Insulation Aging Research Articles

The Simulation Study on Temperature Field Distribution of 220 kV Gas Insulated Switchgear

Under working conditions, the conductive rods in the GIS flow through the power frequency alternating current. Due to the coupling effect of the magnetic field and electric field between the metal aluminum shell and the conductive rod, induced eddy currents are generated in the metal shell of the GIS. The heat generated by the current heating effect of the GIS conductive rod and the eddy current loss of the metal casing will cause the temperature rise of GIS equipment. Due to the limited volume, the heat dissipation capacity of GIS is poor. Excessive temperature rise will accelerate the insulation aging of GIS equipment, and even damage its insulation, which will affect safe operation. In order to obtain the temperature change law of GIS, related influencing factors such as eddy current loss, skin effect, proximity effect, convective heat transfer of SF6 gas, and gravity of SF6 gas are comprehensively considered. The finite element analysis is used to research and discuss GIS magnetic field distribution, eddy current, temperature distribution and SF6 gas velocity. The initial value of the temperature of each part is set to 293.15 K (20 °C), and the temperature in the GIS is calculated to gradually decrease from the inside to the outside under the rated AC current of 3150 A. The temperature at the conductive rod position is the highest at 335.32 K, and the temperature at the housing position is the lowest at 294.65 K.

Aging characterization of 500-kV field-serviced silicone rubber composite insulators with self-normalized photothermal radiometry

Aging of silicone rubber composite insulators widely used in high-voltage transmission lines is a dynamic process starting from surface and gradually forming an aged surface layer with reduced thermal diffusivity. Based on the demodulation of the modulated infrared radiation signal and the subsequent data analysis, self-normalized photothermal radiometry (PTR) is used to quantitatively characterize the aging degree of field-serviced composite insulators. The ratio of the thermal diffusivity of the aged layer normalized by the substrate thermal diffusivity is proposed to characterize the aging degree to eliminate the influence of the wide variation of the initial thermal diffusivities of fresh silicone rubber composite insulators which are normally not known. Experimental results show that the thermal diffusivity ratio decreases and the aged layer thickness increases monotonically with the increasing field-service years, in consistent with the measurements of hydrophobicity class. The thermal diffusivity ratio and thickness of the aged layer are expected to be used as numerical indices for understanding the aging mechanism and estimating the remaining lifetime of silicone rubber composite insulators in field service.

Aging of Cross-Linked Polyethylene Insulation Cable Lines

Abstract. Preference is given to cable lines with cross-linked polyethylene insulation in electrical networks with a nominal voltage of 10 kV during reconstruction of existing and construction of new industrial enterprises. The standard service life of such cables is at least 30 years (subject to the conditions of storage, transportation, installation and operation), and the actual one is determined by the technical condition of the cable. The service life of a cable line depends on the state of its insulation, the aging of which occurs under the influence of several factors. Conventionally, all factors influencing one or another degree on the cable insulation resource can be divided into thermal, electromagnetic, climatic, mechanical and operational. The most significant reason for the insulation aging is high temperature, which accelerates the reaction of thermo-oxidative destruction, during which high-molecular polymer compounds decompose. In fact, cables are operated at temperatures below the long-term permissible values, and, therefore, the aging of the insulation is slower, and the actual service life will be longer than the standard. At present, condition of the insulation is monitored with the use of high voltage tests related to destructive testing methods. When designing and operating cable lines, it is necessary to estimate the duration of the actual service life under various operating conditions. In theory, there are several expressions for calculating the service life of a cable line when exposed to temperature, humidity, electric field and aggressive environments, but all of them are not applicable in practice due to the presence of a large number of coefficients whose values are unknown. The paper presents an analytical expression obtained for determining the service life of power electric cables, taking into account the aging of the insulation under the influence of temperature and electric field.

