Insulation Degradation Research Articles

Fabrication Aspects and Performance Characterization of α-Al2O3/AlPO4 Based Sandwich Configuration Flow Channel Inserts and Coatings for High Temperature Liquid Metal Applications

Abstract Liquid metals (LMs) exhibit several key characteristics justifying their utilization as coolants and breeders for nuclear fusion reactors and advanced fission reactors. In fusion reactors, the LMs confront an exorbitantly high flow retarding force, due to the magneto-hydro-dynamics (MHD) effect, imposing significant demands on the pumping power and designs of ancillary coolant systems. Corrosion of structural materials leading to activated corrosion products and coolant chemistry control are some of the vital issues common to both fusion and fission reactors employing liquid lead (Pb) and its alloys. To address these concerns, different technological solutions such as flow channel inserts (FCIs) and high temperature compatible corrosion resistant coatings are being investigated to provide a chemical and/or electrical isolation between the LM and structural material for advanced reactors. In this study, three different prototype geometries (circular, square, and 90 deg bend) of steel-insulator-steel sandwich FCIs are fabricated for fusion reactor applications and an extensive characterization of the electrical insulation is performed over an operating temperature range of 100 °C–600 °C. Welding trials and pneumatic pressure tests up to 10 kg/cm2 (g) are performed on the assemblies to validate the electrical and mechanical integrity over typical fusion reactor operational regime. This paper presents detailed fabrication aspects along with quantitative estimations of insulation filling density, electrical insulation performance and, for the first time, a detailed systematic study of insulation degradation resulting from the combined effects of tungsten inert gas (TIG) welding, exposure to pressure and machining operations on these FCIs. The paper also provides critical details derived from the metallurgical examinations and visual observations from the destructive tests executed on the prototypes. Further, from an implementation perspective toward Lead-cooled Fast Reactors (LFRs), a preliminary feasibility assessment of the α-Al2O3/AlPO4 coating is performed through thin film deposition trials on planar and non-planar substrates followed by mechanical characterizations, such as coating thickness, surface roughness, adhesion strength and microhardness. Metallurgical analyses are presented and discussed to assess Pb ingress after 700 h of continuous exposure to molten Pb alloy at 300 °C–400 °C.

An Adaptive Reclosing Scheme for All-Parallel Autotransformer Traction Network of High-Speed Railway Based on Multisource Information

In high-speed railway (HSR), directly reclosing a circuit breaker after a fault trip can result in a large second overcurrent, causing insulation degradation, equipment damage, and even system instability. Meanwhile, existing direct reclosing schemes fail to simultaneously identify the faulty line, fault property, and fault type, especially for all-parallel autotransformer traction networks (AATNs). Therefore, an adaptive reclosing scheme for AATNs based on multi-source information is proposed in this paper, including two critical methods of faulty line identification, and fault property and type identification. Specifically, a multi-source fault impedance angles integrated faulty line identification method is proposed to effectively distinguish the faulty line and healthy line. Moreover, a novel approach is developed to incorporate multi-source line-to-line induced voltages obtained by reclosing the healthy line, which facilitates the fault property and type identification. The effectiveness and superiority of the proposed reclosing scheme are demonstrated using field tests and simulation experiments. In addition, the proposed scheme not only shortens the power outage time of healthy line, but also completely avoids second damages to equipment, and prevents power outages when the feeder is permanently faulted to the ground, presenting its remarkable robustness benefiting from multi-source information.

Cross-linking tendency and photo-oxidation degradation in Silane-grafted LDPE insulation under accelerated cyclic weathering aging

