Conference Agenda

Short-circuit forces on Cleats and Leads (C&L) conductors in power transformers can cause serious faults and finan-cial losses, yet there is no standard design method to address these issues. This publication presents a modelling approach using Multiphysics simulation and parametric electromagnetic field analysis, integrated with structural as-sessment to evaluate C&L conductor performance under short-circuit conditions.

The method further automates the generation and evaluation of study cases via the PyAEDT library, creating simpli-fied FEM arrangements for various configurations. Electromagnetic forces are automatically extracted and applied as loads in a homogenized mechanical model to identify critical cases.

In the next phase, conductor stiffnesses were experimentally measured to build accurate cable models. Power trans-former cables, made of stranded copper wires wrapped in paper insulation, display high bending flexibility and ten-sion stiffness. Insulation thickness significantly affects bending stiffness, which was quantified through three-point bending tests, enabling the creation of reliable numerical beam models for advanced simulations.

Simulations revealed deformation and stress patterns during short-circuit events that matched experimental data, validating the model's predictive capabilities. This approach helps inform better design guidelines and improves transformer reliability under extreme conditions.

The accelerated thermal aging of Type I electrical insulation components and systems to predict the life span of rotating machines is well established in the literature. Accelerating aging tests on motorettes are usually performed for machines design by monitoring turn-to-turn, phase-to-phase and phase-to-ground configurations. However, make the link between motorettes aging behavior and simple samples as enameled wires or insulating papers coupons degradation behavior is a difficult task. Enameled wire twisted pairs, is the only standardized intermediate object representing the turn-to-turn configuration, which can be used for degradation understanding studies. However, no standardized intermediate object exist to represent the phase-to-ground configurations, which represent the most critical failures in rotating machines and involving all insulation layer of the EIS (enamel, paper and impregnation resin). This study presents a new intermediate test vehicle called “Binos” demonstrating the validity of this object to represent the phase-to-ground configurations of motorettes and showing the interest of this new intermediate sample for the EIS thermal aging behavior understanding.

The experimental work is mainly based on thermal aging monitoring of partial discharge inception voltage (PDIV) and capacitance measurements of Motorettes and Binos with additional Binos micro-sections to support the discussions. Accelerated thermal aging was done during 2 000 h at three different isotherms (200°C, 220°C and 240°C) to observe the drift of properties.

PDIV results shown similar steady trends and similar breakdown kinetics highlighting the good correspondence between both test vehicles. Impregnated and non impregnated Binos allow to determinate that the origin of the breakdown during PDIV test is probably induced by a mechanical stress increase due to impregnation aging, supported by Binos micro-section.

Capacitance results show a decrease during the thermal aging in accordance with the aging temperatures in both test vehicles. Motorettes capacitance loss appeared to be faster and more important. This drift of properties has been highlighted by Binos to be induced by increased air gap appearance in the phase-to-ground configuration due to winding movements.

The good repeatability of the PDIV and capacitance results of Binos combined with a good correspondence with motorettes results confirm that Binos can be considered and used as a new intermediate test vehicle of reference for EIS thermal aging understanding to link motorette phase-to-ground configuration behavior to Binos behavior.

The continuous demand for higher power densities in electric vehicles raises significant reliability concerns, particularly for rotary machines. While thermal endurance remains a major issue in this field, increasing attention is being paid to the thermal cycling aging of Electrical Insulation Systems (EIS). Indeed, certain mission profiles require enduring a large number of ON/OFF duty cycles and abnormal transients that can induce thermomechanical fatigue in the winding components. Differential mechanical stresses develop between materials due to mismatched coefficient of thermal expansion and the complex geometry of the assembled system, for which the understanding of failure mechanisms and the availability of accurate design tools remain limited.

Active temperature cycling tests are carried out on test specimens known as motorettes, by periodically applying current pulses through the conductors to induce Joule heating. This approach enables rapid heating of the windings within seconds, followed by cooling through air vents and a cold plate onto which the stack is clamped.

The accelerated aging tests involve applying one or more stresses beyond their nominal in-service levels. In this study, only the temperature cycle amplitude (ΔT) is varied, while the cycling frequency is kept constant. Due to the chosen heating and cooling methods, the average temperature — which can be regarded as a steady-state thermal constraint — is not yet fully controlled. The experimental campaign produced results for three thermal cycle amplitudes and temperature profiles applied to three batches of motorettes representative of automotive EIS configurations.

