TRANSFORMER BUSHINGS FOR SPECIAL APPLICATIONS


High-Altitude Applications
Bushings intended for application at altitudes higher than 1000 m suffer from lower air density along the outer insulator. Standards specify that, when indicated, the minimum insulation necessary at the required altitude can be determined by dividing the standard insulation length at 1000 m by the correction factor given in Table 3.2.2.

For instance, suppose that the required length of the air insulator on a bushing is 2.5 m at 1000-m altitude. Further, suppose that this bushing is to be applied at 3000 m. Hence, the air insulator must be at least 2.5/0.8 = 3.125 m in length.

The air insulator on the bushing designed for 1000 m must be replaced with a 3.125-m-long insulator, but the remainder of the bushing, i.e., the central core and the oil insulator, will remain the same as the standard bushing because these parts are not affected by air insulation. These rules do not apply to altitudes higher than 4500 m.

Highly Contaminated Environments
Insulators exposed to pollution must have adequate creep distance, measured along the external contour of the insulator, to withstand the detrimental insulating effects of contamination on the insulator surface. Figure 3.2.2 shows the undulations on the weather sheds, and additional creep distance is obtained by adding undulations or increasing their depth. Recommendations for creep distance are shown in

Table 3.2.3 according to four different classifications of contamination. For example, a 345-kV bushing has a maximum line-to-ground voltage of 220 kV, so that the minimum creep is 220 X 28 = 6160 mm for a light contamination level and 220 X 44 = 9680 mm for a heavy contamination level. The term ESDD (equivalent salt-density deposit) used in Table 3.2.3 is

TABLE 3.2.2 Dielectric-Strength Correction Factors for Altitudes
Greater than 1000 m
Altitude, m Altitude Correction Factor for Dielectric Strength
1000 1.00
1200 0.98
1500 0.95
1800 0.92
2100 0.89
2400 0.86
2700 0.83
3000 0.80
3600 0.75
4200 0.70
4500 0.67
Source: ANSI/IEEE, 1997 [1]. With permission.

TABLE 3.2.3 Recommended Creep Distances for Four Contamination Levels
Contamination Level
Equivalent Salt-Deposit
Density (ESDD), mg/cm2
Recommended Minimum Creep
Distance, mm/kV
Light 0.03–0.08 28
Medium 0.08–0.25 35
Heavy 0.25–0.6 44
Extra heavy above 0.6 54
Source: IEEE Std. C57.19.100-1995 (R1997) [8]. With permission.

the conductivity of the water-soluble deposits on the insulator surface. It is expressed in terms of the density of sodium chloride deposited on the insulator surface that will produce the same conductivity.

Following are typical environments for the four contamination levels listed:

Light-contamination areas include areas without industry and with low-density emission-producing residential heating systems, and areas with some industrial areas or residential density but with frequent winds and/or precipitation. These areas are not exposed to sea winds or located near the sea.

Medium-contamination areas include areas with industries not producing highly polluted smoke and/ or with average density of emission-producing residential heating systems, areas with high industrial and/or residential density but subject to frequent winds and/or precipitation, and areas exposed to sea winds but not located near the sea coast.

Heavy-contamination areas include those areas with high industrial density and large city suburbs with high density emission-producing residential heating systems, and areas close to the sea or exposed to strong sea winds.

Extra-heavy-contamination areas include those areas subject to industrial smoke producing thick, conductive deposits and small coastal areas exposed to very strong and polluting sea winds.

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