Si-g-LDPE (Silane-grafted LDPE) is the based material of silane-crosslinked polyethylene (Si-XLPE) nominated to be used in renewable energy (photovoltaic) structures for the reason of its good behavior under weathering circumstances. The present study has an attempt to follow up the cross-linking process of Si-g-LDPE under cyclic accelerated weathering aging and to track its photo and thermo-oxidative degradation at both microscopic and macroscopic scales. The realized experiments consist to carry out cyclic accelerated weathering aging of 1152 hours, using QUV chamber, on films of Si-g-LDPE. The process of cross-linking is checked by using Hot-Set-Test measurements and Fourier Transform Infrared (FTIR) spectroscopy. Evolution of mechanical properties and carbonyl index are used as relevant precursors of photo and thermo-oxidation degradation. Additional information on the microstructural changes (crystallinity) and on the optical properties is gained by using X-ray difractometry and UV-visible measurements. Our findings lead to a result that Si-g-LDPE has a big ability to crosslink under cyclic weathering aging. The elastic behavior of the material was enhanced gradually with increasing aging time. The elastic behavior enhancement is a consequence of crosslink network density increase. FTIR results support perfectly the Hot-Set-Test results by the increase of single or multi-siloxane linkages absorption bands. These linkages modify the digital fingerprint of the material. Our results ascertained also that we can relate the elongation at break and tensile strength changes to the changes in the carbonyl index. Both characteristics are related to the photo and thermo-oxidation degradation. Photo-oxidation degradation leads to the decrease of mechanical properties and to the increase of carbonyl index. An abrupt and sharp increase at the beginning of aging in the crystallinity of the material followed by a level-off state was observed. Finally, the optical properties (band gaps, Urbach energy and dielectric constant) are greatly affected by the weathering aging process.

Developments in an acoustic resonance test for the detection of manufacturing anomalies in hydroelectric generator stator windings

To meet the ever-increasing demand for electricity, Hydro-Québec (HQ) is seeking to simultaneously increase the power of its generating stations while improving its service quality. Thus, the company has tasked its research institute to investigate innovate methods to meet the aforementioned goals. Of interest in the current study is the development of an acoustic resonance test (ART) to improve the quality control (QC) in the manufacturing of hydroelectric generator stator windings. Since HQ and its suppliers are investigating new fabrication methods for stator windings to meet the required timelines and increased power requirements, QC is required to ensure the service quality of its new hydroelectric generators. Typical manufacturing anomalies found in stator windings are delamination and air pockets between insulation layers. Such anomalies can result in an acceleration in the degradation of the winding insulation, which reduces the service quality of hydroelectric generators. To benchmark the ART method, the results of the suspected locations of the anomalies along the stator windings were compared with an acoustic camera and the locations were dissected and inspected under microscope for validation. Ten different stator windings were tested and two metrics (variations in both force and frequency responses) were found to indicate the location of delamination sites in the stator windings and corroborated with the result of both the acoustic camera and the dissections.

Determination of Hotspot Temperature Margin for Rectangular Wire Windings Considering Insulation Thermal Degradation and Partial Discharge

Increasing current density is a common way to improve the power density of electrical machines (EMs) which also boosts the winding temperature. Higher winding temperature accelerates the thermal degradation of insulation and also increases the risk of partial discharge (PD). Commonly, the winding hotspot temperature constraint is decided based on an over-engineering approach (i.e., the winding hotspot temperature is 30°C below the wire thermal class) constraining the EM’s performance. In this paper, the margin of winding hotspot temperature is precisely determined focusing mainly on the insulation degradation issues through experimental investigation. The partial discharge inception voltage and partial discharge extinction voltage have been measured under different temperature conditions for samples with different thermal aging statuses. Rectangular wires which find application in automotive motors are selected as test samples. The results link the temperature with the PD risk and the thermal lifetime of the insulation, and the winding hotspot temperature margin is accordingly determined. An automotive case study is presented to prove the applicability of the investigation outcomes.

Modeling the impact of high thermal conductivity paper on the performance and life of power transformers

Degradation of insulation paper is a key contributor to the failure of power transformers. Insulation degradation accelerates at elevated temperatures, which highlights the potential for better thermal management to prolong life. While several studies have analyzed the benefits of high thermal conductivity oil for reducing temperatures inside a transformer, this study is an initial assessment of the benefits of high thermal conductivity paper on transformer life. Blending particulates with cellulosic fibers offers a pathway for high thermal conductivity paper (with good dielectric properties), which can reduce internal temperatures. Presently, life extensions that can be achieved by the use of such thermally conducting papers were estimated, with the thermal conductivity of the paper being the key parameter under study. The analytical-numerical thermal model used in this study was validated against experimental measurements in a distribution transformer, adding confidence to the utility of the model. This model was then used to provide estimates of hot-spot temperature reduction resulting from the use of papers with higher thermal conductivity than baseline. Transformer life was predicted conventionally by tracking the degree of polymerization of paper over time, based on an Arrhenius model. Results indicate that increasing the thermal conductivity of paper from 0.2 W/mK (baseline) to 1 W/mK reduces the hot spot temperature by 10 °C. While degradation significantly depends on the moisture and oxygen content, the model shows that such a temperature reduction can increase life for all conditions, by as much as a factor of three.

