U.S. patent application number 14/128986 was filed with the patent office on 2014-08-14 for voltage surge protection device and high voltage circuit breakers.
This patent application is currently assigned to ABB TECHNOLOGY AG. The applicant listed for this patent is Dag Andersson, Jurgen Hafner, Jan Lundquist, Leif Skold, Hakan Wieck. Invention is credited to Dag Andersson, Jurgen Hafner, Jan Lundquist, Leif Skold, Hakan Wieck.
Application Number | 20140226244 14/128986 |
Document ID | / |
Family ID | 44629344 |
Filed Date | 2014-08-14 |
United States Patent
Application |
20140226244 |
Kind Code |
A1 |
Lundquist; Jan ; et
al. |
August 14, 2014 |
VOLTAGE SURGE PROTECTION DEVICE AND HIGH VOLTAGE CIRCUIT
BREAKERS
Abstract
A voltage surge protection device for protection of a high
voltage device includes a varistor having a first part and a second
part separated by varistor material. The voltage surge protection
device further includes an expandable member arranged to act on a
movable electrical contact for short-circuiting the voltage surge
protection device upon a threshold voltage being applied between
the first part and the second part of the varistor. A high voltage
circuit breakers includes one or more semiconductor devices
connected in series and the voltage surge protection device
connected in parallel thereto.
Inventors: |
Lundquist; Jan; (Ludvika,
SE) ; Hafner; Jurgen; (Ludvika, SE) ; Skold;
Leif; (Ludvika, SE) ; Wieck; Hakan; (Ludvika,
SE) ; Andersson; Dag; (Ludvika, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lundquist; Jan
Hafner; Jurgen
Skold; Leif
Wieck; Hakan
Andersson; Dag |
Ludvika
Ludvika
Ludvika
Ludvika
Ludvika |
|
SE
SE
SE
SE
SE |
|
|
Assignee: |
ABB TECHNOLOGY AG
ZURICH
CH
|
Family ID: |
44629344 |
Appl. No.: |
14/128986 |
Filed: |
June 27, 2011 |
PCT Filed: |
June 27, 2011 |
PCT NO: |
PCT/EP2011/060718 |
371 Date: |
April 28, 2014 |
Current U.S.
Class: |
361/91.1 |
Current CPC
Class: |
H01C 7/12 20130101; H01H
37/74 20130101; H02H 9/041 20130101 |
Class at
Publication: |
361/91.1 |
International
Class: |
H02H 9/04 20060101
H02H009/04 |
Claims
1-12. (canceled)
13. A voltage surge protection device, the voltage surge protection
device comprising a varistor having a first part and a second part,
separated by varistor material configured to produce heat and/or
gas when subjected to a threshold voltage applied between the first
part and the second part of the varistor, wherein an expandable
member is arranged to act on a movable electrical contact for
short-circuiting the voltage surge protection device upon the
threshold voltage being applied between the first part and the
second part of the varistor, wherein the expandable member is
configured to expand due to the heat and/or gas produced by the
varistor material, the expandable member thereby breaking down
thermally and producing gases through chemical decomposition, said
gases of the expandable member having a gas pressure that moves the
movable electrical contact.
14. The voltage surge protection device as claimed in claim 13,
wherein the movable electrical contact is arranged to be movable
between a first position in which the voltage surge protection
device is non-activated, and a second position in which the voltage
surge protection device is activated, wherein the first part and
the second part are electrically connected by the movable
electrical contact when in its second position.
15. The voltage surge protection device as claimed in claim 13,
wherein the movable electrical contact comprises a movable helical
spring or a movable metal ring.
16. The voltage surge protection device as claimed in claim 13,
wherein the varistor comprises an electrically insulating housing
enclosing the varistor at least partly in a lengthwise direction of
the voltage surge protection device, and wherein the movable
electrical contact is arranged to be movable along the electrically
insulating housing between a first position in which the voltage
surge protection device is non-activated, and a second position in
which the voltage surge protection device is activated, wherein the
first part and the second part are electrically connected by the
movable electrical contact when in its the second position.
17. The voltage surge protection device as claimed in claim 13,
comprising a movable electrically insulating ring arranged between
the expandable member and the electrical contact, and wherein the
expandable member is arranged to act on the movable electrical
contact via the movable electrically insulating ring.
