[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

1.6 cm. This means that most of the current will flow in the region from the outer portion of the conductor
and radially inward to a depth of the skin depth . Second, the losses generated within a conductor will be:
P loss = I 2 R = I 2 L/A = 4I 2 L/(D 12 – D 02 ) (3.2.3)
where
I = rated current
= resistivity of the conductor material, ohmm
= 1.7241 10 –3 ohmm for copper with 100% IACS (international annealed copper standard)
= 3.1347 10 –3 ohmm for electrical aluminum alloy with 55% IACS
L = length of conductor, m
A = cross section of conductor = (D 12 – D 02 )/4
D 1 = outside diameter of conductor, m
D 0 = D 1 – , m
skin depth of the conductor in the case that a tubular conductor is used
0.0127 m for copper, 0.0159 m for aluminum, both at 60-Hz frequency
It can be seen from Equation 3.2.3 that P loss decreases as D 1 increases. Hence, design practice is to
increase the outside diameter of the conductor for higher current ratings and to limit the wall thickness
to near the skin depth. There are other technical advantages to increasing the outside conductor diameter:
First, from Equation 3.2.2, observe that electric-field stress reduces as d = D 1 increases. Therefore, a larger
diameter conductor will have higher partial-discharge inception and withstand voltages. Second, the
mechanical strength of the conductor is dependent on the total cross-sectional area of the conductor, so
that a larger diameter is sometimes used to achieve higher withstand forces in the conductor.
FIGURE 3.2.4 Pressurized SF 6 gas bushing.
rated through 345-kV system voltage, and partial-discharge limits as well as limits for power factor
and capacitance change from before to after the standard electrical tests.
3. IEEE Std. C57.19.03, Standard Requirements, Terminology and Test Procedures for Bushings for
DC Applications [7]. This standard gives the same type of information as ANSI/IEEE Std.
C57.19.00 for bushings for direct-current equipment, including oil-filled converter transformers
and smoothing reactors. It also covers air-to-air dc bushings.
4. IEEE Std. C57.19.100, Guide for Application of Power Apparatus Bushings [8]. This guide recommends
practices to be used (1) for thermal loading above nameplate rating for bushings applied
on power transformers and circuit breakers and (2) for bushings connected to isolated-phase bus.
It also recommends practices for allowable cantilever loading caused by the pull of the line
connected to the bushing, applications for contaminated environments and high altitudes, and
maintenance practices.
5. IEC Publication 137 [9], Bushings for Alternating Voltages above 1000 V. This standard is the IEC
equivalent to the first standard listed above and is used widely in European and Asian countries.
3.2.4 Important Design Parameters
3.2.4.1 Conductor Size and Material
The conductor diameter is determined primarily by the current rating. There are two factors at work
here. First, the skin depth of copper material at 60 Hz is about 1.3 cm and that of aluminum is about
3.2.4.2 Insulators
Insulators must have sufficient length to withstand the steady-state and transient voltages that the bushing
will experience. Adequate lengths depend on the insulating media in which the insulator is used and on
whether the bushing is capacitively graded. In cases where there are two different insulating media on
either side of an insulator, the medium with the inferior dielectric characteristics determines the length
of the insulator.
3.2.4.2.1 Air Insulators
Primary factors that determine the required length of insulators used in air at atmospheric pressure are
lightning-impulse voltage under dry conditions and power-frequency and switching-impulse voltages
under wet conditions. Standard dry conditions are based on 760-mm (Hg) atmospheric pressure and
20C, and wet conditions are discussed in Section 3.2.6.1, High-Altitude Applications.
Bushings are normally designed to be adequate for altitudes up to 1000 m. Beyond 1000 m, longer
insulators must be used to accommodate the lower air density at higher altitudes. Under clean conditions,
air insulators for capacitively graded bushings are normally shorter in length than insulator housings
without grading elements within them. However, as the insulator becomes more contaminated, the effects
of grading elements disappear, and the withstand characteristics of graded and nongraded insulators
become the same over the long term (15 to 30 min) [10]. Further guidance on this subject is given in
Section 3.2.6.2, Highly Contaminated Environments.
3.2.4.2.2 Oil Insulators
Since mineral oil is dielectrically stronger than air, the length of insulators immersed in oil is typically
30 to 40% the length of air insulators. In equipment having oil with low contamination levels, no sheds
are required on oil-immersed insulators. In situations where some contamination exists in the oil, such
as carbon particles in oil-insulated circuit breakers, small ripples are generally cast on the outer insulator
surface exposed to the oil.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC