02_ageing4 - IEEE Transactions on Dielectrics and...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October2002 697 Aging and Polarization Phenomena in PE under High Electric Fields , Jean-Pierre Crine Technology.consultant Brassard, Qc, Canada ABSTRACT The influence of high electrical dc fields (>20 kVlmm) on aging, polarization and on the mor- phology of polyethylene (PE),is discussed. Infrared and positron annihilation spectroscopy measurements as well as capacitance measurements tend to suggest that the polymermorphol- ogy is modified by high fields. We show that the accelerated electrical aging characteristics of PE are linked directly to the morphology changes induced by the field. Below a so-called crit- ical field, the activation volume of the aging process is dependent on the field-induced strain. Above the critical field, the amorphous phase is deformed significantly, and weak van der Waals bonds are broken, leading to another, faster, aging regime. There is an excellent agree- ment between the proposed model and experimental data obtained with various PE samples. The possible relation between the submicrocavity formation proposed in our aging model and various polarization measurements is discussed. It is our contention that strong charge injec- tion occurs only after submicrocavity formation, Le. after weak bond breakage. As is well known, the polarization currents obtained under high fields are controlled by space charges. It seems that the wave packets and the negative resistance observed at >lo0 kV/mm in PE are as- sociated with a steady state in field-induced defect formation. This suggests that space charges are related to the formation of submicrocavities and, therefore, are a consequence, not a cause, of high field aging. 1 INTRODUCTION Electrical aging of polymers is still poorly known and often it has been assumed that there is a power law relation between aging time t and breakdown field F, such as t = CF-", where C and n are constants, characteristic of the dielectric. Accelerated aging results plot- ted on log field us. log time graphs are expected to fit a straight line. We have shown elsewhere [l] that, for ac fields, life extrapolations for crosslinked polyethylene (XLPE) cables made from such graphs are, at best dubious, since the linear relation between log field and log time does not hold. In fact, results fit much better to a semi-log plot be- tween F and logt, in agreement with our previous aging model ll] or with the Dissado-Montanari 131 or the Lewis [4,5] models. In this paper, we present an improved dc-field aging model partially based on Lewis et al. suggestion [4,5] that the field-induced strain deforms the insulation. The predictions of our new model describe very well actual accelerated aging data for aged under dc fields in air at 22°C. The model relies on the activation energy and volume parameters, which depend on morphological and structural properties of the dielectric. We also have investigated some of the influence of high dc fields on these properties in order to relate aging and polymer morphology The re-
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

Page1 / 7

02_ageing4 - IEEE Transactions on Dielectrics and...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online