18. The voltage surge protection device as claimed in claim 17,
wherein the movable electrically insulating ring is arranged to be
movable along an electrically insulating housing enclosing the
varistor at least partly in a lengthwise direction of the voltage
surge protection device.
19. The voltage surge protection device as claimed in claim 14,
wherein the movable electrical contact is arranged between the
expandable member and an electrically insulating ring, and wherein
the movable electrical contact contacts a helical spring when in
the second position, the helical spring being arranged on the
second part.
20. The voltage surge protection device as claimed in claim 13,
wherein the movable electrical contact is arranged in a bore
through an electrically insulation housing enclosing the varistor
at least partly in a lengthwise direction, one end of the movable
electrical contact being in contact with the expandable member
arranged as a shell around the varistor material and partly in the
bore, the movable electrical contact being arranged to move in a
radial direction.
21. The voltage surge protection device as claimed in claim 13,
wherein the expandable member comprises a material expandable when
subjected to temperature rises and/or pressure.
22. The voltage surge protection device as claimed in claim 13,
wherein the expandable member comprises silicone gel, silicone
paste or silicone grease.
23. A circuit breaker comprising one or more semiconductor unit(s)
connected in series, wherein a voltage surge protection device as
claimed in claim 13 is connected in parallel to each such
semiconductor unit(s).
24. A circuit breaker comprising two or more semiconductor units,
wherein the semiconductor units are connected anti-serially
pairwise, and wherein a voltage surge protection device as claimed
in claim 13 is connected in parallel to each such pair of
semiconductor units.
25. The voltage surge protection device as claimed in claim 14,
wherein the movable electrical contact comprises a movable helical
spring or a movable metal ring.
26. The voltage surge protection device as claimed in claim 14,
wherein the varistor comprises an electrically insulating housing
enclosing the varistor at least partly in a lengthwise direction of
the voltage surge protection device, and wherein the movable
electrical contact is arranged to be movable along the electrically
insulating housing between a first position in which the voltage
surge protection device is non-activated, and a second position in
which the voltage surge protection device is activated, wherein the
first part and the second part are electrically connected by the
movable electrical contact when in its the second position.
27. The voltage surge protection device as claimed in claim 15,
wherein the varistor comprises an electrically insulating housing
enclosing the varistor at least partly in a lengthwise direction of
the voltage surge protection device, and wherein the movable
electrical contact is arranged to be movable along the electrically
insulating housing between a first position in which the voltage
surge protection device is non-activated, and a second position in
which the voltage surge protection device is activated, wherein the
first part and the second part are electrically connected by the
movable electrical contact when in its the second position.
28. The voltage surge protection device as claimed in claim 14,
comprising a movable electrically insulating ring arranged between
the expandable member and the electrical contact, and wherein the
expandable member is arranged to act on the movable electrical
contact via the movable electrically insulating ring.
29. The voltage surge protection device as claimed in claim 15,
comprising a movable electrically insulating ring arranged between
the expandable member and the electrical contact, and wherein the
expandable member is arranged to act on the movable electrical
contact via the movable electrically insulating ring.
30. The voltage surge protection device as claimed in claim 16,
comprising a movable electrically insulating ring arranged between
the expandable member and the electrical contact, and wherein the
expandable member is arranged to act on the movable electrical
contact via the movable electrically insulating ring.
31. The voltage surge protection device as claimed in claim 16,
wherein the movable electrical contact is arranged between the
expandable member and an electrically insulating ring, and wherein
the movable electrical contact contacts a helical spring when in
the second position, the helical spring being arranged on the
second part.
32. The voltage surge protection device as claimed in claim 14,
wherein the movable electrical contact is arranged in a bore
through an electrically insulation housing enclosing the varistor
at least partly in a lengthwise direction, one end of the movable
electrical contact being in contact with the expandable member
arranged as a shell around the varistor material and partly in the
bore, the movable electrical contact being arranged to move in a
radial direction.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to the field of high voltage
devices, and in particular to protection of such high voltage
devices.
BACKGROUND OF THE INVENTION
[0002] Most electrical systems and devices are occasionally subject
to excessive transient voltages being applied and such voltage
surges, e.g. due to lightning strikes, may result in very costly
damages and subsequent service interruption. Means for protecting
the electrical systems and devices are therefore needed.
[0003] Voltage dependent resistors, varistors, are frequently used
for protecting electrical devices from voltage surges. The varistor
is also referred to as non-linear resistor as it has nonlinear
current-voltage characteristics. If an applied voltage is less than
a certain voltage, the varistor is essentially an insulator. If the
voltage applied is above the certain voltage, the switching
voltage, the varistor resistance drops and allows an increased
current to flow through it. The varistor is connected in parallel
to the device to be protected and arranged to, when triggered by an
overvoltage, shunt the current created by the high voltage away
from the device.
[0004] Further, various kinds of circuit breakers are available for
interrupting fault currents. However, it is difficult to design
circuit breakers for DC currents due to the high currents without
any zero current crossings at which to break the current.
[0005] One known high voltage direct current (HVDC) circuit breaker
based on insulated-gate bipolar transistors (IGBT) technology
comprises a number of stack-mounted IGBT units. Each unit comprises
several IGBT modules connected in parallel, each module in turn
comprising a number of paralleled semiconductor chips which
normally carry only a fraction of a line voltage. If one such
semiconductor chip (IGBT chip) is destroyed by overvoltage, it is
short-circuited by an internal device which has to withstand the
full line current. The short-circuiting device has a limited
current carrying capability and the complete IGBT unit has to be
replaced within short.
[0006] For DC breaker applications, having a large continuous DC
current without any zero current crossings which would enhance the
transitions from one failed IGBT chip to another, the failed IGBT
chip cannot stay in the short circuit mode for very long. The DC
breaker thus has to be tripped and the failed device has to be
replaced.
[0007] There is thus need for improvements in this regard.
SUMMARY OF THE INVENTION
[0008] An object of the invention is to provide a voltage surge
protection device enabling a safe short-circuit function for high
voltage devices, such as circuit breakers.
[0009] The object is according to a first aspect of the invention
achieved by a voltage surge protection device for protection of a
high voltage device. The voltage surge protection device comprises
a varistor having a first part and a second part, separated by
varistor material. The voltage surge protection device comprises an
expandable member arranged to act on a movable electrical contact
for short-circuiting the voltage surge protection device upon a
threshold voltage being applied between the first part and the
second part of the varistor. The invention provides a voltage surge
protection device that uses thermal energy of a varistor to trigger
a bypass process, whereby neither an external control nor an
auxiliary power supply is required to form the bypass.
[0010] In an embodiment, the varistor material, when subjected to
the threshold voltage, produces heat and/or gas expanding the
expandable member.
[0011] In an embodiment, the movable electrical contact is arranged
to be movable between a first position in which the voltage surge
protection device is non-activated, and a second position in which
the voltage surge protection device is activated. The first part
and the second part are electrically connected by the movable
electrical contact when it is in its second position.
[0012] In an embodiment, the varistor comprises an electrically
insulating housing enclosing the varistor at least partly in a
lengthwise direction of the voltage surge protection device. The
movable electrical contact is arranged to be movable along the
electrically insulating housing between a first position in which
the voltage surge protection device is non-activated, and a second
position in which the voltage surge protection device is activated,
wherein the first part and the second part are electrically
connected by the movable electrical contact when it is in its
second position.
[0013] In an embodiment, the movable electrical contact comprises a
movable helical spring or a movable metal ring.
[0014] In an embodiment, the voltage surge protection device
comprises a movable electrically insulating ring arranged between
the expandable member and the electrical contact. The expandable
member is arranged to act on the movable electrical contact via the
movable electrically insulating ring. The movable electrically
insulating ring may be arranged to be movable along an electrically
insulating housing enclosing the varistor at least partly in a
lengthwise direction of the voltage surge protection device.
[0015] In an embodiment, the movable electrical contact is arranged
between the expandable member and an electrically insulating ring.
The movable electrical contact contacts a helical spring when in
the second position, the helical spring being arranged on the lower
part.
[0016] In an embodiment, the movable electrical contact is arranged
in a bore through an electrically insulation housing enclosing the
varistor at least partly in a lengthwise direction, and into the
varistor material, one side end of the movable electrical contact
being in contact with the expandable member arranged as a shell
around the varistor material and partly in the bore, the movable
electrical contact being arranged to move in a radial
direction.
[0017] In an embodiment, the expandable member comprises a material
expandable when subjected to temperature rises and/or pressure.
[0018] In an embodiment, the expandable member comprises silicone
gel, silicone paste or silicone grease.
[0019] The object is according to a second aspect of the invention
achieved by a high voltage circuit breaker comprising one or more
semiconductor unit(s) connected in series, wherein a voltage surge
protection device as above is connected in parallel to each such
semiconductor unit(s).
[0020] The object is according to a third aspect of the invention
achieved by a high voltage circuit breaker comprising two or more
semiconductor units, wherein the semiconductor units are connected
anti-serially pairwise, and wherein a voltage surge protection
device as above is connected in parallel to each such pair of
semiconductor units.
[0021] Further features and advantages thereof will become clear
upon reading the following description and the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIGS. 1a and 1b illustrate a voltage surge protection device
in accordance with a first embodiment of the invention, before and
after activation thereof.
[0023] FIG. 2 illustrates an upper part of a varistor of the
voltage surge protection device of FIGS. 1a and 1b.
[0024] FIGS. 3a and 3b illustrate a voltage surge protection device
in accordance with a second embodiment of the invention, before and
after activation thereof.
[0025] FIG. 4 illustrates a feature of an embodiment of the
invention.
[0026] FIGS. 5a, 5b and 5c illustrate a voltage surge protection
device in accordance with a third embodiment of the invention, in
different views before and after activation thereof.
[0027] FIG. 6 illustrates a first embodiment of a HVDC circuit
breaker comprising a protection device.
[0028] FIG. 7 illustrates a second embodiment of a HVDC circuit
breaker comprising a protection device.
DETAILED DESCRIPTION OF EMBODIMENTS
[0029] In the following description, for purposes of explanation
and not limitation, specific details are set forth such as
particular designs, etc. in order to provide a thorough
understanding of the invention. However, it will be apparent to
those skilled in the art that the invention may be practiced in
other embodiments that depart from these specific details. In other
instances, detailed descriptions of well-known devices and circuits
are omitted so as not to obscure the description of the invention
with unnecessary detail. Like numbers refer to like elements
throughout the description.
[0030] FIGS. 1a and 1b illustrate a voltage surge protection device
in accordance with a first embodiment of the invention, before and
after activation thereof. The voltage surge protection device 1, in
the following denoted protection device 1, is illustrated in FIG.
1a in a cross-sectional view before activation. The voltage surge
protection device 1 is suitable for protecting high voltage
devices, high voltage direct current devices as well as high
voltage alternating current devices.
[0031] The protection device 1 comprises a varistor 2 having a
sealed insulating housing 3 preferably of a cylindrical shape. The
varistor 2 comprises a first part 2a, in the following denoted
upper part 2a, and a second part 2b, in the following denoted lower
part 2b. The upper and lower parts 2a, 2b are made of metal and
constitute electrodes. The upper part 2a and the lower part 2b are
separated by varistor material 4. The varistor 2 may for example be
a metal oxide varistor (MOV). In a length direction L of the
protection device 1, the varistor 2 thus comprises the upper part
2a, the varistor material 4 and the lower part 2b.
[0032] The varistor material 4 and the upper part 2a of the
varistor 2 are surrounded, circumferentially and in the length
direction L, by an expandable member 5. The expandable member 5 is
thus arranged, in a radial direction r of the protection device 1,
between at least part of the surrounding cylindrical insulating
housing 3 on one side and the varistor material 4 and the upper
part 2a of the varistor 2 on another side. The expandable member 5
comprises a material having a voltage withstand exceeding the
protective level of the varistor 2. The expandable member 5 is
thermally, chemically and electrically stable prior to varistor 2
failure and is chosen so as to withstand temperatures that appears
when the varistor 2 absorbs its rated energy, for example
250.degree. C. The heat generated by the varistor material 4 upon
failure of the varistor 2 is quickly and efficiently transferred to
the expandable member 5.
[0033] Upon failure of the varistor 2, defined for example by a
varistor temperature exceeding 300.degree. C. or by arcing
occurring inside or along a collar of the varistor 2, the
expandable member 5 breaks down thermally, thereby producing gases
through chemical decomposition. The expandable member 5 surrounding
the varistor material 4, has a viscosity that enables a gas
pressure to activate an electrical contact device, which will be
described more in detail later.
[0034] The upper part 2a of the varistor 2 is designed so as to
comprise, at a radial distance from the center of the protection
device 1, a ring shaped first channel 8 having a suitable first
width w1 and being partly filled. The first channel 8 is
illustrated separately in FIG. 2 for clarity, and it can be seen in
FIGS. 1a and 1b that the expandable member 5 is arranged in the
first channel.
