[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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3.2.4.2.3 Pressurized SF 6 Gas Insulators<br />
Since various pressures can be used for this application, the length of the insulator can be equal to or<br />
less than an insulator immersed in oil. Since particles are harmful to the dielectric strength of any<br />
pressurized gas, precautions are generally taken to keep the SF 6 gas free of particles. In such cases, no<br />
sheds are required on the insulators.<br />
3.2.4.3 Flange<br />
The flange has two purposes: first, to mount the bushing to the apparatus on which it is utilized, and<br />
second, to contain the gaskets or other means of holding the insulators in place located on the extreme<br />
ends of the flange, as described in Section 3.2.4.5, Clamping System. Flange material can be cast<br />
aluminum for high-activity bushings, where the casting mold can be economically justified. In cases<br />
where production activities are not so high, flanges can be fabricated from steel or aluminum plate<br />
material. A further consideration for high-current bushings is that aluminum, or some other nonmagnetic<br />
material, is used in order to eliminate magnetic losses caused by currents induced in the flange by the<br />
central conductor.<br />
3.2.4.4 Oil Reservoir<br />
An oil reservoir, often called the expansion cap, is required on larger bushings with self-contained oil for<br />
at least one and often two related reasons: First, mineral oil expands and contracts with temperature,<br />
and the oil reservoir is required to contain the oil expansion at high oil temperatures. Second, oilimpregnated<br />
insulating paper must be totally submerged in oil in order to retain its insulating qualities.<br />
Hence, the reservoir must have sufficient oil in it to maintain oil over the insulating paper at the lowest<br />
anticipated temperatures. Since oil is an incompressible fluid, the reservoir must also contain a sufficient<br />
volume of gas, such as nitrogen, so that excessive pressures are not created within the bushing at high<br />
temperatures. Excessive pressures within a bushing can cause oil leakage.<br />
On bushings for mounting at angles up to about 30 from vertical, the reservoir is mounted on the<br />
top end of the bushing. On smaller, lower-voltage bushings, the reservoir can be within the top end of<br />
the upper insulator. Oil-filled bushings that are horizontally mounted usually have an oil reservoir<br />
mounted on the flange, but some have bellows, either inside or outside the bushing, which expand and<br />
contract with the temperature of the oil.<br />
For the purpose of checking the oil level in the bushing, an oil-level gauge is often incorporated into<br />
the reservoir. There are two basic types of oil gauges, the clear-glass type and the magnetic type. The<br />
former type is cast from colored or clear glass such that the oil level can be seen from any angle of rotation<br />
around the bushing. The second type is a two-piece gauge, the part inside the reservoir being a float<br />
attached to a magnet that rotates on an axis perpendicular to the reservoir wall. The part outside the<br />
reservoir is then a gauge dial attached to a magnet that follows the rotation of the magnet mounted<br />
inside the reservoir. This type of gauge suffers a disadvantage in that it can only be viewed at an angle<br />
of approximately 120 around the bushing. For this reason, bushings with this type of gauge are normally<br />
rotated on the apparatus such that the gauge can be seen from ground level.<br />
3.2.4.5 Clamping System<br />
The clamping system used on bushings is very important because it provides the mechanical integrity<br />
of the bushing. A thorough discussion and excellent illustration of different types of clamping systems<br />
used for all insulators, including those used on bushings, is given in Section Q.2.2 of Appendix Q of the<br />
IEEE 693-1997, Recommended Practice for Seismic Design of Substations [11].<br />
Two types of clamping systems are generally used on bushings, and a third type is used less frequently.<br />
The first, the mechanically clamped type, uses an external flange on the end of each insulator, and bolts<br />
are used to fasten them to mating parts, i.e., the mounting flange and the top and bottom terminals. A<br />
grading ring is often placed over this area on higher-voltage designs to shield the bolts from electric<br />
fields. The mechanically clamped type is economical and compact, but it has an increased potential for<br />
