Note: Descriptions are shown in the official language in which they were submitted.
CA 02205539 2000-08-02
77326-65D
Title - TRANSFORMER TESTING METHOD
This is a division of our copending Canadian Patent
Application No. 2,004,444 filed December 1, 1989.
1. Field of the Invention
This invention relates to devices for protecting
single and multiphase high voltage apparatus to provide over-
voltage protection and apparatus test, and more particularly,
relates to the means for testing single and multiphase high
voltage apparatus equipped with surge arresters or surge
protectors and/or test the surge arrester without the
necessity of disassembling the apparatus. wen more
specifically, this invention relates to the testing of an oil
filled apparatus such as a transformer having a surge arrester
of the metal oxide varistor (MOV) type designed for under oil
mounting and an external isolator.
2. Background
Very often, it is necessary to test for any one of
a number of reasons both in the field and prior to being
shipped the single phase and multiphase voltage apparatus,
i. e., transformer and/or its surge arrester. For example,
after a transformer is subjected to high voltage transients,
which could damage or destroy it, it may be necessary to
conduct tests in order to determine whether a part should be
replaced. During manufacture there could be faulty connections
or the like. Therefore, it may be necessary to test the
transformer of ter manufacture and before shipment. It may also
be desirable, and often necessary, to conduct routine tests on
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CA 02205539 1997-06-16 '
the transformer in order to determine that it is in good
working order.
Oil filled transformers and metal oxide varistor
arresters are known. Generally, it is necessary to provide an
arrester or a surge protector which protects the transformer
against high voltage transients. For this reason, it is common
practice to connect an arrester which will conduct transients
from a power line to ground ahead of or at the transformer when
high voltage occurs. The surge arrester may be mounted within
the transformer tank.
High voltage surges actuate the arrester so that
damaging electrical potentials are shunted to ground via the
arrester before the transformer can be destroyed. Since the
internally mounted arrester provides a path for shunting high
voltage to ground, it also prevents a valid dielectric test
of the transformer insulation system. Thus, it is not possible
to test the transformer without disconnecting the internal
arrester.
Therefore, the common practice is to disconnect the
a.rrester, dielectrically test the transformer, and then
reconnect the arrester or surge protector. In the case of an
oil~filled transformer which has an arrester mounted therein,
it is both awkward and costly to test the transformer and/or
arrester. The transformer tank must be opened to so disconnect
and reconnect the surge protector. This therefore substant-
Tally eliminates field and/or installation evaluation of the
transformer.
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Still another condition which leads to cost problems
and design restrictions is the need heretofore wherein an
arrester failure should result in an open circuit fault. For
example, most arresters are designed to melt open an isolating
.fuse link or to fracture and result in an open circuit
condition when a transient persists for a period of time.
Thereafter, it is necessary to disassemble the transformer
and clean and remove all of the arrester parts from the trans-
former housing. This is especially difficult when the
transformer housing is filled with oil. Further, when an
under oil arrester mounted in an oil filled transformer fails,
- t=here is no readily visible means to indicate the arrester
failure .
'Summa~.y .o.f '.the. .Tri~enti:on
According to the present invention, we provide a '
method of testing a transformer in an oil filled housing,
comprising the steps of: (a) providing an external arrester
i.nsulator/isolator penetrating said housing for making an
electrical connection through the housing while preventing a
leaking of oil from said housing; (b) electrically coupling
a.n arrester inside said housing between an interior end of
said arrester isolator and a predetermined potential point
within said housing; (c) removably and electrically coupling
an exterior end of said arrester isolator to a ground potential
point; (d) removing said ground coupling from said exterior
end of said arrester isolator; (e) testing said transformer by
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CA 02205539 2000-08-02
?7326-65D
measuring electrical characteristics on transformer connections
emerging from said housing; and (f) reconnecting said ground
coupling to said exterior end of said arrester isolator after
said testing is completed. If required, an insulating cap may
be applied to the external isolator to provide additional
withstand capability to external flashover during the tests.
The arresters or surge protectors of the oil filled
high voltage apparatus do not have to be disconnected in order
to test the transformer either in the factory after manufacture
and before shipment or in the field during operation. There
preferably is included means for remotely separating or
isolating the apparatus from the surge protector.
It is desirable to provide a visual indication of the
destruction of a surge arrester mounted within a transformer
housing so that workers in the field can quickly tell if a
transformer must be tested and repaired or replaced. An
internally mounted arrester no longer has to fail in an open
circuit condition. Here, an arrester may fail in a short
circuit mode and still give ample "open circuit" isolation.
An oil filled transformer has suitably mounted in its
transformer housing a surge arrester of the metal oxide
varistor type. An isolator is externally mounted on the
transformer housing and has an external visible link thereon.
