Here are a few supplementary notes. Since our text was written by persons working with high voltage electric power systems, much of the terminology is derived from those systems. To introduce a few of the terms please refer to this figure:
In this figure of voltage as a function of time, I show that normally the signal oscillates in a sinusoidal fashion between one "per unit" and minus one "per unit" where these normalized units are used so that the plot applies to any power system voltage. Note that the lightning overvoltage (or impulse) has a very fast rise time (about a microsecond) and a slower fall time (about 50 microseconds to fall to the half-of-maximum level). The switching overvoltage is similarly on a microsecond time scale but is usually characterized by damped oscillations. The power frequency overvoltage could result from sudden loss of load that a generator is supplying and for a few cycles, the remaining load might be exposed to this power frequency overvoltage. High voltage techniques must be used to design the power system to withstand not only the nominal 1 per unit high voltages but also the transient overvoltages that are shown. These same concepts can be used to design particle accelerators, television picture tubes, x-ray generators, laser power supplies, pulsed electric field treatment chambers for food pasteurization, chemical plasma reactors for production of microelectronic circuits, and many other applications. As the course progresses, please let me know of specific aspects of high voltage engineering that are of interest to you. Let's start with a qualitative definition of ** Dielectric Breakdown Strength (DBS): Electric field intensity in V/m that causes an "insulator-to-conductor" transition in a material (solid, liquid, or gas). This transition is called dielectric breakdown of the material. Often times dielectric breakdown is followed immediately by the conduction of electrical current (from fractions of an ampere to millions of amperes depending on circumstances) through the newly formed conducting region. If conduction takes place in a hot ionized gas (plasma) then this post-breakdown conduction is called an electrical "arc" or "spark". Next are brief comments on the 3 states of matter as they relate to high voltage engineering: 1. Gases. (DBS < 25 MV/m). Atmospheric air is the most commonly used gaseous dielectric material. A few other common gases used in high voltage engineering are carbon dioxide, nitrogen, and sulfur hexafluoride. The oxygen in air and the fluorine in sulfur hexafluoride are important because they have a high cross section for attachment (they trap mobile free electrons to form relatively immobile negative ions.) As we will learn soon, it is free electrons that are instrumental in initiating dielectric breakdown. Gases may be self healing after dielectric breakdown. This means that if the voltage is removed or reduced then the dielectric strength may be recovered. The ability to self heal is sensitive to the formation of contaminants that might have formed during previous breakdown events. 2. Liquids. (DBS < 100 MV/m). Liquids provide cooling as well as dielectric strength as for the oil in large transformers. As for gases, liquids can be self healing with the caveat that the build up contaminants from previous breakdown events may degrade the dielectric strength of the liquid. Deionized water and a large variety of oils have been used as liquid dielectrics in high voltage systems. In the case of pulsed electric field pasteurization of liquid foods, the liquid dielectric may be exotic items such as milk, apple juice, orange juice, soup, or liquid eggs to name a few. Deionized water is used for its high electrical relative permittivity (about 80) while oil has a smaller value for this constant (about 3). Note that a measure of relative permittivity is NOT a measure of dielectric strength. Castor oil is a popular oil dielectric. In the past, oils with polychlorinated biphenyls (PCB's) were used as a fire resistant liquid but PCB's have been found to be very toxic and non biodegradable. There are VERY stringent standards for disposal of material containing PCB. 3. Solids. (DBS < 1000 MV/m). Surface flashovers may be self healing; however, severe surface tracking, which in its simplest form is the formation of a carbon track along a previous arc channel, can create permanent loss of dielectric strength. Breakdown and arcing through the center of a solid will not be self-healing. A popular solid dielectric is epoxy poured as a liquid (under vacuum conditions to remove gas bubbles) which is allowed to harden before high voltage is applied. Since control of electric field intensity, E, is essential in high voltage designs, I will remind you of important equations that describe the nature of E:
Recall that electrical permittivity accounts for bound space charge that exists in dielectrics. This bound space charge is restricted to move distances that are comparable to atomic dimensions (say 5 angstroms or so) while free space charge accounts for free electrons and ions that can move large distances when compared to atomic dimensions. E is the most important field since according to the Lorentz force equation, it is E that will accelerate free electrons. If E is strong enough, these accelerated free electrons can have an ionizing collision with a neutral species, and create another free electron. If this process continues, the concentration of free electrons can increase exponentially with distance and we have an "electron avalanche" which may lead to dielectric breakdown which may be followed by an electrical spark. Before listing homework assignments, I will end this section with a qualitative definition of ** Electrical Corona: Localized dielectric breakdown of gas insulation (usually air) due to locally large E field. The large E field is present only in some regions. Most of the region is filled with small E field and consequently catastrophic dielectric breakdown (spark) of the entire "gap" does not take place. Corona in power systems creates power loss, audible noise (if you stand under an energized transmission line, you may hear the snap-crackle-pop of corona), and radio interference especially on AM radio stations. When applied voltage in high voltage apparatus is increased slowly to produce spark over, corona usually precedes the spark and is an integral part of the spark over event. Similarly, in the case of lightning strikes, corona precedes the lightning flash.
End of Introduction to High Voltage Engineering.