David Malec

Electrical Aging of the Insulation of Low-Voltage Machines: Model Definition and Test With the Design of Experiments

The aim of this paper is to present a method for modeling the life span of insulation materials in a partial discharge regime. Based on the design of experiments, it has many advantages: It reduces the number of time-consuming experiments, increases the accuracy of the results, and allows life span modeling under various stress conditions, including coupling effects between the factors. Accelerated aging tests are carried out to determine the life span of these materials. The resulting model presents an original relationship between the logarithm of the insulation life span and that of electrically applied stress and an exponential form of the temperature. Results show that the most influential factors can be identified according to their effects on the insulation life span. Moreover, the life span model validity is tested either with additional points which have not been used for modeling or through statistical tests. Finally, it is shown that fractional plans are not suitable to reduce the number of experiments. This application of the experimental design is best used during the initial phase, before the final drive has been built and any online diagnosis.

Improvements on lifespan modeling of the insulation of low voltage machines with response surface and analysis of variance

The aim of this paper is to present some improvements of the method for modeling the lifespan of insulation materials in a partial discharge regime. The first step is based on the design of experiments which is well-known for reducing the number experiments and increasing the accuracy of the model. Accelerated aging tests are carried out to determine the lifespan of polyesteimide insulation films under different various stress conditions. A lifespan model is achieved, including an original relationship between the logarithm of the insulation lifespan and that of electrically applied stress and an exponential form of the temperature. The significance of the resulting factor effects are tested through the analysis of variance. Moreover, response surface is helpful to take into account some second order terms in the model and to improve its accuracy. Finally, the model validity is tested with additional points which have not been used for modeling.

Using the design of experiments (DoE) method to elaborate an electrical ageing model for the insulation of low voltage rotating machines fed by inverters

A large amount of parameters related to both operating conditions and material design affects the electrical ageing of the low voltage rotating machine insulation. Accelerated ageing tests are usually undertaken in order to develop a theory to describe the electrical ageing process and to determine a lifetime model of these materials. However, to the best of our knowledge, there is no complete model allowing the prediction of an insulation lifetime from accelerated ageing tests, since there are many possible failure mechanisms and various synergetic effects between them. Another problem with accelerated ageing tests is that results of the tests tend to have a great deal of scatter. In the present work, we propose the use of the design of experiments (DoE) method, which is a useful statistical approach that would lead to a reliable and significant interpretation of the different ordering parameters of the insulation ageing process. Using the DoE method, the analysis of accelerated ageing test results allows identifying the factors that most influence the results, and those that do not, as well as details such as the existence of interactions and synergies between these factors. In the following, results from accelerated ageing tests on PEI varnishes, largely used in rotating machines insulation, are presented and analyzed with the DoE method.

Electrical ageing modeling of the insulation of low voltage rotating machines fed by inverters with the design of experiments (DoE) method

In the context of more electric aircrafts, the reliability of the low voltage insulation systems in rotating machines is an important issue. But, modeling and understanding their electrical ageing is a complex phenomenon especially when they are fed by PWM inverter. They involve a large amount of parameters related to both operating conditions and material design, which act together. Accelerated ageing tests are usually run in order to describe the electrical ageing process and to determine the lifetime of these materials. Nevertheless, full ageing tests with all the various parameters for different samples become rapidly time consuming. Moreover, there is not any complete model for insulation lifetime prediction under accelerated ageing tests. In this paper work, we propose a method based on the design of experiments (DoE) method, which is a useful statistical approach that would lead to a reliable and significant interpretation of the different ordering parameters of the insulation ageing process. Moreover, this method will help to reduce the number of required experimental or numerical trials and will, take into account the possible failure mechanisms and the various synergetic effects between them. Using the (DoE) method, a preliminary model of the insulation “lifetime” is presented with respect to the most important parameters at play and interactions between them.

Lifespan and aging modeling methods for insulation systems in electrical machines: A survey

This paper deals primarily with modeling lifespan and, to a lesser degree, aging in insulation systems of electrical machines. The different aging processes involved, including partial discharges, are described. Previous works, in conjunction with the current study, are described and lead to the conclusion that there is no single method, even with accelerated lifetime tests, able to provide lifetime models taking into account all the stress factors in a simple way. Consequently, the principles of different regression methods are described (design of experiments, response surface, multilinear and robust regression) and applied with a limited number of experiments, thus reducing experimental cost. Several types of insulation material were tested in order to confidently recommend one in particular.

Ozone concentration impact on the lifespan of enameled wires (conventional and corona-resistant) for low voltage rotating machines fed by inverters

Since the introduction of the power electronic power supplies, the electrical insulation of low voltage motors faces new hazards. Especially the primary insulation of the magnet wire is endangered due to the use of frequency inverters. As an effect, partial discharges may appear causing degradation of the electrical insulation to lead finally to a premature breakdown of the machine. This study shows the impact of ozone, generated during partial discharge activity, on the lifespan of samples used to model the winding of the machine. Different types of magnet wires, composed of different enamel polymer, both conventional and corona-resistant, were tested in this paper. The results state that the mean time to failure is up to 25% shorter if the ozone is not evacuated from the test chamber and samples are exposed to its degrading properties. This is the case for close machines, in which the air embedded between the stator and the rotor and within the slot is not ventilated, thus allowing ozone to accumulate to significant concentrations.