The visible ground wire connected to the isolator is connected
to a ground point outside the housing. The other end of the
isolator is connected to the arrester inside the housing. Thus,
the arrester is connected from a ground point outside
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CA 02205539 1997-06-16
the transformer housing through the isolator. High voltage
transients are conducted to ground via the arrester. As stated
above, the arrester may be activated or destroyed by high
voltage and high current transients. When the energy level of
the transient is sufficient to damage the arrester, it will
also be sufficient to blow off the visible ground lead and
disconnect the external ground connection. Thus a man in the
field can readily see when the arrester internally mounted in
a transformer tank has been damaged.
Further, the externally mounted isolator enables the
i=ransformer to be readily tested. If required, an insulating
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CA 02205539 1997-06-16
E r
cap can be placed over the isolator to provide greater
insulation to ground during the dielectric tests.
~3RIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown in the
attached drawings, wherein:
Fig. 1 schematically illustrates a two coil transformer
in an oil filled housing with an under oil arrester in a
veri~ical position between transformer coils;
Fig. 2 is a cross-section taken along line 2-2 of Fig. 1;
Fig. 3 is a cross-section which shows a wall or cover
mounted feed-through bushing isolator and arrester insulator;
and
Fig. 4 is a schematic illustration of a single coil
transformer having a horizontal surge arrester;
Fig. 5 is a cross-section taken along line 5-5 of Fig. 4;
and
Fig. 6 is a graph showing a typical arrester isolator
disconnect characteristics that are used by the invention. .
DETAINED DESCRIPTION OF THE INVENTION
Figs. 1 and 2 show a transformer housing 20 coupled to a
high voltage line 22 via a primary insulated bushing 24. The
housing is substantially full of oil, up to level 26 or other
insulating;media. Oil is commonly used within the housing to
provide the required dielectric strength.
Inside the housing, an oil insulated transformer is
provided in any conventional design. We illustrate here two
coils 28, on a core 2.9 in a conventional manner. The exact
design of the transformer is not relevant to our invention.
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The transformer has a primary wire 22 connected to an
insulated bushing 24 on the exterior of tank 20. The external
connections 22 and point G are available for conducting
specified dielectric tests.
Enclosed within tank 20 is any suitable and known
arrester or surge protector 30 which is designed to protect
the transformer from high voltage. The preferred arrester is
a mental oxide varistor type which provides a non-linear
resistance that decreases under over voltage conditions. If
the voltage transient is high enough, the resistance of
arrester 30 significantly decreases, limiting the over voltage
applied to the potential point P1, thereby protecting the
transformer winding 28.
The arrester 30 is preferably positioned within the tank
20 in a position which minimizes the length of the lead lines
31, while placing the arrester in a mechanically safe and
fully protected position. The short lead lines are desirable
primarily to prevent their impedance from increasing the. ~-
overvoltage stress on the winding 28.
The arrester 30 is grounded at point G, via a through the
wall. external insulator/isolator 32 and connected, to a high
potential point P1. High potential point P1 is between
primary bushing 24 and transformer coil 28.
Under normal operating conditions, the arrester
resistance is high and has no significant effect upon the
potential at point P1. However, if lightning, for example,
should strike the primary feed line 22, the resulting high
voltage transient significantly reduces the resistance of
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CA 02205539 1997-06-16
.. .
arrs~ster 30 in order to conduct the transient to ground and
remove the current surge that might damage the coil 28. Upon
cessation of the overvoltage, a sharp increase in resistance
of i~he arrester takes place and the current through the
arrester 30 returns to the magnitude typical for normal
service.
Referring to Fig. 3, the external wall or cover mounted
arrester insulator/isolator 32 is shown in a partial cross
seci~ional view as mounted on the transformer tank 20. The
insulator/isolator 32 may have any appropriate configuration.
We use an insulator/isolator 32 having a feed-thru bushing
insulator 40 with an appropriate isolator 51 mounted thereon.
The isolator has an appropriate disconnector 52 releasably
mounted thereon. The insulator bushing 40 is made of any
appropriate insulating material. The insulating material and
insulating characteristics of the insulator bushing are such
that, with the ground lead removed from the isolator, standard
dielectric high potential tests may be run on the transformer.
In some cases it may be desirable to encase the external
isolator 51 with an insulating cap (not shown) to provide
additional voltage withstand capability.
