ABSTRACT
Electrical properties of interest for insulation materials can be classified into two major categories:
• Those of significance at low voltage operating stresses
• Those of significance at high voltage operating stresses
At low stresses, the properties of interest relate to dielectric constant, dissipation or power factor, and conductivity (resistivity). From an insulation materials perspective, dielectric constant can be visualized as representing the ability of the insulation to "hold charge." Dissipation factor represents a measure of the amount of energy lost as heat rather than transmitted as electrical energy. A good dielectric (insulation) material is one that holds little charge (low dielectric constant) and has very low losses (low dissipation factor). Polyolefins such as polyethylene (PE), cross-linked polyethylene (XLPE) and non-mineral-filled ethylene co-and ter-polymers (EP or EPR) represent examples of polymers that possess excellent combinations of these properties. This is discussed in depth in Chapter 5. [In this chapter, the terms 'power factor' and 'dissipation factor' will be used interchangeably.]
At high stresses (greater than operating stress), the characteristic of importance is dielectric strength. Here, the insulation must be resistant to partial discharges (decomposition of air in voids or microvoids within the insulation). Also of interest is the inherent ability of the polymeric insulation material to resist decomposition under voltage stress. Unfortunately, the data generated from the dielectric strength measurement does not provide constant values, but can vary depending upon how the measurement is performed. This will be discussed later in this chapter. In any event, the dielectric strength must be "high" for the insulation to be functional. This chapter will review factors that influence electrical properties at both low and high voltage stresses.