Mitigation of common mode voltage in ultra sparse matrix converters using auxiliary shoot‐through switches for wind energy systems

SummaryThis paper targets to minimize the peak common mode voltage (CMV) in ultra sparse matrix converters (USMC) by applying new zero vectors for the space vector modulation technique (SVM). Conventional zero vectors in the SVM raise the peak CMV by 42.23%. An increased magnitude of CMV is associated with the occurrence of leakage current, hence resulting in insulation degradation. So, the reduction of peak CMV is necessary for USMC. The proposed method offers suitable switching arrangements using auxiliary shoot through (AST) switches to minimize the peak‐to‐peak CMV. This proposed method holds the advantages of the conventional method, like a wide range of modulation index and zero current switching at the rectifier side, and also reduces the CMV problem. This paper also provides a comparative analysis of the existing reported methods and the proposed method. The effectiveness of the proposed method is validated through both simulation and experimental results.

Experimental study of time-dependent dielectric degradation by means of random telegraph noise spectroscopy

Time-dependent dielectric breakdown (TDDB) is commonly used to assess dielectric failures. However, TDDB provides limited insights into the physics of dielectric degradation. In this paper, we explore the potential of random telegraph noise (RTN) spectroscopy to study the physics of dielectric breakdown. RTN is a fluctuation in the dielectric leakage current due to capture/emission of injected electrons by dielectric traps. We report an RTN study of large-area alumina (Al2O3) thin films. A stress experiment is performed on a fresh sample, where RTN is measured before, during and after stress. Important degradation signatures are identified in the RTN spectra. The degradation imposed by the applied stress is observed as a consistent transition between two distributions, where the RTN transitions from an initial pre-stress Gaussian, to a final post-stress exponential. A calculation of the noise entropy, which generally increases with growing material disorder, confirms the transition to an exponential distribution. Finally, we relate the RTN distribution parameters to the defectivity of the dielectric.

Evaluation method for insulation degradation of power transformer windings based on incomplete internet of things sensing data

AbstractThis paper proposes a novel evaluation method to address the challenge of evaluating insulation degradation in power transformer windings based on incomplete online Internet of Things (IoT) sensing data. The method leverages the Wasserstein Slim Generative Adversarial Imputation Network with Gradient Penalty algorithm to fill the irregularly missing power transformer IoT perception data, including voltage, current, temperature, and partial discharge. Subsequently, electrical, thermal, and mechanical performance degradation damage indicators for transformer winding insulation are constructed using the filled and complete IoT perception data. By applying the tensor fusion algorithm, the characteristics of these degradation damage indicators are fused, leading to the development of a comprehensive degradation evaluation index for the winding insulation. The evaluation of the winding insulation degradation state is achieved through the minimum quantization error method. The proposed method is validated using the real‐world transformer IoT perception data, and the experimental results demonstrate its ability to accurately assess the degree of winding insulation degradation, regardless of the presence of random or continuous irregularities in IoT sensing data.

WITHDRAWN: Design of partial discharge measurement model for internal cavities in high voltage power cables

The phenomenon of partial discharge (PD) in power cables can result in insulation degradation and eventual failure if left undetected and unaddressed. As a result, the measurement of PD is key to evaluating the performance of insulation diagnostics in the power cables. This paper presents a PD measurement model for internal cavities in XLPE insulating material for medium voltage power cables. The proposed PD measurement model is designed using the COMSOL Multiphysics software as a finite element analysis (FEA) tool added to MATLAB environment, specifically tailored for power cables. The proposed PD measurement model uses the multiphysics capabilities of COMSOL to simulate the electrical discharges behavior within power cables. The model considers factors such as cable geometry, material properties, cavity size, applied voltage and numerical conditions to accurately capture the PD characteristics. Distributions of both electric potential and electric field were studied before and after occurrence of PD across the internal cavity. According to the results, it has been noticed that the discharge magnitude mainly depends on the cavity diameter; as increasing cavity diameter leads to increasing the magnitude of the measured PD. Moreover, the leakage current magnitude is recorded during the PD event period. The proposed model also enables the evaluation of different cable designs, insulation materials, and operating conditions to optimize cable performance and mitigate PD risks.

Novel Method for Insulation Degradation Location of Rail Transit System Based on Fiber Grating

Novel Method for Insulation Degradation Location of Rail Transit System Based on Fiber Grating

Research on a Partial Discharge Expert System for the Diagnosis of Damaged Transformation Equipment

Partial discharge (PD) characteristics are very important for the diagnosis of damaged transformation equipment. If the power transmission and transformation equipment fails, it will cause large economic losses, and thus prevention is better than treatment. One of the effective methods for high-voltage insulation degradation detection is to observe the phenomenon of partial discharge of equipment, which is the earliest characteristic. Prevention can be carried out in advance, and trend observation is better than periodic inspection. Long-term observation can effectively reduce the probability of misjudgment. This project intends to develop a high-speed data-acquisition device to acquire the original discharge waveform data of partial discharge for noise suppression. In order to improve the diagnostic efficiency and accuracy of the diagnostic system, it is necessary to suppress the noise of the measurement data, so as to carry out the fault identification of the discharge type. Through this method, the actual operating data of the system can be recorded as the original discharge waveform, in order to understand the original discharge waveform conditions of the power transmission and transformation equipment in multiple partial discharge measurements, and then after noise suppression, it can be regarded as a type of partial discharge identification.

Investigation of partial discharge characteristics in XLPE cable insulation under increasing electrical stress

Insulation degradation in power components is a looming issue that can have a significant impact on the reliability of power system if not attended timely. Partial discharge (PD) diagnostics is a key technique used for the condition monitoring and health assessment of insulation. Inception of PD is a primary indicator which shows degradation and decreased local dielectric strength of the insulation. PD magnitude can increase with time and level of stress, which may result in failure of insulation. This article investigates the behavior of PD characteristics in a medium voltage (MV) cross-linked polyethylene (XLPE) cable sample, containing a defect at the cable termination, under elevated electrical stresses. PD behavior is studied by investigating the key PD characteristics, such as partial discharge inception voltage (PDIV), phase of occurrence (ɸ), mean amplitude (Vmean), pulse repetition rate (PRR), and phase span (ɸspan). These quantities are extracted from a phase-resolved partial discharge (PRPD) pattern using a pulse detection algorithm. Later, these characteristics are analyzed in correlation with the electrical stress. Various regression models are implemented and compared to find the best fit model that represents trending attributes of major PD characteristics. The presented study can be used as an effective process to analyze the measured PD data for evaluation of the insulation condition in the power components.

Insulation Condition Assessment in Inverter-Fed Motors Using the High-Frequency Common Mode Current: A Case Study

The use of the common mode current for stator winding insulation condition assessment has been extensively studied. Two main approaches have been followed. The first models the electric behavior of ground-wall insulation as an equivalent RC circuit; these methods have been successfully applied to high-voltage high-power machines. The second uses the high frequency of the common mode current which results from the voltage pulses applied by the inverter. This approach has mainly been studied for the case of low-voltage, inverter-fed machines, and has not yet reached the level of maturity of the first. One fact noticed after a literature review is that in most cases, the faults being detected were induced by connecting external elements between winding and stator magnetic core. This paper presents a case study on the use of the high-frequency common mode current to monitor the stator insulation condition. Insulation degradation occurred progressively with the machine operating normally; no exogenous elements were added. Signal processing able to detect the degradation at early stages will be discussed.

Performance Analysis of Methods at Estimating Insulated Cables Degradation

The quality of electricity supply, among other issues, deals with continuity indicators as one of the most important factors for the characterization of an adequate service as far as power quality is concerned. In this context, one of the major problems responsible for the unscheduled interruptions is related to the degradation of electrical cables insulation and the well-known water tree effect emerges as an important cause for the deterioration of their insulation layer. If this phenomenon happens, a large loss of insulating properties of dielectric material used to build up such component will occur. In light of these facts, the development of procedures to evaluate, predict, and indicate the operational status of operating cables shows itself highly attractive in order to improve the quality and reliability of energy supply. Focusing on this subject, this paper presents a synthesis of existing methods and a new trend towards the mentioned matter. By using experimental measurements, carried out in medium voltage commercial operating cables in a distribution utility throughout non-invasive approach, applied voltage and insulating leakage currents are obtained to support the analysis. The data are used to obtain degradation indexes to estimate the cables physical operational conditions and the indicators found are further considered in the view of the effectiveness of the strategies for the diagnosis purposes