A Novel Diagnosis Method for Void Defects in HVDC Mass-Impregnated PPLP Cable Based on Partial Discharge Measurement

Mass Impregnated PPLP cable, which is applied to various high-voltage direct current (HVDC) projects due to its excellent dielectric and temperature properties, has a problem wherein voids are formed inside the butt-gap due to cavitation. However, there has been no previous research into technology for void defect identification and insulation diagnosis on HVDC MI-PPLP cables. In this paper, to propose an insulation diagnosis method for void defects in HVDC MI-PPLP cable, the direct current (DC) void discharge patterns were analyzed according to the specimen temperature and the magnitude of applied voltage using the pulse sequence analysis method. In addition, to confirm the pre-symptoms of dielectric breakdown in MI-PPLP cable due to DC void discharge, partial discharge patterns were analyzed continuously until dielectric breakdown occurred. From the experimental results, DC void discharge patterns of the same shape were obtained regardless of the specimen temperature and the magnitude of applied voltage. In addition, it was confirmed that new insulation aging patterns were generated as electrical and thermal aging occurred due to the continuous DC void discharge. Therefore, it is demonstrated that identification and insulation diagnosis of void defects in HVDC MI-PPLP cable is possible through the obtained DC void discharge and insulation aging patterns.

Probabilistic analysis of the failure of high-voltage insulators based on compositional analysis

High-voltage insulators operating at 154 kV and 25,000 lbs and having service periods ranging from 0 to 53 years were considered to study the failure mechanism based on the composition of ceramic materials. Thermal and mechanical load tests (T&M) were conducted to accelerate the aging and determine the remaining mechanical strength of the aged insulators. Based on load tests (M&E), it was observed that 38 cristobalite samples approaching or completing 50 years of service failed the mechanical load test and exhibited degraded microstructures and insulation resistances of 1200 and 970 MΩ, respectively. Alumina insulators showed a 4% improvement in mechanical strength and 11% increase in hardness compared with the cristobalite samples. A compositional study (X-ray fluorescence) was performed to determine the elemental composition of both the superior and inferior samples to correlate their probabilities of failure P(F) with the wt (%) of SiO2, Al2O3, and Fe2O3 present in the ceramic insulators.

Dielectric response of resin-rich insulation ageing characteristic for large generator

This paper describes the research on the electrical ageing mechanism of the resin-rich molding (RRM) insulation by the dielectric response method for large generator. The dielectric properties of the RRM insulation materials with different ageing time were measured and analysed by dielectric temperature spectrum, frequency-domain spectrum (FDS) and thermally stimulated depolarization current (TSDC). The microscopic properties of the main insulation material were also analysed by scanning electron microscope and dynamic mechanical analysis. The results of the FDS measurements showed that the dielectric loss factor $$\tan \delta$$ and the real part e of the complex dielectric constant of the main insulation material increased significantly in the frequency range of 10−2–10 Hz as the ageing time increased; the fitting results of the double relaxation HN (Havriliak–Negami) model indicated that DC conductivity and interfacial polarization increased by the electric ageing effect in the main insulation. When the test temperature exceeded the glass transition temperature, the relaxation process of turning-direction polarization changed in the main insulation system; the TSDC tests indicated that the energy levels of the deep and shallow traps were slightly reduced during the main insulation's ageing, but the number of traps increased; therefore, the ability of capturing and releasing charges was enhanced in the main insulation.

Volume Resistivity of Viton Polymer under Thermal Aging

This study examines the influence of various electrical parameters on the volume resistivity of the Viton fluoroelastomer. The transient current, the temperature dependence of volume resistivity, the voltage dependence of resistivity, and the surface morphology of Viton insulators are investigated for new and aged specimens. An accelerated aging process has been employed in order to simulate the natural aging of insulators in service. A detailed comparison between the new and aged samples is presented. The transient effect, which is a challenge to the resistivity measurement of insulators, has been investigated. The first 60 s of the resistivity measurement test showed a significant influence from the transient effect and should be excluded from the data. The volume resistivity of both new and aged samples decreased when the temperature increased. However, the resistivity of the aged sample was lower than the new one at all tested temperatures. When the temperature increased from 35 to 190 °C, resistivity decreased from 4.77 × 1010 to 6.99 × 108 Ω-cm for the new sample and from 2.6 × 1010 to 6.68 × 108 Ω-cm for the aged sample under 500 V. Additionally, the results from this study showed that the volume resistivity is inversely proportional to the applied voltage. Finally, scanning electron microscope (SEM) micrographs/images allowed us to closely examine the surface morphology of new and aged Viton samples. The surface of aged samples has been recognized with higher surface roughness and more significant surface cracks leading to poor performance under high voltage applications.

New device for the quality control of the high-voltage electrical insulator

During the operation of high-voltage electrical equipment, its insulation ages, its properties deteriorate, and the electrical strength decreases. To avoid sudden breakdowns of insulation and maintain the necessary level of reliability of high-voltage electrical equipment, the state of its insulation must be periodically monitored. The device described in the article is intended for preventive testing and diagnostics of heterogeneous high-voltage insulation of electric machines, power oil transformers and cables with paper-oil insulation. In particular, the integral assessment of high-voltage insulation aging, its humidification and remaining operational life can be based on the use of the absorption phenomenon (the accumulation of internal absorbed charge). Self-discharge and return voltage are measured to assess the insulation condition. A block diagram of the device for monitoring the electrical insulator quality is given as well as the formulae for determining the spent and remaining operational life of the tested insulation. The proposed device will increase the operational reliability of high-voltage electrical equipment.

Study on Influence of Aging on Vibration Characteristics of T ransformer

It is important to assess the insulation aging state of transformers, which can not only avoid equipment faults, but also improve the economic benefits of transformer operation. The main negative effect of transformer insulation aging is the decline of insulation mechanical properties, which will eventually lead to transformer fault. In this paper, the transformer vibration characteristic test platform is built and the vibration characteristic variables are defined to describe the vibration characteristic quantitatively. Then, the vibration characteristics and vibration variables of transformer tank surface under different aging time are studied by accelerated aging test in the laboratory. Finally, the vibration signals of transformers in service under different operation time are measured. And the vibration characteristics and vibration variables of in-service transformers under different operation time are analyzed and studied. This paper directly aims at the mechanical characteristics of insulation, and studies the relationship between insulation aging state and vibration of transformer. It provides a basis for more effective and direct monitoring of insulation aging state.

Quantitative prediction of aging state of oil-paper insulation based on Raman spectroscopy

Accurate monitoring of insulation aging of oil-paper insulation power equipment such as oil-immersed transformers is a key and difficult point in the field of high voltage research. In this paper, a method based on Raman spectroscopy to diagnose the aging degree of oil-paper insulation is discussed. Raman detections of the samples were carried out on a self-built Raman detection platform. The partial least squares method was used to extract and analyze the spectral features. The aging time of the sample was used to supervise the feature extraction of oil-paper insulation Raman data, and the intrinsic mathematical relationship between the Raman features of oil-paper insulation and the aging was excavated. Finally, a quantitative aging diagnostic model based on Raman spectral features of oil-paper insulation to predict its aging state was built with the assistance of the support vector regression method. The results of aging time prediction for 30 test samples show that the mean square error is 0.0123 and the square of correlation coefficient is 0.987. The proposed method provides a new idea for Raman aging diagnosis of oil-paper insulation.

Simulation Research on Transformer Pre-test High Voltage Electrical Insulation Parameters

This article uses an image processing method to study the hydrophobicity of transformer composite insulators, and then determine the surface aging performance of transformer composite insulators. In order to meet the requirements of the randomness and ambiguity of the hydrophobic image, this paper uses a series of algorithms to process the hydrophobic image of the composite insulator: the enhancement of the hydrophobic image based on adaptive histogram equalization, the hydrophobic image based on wavelet transform Filtering and segmentation of the hydrophobic grade classification of composite insulators. Taking characteristic parameters such as the largest water droplet shape factor and area ratio as input, and the surface insulation aging performance of composite insulators as output, the relationship between insulation parameters and insulation aging performance is established. The simulation results show that the method can judge the surface insulation aging performance of composite insulators into three states: good, normal, and bad. Especially when the maximum water drop area ratio is input, the classification accuracy rate reaches 933%.

Power Transformers Thermal Modeling using an Enhanced Set-Membership Multivariable Gaussian Evolving Fuzzy System

In this work we present two new models based on Evolving Multivariable Gaussian approach and on the Set-Membership/Enhanced Set-Membership adaptive filtering framework to model the thermal behavior of power transformers. In these new models, adaptive filtering approaches work to adjust the learning rates of the evolutionary model, while the use of multivariable Gaussian membership functions, instead of single-variable as evolving models in general, makes information about interactions between input variables preserved and used in the training process. In addition, the evolving structure of the proposed models make these models more adaptable to changes in the operational conditions of power transformers (like insulation aging, environmental changes, load profile changes, among others) than its non-evolving counterparts. To evaluate the performance of the proposed models they were applied to two benchmark problems and to the thermal modeling of real power transformers problem under two load conditions: with and without an overload condition. The obtained results are compared with the performance of the original evolving Multivariable Gaussian and with other classical models suggested in the literature. Both proposed models obtained significantly higher performances than all the other tested models, suggesting that the models are flexible and efficient approaches in these scenarios representing a promising approach in the modeling of power transformers, especially for real-time applications.

A Novel DC Bias Suppression Strategy for Single-Phase Full-Bridge DC-DC Arc Welding Converter

The high frequency transformer in single-phase full-bridge DC-DC converter is prone to saturation because of the asymmetry of circuit parameters. Transformer saturation will increase power consumption, accelerate the aging of winding insulation, and even damage power switches. In order to prevent this risk, a DC bias suppression strategy is presented in this article, and the main advantage of this strategy is that the problem of transformer saturation can be completely eliminated. In this article, firstly, the DC bias and saturation mechanism of single-phase full-bridge DC-DC converter are analyzed in detail, and the Maximum Integral Value of Volt-Second Product error (MIVVSPE) is derived. Secondly, aiming at the saturation problem of single-phase full bridge DC-DC converter, a new digital integral circuit is designed to evaluate the DC bias state of transformer, and a DC bias suppression strategy is constructed to suppress the saturation of transformer. Furthermore, different from the traditional current feedback control strategy, the DC bias suppression strategy based on volt-second product error integral can be triggered before the transformer enters the saturation state, and the transformer saturation can be completely suppressed. Finally, a 30 kW single-phase full-bridge DC-DC converter for arc welding is established in lab. The experimental results show that the new DC bias suppression strategy can effectively prevent the transformer from entering the saturation state and improve the operation stability of single-phase full-bridge DC-DC arc welding inverter.

Research of cable insulation types operation life

The paper describes the study of insulation materials’ operation life and remaining life. Insulation aging models, which can be implemented to AC insulated cables, are considered. Based on models and previously proposed approach XLPE, EPR, and PVC insulated cables’ operation lives were evaluated. The evaluation was performed using voltage and current monitoring data of the 6 kV cable power transmission line, which is used in the distributive networks in Russia. The air temperature was obtained from the open-source. The results showed the applicability of all models for remaining life evaluation as models gave the same operation life. It can be explained by the same operation lives according to the cables’ parameters. However, in real conditions, the operation lives of listed insulation types can be different because of their dielectric and structural properties.

Failure of submarine cables used in high‐voltage power transmission: Characteristics, mechanisms, key issues and prospects

This study reviews the failure of high-voltage submarine cables used in offshore power transmission and provides highlights of their failure characteristic, mechanisms, key issues and prospects. High-voltage submarine cables are designed and applied according to the high-voltage alternating current and high-voltage direct current requirements. Inevitably, the fault occurs in HV submarine cables that is different from that of an underground cable. External aggression remains the primary cause of faults, such as fishing and anchors. Most faults continue to occur at a shallow depth (300 m). The optical fiber inside the submarine cables plays a substantial role in the temperature and stress-strain monitoring and diagnosis. However, it is regarded as a weak point for electrical fault. Insulation breakdown is the leading reason for the short fault. The failure mechanism is complicated when associated with marine conditions. Some defects of insulation and extensive voids, water treeing, mechanical stress, partial discharges, overheating, and electrochemical erosion contribute to the insulation breakdown. Several key issues, including anchoring damage, treeing, defects, and thermal-electric ageing, are proposed. Prospects and new methods related to the cable failure, especially for insulation ageing by treeing and electrothermal effects, are also discussed.

Effect of Electrical Trees on the Distribution of Electric Field and Space Charge in Cross-linked Polyethylene Cables

Background: Electrical trees can affect the distribution of electric field and space charge in cross-linked polyethylene (XLPE) cables, and play an important role in insulation aging and breakdown of cables. Therefore, it is important to study the influence of electrical trees in cables. Methods: In this study, the finite element method of second-order tetrahedral element and electromagnetic theory method is used for calculation. A model of XLPE cable with three-dimensional electrical trees is taken as an example for calculation. Results: The results show that the longer the trunk length is, the greater the electric field intensity at the end of the branch is; the farther the electrical trees are from the insulation side of the high voltage, the more the electric field intensity of each location decreases. Conclusion: With the increase of the resistivity of the trees, the electric field intensity and charge density tend to be stable at the end of the tree.

