Insulation Deterioration Research Articles

Improving the Hydrophobicity and Insulation Properties of Epoxy Resins by the Self-Assembly-Induced Coating of Fluorinated Graphene.

Moisture and insulation deterioration are important factors that cause the failure of epoxy packaging materials. Thus, improving the long-term stability of epoxy resins in a hot and humid environment is an important prerequisite for electronic components to adapt to complex working conditions and achieve high power densities. In this study, fluorinated graphene doped with hydroxy-terminated poly(dimethylsiloxane) was prepared and self-assembled into a micro/nanostructure on the surface of an epoxy resin, which effectively improved the surface hydrophobicity of the epoxy resin. In addition, the doping with hydroxy-terminated poly(dimethylsiloxane) modified the fluorinated graphene filler, thereby forming an arch bridge energy band structure inside the epoxy resin and thus regulating carrier migration. The water absorption of the epoxy resin decreased from 1.02 to 0.24%, and the surface water contact angle increased from 93.58 to 133.2°. Moreover, the electrical insulation performance of the modified epoxy resin was greatly improved when the surface resistivity and flashover voltage increased by 50.5 and 36.4%, respectively. Therefore, the proposed method realizes a simultaneous improvement in the hydrophobicity and insulation of epoxy resins.

Digital colorimetric sensing for real‐time gas monitoring for smart green energy system

AbstractIn this study, we demonstrate that a digital colorimetric sensor platform can transform a conventional energy system, such as gas‐insulated switchgear (GIS) into a smart green energy system. Our sensor platform consists of a colorimetric sensor film (CSF), an array of silicon photodiodes, and a low‐power‐driven fiber optical photodiode with a wireless communication protocol integrated into an information network. The photodiode measures the transmitted light of the color‐variable CSF when exposed to sulfur dioxide (SO2) gas, a decomposition by‐product of the sulfur hexafluoride (SF6) gas in GIS. We report the electrical photocurrent measurement through the CSF can measure the concentration of SO2 gas via remote and real‐time monitoring and shows a comparable behavior to UV–Vis absorption measurement. The limit of detection of the sensor, 0.78 ppm is sufficient to analyze the GIS insulation deterioration allowing green energy systems.image

Dynamic Behavior and Residence Characteristics of Space Charge in Oil-Paper Insulation Under Polarity–Reversal Electric Field

As one of the core components of the direct current (dc) transmission system, the converter transformer bears not only the alternate current (ac) electric field but also complex electric fields, such as polarity reversal. In the process of polarity reversal, the phenomenon of space charge residence occurs in the oil-paper insulation inside the converter transformer, which distorts the internal electric field, leading to insulation deterioration or even failure. However, studies on the dynamic characteristics of space charge during the reversal process and the mechanism of insulation degradation are not comprehensive. Therefore, in this study, a test pulse matching circuit was developed to accurately match the test pulse with the polarization voltage of polarity reversal and perform the multi-point rapid test of the space charge transient characteristics during polarity reversal. The space charge tests (of the unaged specimen) were performed at the polarity–reversal time of 60, 30, and 10 s, and the influence of polarity–reversal time on the charge residence characteristics of oil-paper insulation was compared and analyzed. Additionally, space charge tests were performed on oil-paper insulation samples with accelerated electro–thermal–mechanical aging treatment for 0, 2.5, 5, 10, and 15 days to compare and analyze the influence of oil-paper insulation aging degree on charge residence characteristics under the action of multi-physical field coupling. Research results indicate that the shorter the reversal time, the shorter the time required for the charge to dissipate, and the more the charge resides, the stronger its enhancement effect on the electric field. As the aging degree of the sample increases, the dissipation rate of the resident charge becomes slower during the reversal process, and the resident charge increases, so that the maximum electric field intensity inside the sample increases under the same polarization electric field. The study of the charge residence characteristics and the dynamic charge behavior during the polarity inversion process has important guiding significance for clarifying the deterioration mechanism of the converter transformer oil-paper insulation under polarity–reversal electric field.