breakage due to stress concentration present at the bolted clamps.<br />
The second type, the center-clamped type, involves the use of a compression-type spring assembly in<br />
the reservoir located at the top of the bushing, thereby placing the central conductor in tension when<br />
the spring assembly is released. This action simultaneously places the insulators, the flange and gaskets<br />
between these members, and the terminals at the extreme ends of the insulators in compression, thereby<br />
sealing the gaskets. The center-clamped type is also an economical, compact design, but it has the potential<br />
of oil leaks due to cantilever or seismic forces placed on the insulator. The capacitance-graded bushing<br />
shown in Figure 3.2.2 uses a center-clamped type of clamping system.<br />
The third type, the cemented type, uses a metal flange to encircle of the ends of the insulator. A small<br />
radial gap is left between the outer diameter of the insulator and the inside diameter of the flange. This<br />
gap is filled with grout material rigid enough to transfer the compressive loads but pliable enough to<br />
prevent load concentrations on the porcelain. As with the mechanically clamped type, bolts are used to<br />
fasten them to mating parts, and grading rings are used at the higher voltages. This type of clamping<br />
system minimizes the potential for oil leakage or breakage due to mechanical stress concentrations, but<br />
the overall length of the insulator must be increased slightly in order to maintain electrical metal–metal<br />
clearances. The pressurized gas bushing shown in Figure 3.2.4 uses the cemented type of clamping system.<br />
Whatever method is used for the clamping system, the clamping force must be adequate to withstand<br />
the cantilever forces that will be exerted on the ends of a bushing during its service life. The major<br />
mechanical force to which the top end of an outdoor bushing is subjected during service is the cantilever<br />
force applied to the top terminal by the line pull of the connecting lead. This force comprises the static<br />
force exerted during normal conditions plus the forces exerted due to wind loading and/or icing on the<br />
connecting lead. In addition, bushings mounted at an angle from vertical exert a force equivalent to a<br />
static cantilever force at the top of the bushing, and this force must be accounted for in the design.<br />
In addition to the static forces, bushings must also withstand short-time dynamic forces created by<br />
short-circuit currents and seismic shocks. In particular, the lower end of bushings mounted in circuit<br />
breakers must also withstand the forces created by the interruption devices within the breaker.<br />
Users can obtain guidance for allowable line pull from IEEE Std. C57.19.100-1995 [8], which recommends<br />
permissible loading levels. According to the standard, the static line loading should not exceed<br />
50% of the test loading, as defined later in Section 3.2.8.3, and the short-time, dynamic loading should<br />
not exceed 85% of the same test loading.<br />
3.2.4.6 Temperature Limits<br />
Temperature limits within bushings depend on the type of bushing and the materials used in them. Solidtype<br />
bushings are made of only the central conductor, the porcelain or epoxy insulator(s), and the sealing<br />
gaskets. These bushings are therefore limited to the maximum allowable temperatures of the sealing<br />
gaskets and possibly the epoxy insulators, if used.<br />
The kraft-paper insulation typically used to provide electrical insulation and mechanical support for<br />
the grading elements in a capacitance-graded bushing is severely limited by temperature. The maximum<br />
temperature that this paper can endure without accelerated loss of life is 105C. Standards [1] have<br />
therefore established the following maximum temperatures for this type of bushing:<br />
Temperature of immersion oil: 95C average over a 24-hr period, with a maximum of 105C<br />
Ambient air temperature: 40C<br />
Top terminal temperature: 70C (30C rise over ambient air)<br />
Bushing hottest-spot temperature: 105C<br />
IEEE Guide C57.19.100 [8] gives a detailed procedure for establishing thermal constants for conductor<br />
hottest spot of bottom-connected bushings with no significant dielectric losses and no cooling ducts.<br />
After the tests have been performed, an estimate for the steady-state temperature rise at any current can<br />
be made with the following equation [12]:<br />
HS = k 1 I n + k 2 o (3.2.4)<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC