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The first non-ceramic (composite polymer) insulators, with fibreglass rods and rubber weather sheds, appeared in the mid-1960s. Non-ceramic insulators have a dirt and water repellent (hydrophobic) surface that reduces pollution accumulation and wetting. Polymeric insulators have been increasingly used for both distribution and transmission voltage ranges for the last few decades as, in general, the contamination performance of non- ceramic insulators is better than those of ceramic equivalents (Karady, 1999; Hackam, 1999; IEEE 1523, 2002; IEC 60815-3, 2008).

1.4.2 Pollution severity

Pollution-induced insulator flashover has been a problem since the first electric power systems (Baker et al., 2009). The phenomenon has been extensively studied (e.g. CIGRE TF 33.04.01, 2000; Farzaneh and Chisholm, 2009 and references within), as have a number of other airborne pollutants which can cause flashover (Table 1.3).

Table 1.3: Common types of airborne pollution capable of causing pollution-induced

flashover on electrical insulators (adapted from IEEE Std 957, 2005).

Contaminant Source and Explanation

Salt

Insulators located near a body of salt water (e.g. sea or ocean) are prone to salt contamination from wind-blown spray. Salt pollution may also originate from highways or elevated roadways where salt is used to melt snow and ice. Substantial salt deposits may accumulate during long periods of dry weather.

Cement/lime

Accumulations of lime or cement easily bond to insulator surfaces and often form a hard crust which is very difficult to remove. Cement plants, construction sites and rock quarries provide sources of ionic-rich contamination in the form of cement and or lime (CaCO3_).

Dust

Earth Earth dust arises from agricultural enterprises (e.g. ploughing _{fields), earth moving at construction sites, etc.} Fertiliser Fertiliser dust is emitted from the application of fertiliser _{during farming and from fertiliser manufacturing plants.} Metallic Metallic dust can stem from various mining and mineral _{handling processes.}

Coal

Coal mining and coal handling operations and industrial burning of coal are major sources of coal dust. Soot and fly- ash resulting from the burning of coal may form compounds that adhere firmly to insulator surfaces.

Feedlot

Provender dust and earth dust stirred by animals in large feedlots can settle on nearby insulators during periods of dry weather.

Smog (combustion emissions)

In urban areas, emissions from combustion automobiles and locomotives (e.g. diesel trains) and other industrial activities introduce a significant amount of particulate matter into the environment and pose a pollution hazard to insulators.

Chemical

Atmospheric pollutants from a wide variety of industrial chemical processes and aerial spraying of agricultural chemicals and fire-fighting chemicals (e.g. borate) can form considerable deposits on insulators. The characteristics of these chemical contaminants vary widely.

Defecation

Bird excrement can create a conductive path between the HV conductor and the earthed tower structure leading to ‘bird streamer’ flashover.

Smoke Industrial and agricultural burning or wildfires can, with other compatible conditions (such as moisture and humidity), cause the resulting contamination to accumulate on HV insulators.

Traditionally, power engineers have used quantitative methods to measure the severity (potential for flashover) of soluble and non-soluble airborne pollutants which accumulate on HV insulators (IEC 60815-1, 2008).

1.4.2.1 ESDD

Equivalent salt deposit density (ESDD), expressed in mg/cm2, is one of the most common parameters used to measure the pollution severity on contaminated insulators. An internationally recognised standard parameter, the ESDD or ‘Solid Layer’ method equates the amount of sodium chloride (NaCl) required to yield the same conductivity as the insulator contaminant when dissolved in the same volume of water (CIGRE TF 33.04.03, 1994). From the conductivity, volume, and temperature of the solution and the area from which the sample was collected, ESDD can be calculated. ESDD values are typically classified into insulator specific site pollution severity indexes provided in international standards, guides or technical brochures and/or bulletins, as shown in Table 1.4.

Table 1.4: General site severity and its definition per CIGRE Technical Bulletin 63 (1991)

and IEEE Std 1243 (1997).

Site Severity ESDD (mg/cm

2₎ CIGRE IEEE None 0.0075-0.015 Very light 0.015-0.03 0-0.03 Light 0.03-0.06 0.03-0.06 Average/moderate 0.06-0.12 0.06-0.10 Heavy 0.12-0.24 >0.10 Very Heavy 0.24-0.48 Exceptional >0.48 1.4.2.2 NSDD

The non-soluble deposit density (NSDD) is another internationally recognised pollution parameter that is used to quantify the amount of non-soluble matter on contaminated insulators (also expressed in mg/cm2_{). The}

sensitivity of salt deposit density is higher on insulator performance than that of NSDD (Ramos et al., 1993). However, other studies have shown that inert, non-soluble pollution can greatly reduce the flashover voltage of HV insulators (Ishii et al., 1996; Sundararajan and Gorur, 1996). Insoluble deposits do not contribute directly to conductivity but can affect (1) the run-off rate of soluble material, (2) the hydrophobicity of the insulator

surface, (3) the evaporation rate of the wetted layer, and (4) the local electric field strength (Farzaneh and Chisholm, 2009).

