[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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