Pierre Bidan

Development of a new off-line test procedure for low voltage rotating machines fed by adjustable speed drives (ASD)

Using adjustable speed drives employing fast switching components to supply low voltage asynchronous motors may be detrimental to their reliability in some cases. The premature failures of the insulating materials are often attributed to the existence of partial discharges (PD) in the windings. Nevertheless, a clear evidence of any PD existence in such systems is far from being established. Also, their location may not be in the turn-to-ground or phase-to-phase insulation but in the turn-to-turn insulation due to the voltage waveforms, and as such, the classical AC test is no longer applicable. The aim of this paper is to present an off-line test procedure capable of overcoming this problem. The results of the test carried out on the representative population of different motors are presented.

Evaluation of Encapsulation Materials for High-Temperature Power Device Packaging

High-temperature power electronics represent an increasing demand. Higher power density or severe ambient temperature applications become the trend, while silicon carbide components with 250-300 °C Tjmax are emerging. Among materials used in high-voltage power module, soft encapsulants play a significant role in improving both semiconductor die and module package voltage ratings, especially under enhanced electrical and thermal constraints. In operation close to their upper temperature limit, two silicone materials were selected among the most thermally stable soft insulators available today. Up to 300 °C, dielectric properties and their stability under isothermal aging in air ambient tests were characterized. The gel, tested using sandwich structures, exhibits cracking of its exposed-to-air face, at an aging temperature as low as 250 °C after less than 100 h. The elastomer, tested as free films, presents no cracking, no degraded electrical characteristics, and a 6 % relative mass loss, after 500 h aging. Moreover, the elastomer insulating properties, at low and high electric field, remains stable up to 300 °C (short-term tests), contrary to the gel which shows a strong increase in dc electrical conductivity. So the elastomer shows promising properties for improved encapsulation performance at 250 °C, to be further investigated in package configurations.

Broadband dielectric spectroscopy of BPDA/ODA polyimide films

Dielectric spectroscopy of a high-temperature photosensitive polyimide was investigated in wide temperature and frequency ranges during heating and cooling cycles (from −150 to 370 ◦ C and from 0.1 to 1 MHz). During the heating phase measurements two sub-glass relaxation processes were observed, noted as γ and β relaxations. The γ relaxation appears at a low temperature (around −60 ◦ C at 1 kHz) with an activation energy of 0.44 eV during the heating phase and disappears during the cooling one, indicating that the peak is initially related to the presence of water in the polyimide films. The β relaxation appears at higher temperatures (around 180 ◦ C at 1 kHz) with a higher activation energy of about 1.5 eV. The β peak location and intensity for low temperatures (between 100 ◦ C and 120 ◦ C) change slightly on comparing the heating and cooling spectra, indicating also the effect of water molecules, which may act as a plasticizer. However, for higher temperatures, the β peak does not show any significant effect of the thermal cycle, and the relaxation is mainly attributed to the non-cooperative relaxation of the polyimide molecules. The ac conductivity (σ � ) values show that the electronic hopping process is influenced by the dynamics of the segmental and macromolecular chains of the polyimide in the γ and β relaxation regions. At high temperatures (>250 ◦ C) a plateau region appears in the ac conductivity allowing the extraction of the dc conductivity values, which are not affected between the heating and cooling measurements. This leads us to conclude that there are no significant morphological or chemical changes in the polyimide even for temperatures higher than its glass transition one under N2 for short periods. For temperatures above 300 ◦ C an increase in the values of relative permittivity is observed and referred to the Maxwell–Wagner–Sillars or to the electrode polarization phenomena. In this range the activation energy of the polarization peak frequency, conductivity relaxation peak frequency and the dc conductivity is the same and equal to 2.4 eV, indicating that those three parameters are governed by the same underlying mechanism. (Some figures may appear in colour only in the online journal)

Partial discharges monitoring in twisted pair fed with PWM inverter using non-intrusive sensors

This paper deals with partial discharge measurements in twisted pair fed by a PWM inverter. Measurements are carried out using antenna sensors. Both D-Dot sensor and self-made sensors are compared when it comes to detect partial discharge in a PWM environment. High-pass filter performance is tested as well with these sensors. Finally, a innovative noise suppression method is demonstrated using two identical antenna sensors. This method could be a useful and simple mean to improve signal to noise ratio before signal processing.

Dielectric properties of colossal permittivity materials: An update

During the last ten years, a lot of interest has been devoted to the so-called colossal dielectric constant (CDC) material. The first materials exhibiting this behavior were the perovskyte-based ceramics based on the CaCu3Ti4O12 composition (CCTO). Relative dielectric permittivity can attain values up to (or even larger than) 105. Nevertheless, their losses are still high, the lower values ranging 10%, in a narrow frequency range, thus limiting their applications. Most work on this type of materials aims to reduce these losses and widen their useful frequency range. On the other hand, the underlying physical mechanisms responsible of the CDC are still under study. While the analysis of broadband impedance spectroscopy measurements leads most of the authors to propose an interfacial polarization mechanisms (at the electrodes or at internal barriers), there is a limited number of complementary electrical characterization techniques, for the moment, they comfort the proposed interfacial polarization mechanisms. In the present work, several characterization techniques like I-V, Sawyer-Tower (ST) measurements, and time-domain polarization are used to characterize these materials. One of our main results is the observation of a non-symmetrical response of these materials related to the direction of the polarization. These results are observed for both macroscopic level on bulk polycrystalline material and within individual grains of the same samples. These results do not fit current accepted models for polarization for CDC materials.

Design of packaging structures for high voltage power electronics devices: Electric field stress on insulation

The use of very high voltage power modules (> 10 kV) requires the implementation of appropriate structures for the device packaging. Particularly the triple point between the insulating substrate, the metallization and the encapsulation is subjected to strong electric field stress. A novel architecture is proposed for the metallized ceramic substrate. Its interest is validated by using finite element method simulations to analyze the electric field distribution inside the module. It is shown that creating a gap between the top electrodes thus forming mesa structures, enables a significant spreading of the equipotential lines. The resulting maximum electric field decreases exponentially versus the height of the mesa structure, with better results than when simply increasing the thickness of the ceramic in a conventional module structure.

Using non-intrusive sensors to detect partial discharges in a PWM inverter environment: A twisted pair example

This paper deals with partial discharge measurements in twisted pair fed by a PWM inverter. Measurements are carried out using antenna sensors. Both D-Dot sensor and self-made sensors are compared when it comes to detect partial discharge in a PWM environment. High-pass filter performance is tested as well with these sensors. Finally, a blind source separation method is presented.

Identification of dielectric relaxations: a suitable infinite-dimensional state-space approach

The concept of diffusive representation is used in this paper for modeling purposes in the field of electrical engineering, namely when some non-rational dynamics are involved. Diffusive formulations are convenient for analysis and cheap numerical approximations. We study the problem of time-domain identification of the so-called diffusive symbol /spl eta/(/spl xi/) associated to the permittivity /spl epsiv/(p) of dielectric material. The so obtained state-space models allow simple and precise time-domain simulation of such dynamical components.