Étude de la contrainte électrique subie par l'isolation de spires dédiée des bobines statoriques des grandes machines hydroélectriques

Abstract

The production and consumption of electrical energy are essential for the proper functioning of modern society. Electrical machines play a crucial role in the performance of electrical systems, especially in power generation, emphasizing the importance of maintaining them in good condition. However, failures in alternator coils are often related to the dedicated turn insulation (DTI) and, in such cases, result in significant damage. DTI failure is generally caused by impulses from the electrical network, such as transient phenomena related to equipment switching or lightning strikes. This failure occurs when DTI is degraded or limited to maintain an acceptable level of insulation. Currently, there are few studies on the degradation phenomenon of coil insulation, and there is presently no standardized test to evaluate the long-term performance of this type of insulation. Despite the lack of knowledge, manufacturers propose the use of coils without dedicated turn insulation (DTI), which are less expensive to manufacture and could increase the alternator's power output. This type of coil is currently used in Europe but is rarely found in North America because its long-term performance has not yet been demonstrated. As part of the alternator refurbishment project, Hydro-Québec is required to make decisions regarding the purchase of different types of coils offered by manufacturers. The project to study the long-term performance of coils with and without DTI will allow Hydro-Québec to make informed decisions based on valid data regarding the performance of coils without DTI and to validate whether it is comparable to coils with DTI. The first part of the study aimed to investigate the ability of the SFRA (Sweep Frequency Response Analysis) measurement technique to characterize the degradation of the DTI insulation system. Conventional diagnostic tools, such as partial discharge measurements and dissipation factor measurements for mass insulation diagnosis, are not effective for testing dedicated turn insulation. This is why the SFRA measurement technique, commonly used for power transformers, was of interest and proved effective in detecting DTI insulation defects. Its advantages lie in its portability (battery-powered equipment), speed in detecting various defects, and safety (application of low voltages). To date, this technique has only been tested experimentally for electrical machines, and no standardization projects are in progress. The evaluation of the SFRA measurement was carried out by inserting fault resistances between turns on an individual coil in the laboratory. The second part of the study aimed to develop an RLC model representative of a stator coil developed in the MATLAB environment. The purpose of developing a coil model was to create a mathematical and physical representation of the coil that would allow us to evaluate how the voltage is distributed inside it. This model will subsequently contribute to simulating the behavior of the coil under electrical stress, specifically overvoltages responsible for the ultimate failure of stator coils. Two types of models were evaluated, namely the distributed parameter transmission line model and the lumped parameter model. The latter was further developed as it is more representative of the behavior of a stator coil. SFRA (Sweep Frequency Response Analysis) measurements were used to validate this model. In other words, the results of SFRA measurements were compared to the simulation results obtained from the developed model. This validation ensured that the model was accurate and faithfully reflected the real behavior of the coil.

Date	11 Dec 2023
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Éric David (Supervisor) & Hélène Provencher (Co-supervisor)