[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

3.2.5.2 Bushing Current Transformer Pockets
The bushing flange creates a very convenient site to locate bushing current transformers (BCTs). The
flange is extended on its inner end, and the BCTs, having 500 to 5000 turns in the windings, are placed
around the flange. This location is called the BCT pocket and is shown in Figure 3.2.2. In this case, the
bushing central conductor forms the single-turn primary of the BCT, and the turns in the windings form
the secondary. Bushings built with standard dimensions have standard lengths for the BCT pocket [6].
3.2.5.3 Lower Support/Lower Terminal
Bushings that do not use draw-through leads, described in Section 3.2.7.3, must have a lower terminal
in order to connect to the transformer winding or the circuit-breaker internal mechanism. This terminal
can be one of any number of shapes, e.g., a smooth stud, threaded stud, spade, tang, or simply a flat
surface with tapped holes for an additional terminal to be attached. Standards [6] prescribe several of
these for various sizes of bushings.
One lower terminal specified by standards for bushings with voltage ratings 115 through 230 kV
incorporates the lower-support function of the bushing with the lower terminal. The lower support is
an integral part of the second type of clamping system (compression) described above, and in this
function, it helps create the required forces for compressing the seals. The lower surface of the flat support
has tapped holes in it so that the desired lower terminal can be attached. Two lower terminals specified
by standards have a spherical radius of 102 mm (4.0 in.) machined into their bottom surfaces. This
spherical radius enables the use of an additional lower terminal with a suitably shaped matching surface
to be attached via the tapped holes, but at a small angle with the bottom of the bushing. This feature is
useful for attaching rigid leads that are not always perfectly true with respect to the placement of the
bushing, or when bushings are mounted at a small angle from vertical.
3.2.5.4 Lower-End Shield
It can be seen from Figure 3.2.3 that all regions of the lower end of air-to-oil bushings experience high
dielectric stresses. In particular, the areas near the corners of the lower support and terminal are very
highly stressed. Therefore, electrostatic shields with large radii, such as the one shown in Figure 3.2.2
and Figure 3.2.7, are attached to the lower end of these bushings in order to reduce the electric fields
that appear in this area. Shields also serve the purpose of shielding the bolted connections used to connect
the leads to the bushing. Since shields with a thin dielectric barrier are somewhat stronger dielectrically,
crepe paper is wrapped, or molded pressboard is placed on, the outer surfaces of the shield.
3.2.6 Bushings for Special Applications
3.2.6.1 High-Altitude Applications
Bushings intended for application at altitudes higher than 1000 m suffer from lower air density along
the outer insulator. Standards [1] 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.
3.2.6.2 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 [8] 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 28 = 6160 mm for a light contamination level and 220 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/cm 2
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
FIGURE 3.2.7 Lower-end shield.
Source: IEEE Std. C57.19.100-1995 (R1997) [8]. With permission.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC