WL Vosloo (March 2001)

36-WG11/Renard/62 Contribution to paragraph 6: Rapid events

IDENTIFICATION OF TYPES OF POLLUTION There are two main forms of insulator pollution that can lead to flashover: predeposited and instantaneous flashover occurs mainly at 50 Hz service voltage (Un-Umax). However, a switching Impulse may be a factor but this would be very rare. The maintenance/remedy to prevent such flashovers will depend mainly on the mode of pollution. Thus the user needs to know which mechanism is at work. PRE-DEPOSIT Pre-deposit pollution is classified into two main categories, namely active pollution that forms a conductive layer, and inert pollution that forms a binding layer for the conductive pollution [2]. These categories are described below. Active pollution: High solubility salts: NaCl, MgCl, NaSO4 etc. Low solubility salts: Gypsum, fly ash etc. Acids: SO2, SO3, NOx etc. Active pollution is subdivided into conductive pollution (which is permanently conductive i.e. pollution with metallic conductive particles), high solubility salts (ie, salts that dissolve readily into water), and low solubility salts (that need a large volume of water to dissolve). Active pollution is measured in terms of an Equivalent Salt Deposit Density (ESDD) in mg/cm2 [3]. Inert pollution: Hydrophilic pollution: Kaolin, clay, cement, etc. Hydrophobic pollution: Silicone grease, oil, etc. Inert pollution is classified as either hydrophilic (when it absorbs water) or hydrophobic grease (when it repels water). Inert pollution is measured in terms of Non-soluble Deposit Density (NSDD) in mg/cm2. Possible sources of insulator pollutants and their effective distances of influence are given below. The sea (about 20 km from the coastline).

Factories emitting contaminants such as SO2 that can dissolve to form conductive layers during acid rain conditions (up to 15 km). Mining activities that produce dust-containing substances such as gypsum or Illmenite (up to 15 km). Agricultural activities such as crop spraying or ploughing (up to 2 km). Bird droppings which are solidified or partially wet. For ease of understanding the pre-deposit pollution flashover process, it is divided into six phases described separately below. In nature these phases are not distinct but may tend to merge. The process is described as encountered on hydrophilic surfaces, such as ceramic materials, [1,4,5]. Phase 1: The insulator becomes coated with a layer of pollution. If the pollution is non-conductive (high resistance) when dry, some wetting process (phase 2) is necessary before flashover will occur. Phase 2: The surface of the polluted insulator becomes wetted. The wetting of an insulator can occur in the following ways: by moisture absorption, condensation and precipitation. Heavy rain (precipitation) may wash away the electrolytic components of part or the entire pollution layer without initiating other phases in the breakdown process, or it may promote flashover by bridging the gaps between sheds. Moisture absorption occurs during periods of high relative humidity (>75%RH) when the temperature of the insulator and ambient air are the same [6,7]. Condensation occurs when the moisture in the air condenses on a surface whose temperature is lower than the dew point [6]. This condition usually occurs at sunrise or just before. Phase 3: Once an energised insulator is covered with a conducting pollution layer, surface leakage currents flow and their heating effect starts within a few power frequency cycles to dry out parts of the pollution layer. This occurs where the current density is highest i.e. where the insulator is at its narrowest. These result in the formation of what are known as dry bands. Phase 4: The pollution layer never dries uniformly, and in places the conducting path becomes broken by dry bands which interrupt the flow of leakage current. Phase 5: The line-to-earth voltage appearing across dry bands (which may be only a few millimetres wide) causes air breakdown and the dry bands are bridged by arcs which are electrically in series with the resistance of the undried and conductive portion of the pollution layer. This causes a surge of leakage current each time the dry bands on an insulator spark over. Phase 6: If the resistance of the undried part of the pollution layer is low enough, the arcs bridging the dry bands are sustained and will 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, it is completely bridged and a line-to-earth fault (flashover) is established. One can summarise the whole process as an interaction between the insulator, pollutants, wetting conditions, and applied voltage (and source impedance in laboratory conditions). The likelihood of flashover increases with higher leakage current, and it is mainly the surface layer resistance that determines the current magnitude. It can therefore be concluded that the surface layer resistance is the underlying factor determining whether an insulator will flash over or not, in terms of the above model. High NSDD This is a low conductivity pollution that builds up in thick layers, e.g. cement dust and fly ash, is termed as ‘high NSDD’. Low NSDD This is a high conductivity ‘thin’ pollution layer, e.g. marine salt and SO2, is termed as ‘low NSDD’. INSTANTANEOUS POLLUTION Conductive Fog ‘Instantaneous pollution’ refers to a contamination of high conductivity which quickly deposits on insulator surfaces, resulting in the condition where the insulator changes from an acceptably clean, low conductive state to flashover in a short (< 1 hour) time and then returns to a low conductive state when the event has passed. For ease of understanding instantaneous pollution flashover the same process as described in section 6.1 applies. However, the instantaneous pollution is normally deposited as a highly conductive layer of liquid electrolyte, e.g. salt spray, salt fog or industrial acid fog, thus phases 3 to 6 above may happen immediately. In nature these phases are not distinct but they do merge. These only refer to hydrophilic surfaces. Areas most at risk are those situated close chemical plants, or areas close to the coast with a known history of temperature inversions. Bird Streamer A particular case of ‘instant’ pollution is bird streamer. This is a type of bird excrement, which, on release, forms a continuous, highly (20-40 kΩ/m)

conductive stream of such length that the air gap is sufficiently reduced to cause flashover. In this case, the insulator geometry and characteristics play little or no role [8]. REFERENCES [1]

JST Looms, “Insulators for High Voltages”, IEE Power Engineering series 7, 1988.

[2]

RS Gorur, EA Cherney, JT Burnham, “Outdoor Insulators”, Ravi S Gorur Inc, Phoenix, Arizona, 1999.

[3]

IEC 815, “Guide for the Selection of Insulators in Respect of Polluted Conditions”, Geneva, 1986.

[4]

JP Reynders, “Guide to the Choice of Outdoor Insulators for AC Systems under Polluted Conditions, NEC, Pretoria, 1993.

[5]

WL Vosloo, “Introduction to Insulator Pollution Monitoring in Distribution and Transmission Power Networks: A Theoretical and Practical Workshop”, Eskom internal document, November 1996.

[6]

FF Bologna, “Development of an Insulator Pollution Site Severity Monitor based on a Thermo-Electric Cooler, Masters Dissertation, Port Elizabeth Technikon 1999.

[7]

G Riquel et al, “Wetting Process of Pollution Layer on High Voltage Glass insulators, 9th International Symposium on High-Voltage Engineering, Montreal, 1997.

[8]

PV Taylor et al, “Unknown” Category of MTS Line Faults; Bird Streamers as a Cause of Transient Earth Faults”, Eskom Progress Report, July 1999.