Nonlinear constitutive model of CFRP composites under constant direct current: Combined effects of thermal damage and dielectric degradation

Prolonged exposure to low-density direct current (DC) can lead to mechanical degradation of carbon fiber-reinforced polymer (CFRP) composites, posing significant risks to material safety and reliability. This paper presents a nonlinear constitutive model to elucidate the mechanical degradation of CFRP composites when subjected to DC influences. Grounded in the principles of non-equilibrium thermodynamics, this model introduces two internal variables to account for the impact of thermal damage and dielectric degradation on the Helmholtz free energy. Furthermore, specialized dissipation functions are employed to derive the evolution equations governing these internal variables. Utilizing the model, we analyze the damage progression in carbon fiber–epoxy laminates subjected to constant DC loading. The theoretically projected resistivity and elastic modulus align closely with available experimental data in literature, thus confirming the rationality and accuracy of the proposed model. This model holds the potential to forecast the long-term evolution of mechanical properties in unidirectionally reinforced composite materials with varying carbon fiber contents under the influence of DC, thereby furnishing a theoretical foundation for enhancing the reliability design of CFRP composites in electrically-charged environments.

Detection of insulation degradation by-products in transformer oil using ZnO coated IDC sensor

Abstract Condition monitoring of oil-immersed in-service transformers to facilitate preventive maintenance is still a challenge. Monitoring of 2-Furfuryldehyde (2-FAL), released in the transformer oil as a result of paper insulation degradation, and moisture ingress can provide insight into the health of the insulation of transformers. Since 2-FAL and moisture are high dielectric constant contamination, capacitive sensor-based detection is a potential solution. A novel Inter digital Capacitive (IDC) sensor is reported in this paper to measure the concentration of 2-FAL and moisture uses Zinc Oxide (ZnO) as a sensing film. The sensor shows good sensitivity, approximately linear characteristics, and low characteristic drift.

Markers of cellulose insulation ageing: Calculation of total methanol quantity in the internal insulation of power transformers

The methods of calculation of methanol quantity in the internal insulation of power transformers (PT) based on the values of its concentration in transformer oil (TO) are considered. The total methanol quantity formed during the operation of PTs allows estimating the degree of degradation of cellulose insulation. We are considering the models for calculation of the total methanol quantity in PT on the base of: literature data characterizing the methanol balance in the cellulose-oil system; data on the reduction of methanol concentration after TO replacement during total overhaul. The errors analysis on the base of the results of calculations using the models showed that their value can reach 30%. Reduction of errors in the calculation results should be based on a deeper study of the kinetics of methanol sorption processes by cellulose and taking into account the structural features of different types of cellulose insulation in the particular PT. The considered models can be recommended for estimation of total quantity of any dissolved decomposition products of cellulose insulation in TO, which can become an important indicator of technical condition of PT.

A search for suitable mother wavelet in discrete wavelet transform based analysis of acoustic emission partial discharge signals

Signal processing helps monitor the condition of power equipment. Partial discharge (PD) signals used in condition-based maintenance give crucial information in the diagnosis of degradation of insulation. The acoustic emission technique (AET) is one of the most widely used techniques in PD signal analysis due to its inherent advantages. Analyzing acoustic emission partial discharge (AEPD) signals in the wavelet-domain provides critical insights into the location and type of the sources of PD. Selection of the most suitable mother wavelet in applying discrete wavelet transform (DWT) on AEPD signals is important as it will directly impact the outcome. For this selection, 36 wavelets belonging to the Daubechies, Symlets, Coiflets, and Bi-orthogonal families are investigated. For this purpose, five experimentally collected AEPD test signals are used. The selection is based on the ?accuracy of wavelet decomposition results? in this work, probably for the first time. One mother wavelet from each family is individually shortlisted for all three performances, namely (a) reconstruction, (b) denoising, and (c) compression, by computing and comparing their commonly used metrics. Further, based on percentage energy criteria, the most suitable mother wavelets are identified as coif3, coif4, and coif5, respectively, for the three performances.

Gaussian Process Regression for an Accurate and Uncertainty-Aware Winding Insulation Degradation Prediction

Gaussian Process Regression for an Accurate and Uncertainty-Aware Winding Insulation Degradation Prediction