A study on the weak points of stator windings of high voltage rotating machinery for ships

ABSTRACT Reliability studies of existing rotating machine insulation systems mainly focused on reliability evaluation for diagnosis and life prediction of generators and motors of various onshore power plants. However, with the increase in commercial rated voltage of marine rotating machine, accidents related to insulation systems occur frequently, and since many difficulties are caused in repair and replacement, the demand for improved reliability of marine high-voltage rotating machine is increasing. Accordingly, several international classification societies have begun to require tests to confirm the reliability of the insulation system of high-voltage rotating machine for ships. For the first time among them, in accordance with Rule Proposal No.2015/EL01 (Rules and Regulations for the Classification of Ships, 2016) at Lloyd, international classification for generators and motors contracted after July 2016 is required. It was requested to present the insulation deterioration reliability results of the rotating machines insulation system according to the standards (IEC 60,034-18-31, IEC 60,034-18-32, IEC 60,034-18-33, IEC 60,034-18-34). In this paper, in order to check the weak points of the high-voltage rotating machinery of a ship against voltage deterioration among the above four standards, a test procedure was developed in accordance with the IEC 60,034-18-32 standard, and the weak points were identified and analyzed through testing did.

Thermal-hydraulic model for temperature prediction on oil-directed power transformers

The temperatures of a power transformer have direct influence on the windings insulation deterioration, determining how long the equipment will be able to remain in operation. In order to better predict and study the transformer thermal behavior, a thermal-hydraulic model (TH) for oil-directed (OD) power transformer is proposed. From the constructive dimensions and the operative characteristics of the transformer, the proposed model is able to determine the complete oil flow and temperature distributions over the magnetic core, windings and radiators by iteratively solving an algorithm composed of both hydraulic and thermal resistance networks. A transformer prototype was assembled and equipped with 29 optical fiber sensors as part of a measurement system in order to validate the approach. The details of the active part, tank and radiators modelings are presented in a generalized way, being applicable to different constructive arrangements. The validation results show a maximum relative error of 1.31% for temperature, accurately describing the flow and temperature fields of the prototype, indicating that the model can be satisfactorily used for predicting the hydraulic and thermal behaviors of electric transformers.

Research on Evaluation Method of Cable Line Operation State Based on Partial Discharge Signal Characteristics

To accurately evaluate the operation status of cable lines, an evaluation method based on the characteristics of partial discharge (PD) signals is proposed. In this method, the operation state of the cable line is divided into four levels, amount of PD and energy of PD in online monitoring data of the cable line are utilized, the development trend of cable insulation deterioration is considered, cable line state evaluation indexes are firstly established, then the state membership of each evaluation index is calculated by utilizing a membership function, proper operation operators are selected, Finally, the evaluation system of the operation state of the cable line based on the characteristics of partial discharge signals is constructed. The correctness and effectiveness of the proposed evaluation method are verified by setting three operating conditions of cable lines, which provides a decision-making reference for the maintenance of cable lines.

Estimation of Hot-Spot Heating in OIP Transformer Bushings Due to Geomagnetically Induced Current

This paper studies the temperature distribution in the transformer bushing in the presence of Geomagnetically Induced Current (GIC). The thermal stress can significantly affect the transformer bushing reliability and lifespan. However, the existing research works have focused on thermal modeling of transformer bushing due to loading, and the impact of GIC on this valuable apparatus has not been investigated yet. In this paper, the hot-spot temperature (HST) in an oil-impregnated paper (OIP) bushing is studied during the GIC. The harmonic currents at various GIC levels are obtained from the EMTP-RV time-domain simulations using a detailed topological transformer model. Based on a frequency-dependent model of the bushing resistance, the total power loss is calculated by taking into account the GIC, load current, and harmonic currents. Subsequently, the temperature distribution in the modeled bushing is obtained from the finite-element method (FEM), considering all thermal mechanisms, including fluid flow, internal convection, and thermal conduction. In addition, a Thermal Equivalent Circuit (TEC) is developed for the bushing that replicates the FEM results. The simulation results reveal that the bushing HST may exceeds the recommended permissible temperature of the IEEE bushing standard. This situation may result in bushing insulation deterioration and reduction of the bushing lifespan.

Multiple Sulfur Corrosion Simulation on the Oil–Paper Insulation System Under Synergistic Electric and Thermal Fields

Sulfur corrosion in transformers results in insulation deterioration failures. Therefore, in this study, multiple sulfur corrosion was simulated on the oil–paper insulation system. The mechanical properties and hydrogen bond of the insulating paper as well as the mean square displacement and shear viscosity of the insulating oil were investigated under different combinations of electric and thermal fields. The electric field could promote the degradation of the elastic modulus of cellulose. The influence of the AC electric field on the modulus is not as obvious as that of the DC electric field. Temperature has a significant effect on the modulus parameters. With the increase in the temperature, the elastic modulus decreases considerably. The radial distribution function of the model was also investigated. Cellulose can easily form intramolecular hydrogen bonds under AC electric fields. Furthermore, intermolecular hydrogen bonds are easily formed under a DC electric field. The shear viscosity of insulating oil decreases with the increase in the temperature. The presence of the electric field could increase shear viscosity. The results of the study can provide some reference for the microscopic mechanism of multiple corrosive sulfides and investigate effective protection techniques against sulfur corrosion.

Improved dispersion and interfacial interaction of SiO2@polydopamine fillers in polytetrafluoroethylene composites for reduced thermal expansion and suppressed dielectric deterioration

Copper-clad laminate (CCL) comprised of copper foil and polytetrafluoroethylene (PTFE) faces severe thermal expansion mismatch due to the discrepancy in coefficient of thermal expansion (CTE) between the two components. Incorporating inorganic fillers with low CTE into PTFE has been proved to be a promising way to achieve the goal. However, it is a challenge to achieve homogeneous distribution and good interfacial interaction of fillers in PTFE composites owing to the characteristics of PTFE emulsion. In this work, core@shell structured SiO2@polydopamine fillers (SiO2@PDA) were synthesized and incorporated into PTFE matrix to form SiO2@PDA/PTFE composites. Due to the presence of PDA shell, SiO2@PDA exhibited improved dispersion and interfacial interaction, contributing to the reduced CTE and suppressed dielectric deterioration of SiO2@PDA/PTFE composites. With 40 vol% of filler, the CTE of SiO2@PDA/PTFE composite was efficiently reduced (50%), coupled with a limited sacrifice of only 2% and 40% of increments for dielectric constant (Dk, 2.3) and dielectric loss (Df, 2.4 × 10−3), respectively (@40 GHz), as compared with that of the corresponding SiO2/PTFE composite. The fillers and composites were comprehensively characterized to verify the mechanism of CTE and dielectric properties of the composites.

A Study on the Development of New Silicone based Polymeric Outdoor Insulator Material for Enhanced Properties

Polymeric insulators have been increasingly popular in recent years as a result of their superior performance in contaminated environments due to their hydrophobic properties. However, research into the ageing condition of polymeric materials and their practicality for large-scale use is currently ongoing. Insulator deterioration is caused by environmental tracking and erosion factors. As insulators age, they develop immature failures and inconsistencies in their functioning. pollution performance of polymeric insulators is a vital factor in the quality and reliability of the power system. Over some time, dry band arcing can initiate the flashover and it causes degradation in the form of erosion and tracking. Polymeric insulators’ performance is heavily influenced by the constituent materials and their properties. There is a critical need to investigate innovative filler materials that can be combined with existing polymeric base materials to form composites. In this context the proposed research use silicone rubber as a base polymeric material, to which additives are added to produce three distinct composites by varying the filler concentration. Preliminary studies were made to evaluate the hydrophobicity, dielectric strength, hardness, specific gravity, tensile strength, ultimate elongation and tear strength properties of this HTV silicone rubbed-based composites by using ASTM standards and IEC 60587 requirements. Studies were also made by accelerated ageing on sample material by using the IPTE test. The results show substantial improvement in the electrical and ageing properties.