[0035] A movable insulating ring 6 is arranged as a collar around
the insulating housing 3. The insulating ring 6 is movable between
a first and a second position in the length direction 1 of the
protection device 1, i.e. along the insulating housing 3. An upper
surface of the insulating ring 6 is in contact with the expandable
member 5. The expandable member 5 is thus surrounded by the upper
part 2a of the varistor, the varistor material 4 and the insulating
ring 6.
[0036] The insulating housing 3 may have a stepped shape as
illustrated in FIGS. 1a and 1b, having at an upper end a projecting
part 3a lying on the insulating ring 6. The insulating ring 6 has a
first position at which parts of its upper surface abuts the
projecting part 3a. The expandable member 5 surrounds the
projecting part 3a of the insulating housing 3. The stepped shape
of the insulating housing has a lower projecting part 3b surrounded
by the lower part 2b of the varistor 2, leaving part of the lower
part 2b accessible from a second channel 9.
[0037] In the radial direction r, a second ring shaped channel 9 is
formed in the first channel 8. The second channel 9 is formed
between the insulating housing 3 and the outermost part of the
upper part 2a. The second ring shaped channel 9 has a second width
w2. The lower part 2b of the varistor 2 constitutes, together with
the insulating housing 3, a first wall of the second channel 9. The
outermost part of the upper part 2a of the varistor 2 constitutes
an opposite second wall of the second channel 9.
[0038] In the second channel 9 a movable electrically conductive
contact device 7, e.g. a helical spring, is arranged. The contact
device 7 is movable between a first position P1 and a second
position P2 in the length direction L of the protection device 1,
i.e. partly along the insulating housing 3. In the lengthwise
direction L of the protection device 1, the contact device 7 is
located below the insulting ring 6. The contact device 7 is in the
following exemplified by the helical spring for describing the
invention, but it is to be noted that the contact device 7 can be
any suitable device enabling electrical contact to the made, e.g. a
metal ring. The helical spring 7 provides a stable short-circuit as
it has several contact surfaces with both the upper part 2a and the
lower part 2b in the second position P2. Further, the helical
spring 7 is easy to put in place, giving a simple manufacture of
the protection device 1.
[0039] FIG. 1b illustrates the protection device 1 in its activated
state. The insulating ring 6 is, as mentioned, movable and has an
upper position (illustrated at P1 in FIG. 1a) in its non-activated
state and a lower position P2 (illustrated in FIG. 1b) which it
takes when the protection device 1 is activated. The helical spring
7 is movably arranged below the insulating ring 6 and when the
insulating ring 6 is acted on by the expandable member 5, thus
moving downwards, also the helical spring 7 is moved downwards.
[0040] In normal operation, illustrated in FIG. 1a, the insulating
ring 6 and the helical spring 7 are in their upper positions P1,
leaving an air clearance between the upper part 2a and the lower
part 2b of the varistor 2. The air clearance comprises the width w2
of the second channel 9.
[0041] When the varistor 2 fails, the varistor material 4 produces
heat and possibly gases. The expandable member 5 then expands due
to the increased temperature and gas pressure. As mentioned, the
expandable member 5 thus acts on the insulating ring 6, which moves
the helical spring 7, both then taking their lower positions P2. In
the lower position P2, the helical spring 7 is in electrical
contact with the upper part 2a of the varistor 2 and also in
electrical contact with the lower part 2b of the varistor 2. The
helical spring 7 thus short-circuits the protection device 1 by
providing electrical contact between the upper part 2a and the
lower part 2b.
[0042] FIGS. 3a and 3b illustrate a voltage surge protection device
21 in accordance with a second embodiment of the invention, before
and after activation thereof. The voltage surge protection device
21, in the following protection device 21, is similar to the first
embodiment, comprising a varistor 22 having a first part 22a, in
the following denoted upper part 22a, and a second part 22b, in the
following denoted lower part 22b, separated by varistor material
24.
[0043] In the second embodiment, the electrical contact 27
comprises a movable metal ring 27 acted on by an expandable member
25, in a similar way as in the above described first embodiment.
