Note: Descriptions are shown in the official language in which they were submitted.
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:1. BACI~G~OUND OE~` TEIE INVE.N~`ION
This invention relates to devices for mounting a
bushing of the type used with the lead wire portion of a
power trans~ormer or tank -type switching device, and more
particularly to a device for mounting a large bushing on
high voltage electrical equipment, e.g. of the 400 KV class
and above.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an explanatory diagram showing a bush-
ing mounted on electrical equipment;
Fig. 2 is a sectional view showing one example
of a conven-tional bushing mounting devicei
Fig. 3 is an e~planatory diagram for describing
the operation of the device shown in Fig. 2;
Fig. 4 is a view, partly in cross section, of a
bushing mounting part, showing one embodiment of this
invention;
Fig. 5 is a sectional view, similar to Fig. 4,
showing another embodiment of the invention; and
Fig. 6 is an explanatory dia~ram illustratlng
the operation of the devices of Figs. 4 and 5.
In the case where high voltage electrical equip-
ment is used in an environment where damage due to airborne
pollutants such as salt and dust is high, bushings are used
~5 which have long porcelain tubes with the surface leakage
distances increased, so that the electrical equipment can
withstand the environment. ~hen such equipment is employed
1 :in a re~ion havincJ Ere(Iuent earthquakes, for example in
Japan, it is designed with emphasis on aseismatic strenc~-th.
When the bushing installed on the electrical equipment
encounters an earthquake, the amplification of the earth-
quake, as experienced by the bushing, is affected by the
position of the installation, the foundation and tank
portions of the equipment, and sometimes the bushing is
vibrated more than expected. The natural frequency of the
bushing is determined by the weight distribution and rigid
ity of various parts of the equipmen-t. When the natural
frequency of the bushing appro~imates or equals the pre-
vailing frequency of the earthqua]~es, then a resonant
phenomenon is developed such that the vibrations are amplified
to a very ~arge magni.tude. Such amplified vibrations may
exceed the breaking strength of the bushing, resulting in
the breaking of the latter.
It is generally said that the greater part of the
prevailing frequency of an earthquake ranges from one to ten
hert2. Bushin~s mounted on electrical equipment of 220 KV
or higher classifications may have a natural freqency less
than -ten hertz. Thus, the bushings of these classifications
should have a sufficient dynamic aseismatic strength.
The bushings should be desiyned taking in-to con-
sideration the biggest earthquake which may occur in the
future instead of the greatest earthquake which has previously
occured. A bushing with a small porcelain tube is high in
natural frequency~ and thus no resonant phenomenon is
developed.
~9o~
:l In general, the vibration oE the largest earth-
quake will no-t exceed the breaking stress of a porcelain
tube having a height of up to 3 to ~ m. As the height of
the porcelain tube increases, the diameter is also increased
and the weight is increased proportiona-tely. Accordingly,
the mechanical stress occurring upon vibration is increased.
~specially in the case of bushing which is resistive against
damage due to salt, and which is used with electrical equip-
ment of the 500 KV classification, the height of the upper
porcelain tube may be 6 to 8 m, and the natural frequency
is on the order of 3 to 5 Hz. Therefore, during an earth-
quake, the vibration may exceed the breaking stress of the
procelain tube, with the aid of the resonant phenomenon.
In a bushing for electrical equipment of the UHV (1000 KV)
classification, the porcelain tube is as long as about 12 m.
Thus, this bushing is more li.l~ely to be damaged in the
event of an earthquake.
A bushing is mounted on a mounting seat installed
on the equipment as shown in Fig. 1. More specifically,
the mounting flange 3 of a bushing 2 is fixedly secured to
a bushing mounting seat 1 of the equipment.
When the electrical equipment encounters an earth-
qua~e, the vibration of the ground surface is amplified
through the foundation and tank portions of the electrical
equipment, as a result of which the amplitude of the vibra-
tion is increased several t~mes. Accordingly, stress
developed in the porcelain tube may exceed the breaking
--3~
j~
L s-tress, thus damaginc3 the bushing. In order to increase
the aseismatic streng-th oE the bushing, a variety of devices
have been proposed in which the vibration is greatly damped
at the mounting portion to thereby decrease the accelera-tion
response thereof.
One example of such a device is shown in Fig. 2.
The operating range of an oil damper 5, having coil compres-
sion springs 4 arranged at equal, intervals on the circular
periphery of a mounting part, is limited by a mounting
guide 6, so that the oil damper 5 is not activated before
the stress developed in the porcelain tube e~ceedS the
breaking stress. That is, the damper operates only when a
great vibration occurs, to decrease the shock of the vibra-
tion, i.e. the accelera-tion response of the bushing, to
thereby increase the aseismatic strength.