Short-Time Adaline Based Fault Feature Extraction for Inter-Turn Short Circuit Diagnosis of PMSM via Residual Insulation Monitoring

Inter-turn short circuit (ITSC) is a common fault in electrical machines, which may result in further devastation to the whole winding and magnets. In this article, a method for real-time residual insulation capacity monitoring of ITSC fault in permanent magnet synchronous machines (PMSMs) is proposed, which is robust to speed/torque variation and independent of machine parameters and extra sensors. It is based on the theory that the ITSC fault initiates as an insulation deterioration among adjacent turns and the fault severity can be described by the residual insulation capacity being relevant to the 2nd harmonic component in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -axis currents. Thus, a short-time Adaline-based harmonic extraction method is proposed for the separation of the needed 2nd harmonic component from time-varying current signals, being used as the fault indicator afterward. Finally, the performance of the proposed method is experimentally evaluated on a PMSM and shows quite good performance in fast-tracking and detection of faults in various stages.

A Method to Characterize the Shrinking of Safe Operation Area of Metallized Film Capacitor Considering Electrothermal Coupling and Aging in Power Electronics Applications

Metallized film capacitor (MFC) selection is a key step to ensure the safe and reliable operation of high-capacity power electronic equipment. However, as a crucial factor leading to overstress failure, the changing of stress tolerance boundary caused by parameter drift and insulation deterioration has not been fully considered in capacitor design. Therefore, the establishment method of MFC lifecycle safe operation area (SOA) is proposed innovatively in this article, which characterize the shrinking of safe operating boundaries caused by electrothermal coupling and capacitor aging during long-term operation. First, the failure mechanisms and electrothermal coupling of MFC are discussed and the modeling process of initial SOA is proposed based on the datasheet and dc withstand voltage experiments. In addition, considering the increase of the hot-spot temperature and the decrease of breakdown strength due to equivalent series resistance parameter drift and dielectric insulation degradation in the whole lifecycle, the shrinking of MFC operation boundary is characterized and the modeling process of lifecycle SOA is proposed based on aging model and lifetime tests. Finally, taking an MMC system as example, the MFC selection based on practical operating condition and lifecycle SOA is performed to verify the validity of the method.

Insulation Properties and Interfacial Quantum Chemical Analysis of Cross-Linked Polyethylene Under Different Degassing Time for HVDC Cable Factory Joint Applications

Degassing treatment is critical to remove cross-linking by-products in cross-linked polyethylene (XLPE) cable factory joints and prevent early insulation deterioration. In this article, 500-kV high-voltage direct-current (HVDC) cable factory joint samples were prepared, and the physicochemical and temperature-dependent electrical properties of the samples under different degassing times were investigated. The results show that prolonged degassing treatment increases the crystallinity and reduces the tensile properties of the XLPE samples. The samples without degassing have visible micropores near the interface, and these micropores can be eliminated after a longer degassing time. In addition, degassing treatment can reduce the current density, increase the threshold field strength, and enhance the breakdown strength of the samples. A significant negative charge accumulation appears at the interface of the sample without degassing, which dramatically distorts the electric field. Long-term degassing treatment can improve this phenomenon, and the accumulated charge at the interface at elevated temperatures is significantly reduced. Furthermore, a model of the molecular chain microstructure at the interface before and after degassing is proposed. The molecular chain’s energy band structure and electron cloud distribution at the interface are analyzed based on quantum chemical calculation. It is found that cable makers can improve the insulation properties of cable factory joints by appropriately extending the degassing time to avoid interface deterioration.

