.

The performance of power cables with fully synthetic taped insulation

The testing of oriented polyethylene tape with an embossed surface design for use as oil-impregnated insulation for flexible power cables is described. The development was carried out with small samples in the form of sheets, small cylindrical samples about 1 m long, and long cables either 15 m or 100 m long. Tests with these experimental configurations included investigating: (1) breakdown stress and statistics of breakdown; (2) dielectric loss measurements; (3) design of screens; (4) design of splices; (5) mechanical problems of taping and installing cables; and (6) oil impregnation and type of oil. The results point the way to the next development steps. These are: (1) development of thinner embossed tapes to exploit the potential of the high dielectric strength inherent in synthetic taped insulation; (2) development of accessories such as splices and terminations; and (3) complete system design to avoid a dramatic increase in dielectric loss caused by oil contamination. If the integrity of the insulation homogeneity can be maintained, it appears the high voltage breakdown stresses will be adequate to permit economical operation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Low-Loss 765kV Pipe-Type Power Cable - Part II

This paper describes the completion of laboratory development work on a 765kV low-loss pipe-type AC transmission cable, employing oil-impregnated laminar insulation, comprising cellulose papers with a polypropylene film (PPP). Splice design and testing are emphasized. Additional testing of Sample 1 is described, as well as completed testing of Sample 2 involving two different splice designs. The economic impact of this new cable is considered. Test results demonstrate the quality and stability of the design.

Dielectric Properties of Oil-Impregnated all Polypropylene Film, Power Capacitor Insulation System

We have investigated the synthetic oil-impregnated power capacitor insulation system of all polypropylene film dielectrics which is currently used in company products. The results of dielectric measurements in low electric fields can be interpreted by chemical interaction of the polypropylene film with the aromatic synthetic fluid. These investigations' have proved to be quite helpful in the manufacture of power capacitors with superior dielectric properties for service over a wide range of applied voltages.

Low-Loss 765 KV Pipe-Type Power Cable

A 765 kV Low-Loss Pipe-Type AC Transmission Cable with oil-impregnated laminar insulation of cellulose paper and poly-propylene film has been developed. The development of the prototype sample (using specially treated conventional oil), bending tests and EHV tests on one specimen are described. Test results indicate the integrity and reliability of this design.

Gas-Filled Cavities in Solid-Type Cables

Shape, size, gas pressure, and distribution of cavities are inivestigated in the oil-impregnated insulation of solidtype cables for the purpose of estimating dielectric strength. Possible cavity locations and the influence which acting forces and gas solution in the oil have on the cavities, are discussed first. Gas solution is treated as a diffusion process and the time variation of pressure in the cavities calculated. Cavity properties vary continually and the changes are discussed from manufacture to first operation, when the cable is put on load, and when it is cooling. Cavitation on cooling is treated, growing rate of new cavities and pressure drop across the insulation calculated, and the influence of cavitation upon the temperature rise limit considered. Corona occurrence is estimated with the help of this analysis of cavity properties. Means which diminish corona are discussed and compared with current practice in cable design. It is thought that cavity analysis might be usefully applied also to other apparatus with impregnated insulation, e. g., transformers and capacitors.