Accelerated electro-thermal endurance testing and lifetime modelling of biaxially oriented polypropylene insulation films
Haapamäki, Jaakko (2020)
Haapamäki, Jaakko
2020
Sähkötekniikan DI-tutkinto-ohjelma - Degree Programme in Electrical Engineering, MSc (Tech)
Informaatioteknologian ja viestinnän tiedekunta - Faculty of Information Technology and Communication Sciences
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Hyväksymispäivämäärä
2020-08-12
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202007026314
https://urn.fi/URN:NBN:fi:tuni-202007026314
Tiivistelmä
This thesis was carried out as part of the EU-funded GRIDABLE project whose one objective is to develop and produce novel silica-BOPP (biaxially oriented polypropylene) electrical insulation films. The contribution of this thesis is to study how well the lifetime of these new, experimental materials will compare with established materials under long-term DC electro-thermal (E-T) endurance tests. Experiments were carried out for self-constructed metallized film capacitors insulated with the BOPP film under test, incorporating two separate sheets of metallized films as electrodes. The real operating conditions for commercial-grade metallized film capacitors were simulated by conducting endurance testing at elevated temperatures in an inert, oxygen-free ambient environment utilizing the self-healing capability of the metallized films.
The experiments had two objectives. The first was to conduct and compare endurance tests for different materials, and based on the results, perform lifetime modelling in order to evaluate their lifetime at lower E-T stress levels. The lifetime of a film was modelled separately as a function of an electric field (inverse power law) and temperature (Arrhenius model), and by their concurrent effects (multi-stress model). The second objective was to evaluate whether changes in preconditioning, i.e. the preliminary electro-thermal conditioning used to bring the sample to test conditions, could significantly influence the sample’s lifetime in the ensuing endurance tests. This influence was studied by varying the preconditioning variables and evaluating their effect on the sample’s lifetime in the endurance tests.
The preconditioning tests demonstrated that the life expectancy of the samples tends to increase in the endurance tests. However, this effect depended upon the electrical stress level employed in the endurance test combined with the temperature and duration of the preconditioning period. The effect was noticeable for all the films, including those without nano structuration, at 480 V/µm & 60 °C. However, in the lower stress endurance tests, the preconditioning effect was first observed to decrease (at 380 V/µm & 60 °C), and subsequently, to disappear (at 280 V/µm & 60 °C). It is suggested that the longer lifetimes observed in the high-stress endurance tests may be partly related to the annealing of the polymer film, which may have resulted in, e.g. its increased crystallinity. This could improve the polymer’s resilience to local plastic deformation, and thus explain the longer lifetimes. Since the preconditioning did not influence the sample’s lifetime at an electric field of 280 V/µm, it is plausible that the changes in preconditioning are related to a similar type of annealing effect which occurs during the actual, longer-term endurance testing. Further research is still needed to confirm this hypothesis.
The overall findings of the lifetime modelling imply, firstly, that the inverse power law may also be used to model the lifetime of experimental films and, secondly, that comparable lifetimes were predicted for both nanocomposite and commercial films under real service conditions (∼200 V/µm). However, at the elevated stress levels used in the endurance tests, the commercial pure BOPP reference film had somewhat better performance than the nanocomposite film. The lifetimes for the commercial films predicted by the models were shorter than had been expected, and indeed shorter than what is known. The reasons for this were not thought to be connected to the range of electro-thermal stresses used in the accelerated life experiments, but rather to the structure of the utilized test capacitors, which do not represent all the real properties of commercially-wound capacitor elements, such as winding pressure. Despite these inconsistencies, these kinds of tests can be useful when comparing different insulation films with each other. However, it is recommended that future studies should evaluate how much the endurance performance of polymer film can be improved if the characteristics of the test samples correspond more closely to those of commercial capacitors.
The experiments had two objectives. The first was to conduct and compare endurance tests for different materials, and based on the results, perform lifetime modelling in order to evaluate their lifetime at lower E-T stress levels. The lifetime of a film was modelled separately as a function of an electric field (inverse power law) and temperature (Arrhenius model), and by their concurrent effects (multi-stress model). The second objective was to evaluate whether changes in preconditioning, i.e. the preliminary electro-thermal conditioning used to bring the sample to test conditions, could significantly influence the sample’s lifetime in the ensuing endurance tests. This influence was studied by varying the preconditioning variables and evaluating their effect on the sample’s lifetime in the endurance tests.
The preconditioning tests demonstrated that the life expectancy of the samples tends to increase in the endurance tests. However, this effect depended upon the electrical stress level employed in the endurance test combined with the temperature and duration of the preconditioning period. The effect was noticeable for all the films, including those without nano structuration, at 480 V/µm & 60 °C. However, in the lower stress endurance tests, the preconditioning effect was first observed to decrease (at 380 V/µm & 60 °C), and subsequently, to disappear (at 280 V/µm & 60 °C). It is suggested that the longer lifetimes observed in the high-stress endurance tests may be partly related to the annealing of the polymer film, which may have resulted in, e.g. its increased crystallinity. This could improve the polymer’s resilience to local plastic deformation, and thus explain the longer lifetimes. Since the preconditioning did not influence the sample’s lifetime at an electric field of 280 V/µm, it is plausible that the changes in preconditioning are related to a similar type of annealing effect which occurs during the actual, longer-term endurance testing. Further research is still needed to confirm this hypothesis.
The overall findings of the lifetime modelling imply, firstly, that the inverse power law may also be used to model the lifetime of experimental films and, secondly, that comparable lifetimes were predicted for both nanocomposite and commercial films under real service conditions (∼200 V/µm). However, at the elevated stress levels used in the endurance tests, the commercial pure BOPP reference film had somewhat better performance than the nanocomposite film. The lifetimes for the commercial films predicted by the models were shorter than had been expected, and indeed shorter than what is known. The reasons for this were not thought to be connected to the range of electro-thermal stresses used in the accelerated life experiments, but rather to the structure of the utilized test capacitors, which do not represent all the real properties of commercially-wound capacitor elements, such as winding pressure. Despite these inconsistencies, these kinds of tests can be useful when comparing different insulation films with each other. However, it is recommended that future studies should evaluate how much the endurance performance of polymer film can be improved if the characteristics of the test samples correspond more closely to those of commercial capacitors.