High-voltage switches with arc preventing or extinguishing devic – Arc preventing or extinguishing devices – Vacuum
Reexamination Certificate
1999-11-03
2001-01-09
Scott, J. R. (Department: 2832)
High-voltage switches with arc preventing or extinguishing devic
Arc preventing or extinguishing devices
Vacuum
C218S135000, C218S139000, C218S140000, C218S155000
Reexamination Certificate
active
06172317
ABSTRACT:
TECHNICAL FIELD
This invention relates to high-voltage vacuum interrupter modules. More particularly, this invention relates to switches with vacuum interrupter modules used in electric furnaces and electric utility switching systems. Specifically, this invention relates to refurbished vacuum interrupter modules with improved operating characteristics over the original manufactured modules.
BACKGROUND ART
A high-voltage switch may include a vacuum interrupter module which performs the operation of current interruption. As is well known in the art, electric furnaces require large amounts of power to be under constant control. To properly maintain this control, the switch must be able to withstand repeated interrupting of contact at voltages of up to 138 kV.
An exemplary high-voltage vacuum interrupter module of the prior art is shown in FIG.
1
and generally indicated by the numeral
10
. The module
10
includes an upper terminal pad
12
that provides an electrical and mechanical connection to the furnace or other piece of equipment operating at high voltage. The upper terminal pad
12
is electrically connected to a vacuum interrupter, generally indicated by the numeral
14
, through a stationary stem
18
. The vacuum interrupter
14
includes a vacuum container
20
through which axially extends the stationary stem
18
that is electrically connected to a stationary contact
24
. The stationary contact
24
is mateable with a moving contact
26
.
FIG. 1
shows the contacts
24
and
26
in a closed circuit condition. As those skilled in the art will appreciate, the moving contact
26
connects to and is mated with the stationary contact
24
to complete the current path to the furnace or the like. The moving contact
26
and the stationary contact
24
are sealed within the highly evacuated vacuum container
20
. The moving contact
26
is connected to a moving stem
32
that axially extends from the vacuum container
20
. Attached to the moving stem
32
is a shunt plate
58
. A flexible shunt
34
is connected to the shunt plate
58
and connected at its opposite end to a lower terminal pad
38
. The module
10
is part of a switching mechanism that may include any number of modules.
A cylindrical, hollow housing, which is generally indicated by the numeral
40
, along with the upper terminal pad
12
and the lower terminal pad
38
, enclose the vacuum interrupter
14
and the flexible shunt
34
.
A bellows
41
is incorporated between the moving stem
32
and the proximal end of the vacuum container
20
. The bellows
41
is a very thin flexible metal that allows the contacts
24
and
26
to separate while still maintaining the very high vacuum inside the vacuum container
20
. External air pressure acting on the bellows
41
exerts a force on the moving stem
32
which is proportional to the diameter of the bellows. This external air pressure force has to be overcome by the switch mechanism during the opening of the module
10
. Thus, sizing of the bellows
41
is critical. A dielectric material
42
is interposed and bonded to the housing
40
and the vacuum interrupter
14
to preclude any electrical flashover caused by system transients.
In operation, when the module
10
changes from a closed to an open state, an external lever mechanism moves the moving stem
32
by an insulated pull rod
44
through a pull rod screw
57
. The pull rod
44
axially withdraws the pull rod screw, which is mechanically and electrically attached to the moving contact
26
inside the vacuum interrupter
14
, and separates the contacts
24
and
26
from one another a small amount. Movement of the pull rod screw is biased by the flexible shunt
34
. At the moment of separation, metal in the contacts
24
and
26
is vaporized and forms a conductive plasma. The current continues flowing through this plasma until a current zero is reached. At the current zero, metal vapors are no longer generated at the surfaces of the contacts
24
and
26
. As such, the conductive plasma dissipates, and when the next alternating current cycle occurs, no current flows because there is no conductive material in the gap between the contacts.
Closing of the module
10
initiates a reverse sequence of the above operation. The external lever mechanism pushes on the pull rod
44
which pushes on the pull rod screw which in turn pushes on an over-travel spring
46
contained within the moving stem
32
. Axial movement of the moving stem
32
axially moves the moving contact
26
. Accordingly, the contacts
24
and
26
are connected and the vacuum interrupter module
10
is closed. The spring
46
functions to equalize the force among several vacuum interrupter modules simultaneously operated by the same external switch mechanism.
The switch usually fails because of wear to the vacuum interrupter
14
or some of its attached electrical components. The high-voltage module
10
is then discarded and replaced with a new module. The average useful life of the prior art module is estimated to be in the range of about 60,000 to 125,000 operation cycles. Replacement of the vacuum interrupter modules is quite expensive, especially considering that only one or two of the internal parts have failed.
Another problem with known high voltage modules relates to the dielectric material
42
disposed between the vacuum interrupter
14
and the housing
40
. Since the dielectric material is bonded to both components, it is impossible to simply replace the defective vacuum interrupter without damaging the dielectric material. Accordingly, simple replacement of the vacuum interrupter
14
is not possible.
Another drawback with existing high-voltage switches is that the connection between the upper terminal pad
12
and the vacuum interrupter
14
must be provided with as low a resistance connection as possible. The prior art upper terminal pad
12
is typically made of aluminum that over time might develop a highly resistive aluminum oxide layer. Prior to assembly of the switch, the oxide surface is chemically stripped and an oxide inhibiting grease is applied thereto. The prior art vacuum interrupter module
10
has a copper mounting plate
50
that is mechanically fastened to the aluminum terminal pad with bolts
52
. Thus, maintaining a low electrical resistance joint with this design depends on the mechanical connection between the terminal pad and the mounting plate and the effectiveness of the oxide inhibiting grease. Over time, the effectiveness of either feature could be lost, thereby increasing the joint's electrical resistance. This leads to failure of the vacuum interrupter module
10
.
Yet another drawback of the original vacuum interrupter module
10
is that a single copper moving stem
32
makes both the electrical and mechanical connections between the pull rod screw to the rest of the switch mechanism. In the vacuum interrupter module
10
, a steel pin
56
is inserted through a transverse hole in the copper moving stem and through a slotted hole in the pull rod screw
57
. The copper moving stem
32
is comparatively softer than the steel pin
56
as a result of annealing during the brazing operations used to fabricate the vacuum interrupter. With the numerous opening and closing operations of the vacuum interrupter, a large amount of force is exerted on the steel pin. These repeated cycles on the pin enlarge the hole in which it is retained, thus changing the dimensions of the connecting pieces. When multiple vacuum interrupter modules are controlled by a single lever mechanism, the moving stem holes enlarge at a different rate. As a result, there is a loss of synchronism between the contacts in each vacuum interrupter module
10
as their respective stem holes deform. Prior art vacuum interrupter modules attempt to counteract this enlarging of the stem hole by locating it tangentially to the attached shunt plate
58
. This provides an increased bearing surface on one side of the pin. Alternatively, the module would be cycled through a number of operations prior to synchronizing the switch to cold work
Renner Kenner Grieve Bobak Taylor & Weber
Scott J. R.
Vacuum Electric Switch Co.
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