In general, there is a hole in the wall or cover of the
transformer tank 20 through which the insulator bushing 40 may
pass. Threads T are formed on the inside end of insulator
bushing 40. A compression gasket 42 is trapped between
insulator 40 and tank 20 on one side and a compression nut 44
is i~hreaded onto the -threaded end of insulator 40 on the
inside of the tank 20. When compression nut 44 is tightened,
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_ CA 02205539 1997-06-16
the gasket 42 forms an oil tight seal between tank 20 and '
insulator 40. The oil seal is necessarv to arevent oil
leakage or moisture ingress.
Extending through insulator 40 is a threaded stud 46
which has a threaded receiver hole 49. The isolator 51 has a
threaded terminal 50 that is screwed into the hole 49. An
advantage of this construction is that the isolator 51 and/or
disconnector 52 may be replaced without having to either open
tank 20 or break the oil seal at gasket 42. The arrester 30
is connected to stud 46 via wire 31 (Fig. 1). A ground.wire
56 is connected from ground point G to a stud 58 (Fig. 3) on
the arrester disconnector.
The disconnector 52 may be operated by a blank 22-cal.
cartridge or other means such that the frangible housing of
disconnector 52 is broken and the lead connected to 58 is
disconnected from the arrester. The disconnector 52 is
actuated when enough heat is generated to ignite the powder
(no-t shown) in the disconnector 52. The heat occurs w
responsive to the high current conducted by the arrester
during or after conditions such as voltage transients.
Although we have described the use of a powder charge
disconnector, any suitable thermal type release disconnector
may be used.
In order to test the transformer without involving the
arrester or surge protector 30, the ground wire 56 (Fig. 1) is
disconnected from the lug 58, thereby removing ground from the
arreater, which open.circuits the arrester 30. -The test may
then be carried out by simply measuring the electrical
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CA 02205539 1997-06-16
characteristics on the transformer wire emerging from the
housing 20 and point G. An insulating cap can be placed over
the insulator/isolator so that the open circuited arrester 30
has no effect upon the testing. After the test is completed,
ground wire 56 is reconnected to the lug 58.
Another embodiment is shown in (Figs. 4, 5) where the
insulator/isolator 32 is mounted in the tank wall instead of
in i~he cover. Here, the same reference numerals are used to
identify the same parts that are shown in Figs. 1, 2.
Therefore, they will not be described a second time.
For this type of transformer, the arrester 30 is shown
mounted horizontally on insulated brackets 62, 64 which are
secured to the transformer core/coil assembly 29a.
Heretofore, arresters were usually designed to fail in an
open circuit mode. This requirement caused arresters to be
designed to fall apart or otherwise destroy themselves in
order to be certain that there is a physical gap in the
circuit after a failure has occurred. As a result, after a°
failure, the broken parts of-the perished arrester remained in
the transformer tank.
According to the invention, when a disconnector 52 (Fig.
3) operates, the frangible section ruptures and the ground
wire 56 is-blown off along with the arrester ground stud 58,
thereby producing an open circuit between potential point P1
and ground G. This means that the arrester may now be either
a short or an open circuit. Therefore, it is more probable
that the arrester may. not fall apart. Thus the whole arrester
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may be removed and the transformer placed back in service
after replacing the arrester, and changing the oil.
Fig. 6 illustrates the desired disconnecting fault
curi_-ent-time characteristic for a disconnector 52. Some high
voltage conditions (such as lightning strokes) do not last
long enough to generate a current which is heavy enough to
destroy the arrester 30. Therefore, it would not be either
necessary or desirable to actuate the disconnector 52. On the
other hand, if the energy level is high enough, it might be
desirable to have an instantaneous disconnect.
The horizontal axis of Fig. 6 indicates the root mean
square amperage of the fault current. The vertical axis
indicates the time required to ignite an explosive charge
after the indicated amperage occurs. The operating range or
band 70 indicates the allowable variance V for disconnector
operation.
The advantages of the invention should now be clear. It
is possible to conduct testing upon the transformer after ._
manufacture and before shipment without having to either open
the cover or disconnect the arrester. In the event of
arrester failure, blowing off the ground wire 56 gives a
visual indication to a lineman so that he will know that
maintenance is required, and to take the proper safety
precautions. The failure of an arrester no longer must be an
open circuit failure; therefore, it may not be necessary to
design an arrester to have an internal disconnecting feature.
For convenience of description, this specification
referred to "oil filled transformers°'. However, it should be
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understood that the principles of our inventian may also be
applied to many other types of transformers or other high
voltage devices with other insulating systems which may
require similar protection and testing. Therefore, the
invention is to be construed broadly enough to cover all
equivalent structures including both single and three phase
devices .
Those who are skilled in the art will readily perceive
how to modify the invention. Therefore, the appended claims
are to be construed to cover all equivalent structures which
fall within the true scope and spirit of the invention.
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