Расчет добавочного шунта для диагностики изоляторов

The purpose of the study is to review the existing devices and methods for diagnosing and monitoring the state of high-voltage insulation, which plays an important role in the criteria for the reliability of the power system. One of the main methods for diagnosing insulators is a visual inspection. At the moment, online methods have become widespread, allowing to fix the process of destruction of insulation under voltage. Despite the existence of various methods and devices, the problem of timely detection of insulation deterioration in the operating mode has not been solved at the moment. This problem is solved by using an additional shunt design. When the task is achieved, the method of calculating the strength analysis of the additional shunt is used, and the simulation of this process is performed in the ANSYS program. Mechanical strength sufficient for operation under voltage. Experimental tests are carried out with the developed sample of the shunt. As a result of the study, a number of experiments are carried out with this design of the shunt, which have confirmed its performance. Due to the presence of the shunt, it is possible to fix leakage currents of the order of several tens of microamperes. Thus, the authors have proposed the current sensor, which is made in the form of an additional resistive shunt.

Evaluation of Power Transformer Thermal Performance and Optical Sensor Positioning Using CFD Simulations and Temperature Rise Test

The internal temperatures and the ability to dissipate the heat from active part influence the windings insulation deterioration and determine how long power transformers are able to remain in operation. Due to the great importance of these equipments for the electrical system, studies to avoid failures and early ageing are needed. This paper presents an evaluation of thermal performance of power transformers and the optical fiber sensors positioning to temperature monitoring. For this is performed an experimental and numerical methodology, comparing the measurements during the temperature rise test of single-phase prototype power transformer, under ODAN cooling system conditions equipped with 20 optical fiber sensors along its windings, to the obtained results of computational fluid dynamics (CFD) simulations performed in ANSYS® Fluent software. It was verified that the punctual temperature measured by optical sensors may not accurately represent the critical temperatures along transformers windings and the windings arrangements impact on oil flow and temperature distribution, as the number and positioning of block washers, being necessary to be considered in previous studies for a better positioning of the sensors and temperature monitoring.

The Effect of Thermal and Moisture Stress on Insulation Deterioration Law of Ionic Contaminated High-Voltage Printed Circuit Board of Electronic Power Conditioner

Since the electronic power conditioner (EPC) is a crucial part applied of a Space Travelling-wave Tube Amplifier (STWTA), the reliability issue must be considered. Of all the failure modes of an EPC, the insulation failure of an EPC in thermal and moist environments is the most serious, and needs special attention. By investigating the influence of contamination, humidity, and temperature on surface insulation resistance (SIR) and surface discharge, we focused on the determination of the insulation failure boundary in an EPC. Considering real working conditions, we used the typical circuit applied in the EPC as the test object. The insulation deterioration phenomenon under different thermal and moisture stress was studied. The results show that: (1) SIR of the samples did not change with contamination levels when the relative humidity (RH) was below 70%. When RH was higher than 75%, the SIR began to vary with temperature and ionic contaminant concentration. (2) Even if the samples were not contaminated (the ionic contamination concentration was less than 1.56 μg/cm2), the deterioration of the SIR still occurred at 85 °C/90% RH. (3) The insulation failure boundary caused by surface discharge, and the degree of electrical erosion were related to humidity, pollution, voltage and temperature. To improve the failure caused by insulation, encapsulation was used. Experiments showed that encapsulation is an effective protection method to prevent insulation deterioration.

A Novel Composite Sensor for Overvoltage and UHF Partial Discharge Measurement in GIS

The presence of overvoltage can cause the insulation deterioration and excite the partial discharges (PD) in gas-insulated switchgear (GIS), and it is necessary to detect the overvoltage and PDs simultaneously to evaluate the insulation performance. A novel GIS internal composite sensor for PD detection and overvoltage measurement is proposed in this paper. The proposed sensor is mainly formed with a disc-type monopole UHF sensor and an overvoltage sensor. The effectiveness and sensitivity of the sensor are tested on a designed platform under power frequency voltage, switching impulse, lightning impulse, and oscillating lightning impulse, respectively. Experiment results show that the proposed sensor can be used to detect overvoltage and UHF signals effectively and is expected to help in the GIS condition evaluation.

