Dielectric Materials for Electrical Engineering

Chapter 6

Space Charges: Definition, History, Measurement 1

6.1. Introduction

Under the action of numerous factors (electric field, temperature, humidity, radiation, etc.), the properties of dielectric materials become degraded over time in an irreversible way. The mechanisms causing this phenomenon (called ageing), bring in all of the stresses which the insulator must obey, and are subjected to approaches described later in this chapter. Ageing generates unfavorable effects on electrical components, which can manifest themselves by the appearance of defects under much weaker stresses than those originally considered.

Due to economic and environmental factors, the past several years in the energy and electronic fields have witnessed a race for miniaturization of components and systems, and for the growth of the power densities they convey. This has led to an increase of electrical and thermal stresses to which the insulators are submitted. In order to guarantee the reliability of components and systems, it is of fundamental importance to know, in a more precise manner, the utilization limits of the insulators used and their long-term evolution.

An ideal insulator should not contain charges other than atomic cores, electrons in the internal layers and valence electrons. In reality, however, there is no such thing as an ideal insulator.

Thus, when an insulator is subjected to high stress (high electric field, irradiation, important mechanical efforts and friction, variations of temperature), ...