2
Physical Behavior of Partial Discharges

2.1 Introduction

A partial discharge (PD) is defined as

". localized electrical discharge that only partially bridges the insulation between the conductors and which can or cannot occur adjacent to a conductor. Partial discharges are in general consequences of local electrical stress concentration in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses having duration of much less than 1 µs" [1].

This statement is focused on pulse-like behavior of partial discharges that are generated by ionization processes within the insulation. However, this definition does not consider that in many cases of electrical insulation, there are already charge carriers (mainly ions) below the ionization level1 due to natural radioactivity or cosmic radiation. For example, under (normal) atmospheric air conditions, the number of charged ions of ~(102-103) 1/m3 may be expected [2].

If within the insulation any electric field is acting, those charge carriers move according to physical law toward their counter-electrodes. If there are any insulating interfaces within or near the field-space, the moving carriers could accumulate on their surface for a certain time.

According to the Shockley theorem, each movement of charge carriers leads to an equivalent current in a connected outer circuit [3]. Below the already mentioned physical defined level of self-sustaining ionization process, the value of such currents is minimal, in the range of (p . n) A. In the case of alternating current (AC) electric fields, as is common practice for quality test procedures under AC conditions, the just-mentioned effect of accumulated charges does not play any significant role due to the recombination processes at polarity change of outer electric field. But in case of applied unipolar voltage stress as in a DC or impulse field, some charge accumulation could be considered on existing interfaces within the electrode system. From a physical point of view, such charge is acting as an additional field source and, therefore, could have certain influence on initiating self-sustaining ionization processes by influencing the original (background) field within the electrode system. In addition, there is evidence that pulse-less discharges may occur even at AC electric fields [4]. Nevertheless, the majority of PD phenomena is based on the pulse character, as mentioned, and will therefore be described in later chapters.

For local occurrence of any partial discharge, which is ignited within a certain part of electrical insulation, two basic requirements must be fulfilled:

• Local electrical field strength Eloc (given by voltage stress) in that part must exceed the (intrinsic) dielectric strength Ed of the insulation material and, therefore, fulfill the equation: (2.1)
• Free charge carrier(s), mainly electron(s), enable any ionization process to be initiated.

As defined, a partial discharge leads to a limited breakdown within a volume-part of insulation necessarily associated with an ionization process of gas molecules.2 This event can take place not only in ambient air or other gases but also in gas-filled cavities of solid insulation or in micro bubbles and water vapor of insulation liquids. Likewise, partial discharges may occur on interfaces between different dielectrics as pressboard barrier in liquids, insulator surfaces in gases, or interfacial discharges in slots of solids. Those partial discharges have a typical characteristic related to the connected surface: They "glide" over the participated surface led by electrical field strength, and therefore, they are often called gliding (or creeping) discharges. A simplified overview about such possible "PD-sources" is shown in Figure 2.1, whereby at "homogeneous" air insulation3 those parts are mainly located near electrodes,4 but at other insulation type the condition acc. to (2.1) may be fulfilled also within the insulation by possible imperfections like bubbles, impurities, etc. Note, that the physical nature of partial discharges is mainly based on physics of gas discharges, discussed in this chapter.

Any partial discharge process is dependent on parameters, which are influenced not only by magnitude of electrical field strength E in the insulation caused by electrode configuration (field type, type of imperfect size, and its location) but also by structure and type of insulating material (dielectric parameters, dielectric (intrinsic) field strength), evidence of interfaces, and even by ambient conditions (pressure, humidity, pollution). The requirement of available charged carriers is mainly influenced by type and duration of stressed voltage (alternating, direct, impulse or combined), the type of insulation material (gaseous, liquid, solid, hybrid), and the evidence of any other sources of charged carriers caused by cosmic, natural, or artificial radiation.

Figure 2.1 Typical PD sources within the insulation (schematically) (a) external, (b) internal, (c) gliding-type.

Partial discharges caused by self-sustaining ionization process are accompanied with a large variety of physical phenomena like light emission, mechanical and chemical reactions, as well as acoustic phenomena. All these phenomena will be applied for recognition, detection, and interpretation of PD measurement. It is obvious, that the PD behavior is several for different types of insulating material, electrical field configuration and imperfections, therefore, for a common description a certain classification of PD phenomena seems to be necessary. There are various criteria for such a classification: the location of PD source (imperfection) relating to the insulation (external, internal, surface); the insulating material (gaseous, liquid, solid, hybrid); the initiating electrical field (AC, DC, impulse); physical phenomena (electrical, optical, chemical, mechanical, acoustical); or even the electrical equipment in which the PD occur (e.g. switchgear, transformer, rotating machines, cables, insulators). For this chapter, the first classification will be preferred.

2.2 External Discharges

As defined, external PD occur "outside" of any insulation equipment preferable on sharp edges or points, but also on long electrodes with small curvature (e.g. ropes on overhead lines) or on surfaces of solid insulation (e.g. insulators). Such PD are typical gas discharges and may occur if electrical field strength E is high enough to initiate a self-sustaining ionization process, that value commonly termed as inception value.

The physics of gas discharges were intensively investigated already in last centuries [5-9], covering a large variety of characteristic discharges like glow-, Townsend-, streamer-, leader discharges, etc., sometimes also characterized as Corona discharges. Such PDs occur, if the electrical field has a certain degree of nonuniformity, which might be characterized, for example, by the field efficiency factor ?. The field efficiency factor ? is defined by the ratio of electric field strength E within an electrode arrangement (voltage U, gap distance d) as average value Emean divided by maximum value Emax [10]:

For any evidence of (stable) PD the value of ? should be, for example, in atmospheric air in the range or less than 0.2, mainly dependent on pressure and may be different for various gas type.

Each gas discharge is characterized by movement of charge carriers within the insulation gap, caused by formation of electron avalanches and drift of ions of both polarities. As already mentioned, each movement of charge carriers leads to an equivalent current in a connected outer circuit. Due to a several mobility of electrons and ions and their equivalent current is different as shown by theoretical calculation and practical measurement [11, 12] (Figure 2.2). A typical PD current is a very fast pulse characterized by a rise time in the nanosecond (ns) range , shown in the red dotted ovals) and with longer duration time of ns to µs range shown in green dotted ovals). That time behavior corresponds in the frequency domain with equivalent spectra, which reach up to a few GHz, depending on discharge type (Figure 2.3) [14].

Figure 2.2 Time behavior of PD current pulse (acc. to [11, 12]).

For better explanation of PD behavior, a simple tip-to-plane arrangement with ? < 0.2 in air is considered (Figure 2.4). The expected discharge current will be detected by measuring resistance (e.g. using an oscilloscope).

If the voltage U and, therefore, the electrical field strength E, reaches its inception value, a self-standing ionization process is initiating. That may be described with the generation of electron avalanches caused by collisions of accelerated electrons with neutral gas molecules [7]. Likewise, ions of both polarities will be generated. Note that electrons will be...