In the operation of the device shown in Fig. 2,
the displacement of the bushing is as indicated by the
dotted line 2' in Fig. 3; that is, the displacement takes
place with the periphery oE the mounting flange 3 as the
fulcrum. In this case, the amount of displacement oE the
oil damper 3 having the coil springs is large, and the amount
of compression of the coil springs 4 is also largei that
is, the compression strength varies greatly with the ampli-
tude. Therefore, depending on the magnitude of the amplitude,
the stress developed in the porcelain tube may exceed the
breaking stress thereofO Thus, -this structure is not always
securable against s-trong earthquakes. When the phase of
--4--
:L the busllin~3 is i.nverted by a v:i.brat.ion largc in amp:Litude,
-~he mounting flanc3e 3 of the bushing 2 may collide with
-the mounting guide 6, -thus generating an i~pact s-tress.
This is another drawback of tne conventional device.
U.S. patent 4,267,400, commonly assigned, dis-
closes another type of damped bushing struc-ture somewhat
similar to that just discussed above, but wherein a resil-
ien-t buffer, a bellows seal and dish springs are also used
to advantage. However, this device, like prior devices,
contains only one set of springs so that the displacement
of the bushing is limited as above.
SU~lMARY OF THE INVENTION
An ob~ect of this invention is to eliminate the
above-descri.bed drawbacks accompanying a conventional bush-
ing mounting device. A speciEic feature of a bushing
mounting device of the invention resides in that :Eric-tion
dampers made up of a number of stac~ed annular springs
such as cup springs or ring-shaped springs are installed
on both sides of a bushi.ng flange or bushirlc3 mounting seat.
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1 DETAILED DES.C~IPT~ON OF TH~ P~EFERR~D EM~ODIMENTS
This inventIon ~111 be described with reference
to the embodiments thereof shown in Figs~ 4 and 5. As
shown in Figs. 4 and 5, frIction dampers 10, 11 and 12 are
arranged on the lo~er and upper surfaces of a mounting
flange 3 for a ~ushing 2. The frict1on dampers 10, 11 and
12 are formed by stacking annular springs such as ring-
shaped or cup sprin~s, and are di~posed at equal intervals
in a circle and secured to a mountIng seat 13 with bolts
and nuts 14. The space between the bushing mounting flange
3 and the moun-ting seat 13 is sealed with a bellows lS.
The bushing 2 is mounted as described above~ When
electrical equipment having this mounting structure exper-
iences an earth~ua~e/ the displacement of the bushing mount-
ing part is as indicated by the dotted line 2" in Fig. 6,which is an explanatory diagram of the operation of the
bushing mounting device. That is, ~he displacement of the
bushing is vibration a~out the center of the mounting flange,
and the vertical motion is damped by the friction dampers
10, 11 and 12 arranged on the periphery of the mounting
flange. Thus, a damping effect can ~e o~tained even in a
low level vibration range~ and the shock imparted to the
bushing end can be reduced to a lo~ value.
The damping factors of bushings of 500 KV or UH~
~1000 KV~ classificat~ons are, in general, on the order of
5 to 7 %, and the acceler~tion responses of the ends thereof
are on the order of seven ~o fifteen times~ Therefore~ when
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1. a great earthqu~ke ta~es place, the stre~s in the porcelain
tube may e~ceed the breakage stress value thereo~, Porcelain
tubes of the UllV ~10~0 KV~ classification are 1~ to 12 m
in diameter, and with respect -to aseismatic strength, in a
region where earthquakes occur Erequently, for instance in
Japan, a study has indicaied that the bushings can withs-tand
earthquakes if the acceleration response of the bushing
ends ;s decreased to about five times. Therefore, in the
case of UHV bushings, by provi.ding friction dampers having
a damping factor of about 10 ~, electrî.cal equipment whi.ch
can withstand the largest earthquake can be o~tained,
Another em~odiment of this invention is as shown
in Fig. 5. In this embodiment~ friction dampers lla and 12a
are arranged on the upper and lower surfaces of a ~ushing
lS mounting seat 13a, respectively, and are coupled to a bush-
ing mounting flange 3. The space between -the bushi.ng mount-
ing flange 3 and the bushing mounting seat .13a is sealed
with.a bellows 15a set outside the friction dampers.
Th.erefore, the friction dampers, in this ca~e, can be in-
corpora-ted into the interior of the casing of the electrical
equipment. It will be readily understood that the vibration
damping effect of the device of Fig. 4 is completely equal
to that of the device of Fig. 5.
