Title: Electric switching device
Abstract: An electric switching device for alternating current comprises two branches (2, 3) connected in parallel in a current path and having each at least two contact members (4-7) connected in series. A semiconductor device (8) is adapted to interconnect the midpoints between the two contact members of each branch. When opening the current path a first contact member (5) of one, first branch located before said midpoint as seen in the current direction existing is controlled to open and a second contact member (6) of the second branch located after the midpoint as seen in the current direction is controlled to open for transferring the current to a temporary current path through the semiconductor device. The current path through the switching device is then broken when the semiconductor device is in a blocking state by opening a contact member (4, 7) of the switching device arranged in the temporary current path.
Patent Number: 6,960,844 Issued on 11/01/2005 to Al-Hosini, et al.
| Inventors:
|
Al-Hosini; Falah (Västerås, SE);
Smede; Jan (Västerås, SE);
Nygren Nolemo; Jan-Anders (Västerås, SE);
Backman; Magnus (Västerås, SE);
Kjaer; Philip (Västerås, SE);
Balgård; Lennart (Arboga, SE);
Larsson; Per (Västerås, SE);
Albertsson; Ove (Västerås, SE);
Jonsson; Lars (Västerås, SE);
Johansson; Jan (Arboga, SE)
|
| Assignee:
|
ABB Technology AG (Zurich, CH)
|
| Appl. No.:
|
130351 |
| Filed:
|
November 20, 2000 |
| PCT Filed:
|
November 20, 2000
|
| PCT NO:
|
PCT/SE00/02273
|
| 371 Date:
|
August 28, 2002
|
| 102(e) Date:
|
August 28, 2002
|
| PCT PUB.NO.:
|
WO01/37298 |
| PCT PUB. Date:
|
May 25, 2001 |
Foreign Application Priority Data
| Current U.S. Class: |
307/131; 307/141.8; 361/3 |
| Intern'l Class: |
H01H 083/00 |
| Field of Search: |
307/130,131,138,141.8,126
361/3,8
|
References Cited [Referenced By]
U.S. Patent Documents
| 3864604 | Feb., 1975 | Pfanzelt.
| |
| 4203040 | May., 1980 | Abbondanti et al.
| |
| 4754360 | Jun., 1988 | Nakada.
| |
| 5040417 | Aug., 1991 | Rowlette.
| |
| 5566041 | Oct., 1996 | Rumfeldt.
| |
| Foreign Patent Documents |
| 2209500 | Jun., 1973 | DE.
| |
Primary Examiner: Sircus; Brian
Assistant Examiner: Squires; Brett
Attorney, Agent or Firm: Dykema Gossett PLLC
Claims
1. An electric switching device for alternating current comprising at least two
contact members arranged in a current path through the switching device and a semiconductor
device able to block current therethrough in at least a first blocking direction
and a unit adapted to control opening of a current path through the switching device
by controlling a first of the contact members to open for transferring the current
through the switching device to the semiconductor device when this is in or going
into the conducting state and then the second contact member to open when the semiconductor
device is in a state of blocking current therethrough for breaking the current
through the switching device, wherein the total number of contact members of the
switching device are at least four with two connected in series in each of two
branches connected in parallel in said current path, the semiconductor device being
arranged to connect the midpoints between the two contact members of each branch
to each other, the switching device comprises at least a member adapted to detect
the direction of the current through the switching device, the control unit being
adapted to control opening of the current path by controlling the first contact
member located before said midpoint with respect to the current direction prevailing
of one first branch to open and a second contact member of the second branch located
after the midpoint with respect to the current direction to open for transferring
the current to a temporary current path through the semiconductor device when this
is in or going into the conducting state and then break the current path through
the switching device when the semiconductor device is in a state of blocking current
therethrough by opening at least one contact member of the switching device arranged
in the temporary current path through the semiconductor device, and the control
unit being adapted to chose which branch shall be the first one on the basis of
information from the detecting member.
2. A switching device according to claim 1, wherein the control unit is adapted
to open at least one of a) the second contact member of the first branch and b)
the first contact member of the second branch after the transfer to the temporary
current path for breaking the current path through the switching device.
3. A switching device according to claim 1, further comprising: at least an additional
contact member having two contacts movable with respect to each other arranged
between one of said midpoints and the branch between the two midpoints in which
the semiconductor device is arranged, and wherein the unit is adapted to control
opening of this additional contact member for breaking the temporary current path
through the switching device.
4. A switching device according to claim 3, further comprising two said additional
contact members arranged between one of said midpoints each and the branch including
the semiconductor device.
5. A switching device according to claim 1, wherein the contact members comprise
mechanical contact members each having least two contacts movable with respect
to each other.
6. A switching device according to claim 5, further comprising: at least one
movable contact part arranged to establish a galvanic connection between two fixed
contacts of the respective contact member and break this connection for closing
and opening, respectively, the contact member.
7. A switching device according to claim 6, wherein it has one single said movable
part for all contact members arranged along one and the same of said branch connected
in parallel, the movable part being adapted to close all the contact members of
the branch in question in the closed state of the switching device, and a unit
adapted to control this movable part to carry out one single mechanical movement
for opening or closing the contact members of the respective branch.
8. A switching device according to claim 7, wherein the two movable parts are
interconnected for opening and closing, respectively, the current through the switching
device by one single mechanical movement of one unit in which the two movable parts
are included.
9. A switching device according to claim 6, wherein it has two movable parts,
one for each couple of first contact members of one branch and second contact members
of the opposite branch, each movable part adapted to close all contact members
associated therewith in the closed state of the switching device, and the unit
adapted to control each movable part to carry out one single mechanical movement
for opening or closing the contact members associated therewith.
10. A switching device according to claim 6, wherein the contact means are adapted
to be closed by said movable part through a relative movement of a male and a female
means into engagement with each other.
11. A switching device according to claim 10, wherein the movable part forms
said male means, and that a female contact means is adapted to come to bear therearound
when the movable part is moved into the female means.
12. A switching device according to claim 6, wherein the control unit is adapted
to control the movable parts to move from the closed state of the switching device
in one or the other direction along the movement path thereof depending upon the
current direction detected by the detecting member.
13. A switching device according to claim 5, further comprising a driving member
being electrically controlled and adapted to carry out movement of the movable
part of the switching device for opening or closing contact members included therein.
14. A switching device according to claim 13, wherein the driving member comprises
at least one of an electromagnetic machine and an electric machine.
15. A switching device according to claim 10, further comprising: a control unit
in the form of an electronic unit adapted to control said driving member.
16. A switching device according to claim 1, wherein the contact members belonging
to one and the same of said two branches are arranged along an arc.
17. A switching device according to claim 1, wherein the contact members belonging
to said two branches are arranged along one and the same circle.
18. A switching device according to claim 1, wherein the contact members belonging
to one and the same of said two branches are arranged along a straight line.
19. A switching device according to claim 18, further comprising: two movable
parts to be controlled by the control unit to move in a rectilinear movement substantially
in parallel with each other in at least one of the same direction and opposite
directions for closing and both in the opposite direction to the closing direction
for opening the contact members of each branch.
20. A switching device according to claim 18, wherein the contact members of
the two branches are arranged along one and the same substantially straight line
with the contact members of the respective branch following upon each other, and
that the two movable parts for the respective branch are connected in series and
the control unit is adapted to control them to move in one and the same direction
along said substantially straight line for closing and both in the opposite direction
to the closing direction for opening the current path.
21. A switching device according to claim 1, wherein said detecting member is
adapted to substantially continuously detect the direction and the magnitude of
the current through the switching device and send information thereabout to the
control unit.
22. A switching device according to claim 1, further comprising: a plurality
of said semiconductor devices connected in series and adapted to together hold
a voltage across the switching device in the blocking state.
23. A switching device according to claim 22, wherein each semiconductor device
has a said voltage limiting device each connected in parallel therewith for a substantially
even distribution of the voltage across the series connection of the semiconductor
devices onto the individual semiconductor devices.
24. A switching device according to claim 1, further comprising: means for connecting
it in series with another such switching device for obtaining a series connection
of such electric switching devices adapted to together hold a voltage across the
series connection in the broken state of the switching devices.
25. A switching device according to claim 1, further comprising: a plurality
of said semiconductor devices connected in parallel and adapted to together take
care of the current through the switching device after opening the first contact member.
26. A switching device according to claim 1, wherein a voltage limiting device
is connected in parallel with the semiconductor device, and said device being adapted
to start conducting at a voltage thereacross close to the maximum voltage withstood
by the semiconductor device.
27. A switching device according to claim 26, wherein the voltage limiting device
comprises a varistor.
28. A switching device according to claim 1, further comprising means adapted
to act increasingly upon the voltage when separating two contacts in connection
with opening the first contact member.
29. A switching device according to claim 28, wherein said means comprises a
plurality of first contact members connected in series adapted to be opened substantially
simultaneously for transferring the current to the semiconductor device.
30. A switching device according to claim 28, wherein said means comprises one
or a plurality of components adapted to increase the resistance between said two
contacts at the beginning of said separation thereof while allowing a current between
these contacts therethrough.
31. A switching device according to claim 30, wherein said component increasing
the resistance is formed by a semiconductor device being controllable to be turned
off so as to increase the voltage between said two contacts.
32. A switching device according to claim 30, wherein said component increasing
the resistance is formed by a resistor having a controllable resistance and adapted
to have an unimportant resistance in the closed state of the switching device and
be controlled to get a substantial resistance for increasing the voltage between
the two contacts.
33. A switching device according to claim 28, wherein said component increasing
the voltage comprises a charged capacitor adapted to be switched in between said
two contacts of the first contact member when this is to be opened.
34. A switching device according to claim 28 wherein said components are formed
by the fact that the contacts included in the first contact member have at least
a part of ablating material adapted to be heated and evaporated to gases for gas
blowing on an arc when separating the two contacts when opening the first contact member.
35. A switching device according to claim 1, wherein the semiconductor device
comprises at least one of a diode, a controllable device, a turn-off type device
and a thyristor.
36. A switching device according to claim 35, wherein the semiconductor device
is bi-directional.
37. A switching device according to claim 1, wherein the semiconductor device
comprises a material having an energy gap between the valence band and the conduction
band exceeding 2.5 eV, such as SiC and diamond.
38. A switching device according to claim 1 being operable at a system voltage
between 1-52 kV.
39. A switching device according to claim 1 being operable, to withstand at least
an operation current of 1 kA, preferably at least 2 kA, in the closed state.
40. A method for breaking a current path through an electric switching device
for alternating current, in which a main current path through the switching device
is opened and the current is transferred to a temporary current path through a
semiconductor device able to block current therethrough in at least a first blocking
direction when this is in or going into the conducting state and the temporary
current path is then broken and the current through the switching device is by
that broken, wherein the current path has two branches connected in parallel between
a first and a second end of the switching device and cross-linked to each other
through the semiconductor device, the direction and the magnitude of the current
through the switching device is detected, for said breaking of the current path
through the switching device firstly both branches are opened, one of them before
as seen from said first end and the other after as seen from the first end the
connection of the respective branch to the semiconductor device, wherein which
of the branches is opened before and which is opened after said connection is made
dependent upon the detection of the current, so that the current is transferred
into a temporary current path between said two ends through one part of one of
the branches, the semiconductor device and one part of the other branch when the
semiconductor device is in or is going into the conducting state and the current
path through the switching device is then broken when the semiconductor device
is in a state of blocking current therethrough by opening said temporary current path.
41. A method according to claim 40, wherein the current through the switching
device is detected and a future zero-crossing of the current is predicted on the
basis of this detection, and the opening of the main current path is controlled
to take place substantially at a zero-crossing of the current.
42. A computer program product adapted to be loaded directly into the internal
memory of a computer and comprising software code portions for instructing a processor
to carry out the steps according to claim 40 when the product is run on a computer.
43. A computer program product according to claim 42 provided at least partially
over a network.
44. A computer readable medium having a program adapted to make a computer control
the steps according to claim 40 recorded thereon.
Description
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to an electric switching device for alternating
current comprising at least two contact members arranged in a current path through
the switching device and a semiconductor device able to block current therethrough
in at least a first blocking direction and a unit adapted to control opening of
a current path through the switching device by controlling a first of the contact
members to open for transferring the current through the switching device to the
semiconductor device when this is in or going into the conducting state and then
the second contact member to open when the semiconductor device is in a state of
blocking current therethrough for breaking the current through the switching device.
Such electric switching devices are usually called hybrid breakers, and it is
characterizing for them that they are able to achieve an arc-free breaking of the
current path through the switching device, since this takes place when the semiconductor
device is in blocking state and no current flows through the switching device.
In switching devices having contact members breaking the current therethrough,
and in which accordingly an arc is generated, the gas pressure inside the breaker
used has to be high for achieving a sufficient insulation and breaking performance
or vacuum has to be provided inside the breaker for the same reason. Quite an amount
of energy is needed in the first case for blowing out the arc, while in the second
case a comparatively high contact pressure for a good contact is needed, which
consumes a not negligible amount of energy. The corresponding amount of energy
may in a switching device according to the introduction having an arc-free breaking
in the way mentioned instead be used for making the breaking more rapid so as to
better protect different types of electrical equipment connected to said current
path upon occurrence of faults and reduce the material wear of contacts included
in the second contact member.
The invention is of course not restricted to any particular range of operation
current through such an electric switching device in the closed state, and neither
to any particular voltage levels existing in said current path, but it may nevertheless
be mentioned that it is particularly useful for intermediate voltage, i.e. corresponding
to 1-52 kV system voltage, in which the operation current in question typically
may be 1 kA, but both lower and higher voltages and currents than these are conceivable.
Such and electric switching device is generally used for obtaining breaking
of a current path upon occurrence of any fault, such as a short-circuit, along
the current path. The fault may for example be caused by cutting off a cable of
an alternating voltage distribution network by a digging machine. It is then important
to break the current rapidly for minimizing damage on persons and material. It
is not necessary, but well possible that the second contact member of such a switching
device accomplishes a breaking visible to the eye, i.e. functions as a disconnector,
which is necessary when the breaking of the current is made for carrying out any
type of maintenance work along the current path, for example after a tree has fallen
down onto a transmission line.
A switching device of this type is particularly well suited to be arranged within
a switch gear for supply of electricity within industries or in distribution or
transmission networks. It may also be mentioned that it may advantageously be used
for being able to rapidly disconnect a generator and other apparatuses from an
alternating voltage network for protecting them against different types of disturbances
or faults on the alternating voltage network.
It is pointed out that "conducting state" above is to be given a broad sense,
and it is not necessary that a component going into or being in the conducting
state really conducts, but this is also intended to cover that it may be brought
to conduct in that moment should that be desired, which could be the case for a
semiconductor device of turn-on type, such as a thyristor, while a passive semiconductor
device in the form of a diode instead always will conduct in the conducting state
as defined here.
Furthermore, it is pointed out that "contact member" comprises all types
of members for opening and closing an electric circuit, in which for example although
not necessarily, physical separation of two parts while forming a gap therebetween
may take place when opening the contact member, and this may for example take place
by moving a movable contact interconnecting two contacts mutually spaced so that
these are no longer in connection with each other or by the fact that a movable
contact bears against a fixed contact and is moved away therefrom. Contact members
without physical separation of contacts when opening are also conceivable.
Electric switching devices of the type mentioned in the introduction already
known, such as for example the one known through U.S. Pat. No. 4,459,629, have
a comparatively costly control electronic so as to accomplish opening of the two
contact members when there is a desire to break said current path or closing the
contact members when re-establishing the current path in a well defined way through
an exact co-ordination required of the control of the two contact members.
Another disadvantage of so-called hybrid breakers already known is that they
leave something to be desired with respect to the rapidity by which the breaking
may take place, since a certain position of the alternating voltage for said current
path has to be waited for before the breaking procedure may be started. It has
been tried to solve this problem by arranging semiconductor devices in different
switching circuits of such an electric switching device for using separate semiconductor
devices in different positions of the instantaneous alternating current in said
current path so as to shorten the time between discovery of a need of breaking
and a completed opening of the current path through the switching device. However,
the semiconductor devices stand for a considerable part of the total cost for such
a switching device, which means that such a solution gets costly.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an electric switching device
of the type defined in the introduction, which shows a possibility to a rapid opening
of said current path when a need thereof arises independently of the instantaneous
position of the alternating current without making the switching device exaggeratedly
costly and at the same time requires only low control energy.
This object is according to the invention obtained by the fact that the total
number of contact members of the switching device is at least four with two connected
in series in each of two branches connected in parallel in said current path, that
the semiconductor device is arranged to connect the midpoints between the two contact
members of each branch to each other, that the switching device comprises at least
a member adapted to detect the direction of the current through the switching device,
that the control unit is adapted to control opening of the current path by controlling
the first contact member located before said midpoint with respect to the current
direction prevailing of one first branch to open and a second contact member of
the second branch located after the midpoint with respect to the current direction
to open for transferring the current to a temporary current path through the semiconductor
device when this is in or going into the conducting state and then break the current
path through the switching device when the semiconductor device is in a state of
blocking current therethrough by opening at least one contact member of the switching
device arranged in the temporary current path through the semiconductor device,
and that the control unit is adapted to chose which branch shall be the first one
on the basis of information from the detecting member.
By this design of the switching device a predetermined breaking sequence may
be
started as soon as a need thereof is detected, although the switching device may
have one single said semiconductor device, since said contact members may always
be controlled so that a temporary current path in one and the same direction through
the semiconductor device may be accomplished independently of the direction of
the alternating current through the switching device. The cost for semiconductor
devices may in this way be at least half as high with respect to other known hybrid
breakers with a similar rapidity, which have two semiconductor devices directed
in opposite directions instead of one.
According to a preferred embodiment of the invention the control unit is
adapted to open at least one of a) the second contact member of the first branch
and b) the first contact member of the second branch after the transfer to the
temporary current path for breaking the current path through the switching device.
By utilizing one of these contact members for breaking the temporary current path
it is possible to keep the number of contacts of the switching device and by that
the cost therefor down.
According to another preferred embodiment of the invention the switching
device comprises at least one movable contact part arranged to establish a galvanic
connection between two fixed contacts of the respective contact member and break
this connection for closing and opening, respectively, the contact member. This
constitutes a simple and reliable way to operate the contact members.
According to a particularly preferred further development of the embodiment
of the invention last mentioned the switching device has one single said movable
part for all contact members arranged along one and the same of said branches connected
in parallel, the movable part is adapted to close all the contact members of the
branch in question in the closed state of the switching device, and the unit is
adapted to control this movable part to carry out one single mechanical movement
for opening or closing the contact members of the respective branch. This results
in the possibility to a very simple control of the separate contact members, so
that no complicated control electronic is required for this. It is then particularly
advantageous if the two movable parts are interconnected for opening and closing,
respectively, the current path through the switching device through one single
mechanical movement of a unit in which the two movable parts are included. The
opening and the closing of the current path through the switching device may by
this take place while perfectly synchronizing the opening and the closing, respectively,
of the different contact members by very simple means. Another advantage is that
one single driving arrangement may be used for achieving all openings by driving
said unit and by that both movable parts to carry out one movement. It is here
pointed out that the two movable parts could be interconnected in such a way that
they in the practice are constituted by one and the same part, but it is then necessary
that the portions of this part forming one of said movable part each are electrically
insulated with respect to each other. By the fact that the opening or closing of
the electric switching device takes place by one single mechanical movement improved
possibilities to make the operation faster are obtained, since only one acceleration
of one movable part is necessary.
According to a very preferred embodiment of the invention the switching
device comprises a driving member being electrically controlled and adapted to
carry out movement of the movable part of the switching device for opening or closing
contact members included therein, and it is particularly advantageous if this driving
member is an electromagnetic machine in the form of an electric motor. By using
such a driving member it gets possible to control the movement of the movable part
for breaking and closing very accurately and for example ensure that a separation
of two contacts takes place at a particular phase position of the alternating current.
By arranging a control unit in the form of an electronic unit adapted to control
the driving member it is then also possible to influence the movement of the movable
part also when this has already been started so as to make adaptations to newly
measured values of parameters, such as current or voltage, and possibly interrupt
the entire procedure, if it is discovered that there is no longer any need thereof
or that the movement should for example rather take place in the opposite direction.
Furthermore, this embodiment is suited for co-ordination with a prediction of the
future development of the current through the switching device, such as a future
zero-crossing of the current so as to co-ordinate a breaking of the current through
the switching device with such a prediction, for example for ensuring that said
component with ability to block current will only conduct current during a so-called
short half wave. By the possibility to in this way ensure that the semiconductor
device, such as a diode, only has to conduct current during a very short time,
in the order of half a current period, this component has not to be dimensioned
for being able to withstand operation currents during a long time, but it may instead
be allowed to be substantially overloaded once it has to conduct, since this only
takes place during a very short time. This means that fewer such semiconductor
devices may be used than otherwise would be the case if they had to withstand the
currents in question over a long time.
According to another preferred embodiment of the invention the contact
members belonging to one and the same of said two branches are arranged along an
arc. This enables a closing or opening of the current path through the switching
device by a rotation of the movable part, which improves both the flexibility and
the possibility to rapidly move the movable part to another position than it had
before, after a certain movement thereof. After opening said current path through
rotating said movable part in one direction if would for example be possible to
close the current path again either by rotating the movable part back to the closed
position in the opposite rotation direction or continue the rotation of the movable
until the closed position is obtained. It also gets simpler to operate the switching
device by for example one electric motor.
According to another preferred embodiment of the invention the contact
members belonging to one and the same of said two branches are arranged along a
straight line, and the contact members are adapted to be closed by said movable
part by a relative movement of a male and a female means for engagement with each
other. This makes it possible to let the contacts of such a contact member in the
closed position obtain a continuously surrounding electric contact to each other
without any interruption, so that problems due to asymmetric contact and current
forces are avoided. It has then turned out to be advantageous to design the movable
part as the male means and make arrangements so that a female contact means is
adapted to come to bear around the movable part at a movement thereof into the
female means.
According to another preferred embodiment of the invention the switching
device comprises members adapted to substantially continuously detect the direction
and the magnitude of the current through the switching device and send information
thereabout to the control unit, which makes it possible for the control unit to
instantaneously react upon irregularities of the current, which could motivate
a breaking of the current path in question.
According to another preferred embodiment of the invention the switching
device comprises a current limiting device connected in parallel with the semiconductor
device, and said current limiting device is adapted to start conducting at a voltage
thereacross close to the maximum voltage withstood by the semiconductor device.
By the fact that in the closed and opened state of the switching device no voltage
will be applied across the semiconductor device and thereby neither across the
current limiting device this is possible, so that this will not be heated by any
leakage currents therethrough. Through the voltage limiting device, which may be
a varistor, the first voltage peak occurring across the semiconductor device through
the returning voltage after opening the first contact member, may be limited, which
in the case of one single semiconductor device makes it possible to dimension it
for being able to hold a lower returning voltage in the blocking direction thereof
and thereby be less expensive then otherwise, but particularly in the case of a
plurality of semiconductor devices connected in series the number of such semiconductor
devices connected in series having a determined voltage withstanding capability
may be reduced through an arrangement of such a varistor in parallel with each
semiconductor device. It is hereby avoided that any individual semiconductor device
gets a higher voltage thereacross than it may withstand, while other semiconductor
devices get a lower voltage thereacross.
According to another preferred embodiment of the invention the switching
device comprises means adapted to influence the voltage to increase when separating
two contacts in connection with opening of the first contact member. The voltage
at the contact separation is normally in the order of 12-15 V, and it drives the
transfer of the current to the semiconductor device connected in parallel therewith.
The higher this voltage the quicker the current may be fed into the semiconductor
device. Less material wear is obtained by the arrangement of this means and the
contact position will also be more stable with respect to the insulation.
According to another preferred embodiment of the invention said means comprises
a plurality of first contact members connected in series and adapted to be opened
substantially simultaneously for transferring the current to the semiconductor
device. The voltage for driving the conduction of the semiconductor device may
be increased through such a series connection of a plurality of contact members,
since this voltage will be formed by an addition of the voltages of the contact
members connected in series with exactly said advantageous result as a consequence.
According to another preferred embodiment of the invention said means are
formed by the fact that the contacts included in the first contact member have
at least a part of ablating material adapted to be heated and evaporated to gases
for gas blowing on an arc when separating the two contacts when opening the first
contact member, which also causes a higher arc-voltage and a faster commutation
of the current to the semiconductor device.
According to a preferred embodiment of the invention the semiconductor
device is a diode, which often will be preferred, since such a solution is inexpensive
with respect to other controllable semiconductor devices and also very reliable.
However, it is also conceivable that the semiconductor device is controllable,
such as a thyristor, and it may also be of turn-off type, such as a GTO or an IGBT,
for enabling a quicker breaking process. It could also in some situations be advantageous
to arrange a bi-directional semiconductor device, i.e. a semiconductor device which
may block and conduct in both directions, such as a BCT (bi-directionally controlled thyristor).
If a semiconductor device of a material having a wide energy gap between the
valence
band and the conduction band is used, i.e. an energy gap exceeding 2.5 eV, such
as SiC and diamond, comparatively high voltages may be handled by the switching
device while utilizing a low number of semiconductor devices.
The invention also relates to advantageous uses of a switching device as above
in accordance with the appended claims, and advantages thereof appear without any
doubt from the discussion above.
The invention also relates to a switch gear for supply of electricity within
industry or in distribution and transmission networks provided with an electric
switching device according to the invention. The method according to the invention
is also excellently suited for being carried out through a computer program provided
with suitable program steps, and the invention also relates to such a program as
well as a computer readable medium on which such a program is recorded.
Further advantages as well as advantageous features of the invention appear
from the following description and the other dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
With reference to the appended drawings, below follows a description of preferred
embodiments of the invention cited as examples.
In the drawings:
FIGS. 1-3 are simplified circuit diagrams illustrating an electric switching
device according to a first preferred embodiment of the invention in a closed,
temporary closed and opened position, respectively,
FIGS. 4-6 are simplified views illustrating an electric switching device according
to a preferred embodiment of the invention in the positions according to FIGS. 1-3,
FIGS. 7-9 are simplified views illustrating an electric switching device according
to a second preferred embodiment of the invention in the positions according to
FIGS. 1-3,
FIGS. 10-12 are simplified views illustrating an electric switching device
according to a third preferred embodiment of the invention in the positions according
to FIGS. 1-3,
FIGS. 13-15 are simplified views illustrating an electric switching device
according to a fourth preferred embodiment of the invention in the positions according
to FIGS. 1-3,
FIG. 16 illustrates very schematically a possible modification of a switching
device according to the present invention,
FIG. 17 illustrates how the current I through and a voltage U across the semiconductor
devices of the embodiment according to FIG. 16 are developed versus time in comparison
with the embodiment according to any of FIGS. 4-15,
FIG. 18 is a simplified circuit diagram illustrating a possible use of an electric
switching device according to the invention for switching in and switching out
capacitors to an alternating voltage network for reactive power compensation,
FIG. 19 illustrates very schematically an additional preferred embodiment of
the invention,
FIG. 20 illustrates very schematically a still further preferred embodiment
of the invention,
FIGS. 21 and 22 illustrate a part of a switching device in two different positions
when breaking the current therethrough,
FIGS. 23 and 24 are schematical circuit diagrams illustrating two possible
ways of arranging electric switching devices according to the invention for start
of an electric motor, and
FIG. 25 is a view corresponding to FIG. 4 illustrating how two electric switching
devices according to the invention may be connected in series.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The general construction of an electric switching device for alternating current
according to the invention is schematically illustrated in FIG. 1 and this is connected
in a current path
1 for being able to rapidly open and close this path.
One such switching device is arranged per phase, so that a three-phase network
has three such switching devices on one and the same location. The switching device
comprises two branches
2,
3 connected in parallel in said current
path and each having at least two mechanical contact members
4-
7
connected in series. A semiconductor device
8 in the form of a diode is
adapted to connect the midpoints
9,
10 between the two contact members
of each branch to each other.
The switching device comprises also a detecting member
11 schematically
indicated and adapted to detect the direction and magnitude of the current in the
current path and send information thereabout to a unit
12 adapted to control
the contact members
4-
7 in a way to be described further below. The
control unit will in this way all the time know what the current instantaneously
look like and be instantaneously able to control the contact members in the way desired.
The function of this electric switching device is as follows: When a desire to
break the current path
1 occurs, for example by the detection of a very
high current in the current path
1 by the detecting member
11, which
may be caused by a shortcircuit therealong, the control unit
12 firstly
decides which two contact members, here the contact members
5 and
6
(see FIG. 2), are to be opened so as to establish a temporary current path through
the semiconductor device
8. Thus, this decision depends upon in which position
the current in the current path is then. In the position according to FIG. 1 the
entire current flows through the switching device through the two branches
2,
3 and nothing through the diode. When now the breaking is to take place
the current shall as rapid as possible be transferred to flow through the diode
instead. The current may be switched in to the diode from a certain direction during
that part of an alternating current period that is located between the time shortly
before the diode gets forward biased until the diode gets reversed biased next
time. This means when a whole period is 20 ms in the practice that an opening of
the contact members according to FIG. 2 may take place for example about 2 ms before
zero-crossing towards the forward biased direction until the next zero-crossing.
When the wrong half-period of the alternating voltage for opening the contact members
5 and
6 according to these conditions prevails, the contact members
4 and
7 may then instead be immediately opened for establishing that
temporary current path instead. Accordingly, this temporary current path may be
established immediately after detecting the need of opening of the switching device.
By using an electrically controlled driving member, an electronic unit for the
control thereof and a prediction of a future zero-crossing of the current the opening
of this first contact member may be controlled to take place substantially at such
a zero-crossing, which means within about 0,5 ms before and about 0,5 ms after
such a zero-crossing. This means that the current to be commutated over to flow
through the diode is small and the commutation may therefore take place quickly
without any high demand on means for increasing the voltage across this contact member.
When the temporarily closed position illustrated in FIG. 2 is obtained by opening
the contact members
5,
6 a small spark is created in the gap between
the contacts of the respective contact member, which results in a voltage of usually
12-15 V, which will drive the transfer of the current through the diode
8.
With reference to FIGS. 4-6 and FIGS. 21 and 22 possible ways to make the transfer
of the current quicker will be described further below.
When then the voltage across the switching device changes direction no current
will flow therethrough, but a voltage will be built up across the diode
8
then reverse biased and at least one of the two other contact members
4,
7 is now opened, so that the temporary current path is broken, in which
this breaking may take place arc-free, since no current flows through the contact
site at the time for the breaking. The completely open position according to FIG.
3 is thereby obtained. It is in this breaking important that it takes place so
fast that the voltage across the diode
8 will not change direction again
and this starts to conduct. By the fact that the frequency of the opening of the
contact members may be controlled in dependence upon the position of the alternating
current when a need of opening of the switching device occurs the switching device
may be brought between the closed position and the completely open position according
to FIG. 3 within a period of time being substantially shorter than a period, usually
always within 15 ms for a frequency of 50 Hz of the alternating voltage.
By the fact that in the closed position of the switching device the current never
flows through the diode
8 the contact members
4-
7 have only
to be dimensioned for the operation current, which may for example be 1000 A, while
the diode is dimensioned for a possible shortcircuit current, which in such a case
could be 25 kA. However, it only has to withstand that current during a very short
time, and the dimensioning of the diode may be made without any considerations
taken to any continuous operation current through the switching device. Furthermore,
the diode has to be dimensioned for a returning voltage that during a short period
of time is applied thereacross after opening the two contact members opened firstly.
This may in the case of a network voltage of 12 kV for example be about 20 kV.
However, the very contact members of the switching device have in the open position
according to FIG. 3 to be able to withstand a considerably higher so-called impulse
voltage, which in this case could be 75 kV.
The switching device may advantageously be arranged in such a way that the breaking
location in the position according to FIG. 3 is visible, i.e. as disconnector,
so that works may be carried out along the current path in this position. The utilization
of the same semiconductor device in the temporary current path independently of
in which direction this takes through the switching device makes great cost savings
possible by reducing the number of semiconductor devices substantially with respect
to switching devices of this type already known.
It is schematically illustrated in FIGS. 4-6 how an electric switching device
for alternating current according to a first preferred embodiment of the invention
and having the function illustrated in FIGS. 1-3 is constructed. This has two movable
contact parts
13,
14, which are adapted to make a galvanic connection
to two fixed contacts of the respective contact member for closing the contact
member. The respective movable part is arranged to close all the contact members
of a branch each of the branches
2,
3 in the closed state of the
switching device. Two additional fixed contacts
9′,
10′
are here also arranged between the two contact members of the respective branch
and a branch
15 between the two other branches, in which the semiconductor
device is arranged, and these fixed contacts are also adapted to be galvanically
connected to each other by the respective movable part
13,
14. The
two movable parts
13,
14 are rigidly connected to each other by being
arranged on one and the same disc
16, which is arranged to be able to rotate
freely around a centre axis
17. It is apparent from the description above
how the electric switching device according to FIGS. 4-6 is transferred from the
closed position according to FIG. 4 to the completely open position according to
FIG. 6, and it is accordingly the state of the current through the switching device
prevailing at the time for detecting a need of opening that decides in which direction
the movable parts
13,
14 shall rotate for the quickest possible opening
of the switching device. An electrically controlled driving member
52 in
the form of an electric motor is adapted to drive the movement of the movable parts
13,
14. The control unit
12 is an electronic unit, so that
the movement of the movable parts
13,
14 may be controlled very accurately
and be adjusted or interrupted as long as it goes on.
Two alternatives to quickly commutate the current to flow through the diode when
the opening of a first contact member has taken place are also shown in FIGS. 4-6.
One alternative is shown in the form of resistance increasing components
53
arranged between the connection of the respective contact to the current path
1
and the contact. This resistance increasing component is intended to be controlled
by the electronic unit
12 to either have a negligible resistance in the
closed state of the switching device according to FIG. 4 or get a comparatively
high resistance for taking a voltage thereacross. The resistance increasing component
could be a resistor having a controllable resistance, such as a powder having a
very low resistance when applying an outer pressure thereonto, but which gets a
high resistance when the pressure is removed, or a controllable semiconductor device,
which has a low on-state voltage, but which may be brought to be turned off so
as to then increase the resistance considerably therethrough.
It is here also illustrated how a voltage increasing means
54 corresponding
to the resistance increasing components
53 is there, which here comprises
a charge capacitor adapted to be switched in between adjacent contacts of the first
contact member of the switching device when this is to be opened so as to quickly
transfer the current through the diode
8. This is only shown for the contact
members
4 and
5, but the corresponding arrangement is preferably
also there for the contact members
6 and
7. By co-ordinating the
separation of the contacts of the first contact member with the control through
the control unit
12 of the resistance increasing component
53 to
increase the resistance thereof or the voltage increasing means
54 to increase
the voltage a voltage may very rapidly be built up across the diode
8 and
the transfer of the current to flow through the diode will by that take place rapidly.
It is schematically illustrated in FIGS. 7-9 how a switching device according
to a second preferred embodiment of the invention is brought between a closed position
(FIG. 7), a temporarily closed position (FIG. 8) and an open position (FIG. 9).