Only the main differences between the embodiments are described in
the following. A movable insulating ring 26 is arranged under the
metal ring 27 and ensures a required voltage withstand. In a first
position P1', when the protection device 21 is non-activated (FIG.
3a) the insulating ring 26 is abutting a helical spring 28 arranged
on the lower part 22b.
[0044] In a second position P2', the movable metal ring 27 comes in
contact with the helical spring 28 arranged on the lower part 22b
of the varistor 22. The helical spring 28 provides a stable
short-circuit as it has several contact surfaces with both the
upper part 22a and the lower part 22b.
[0045] Like in the first embodiment, when the varistor 22 fails,
heat and, gas are produced by the varistor material 24. The
expandable member 25 then expands and moves due to the increased
temperature and gas pressure. The expandable member 25 acts on the
metal ring 27, which moves to its lower position P2'. In the lower
position P2', the metal ring 27 short-circuits the protection
device 1 by providing electrical contact between the upper part 22a
and the lower part 22b of the varistor 22.
[0046] In an embodiment, the voltage surge protection device 1
comprises a movable electrically insulating ring 6 arranged between
the expandable member 5, 25 and the electrical contact 7, 27. The
expandable member 5, 25 is then arranged to act on the movable
electrical contact 7, 27 via the movable electrically insulating
ring 6. The movable electrically insulating ring 6 may be arranged
to move along the electrically insulating housing 3, 23.
[0047] In another embodiment, the movable electrical contact 7 is
arranged between the expandable member 5, 25 and an electrically
insulating ring 26. The movable electrical contact 7 contacts a
helical spring 28 when in the second position P2, P2'. The helical
spring 28 is then arranged on the lower part 2b, 22b.
[0048] FIG. 4 illustrates a feature of an embodiment of the
invention. The varistor 2, 22 may fail by internal arcing. To
ensure that the gas pressure created upon varistor failure will act
on the surrounding expandable member 5, 25 also if gas escapes from
the horizontal surfaces of the upper part 2a and lower part 2b of
the varistor, the surfaces in contact with the varistor material 4,
24 can be machined as illustrated in FIG. 4. That is, the surfaces
comprises protruding parts with a size of a few millimeters thus
creating ways for the gas to act on the expandable member 5, 25
while still providing a reliable electrical contact with the
varistor 2, 22.
[0049] FIGS. 5a, 5b and 5c illustrates yet another embodiment of
the voltage surge protection device 1. In particular, FIGS. 5a and
5b illustrate a voltage surge protection device 31 in accordance
with a third embodiment of the invention, before activation
thereof, and FIG. 5c illustrates the voltage surge protection
device 31 after activation thereof. The voltage surge protection
device 31, in the following protection device 31, is similar to the
previous embodiments in that it comprises a varistor 32 having a
first part 32a, and a second part 32b, separated by varistor
material 34. In contrast to the previous embodiments, a movable
electrical contact 37 is arranged to move in a horizontal direction
instead of in a vertical direction.
[0050] FIGS. 5a and 5b illustrate the protection device 31 in a
non-activated position, FIG. 5a in a cross-sectional side view and
FIG. 5b in a cross-sectional top view. The expandable member 35 is
provided between, as seen in a radial direction, the varistor
material 34 and an electrically insulating housing 33. In a
vertical direction, the varistor material 34 is provided between
the first and second parts 32a, 32b of the varistor 32. The
expandable member 35 is thus arranged as a cylindrical shell around
the varistor material 34, as best seen in FIG. 5b.
[0051] The movable electrical contact 37 is movably arranged
between a first position P1'' in which the protection device 31 is
non-activated and a second position P2'' in which the protection
device 31 is activated. When in the first position P1'', i.e. when
the expandable member 35 has not expanded due to gas and pressure
from the varistor material 34, the movable electrical contact 37 is
arranged partly through the electrically insulating housing 33 that
surrounds the varistor 32, and partly outside the electrically
insulating housing 33, thus protruding from the housing 33.
[0052] An end side 39 of the movable electrical contact 37 is in
contact with the expandable member 35. The movable electrical
contact 37 can be seen as arranged in a bore through the
electrically insulating housing 33. The bore continues all the way
to the shell of expandable member 35, the expandable member 35 thus
filling part of this bore, and thereby being in contact with the
end side 39 of the movable electrical contact 37. The expandable
member 35 can thus act on the movable electrical contact 37 upon
expansion thereof.
[0053] As for the previously described embodiments, when the
varistor 32 fails, the expandable member 35 expands and moves the
movable electrical contact 37 to its second position P2'', in which
the first part 32a and the second part 32b are in electrical
contact by means of the movable electrical contact 37 moving in a
radial direction as mentioned earlier, and thus short-circuiting
the protection device 31.
[0054] The movable electrical contact 37 is thus arranged in the
bore, through the cylindrical electrically insulation housing 33
that encloses the varistor 32 at least partly in a lengthwise
direction. The end side of the movable electrical contact 37 is in
contact with the expandable member 35 that is arranged as a
cylindrical shell around the varistor material 34 and partly in the
bore. The movable electrical contact 37 is arranged to move in a
radial direction, i.e. in a direction perpendicular to the length
direction of the protection device 31.
[0055] Three particular embodiments have been described above with
reference to FIGS. 1a, 1b, 2, 3a, 3b, and 5a, 5b, 5c but in its
most general embodiment, the protection device 1, 21, 31 comprises
a varistor 2, 22, 32 having first part 2a, 22a, 32a and a second
part 2b, 22b, 32b, separated by varistor material 4, 24, 34. The
protection device 1, 21, 31 comprises an expandable member 5, 25,
35 that is arranged to act on a movable electrical contact 7, 27,
37. The voltage surge protection device 1, 21, 31 is
short-circuited when a threshold voltage is applied between the
first part 2a, 22a, 32a and the second part 2b, 22b, 32b of the
varistor 2, 22, 32.
[0056] This can be accomplished by the varistor material 4, 24, 34
producing heat and/or gas(es) when subjected to the threshold
voltage, and the produced heat and/or gas makes the expandable
member 5, 25, 35 expand.
[0057] The movable electrical contact 7, 27, 37 is arranged to be
movable between a first position P1, P1', P1'' in which the voltage
surge protection device 1, 21, 31 is non-activated, and a second
position P2, P2', P2'' in which the voltage surge protection device
1, 21, 31 is activated. In the first position P1, P1' the varistor
and in particular the varistor material 4, 24, 34, is essentially
electrically non-conducting. In the second position P2, P2', P2''
of the movable electrical contact 7, 27, 37 the first part 2a, 22a,
32a and the second part 2b, 22b, 32b are electrically connected by
the movable electrical contact 7, 27, 37.
[0058] The movable electrical contact 7, 27, 37 may be arranged to
be movable along an electrically insulating housing 3, 23, 33 that
encloses the varistor 2, 22, 32 at least partly in a lengthwise
direction L of the protection device 1.
[0059] As described earlier, the movable electrical contact may,
for example comprise a movable helical spring 7 or a movable metal
ring 27.
[0060] FIG. 6 illustrates a first embodiment of a HVDC circuit
breaker comprising a protection device 1, 21, 31 as described. The
HVDC breaker 40 comprises several series-connected semiconductors,
e.g. TGBT units 41.sub.1, 41.sub.2, . . . , 41.sub.n. Each IGBT
unit may comprise several parallel-connected IGBT modules. One
protection device 1, 21, 31 as described is connected in parallel
to each such IGBT unit 41.sub.1, 41.sub.2, . . . , 41.sub.n. It is
to be noted that although IGBT units are used here as an example
for illustrating and describing the invention, other semiconductor
devices can be protected by the inventive concept. Further, yet
other switch applications, and even other devices than
semiconductor devices, can be protected by protection devices in
various aspects of the invention.
[0061] FIG. 7 illustrates a second embodiment of a HVDC circuit
breaker comprising a protection device. The HVDC circuit breaker 50
of this embodiment also comprises a number of series--connected
IGBT units 51.sub.1, 51.sub.2, . . . , 51.sub.n-1, 51.sub.n,
wherein the IGBT units are connected anti-serially pair-wise,
enabling the breaking of current in both current directions. One
protection device 1, 21, 31 as described is connected in parallel
to two such IGBT units.
[0062] The IGBT unit should enter a short-circuit failure mode
(SCFM) upon failure. The protective device 1, 21, 31 provides such
SCFM. When the IGBT unit fails, the protective device 1, 21, 31
changes from the state illustrated in FIGS. 1a and 3a and 5a, 5b to
the activated states illustrated in FIGS. 1b and 3b and 5c, as
described in relation to these figures.