1.5 INSULATOR FLASHOVER

Insulator flashovers result in power outages that are expensive and therefore undesirable. For example, flashover across a line insulator may cause a 250 ms trip-out before an auto-reclose system reconnects the circuit. This is sufficient time to shut-down a paper machine, resulting in hours of down time, possible equipment damage, and up to $50,000 in lost production (IEEE Std 1523, 2002).

Johnston (1997) identified key factors influencing volcanic ash-induced insulator flashover (Figure 1.4). Once a volcanic ash deposit becomes conductive it provides a path for electric ‘leakage’ current to flow between the phase conductor (line) and the ground (earth). This leakage current causes a heating effect which dries out parts of the ash layer, giving rise to ‘dry bands’ on the insulator which interrupt the flow of leakage current. The dry bands are bridged by arcs which cause a surge of leakage current. Ultimately, the arcs or grow in size to span the whole of the insulator surface and a line-to-earth fault occurs (Baker et al., 2009).

Figure 1.5: Factors influencing volcanic ash-induced insulator flashover (from Johnston,

1.5.1 Flashover mechanism

Flashover is produced by the application of voltage wherein the breakdown (flashover) path becomes sufficiently ionized to maintain an electrical arc (IEEE Std 1410, 2004).

1.5.1.1 Hydrophilic surfaces

Hydrophilic surfaces attract and retain water molecules (Starov et al., 2007). The flashover process around or over porcelain and glass (hydrophilic) surfaces is believed to occur in 6 phases (adapted from IEC 60815-1 (2008) and Farzaneh and Chisholm (2009)):

Phase 1: A dry insulator string has been uniformly coated in dry, fine-

grained (e.g. <0.1 mm particle diameter) volcanic ash. As the dry contaminant is non-conducting, no sizeable leakage current will flow over the insulators’ surfaces.

Phase 2: As moisture gathers on the insulators’ surfaces via absorption,

condensation or precipitation, the soluble component of the ash dissolves, forming a conducting solution through which leakage current flows (Figure 1.6a).

Phase 3: The leakage current steadily builds up and, in time, will

generate enough heat in areas of high current density (e.g. near the pin of each cap-and-pin disc insulator in a chain) across the ash layer to induce a rate of evaporation greater than the rate of moisture accumulation. Prolonged evaporation leads to the formation of dry spots (Figure 1.6b). These dry spots offer a much larger resistance than the remaining conducting solution, thus almost the entire applied voltage falls across the dry spot to create an exceedingly non uniform voltage distribution (kV/m) along the insulator string.

Phase 4: The voltage gradient across the dry zones will subsequently

exceed the dielectric strength of the surrounding air to cause small arc discharges. Further discharging across the dry spots increases the

rate of evaporation, which in turn leads to an enlargement or coalescence of the dry spot(s) to form a single dry band (Figure 1.6c).

Figure 1.6: Process of flashover across a hydrophilic surface (from Farzaneh and Chisholm,

2009).

Phase 5: Areas of higher current density result in increased heating

effects and cause the dry band to increase in size until it becomes too wide (critical width) for discharges to exist and a breakdown occurs, forming a localised arc across the dry band (Figure 1.6d). This causes a surge of leakage current each time the dry bands on an insulator spark over.

Phase 6: The arc may migrate laterally along the dry band to an area of

higher electric field stress (Figure 1.6e). If the resistance of the still wet and conductive part of the pollution layer is low enough, the arcs bridging the dry bands are sustained and may finally continue to extend along the insulator, bridging more and more of its surface. This in turn decreases the resistance in series with the arcs, increasing the current and permitting them to bridge even more of the insulator surface. Ultimately, the insulator surface is completely bridged and a line-to-earth fault (flashover) is established. (Figure 1.6f).

1.5.1.2 Hydrophobic surfaces

A different model exists for hydrophobic (water repelling) surfaces such as those on composite polymer insulators or ceramic insulators with a room temperature vulcanising (RTV) silicone rubber coating. In summary, pollution flashover begins with pollution (e.g. volcanic ash) building up on the surface of non-ceramic weathersheds. Wetting produces distinct water droplets (as opposed to a film, as is the case on hydrophilic surfaces) (Figure 1.7a) on the surface and these droplets slowly migrate to the pollutant and subsequently dissolve the soluble material in the contaminant (Figure 1.7b). These wetted areas coalesce and initiate a leakage current (Figure 1.7c). The leakage current dries the insulator surface where areas of current density are highest (e.g. near the pin on standard cap and pin insulators) and increases surface resistance between wet regions. Electric fields between the wet regions increase to form small spot discharges (Figure 1.7d). With time, the increased arcing activity decreases the hydrophobicity of the insulator surface, creating larger wet regions and an intensification of discharges (Figure 1.7e). These discharges continue to increase until flashover occurs (Figure 1.7f).

Further information on the flashover mechanism on hydrophobic surfaces can be found in Karady (1999), Karady et al. (1995) and Farzaneh and Chisholm (2009). However, given the dynamic nature of hydrophobic surfaces and the resulting complex interactions with pollutants and wetting agents, no generally accepted model of pollution flashover exists for hydrophobic insulator surfaces (IEC 60815-1, 2008).