Simulation of Current Pulses and Sound Waves Resulting from Partial Discharges in a Needle-Plane Geometry in Air

Partial discharges can occur in different types of electric equipment and cause progressive insulation deterioration, so there is interest in monitoring partial discharges for assessing the state of the isolation of an electric system and to predict failures. Techniques to detect partial discharges, such as detecting ultrasonic emission, have been proposed in the literature, but as various effects can co-occur during a discharge, identifying characteristics in sound and pressure waveforms and correlating then with discharge characteristics is difficult. Simulations can assist in these correlations by allowing the isolation of the different phenomena. In this work the drift diffusion model, including photoionization, is coupled with the linearized compressible Navier-Stokes equations to simulate ultrasonic waves produced by partial discharges. Previous works have used the incompressible Navier-Stokes equations, so they can simulate ionic wind produced by the Trichel pulses, but no sound. In the literature, simulations have focused either on streamers or Trichel pulses. In this work both discharges and produced sound waves are successfully simulated for the needle-plane geometry in air. The electric current and charge per pulse are compared with experimental results reported in the literature for the same discharge conditions. The simulations have demonstrated that the sound waves depart from the electrode tip for the Trichel pulses, and for the streamers two sound waves are produced, one from the electrode tip and the other from the whole discharge length. Differences in the wave front with respect to the relative position to the electrode tip were analyzed, showing that near the discharge spot the sound wave is not a spherical wavefront. The sound wave for one of the discharges in the Trichel pulse regime was compared with experimental results in the literature. Results are in good agreement with the experimental data found in the literature. Both current and sound waves were successfully predicted and correlated with the discharge, results that can be used to help in the detection of partial discharges.

Motion characteristics and distribution of metal particles in insulating oil under DC voltage

Studying the dynamic behavior of metal particles in insulating oil in case of flow-electric field coupling is important to fully understand the insulation mechanism in transformers, such as partial discharge and insulation deterioration of contaminated insulating oil. In this paper, through experiments, the particle image velocimetry (PIV) technique is applied to study the motion of metal particles in the oil flow under direct current (DC) electric field. According to the observation results of flow field, the effects of voltage level, electric field non-uniformity, and oil flow velocity on the motion and distribution of particles are analyzed in detail. The obtained results show that the applied voltage has an obvious effect on the motion of metal particles in oil flow, and it causes the formation of wavy trajectories of particles, where such effect becomes more obvious as the voltage level increases. Moreover, it is observed that the increase of flow velocity can weaken the influence of electric field, and the electric field non-uniformity affects the trajectory of particles mainly by changing their velocity component in the vertical direction. In addition, the concentration of particles group in the oil flow is sensitive to the external electric field, where the concentration decreases with the increasing electric field. Certainly, the results reported herein are effective in further revealing the effect of impurity particles on the insulating oil.

Molecular Dynamics on Cracking Mechanism of Oil-Pressboard Insulation Under Electric–Thermal Coupling

Transformer oil and insulation paper are subject to the combined effects of the electric field and temperature operations, which significantly impact insulation deterioration. This article designs a comparative test of the electro-thermal combined aging to obtain several characteristic parameters for the aging degree of oil-pressboard insulation. Molecular dynamics are used to establish a mixed molecular model of oil-pressboard insulation to simulate the pyrolysis reactions at high temperatures using the ReaxFF force field. The microscopic effects of the electric field on the aging of oil-pressboard insulation are studied. The rationale of the simulation model is verified from experimental results, and the trend of the reactants and products in the reaction process was considered to determine the influence mechanism of electric–thermal coupling on oil-pressboard insulation. The experimental results show that thermal stress remains the most important factor toward cracking in insulation paper. The acid value and furfural formation rate in oil increase, and the polymerization degree of the insulation paper decreases rapidly after the combined electric and thermal aging. In addition, the molecular dynamics simulations show that pyrolysis increases the bond energy, promotes repulsion between atoms, and leads to fractured bonds. The addition of the electric field increases the susceptibility of the chemical bonds in cellulose molecules to breakage. The morphology of the cellulose chain then changes, and the diffusion of small molecules intensifies. This study reveals the mechanism of insulation aging under electro-thermal coupling through combined macroscopic testing and molecular dynamics simulations.