[0063] Upon varistor failure, the protection device 1, 21, 31
behaves as follows:
[0064] If the varistor temperature exceeds 450-500.degree. C., the
expandable member 5, 25, 35 surrounding the varistor 2, 22, 32
shall break down thermally, thereby producing gases through
chemical decomposition. The gases produced should not be poisonous
or corrosive.
[0065] The expandable member 5, 25, 35 surrounding the varistor
shall have a viscosity that enables the gas pressure to activate
the electrical contact 7, 27, 37.
[0066] The housing shall mechanically withstand the internal gas
pressure created by gassing or arcing.
[0067] The protection device 1, 21, 31, and in particular the
varistor 2, 22, 32 shall be permanently short-circuited by means of
the electrical contact 7, 27, 37 and the gas pressure shall be
relieved.
[0068] The expandable member 5, 25, 35 should, as mentioned earlier
and as is evident from the above, fulfill several requirements. The
following group of polydimethylesilicone (PDMS) materials has been
found to have the electrical, thermal, and viscosity properties
required:
[0069] 1) Silicone gel: two-component, curing at room temperature;
very soft (gel hardness <100 g, penetration .sub.>5 mm);
thermally stable up to +250.degree. C.; during heating in
oxygen-free environment beginning at about 400.degree. C., the
silicone gel is decomposed at into tricyclosilicone (Si(CH3)0)3 and
other cyclic low molecular weight silicones.
[0070] 2) Silicone paste: composed of PDMS and thickeners, for
example amorphous silica (Si02); decomposed in a similar way as
silicone gel; transfers the hydraulic pressure better that silicone
gel
[0071] 3)Silicone grease: composed of silicone oil and a thickener,
for example amorphous silica and other fillers; decomposed in a
similar way as silicone gel; transfers the hydraulic pressure like
silicone paste
[0072] Upon varistor failure, it is most likely that the
decomposing of the expandable member 5, 25, 35 is fast. The
decomposed gas will then mostly consist of low molecular weight
cyclosilicones where tricyclosilicone (TCDMS) constitutes the
largest volume. Because the boiling point of TCDMS is 133.degree.
C. it is gasified at the failing temperature of the varistor 2, 22,
32. The amount of gas (TCDMS) generated can be estimated in order
to determine the amount of material needed to ensure activation of
the electrical contact 7, 27, 37. From that the layer thickness of
silicone gel, paste or grease surrounding the varistor block 2a, 4,
2b can be calculated.
[0073] Theoretically, 1 mole of TCDMS is created for each mole of
PDMS. As a conservative assumption the material consists of 50%
PDMS. Suppose also that 50% of the breakdown products consist of
TCDMS. That means that only 25% of the material is used for gas
generation. In the table below, the required layer thickness is
estimated for the two embodiments Embodiment 1, Embodiment 2
described in relation to FIGS. 1a, 1b and 3a, 3b, respectively.
TABLE-US-00001 Embodiment Embodiment 1 Embodiment 2 Varistor cm2
46.6 46.6 46.6 46.6 envelope area Helical N 400 400 340 340 spring
load Pressure cm2 40 40 9.2 9.2 area Gas kPa 200 200 470 470
pressure on contact ring Expansion cm3 68 68 9.2 9.2 volume
Material g/cm3 1 1 1 1 density Gas K 406 673 406 673 temperature
Moles of 0.00403 0.00243 0.00128 0.00077 gas Proportion % 50 50 50
50 PDMS Proportion % 50 50 50 50 TCDMS units 4 4 4 4 gel/units
TCDMS Mole weight 222 222 222 222 TCDMS Mass of g 3.58 2.16 1.14
0.69 gassing material Layer mm 0.77 0.47 0.25 0.15 thickness
[0074] In the left columns of Embodiment 1 and Embodiment 2 the gas
temperature is equal to the boiling point of TCDMS (the lowest
possible gas temperature). In the right columns the gas temperature
is 400.degree. C. which is a typical thermal breakdown temperature
for the varistor in e.g. HVDC circuit breaker application.
[0075] These examples show that a very thin layer of gassing
material is needed to produce the necessary volume and pressure to
activate the contact device in both designs and to ensure that the
contact can be pressed all the way until fully closed. It is also
shown that the residual gas pressure must be relieved in order to
reduce the gas pressure when the contacts have been closed.
* * * * *