Morphological evolution of polytetrafluoroethylene (PTFE) during thermal-oxidative ageing above and below the melting temperature

The organic electric insulator polytetrafluoroethylene (PTFE) is used in aerospace industry under extreme conditions of temperature and electric field. The melting temperature of PTFE is about 327°C. In our days operating temperature of this kind of insulators could punctually reach 300°C and 350°C for the new generation machines. This last temperature is above the melting temperature of the material. These severe operating conditions can be factors of ageing acceleration and/or degradation of the insulators. In general, when a polymer is exposed to an important thermal stress, chemical degradation reactions may take place, such as the macromolecular chain scissions and the formation of new chemical groups. This may affect significantly the molecular weight, chains arrangement or the size distribution of macromolecules. Hence, all this processes may cause modifications of theirs properties. Our present work is focused on the organic insulator behaviour above and below the melting temperature in order to understand the different mechanisms of thermal-oxidative ageing and degradation in the molten and the solid state. Thin films in PTFE were aged by accelerated method under oxidizing environment (in air) and severe thermal stresses between 250 and 450°C with exposure times from 2 to 300 hours. Before and after each ageing cycle, samples were characterized by weight loss evolution (performed by a microbalance), Fourier transform infrared spectroscopy (FTIR), and Differential scanning calorimetry (DSC).

Dielectric breakdown and morphological evolution of PTFE during thermal-oxidative ageing at temperatures lower and higher than the melting temperature

The organic electric insulator in polytetrafluoroethylene (PTFE) is widely used under extreme conditions of temperature and electric field. Nowadays, operating temperature of this kind of insulators can reach about 350 °C for new generations of machines, which is above than the melting temperature of the material (327 °C). These severe operating conditions can be factors of ageing acceleration and/or degradation of the insulators. Currently, there are very few specifications on the insulators for high temperature and/or high voltage applications, furthermore their ageing and degradation patterns are not well known. Our present work is focused on the organic insulator behavior at high temperature under oxidizing atmosphere in order to characterize the mechanisms of thermal-oxidative ageing and degradation as well as the impact on the dielectric breakdown strength. Thin films in PTFE were aged by severe thermal stresses between 250 and 450 °C under oxidizing environment were characterized by using weight loss measurements, FTIR as well as dielectric breakdown under AC voltage. The experimental results show that irreversible chemical ageing occurs at about 100 °C higher than melting temperature. PTFE losses its thermal stability and electrical performance at 400 °C; no noticeable thermal decomposition was revealed at 340 °C until 300 h of ageing. The degree of crystallinity has tendency to increase at early stage of ageing (340 °C), then it decrease for samples aged at very high temperature during long time (400 °C during 300 h and 450 °C during 50 h).

Electrical ageing of the insulation of low voltage rotating machines fed by inverters: The use of the design of experiments (DoE)

The electrical ageing of the low voltage rotating machine insulation involves complex phenomena since a large amount of parameters related to both operating conditions and material design are acting together. Accelerated ageing tests are usually undertaken in order to develop a theory to describe the electrical ageing process and to determine the lifetime of these materials. To the best of our knowledge, yet there is no complete model allowing the prediction of an insulation lifetime from accelerated ageing tests, since there are many possible failure mechanisms and various synergetic effects between them. Indeed, if one intends to perform a complete ageing test taking into account the various parameters for different samples, the operation becomes financially intolerable and extremely time consuming. In the present work, we propose the use of the design of experiments (DoE) method, which is a useful statistical approach that would lead to a reliable and significant interpretation of the different ordering parameters of the insulation ageing process. Moreover, this method will help to reduce the number of required experimental or numerical trials. Using the (DoE) method, a preliminary model of the insulation “lifetime” is presented with respect to the most important parameters at play and interactions between them.

Overvoltage at motor terminals in SiC-based PWM drives

Key points in the development of More Electrical Aircraft (MEA) are currently DC power distribution in higher voltage levels (540 V) and the use of disruptive technology such as Wide BandGap (WBG) semiconductors in power inverters. Using WBG components (SiC and GaN) increases the power converter mass density. However, fast switching of WBG components (tens of kV/s) induces voltage transient overshoots due to parasitic elements within the inverter. In addition, propagation and reflection phenomena along the harness connected to this inverter, even for small lengths, cause a significant voltage overshoot across the loads. Such overvoltage in Adjustable Speed Drives (ASD: association of inverter, harness and motor) supplied by the new HVDC 540 V aeronautical network could be fatal for the Electrical Insulation System (EIS). This paper proposes a fast and accurate modeling methodology to predict transient overvoltage; it allows us to analyze the impact of SiC inverter technology on overvoltage at motor terminals.