An Advanced Wideband Model and a Novel Multitype Insulation Monitoring Strategy for VSC-Connected Transformers Based on Common-Mode Impedance Response

The insulation monitoring technique for converter transformers has been explored using characteristic harmonics in the voltage source converter (VSC) system by the authors. The previous work enables the monitoring of aged groundwall insulation but cannot cover other insulation components. In order to explore the possibility of multitype insulation aging monitoring, in this article, we first derive an advanced model considering unevenly distributed circuit parameters along transformer windings and their frequency-dependent feature. Model improvements provide an accurate tool to describe multitype insulation in a wide frequency range. Based on the advanced model, a novel wideband common-mode (CM) impedance-response-based monitoring strategy for the VSC-connected transformer is proposed to achieve the evaluation of the aging in different insulation components. An illustrative case is studied on a 15 kVA transformer driven by an industrial inverter. Offline and online experiments are conducted to prove the effectiveness of the advanced model and the proposed strategy. Compared with the existing methods, the CM impedance-response-based strategy enables the online monitoring of multitype insulation components in an effective and noninvasive manner. In addition, the aging severities can be estimated reliably, even at incipient stages.

Influence of Aging on Creepage Discharge Characteristics of Oil-Paper Insulation Under AC-DC Combined Voltage

There is a relative lack of research on the development of creepage discharge in aged oil-paper insulation subjected to AC-DC combined voltage in converter transformers. In this study, based on the physical and chemical properties of oil-paper, we propose a sphere-plate electrode model to study the creepage discharge parameters and characteristic spectra of aged oil-paper subjected to AC-DC combined voltage. The results show that the creepage discharge inception voltage of aged oil-paper is related to the AC voltage component of the AC-DC combined voltage. However, when the DC voltage component is 60 kV, the flashover phenomenon was found to occur in one of the severely aged samples before the AC voltage was applied. Despite being subjected to the same DC voltage, the flashover phenomenon did not occur in three other samples aged to different degrees. Large-amplitude discharges were found to appear earlier in the severely aged samples. At the same creepage discharge stage, as the aging time of the oil-paper increased, the number of large-amplitude discharges was found to rapidly increase, and the time interval between adjacent discharges was found to gradually decrease. At each stage of the creepage discharge, the small-amplitude discharge has a “hump” shaped distribution. The larger the DC voltage component, the shorter the endurance time of aged oil-paper, and the more severe the damage to the oil-paper. The DC voltage component was found to have an accelerating effect on the development of creepage discharge in aged oil-paper insulation.

Aging Assessment of Oil–Paper Insulation Based on Visional Recognition of the Dimensional Expanded Raman Spectra

Detecting insulation performance through testing the Raman spectroscopy of transformer oil is noninvasive and shows great promise in on-site diagnosis. The information, stored in spectra, that may reflect aging states of insulation aims to be effectively unfolded and provides decision support as well. In this article, a transformation method based on intrinsic product arithmetic is used to transform the 1-D Raman spectral data into 2-D image data. With the help of CNN based on the dimensionality expanding method, diagnosis of oil–paper insulation with different aging states based on the Raman spectroscopy was realized. The network-based oil–paper insulation aging diagnosis model has an accuracy of 95% in ten aging categories of 60 test samples and 100% in four aging categories. The result shows the trained CNN was able to identify the differences of transformer oil samples at each aging stage through visualized Raman spectral image. This method provides a more accurate determination method for diagnosing oil-immersed transformers’ aging conditions; it also develops an available method for a more detailed evaluation of insulation aging levels.

Aging Modeling of Low-Voltage Cables Subjected to Radio-Chemical Aging

This article presents the development and application of a modelling approach based on quantities obtained from mechanical and electrical tests to assess the aging of low voltage cable insulation for nuclear applications. In order to obtain experimental data needed for the establishing of the models, accelerated aging is performed on coaxial cables. The first part of this paper focuses on the development of a predictive modelling for mechanical properties; this allows the evaluation of the Dose to Equivalent Damage (DED) as a function of the aging stress (dose rate). However, the use of mechanical tests for cable aging assessment presents some problems, being such tests destructive for the insulation and potentially affected by local defects. The novelty of this work lays on the introduction of a new model, allowing the definition of the end-of-life point in terms of electrical non-destructive tests. The second part of this article presents the development of aging modelling of cable insulation and correlation with experimental data obtained at different stresses, which allow the estimation of the expected life of the cable under test to be derived from diagnostic measurements of electrical properties like, e.g, tan δ. Finally, the application of the proposed life model to two typical real-condition nuclear environments is presented and discussed.