Several electrical markers were periodically monitored during aging: Partial Discharge Inception Voltage (PDIV), leakage current, capacitance, and dissipation factor, to assess how these parameters are affected by EIS degradation (wear, delamination, defects) under the given conditions. Additionally, regular visual inspections were conducted to evaluate the reliability of the electrical markers being monitored.

PDIV and insulation resistance measurements for both phase-to-phase and phase-to-ground configurations showed a decreasing trend with thermal cycling. Capacitance exhibited a non-monotonic response across all specimens. Failures frequently occurred during PDIV measurements throughout the campaign, sometimes correlating with the overall integrity degradation observed during visual inspection. Finally, a Coffin–Manson law is established based on a defined failure criterion using the evolution of electrical markers and breakdown observations.

In the case of Type I (partial discharges free) inverter-fed machines, the Partial Discharge Inception Voltage (PDIV) remains a critical factor to asses thermal endurance. Monitoring its evolution and optimizing design margins through accelerated thermal ageing can be challenging due to significant time required for such tests, expensive testing infrastructure, and the expense of producing representative specimens. To overcome these limitations, experiments are often performed on simplified test vehicles known as motorettes. Although they are cost-effective and reproduce the correct assembly of EIS materials, winding configurations, and impregnation processes, questions remain regarding how accurately motorettes can reflect the behavior of full-scale stators under real operating conditions.

To address these problems, a comparative thermal ageing campaign was carried out on both stators and motorettes with exactly the same EIS. Throughout the ageing process several electrical indicators, such as PDIV, capacitance, and dielectric loss factor were monitored.

The results revealed that in some configurations similar trends were observed for stators and motorettes, in particular for capacitance and average parameter levels. However, a much larger dispersion was noted for stators, and differences appeared in PDIV values. These variations may suggest that motorettes, while useful, may not fully represent some complex interactions present in real stators.

Finally, in order to better understand these differences, a comparative assessment was made aiming to correlate the observed discrepancies with variations in geometry, manufacturing processes, and other system-level parameters that might influence the overall performance of EIS. Those conclusions were confronted with the results of the aging tests on the individual EIS component in order to better understand how to correlate those material properties with the performance of the assembled EIS.

High-voltage transformers and rotating machines form the backbone of modern power systems, where insulation systems play a crucial role in ensuring reliability and operational safety. The performance and lifetime of such electrical equipment are significantly influenced by the quality, design, and condition of their insulation systems. This paper presents a comprehensive review of insulation materials, design strategies, degradation mechanisms, and diagnostic techniques for transformers and rotating machines. The study also discusses recent advancements in nanocomposite insulation materials, thermal aging studies, and condition monitoring methods that enhance dielectric performance and extend equipment life. Future directions emphasizing sustainability and smart insulation systems are also explored.

The aim of this paper is to investigate the impact of the thermally forced accelerated aging process on lightning properties of selected modern dielectric liquids such as GTLs compared to conventional mineral oil. (MO). Research plan implemented consists in affecting liquid with a high temperature of 150°C for a period of 168 hours, which are in accordance with CIGRE and IEEE recommendations, and then cooling the liquid and conducting the tests for lightning impulse inception voltage, breakdown voltage and acceleration voltage. Tests are carried out in a point-sphere electrode system for a 25 mm inter-electrode gap and negative polarity of lightning impulse voltage. Tests are performed on fresh liquid as a reference point and then after at least two aging cycles, 168 hours each. Liquids will also be verified in terms of their fundamental parameters such as kinematic viscosity, dielectric losses, acidity, and interfacial tension, which are the well-known indicators of liquid aging.

Based on the measurements performed, it is possible to correlate the level of aging with changes in lightning properties of liquids under consideration.

The results show the reduction of the lightning impulse breakdown voltage and acceleration voltage after aging in all liquids considered. This happens with a similar level independently of the liquid. This means that the liquids under consideration are affected equally by the thermal aging. However, detailed data, including also fundamental properties of the liquids confirming progress of aging such as moisture content, dielectric dissipation factor and interfacial tension, will be provided in full paper.