Also this switching device has two movable contact parts
18,
19,
which here are of a rod-like design and adapted to function as male means adapted
to be received in female means in the form of contact rings
20 for surrounding
electric contact therewith. The two contact members
18 and
19 are
rigidly connected to each other to one single unit, and they are adapted to be
moved in parallel with each other in one and the same direction for opening or
closing the switching device. The instantaneous direction of the current through
the switching device decides in which direction, in FIG. 7 upwardly or downwardly,
the rod-like components shall move for accomplishing an opening of the switching
device starting from the position according to FIG. 7. When the current direction
is the one shown in FIG. 7 a decision is taken to move the movable parts
18,
19 downwardly for establishing a temporary current path through the diode
8 as quick as possible.
A switching device according to a further preferred embodiment of the invention
is illustrated in FIGS. 10-12 and this differs from the one according to FIGS.
7-9 by the fact that the two movable parts
18,
19 are here mutually
interconnected by a rocker arrangement
21 and they move substantially in
parallel with each other but in opposite directions. The instantaneous position
of the current through the switching device when detecting a need of an opening
decides which of the two movable contact parts
18,
19 is to be moved
upwardly and which is to be moved downwardly starting from the position according
to FIG. 10 for establishing the temporary current path through the diode
8
as quick as possible.
In the embodiment according to FIGS. 13-15 the two movable contact parts
18,
19 have been mechanically connected to each other for being moved together
in one and the same direction along a substantially straight line. The parts are
there electrically insulated with respect to each other. In which direction the
movable parts shall move from the closed position according to FIG. 13 so as to
obtain the temporarily closed position as quick as possible depends upon the position
of the alternating current prevailing at the time for detecting the need of breaking.
Two additional aspects of the present invention are illustrated in FIG. 16, in
which one is based on connecting a plurality of semiconductor devices
22-
25
in series for being able to together take a certain returning voltage after breaking
the current path. Thus, in all embodiments shown above each diode symbol may be
replaced by a number of diodes connected in series in this way. It is here also
possible to choose a material having a wide bandgap between the valence band and
the conduction band, such SiC or diamond, for obtaining a lower number of the semiconductor
devices required for a given voltage.
The other aspect consists in connecting a varistor
26-
29, preferably
of ZnO, in parallel with each semiconductor device, in which the varistor is adapted
to start conducting at a voltage thereacross close to the maximum voltage that
may be withstood by the semiconductor device. This may be accomplished by the fact
that the varistors do not normally conduct any current at all, since no voltage
will be applied thereacross, but they will only receive a voltage thereacross in
connection with the transition between the temporarily closed and the completely
open position. It is illustrated in FIG. 17 how the voltage U over the semiconductor
devices
22-
25 in reverse direction thereof is developed over time
t when the voltage increases thereacross in the temporarily closed position at
the time zero. The dashed line shows how the voltage across the diodes is developed
in the absence of varistors and the solid line with varistors. Thus, it appears
that the varistors cut the first voltage peak off. Would for example four 5 kV-diodes
be connected in series in a system having a network voltage of 12 kV and a normal
returning voltage of 22 kV the varistors may in this way start conducting a small
current during the short period of time (about 10 μs) that the peak of the
returning voltage lasts, so that this voltage peak may be brought down to 18 kV.
This means that 5 diodes connected in series are not required, but only four, for
being able to take care of the returning voltage. The change of the current I is
illustrated to the left of (before) the time
0. By connecting a separate
varistor in parallel with each semiconductor device in this way it is avoided that
any individual semiconductor device gets a higher voltage thereacross than it may
withstand, while other semiconductor devices get a lower voltage thereacross. It
is also possible to arrange resistances or capacitances connected in parallel with
the semiconductor devices for distributing the voltage substantially equally over
the semiconductor devices.
A possible application of a semiconductor device according to the invention for
switching in capacitors
30 to a three-phase alternating voltage network
31 for reactive power compensation is illustrated in FIG. 18. A switching
device according to the invention may then replace two breakers
32,
33,
such as illustrated in FIG. 18. When connecting the capacitor
30 to the
phase in question of the alternating voltage network a breaker
32 may firstly
be closed. Thyristors
51 connecting the breaker
32 to the phase in
question are then turned on so that the capacitor
30 is switched in at a
desired time. The breaker
33 is then closed. The breaker
32 is then
opened, so that the thyristors not have to conduct any longer, but the breaker
33 is closed and the switching in of the capacitor is completed. By switching
in the diode firstly in this way when the capacitor in question is to be switched
in to the network, transient voltages on the network emanating from a certain residual
voltage of the capacitor may be limited.
It is illustrated in FIG. 20 how it is possible to connect semiconductor devices
38,
39 in parallel for being able to take certain shortcircuit current
or just for redundancy reason, so that a switching device may function in a desired
way even if any diode in one so-called package of diodes connected in series gets broken.
It is schematically illustrated in FIG. 21 how it may be possible to arrange
one
movable part
43,
44 crosswisely interconnecting the contact members
4 and
7 as well as
5 and
6, so that each movable part
is adapted to close all contact members associated therewith in the closed position
of the switching device (for the part
43 the contact members
4 and
7), in which each movable part is adapted to carry out one single mechanical
movement for opening and closing the contact members associated therewith.
It is illustrated in FIG. 22 what means adapted to influence the voltage to increase
upon separation of two contacts in connection with opening of the first contact
member may look like. We now assume that the first contact member has two fixed
contacts
40,
41, which are adapted to be galvanically connected through
a movable part
45 in the closed state. The movable part
