Title of Invention	A COMPACT DOUBLE-CHAMBER SWITCHGEAR WITH IMPROVED DIELECTRIC STRENGTH AND VOLTAGE SISTRIBUTION FOR HIGH VOLTAGE APPLICATIONS
Abstract	The invention relates to an electrical switchgear with two switches (2, 3) arranged in the switchgear enclosure (5) and electrically connected in series whereat each of the switches (2,3) comprises a first and second contact (6, 7), at least one of the first and second contact (6,7) of each switch 5 (2, 3) being a mobile contact (6), the first contacts (6) of the two switches (2, 3) are mechanically and electrically connected by means of a connecting means (4), the first contact (6) of a switch (2, 3) is at least partly surrounded by a first electrical conductive shield (10) and the second contact (7) of the switch is at least partially surrounded by a second electrical conductive shield (11), the second shield (11) is electrically 10 connected to the second contact (7) and is at least partially surrounding the first shield (10) which is electrically connected to the first contact (6) so that a shield capacitor (C1") is formed between the first and second shield (10,11), in that the second shield (11) is at least partially surrounding the connecting means (4) so that a further capacitor (C1"') is formed between the second shield (11) and the connecting means (4) and in that a second 15 capacitor (C2) is formed between the, preferably grounded, enclosure (5) of the switchgear (1) and the connecting means (4).

Title of Invention

A COMPACT DOUBLE-CHAMBER SWITCHGEAR WITH IMPROVED DIELECTRIC STRENGTH AND VOLTAGE SISTRIBUTION FOR HIGH VOLTAGE APPLICATIONS

Abstract

The invention relates to an electrical switchgear with two switches (2, 3) arranged in the switchgear enclosure (5) and electrically connected in series whereat each of the switches (2,3) comprises a first and second contact (6, 7), at least one of the first and second contact (6,7) of each switch 5 (2, 3) being a mobile contact (6), the first contacts (6) of the two switches (2, 3) are mechanically and electrically connected by means of a connecting means (4), the first contact (6) of a switch (2, 3) is at least partly surrounded by a first electrical conductive shield (10) and the second contact (7) of the switch is at least partially surrounded by a second electrical conductive shield (11), the second shield (11) is electrically 10 connected to the second contact (7) and is at least partially surrounding the first shield (10) which is electrically connected to the first contact (6) so that a shield capacitor (C1") is formed between the first and second shield (10,11), in that the second shield (11) is at least partially surrounding the connecting means (4) so that a further capacitor (C1"') is formed between the second shield (11) and the connecting means (4) and in that a second 15 capacitor (C2) is formed between the, preferably grounded, enclosure (5) of the switchgear (1) and the connecting means (4).

Full Text	FIELD OF THE INVENTION The present invention relates to an electrical switchgear with two switches arranged in the switchgear enclosure and electrically connected in series whereat each of the switches comprises a first and second contact, at least one of the first and second contact of each switch being a mobile contact, the first contacts of the two switches are mechanically and electrically connected by means of a connecting means, the first contact of a switch is at least partially surrounded by a first electrical conductive shield and the second contact of the switch is at least partially surrounded by a second electrical conductive shield. BACKGROUND OF THE INVENTION Electrical switchgear, e.g. a circuit breaker, must in general provide good dielectric strength in open position in order to avoid breakthrough by arcing between the separated contacts or between a contact and a grounded part of the switchgear, like the grounded switchgear enclosure. To improve the dielectric strength capacitors are often arranged in parallel between the contacts of the switchgear. Due to the required capacitances which make the capacitor big and heavy such switchgear requires a lot of space. For very high voltage applications, e.g. >500kV, two circuit breaker are connected in series for switching such high voltages, i.e. the voltage to be switched needs to be shared by the two switches. For such double chamber circuit breaker each circuit breaker is provided with a capacitor connected in parallel between the contacts of each switch for improving dielectric strength. Such a double chamber circuit breaker is shown in US 3 786 216 A. Some arrangements of prior art show either capacitors made by solid isulators integrated into single-chamber circuit breaker (allowing transitory voltage to be reduced particularly when short-line fault occur) and into two- chamber circuit-breaker (allowing to share the voltage equally by the chambers) or shields, e.g. made by metallic sheets, around the chambers for dielectric purposes. Examples of such switchgears are given in US 5 728 989 A or EP 335 338 A2. US 3 953 693 A shows a vacuum switch with integrated capacitor shields. Such vacuum switches can be used in series using the integrated capacitors to assure proper voltage distribution between the switches. The integrated capacitors are also effective as shields and serve as a labyrinth to shield against diffusions of arc products. To this end a number of shields are arrangend labyrinth-like to form a labyrinth passage which effectively intersects arc particels which are generated on separation of the contacts. To form a labyrinth a great number of such shields are required which leads to a costly design with great dimensions, especially diameters. Each switch is arranged in its own enclosure of insulating material. It is also known from prior art, e.g. from US 3 541 284 A, to employ a capacitor made of two tubular, concentric and partly overlapping shields in parallel to an electrical single-chamber switch to increase the inherent capacitance of the single-chamber switch, and consequently also its dielectric strength. OBJECT OF THE INVENTION Hence, it is an object of the present invention to provide a compact double- chamber switchgear for high voltage applications with improved dielectric strength and good voltage distribution between the two serially connected switches of the switchgear. SUMMARY OF THE INVENTION This object is achieved by arranging the first and second shield such that a shield capacitor is formed between the first and second shield, by arranging the second shield that partially surrounds the connecting means so that a further capacitor is formed between the second shield and the connecting means and in that a second capacitor is formed between the, preferably grounded, enclosure of the switchgear and a connecting means. Such an arrangement increases the dielectric strength of the electrical switchgear signifi-cantly by increasing the natural capacitor between the open contacts of the switch thus reducing the rise of breakthrough and discharges when the switchgear is in open position. Since no bulky capacitors are required to improve the dielectric strength such a switchgear can be of compact design and reduced overall dimensions, espescially of reduced enclosure diameter. This means that the switchgear requires less space which is especially advantageous. Furthermore, since the costs of the shields are small compared to classical capacitors, such a switchgear is also cheaper than conventional ones. The large surface of the shields act also as radiative surface which increases the thermal capability of the switchgear and which is also advantageous for temperature rise tests. The dielectric strength of the switchgear is further increased, if the second shield is at least partially surrounding the connecting means so that a further capacitor is formed between the second shield and the connecting means. The further capacitor is parallel to the shield capacitor and the natural capacitance of the switch and increases consequently directly the capacitance of the switch further. Indeed, according to the example described below, the fact that the second shield (11) is at least partially surrounding the connecting means (4) so that a further capacitor (Ci'") is formed between the second shield (11) and the connecting means (4) is very relevant for the invention, because this increases capacitor CI (being CI +C1" + CI'"), and decreases capacitor C2, and thus improves voltage distribution between the two switching units, while the voltage ratio is C1/(C2+2C1) and thus its value tends towards 1/2. An especially compact design can be achieved when the connecting means is at least partially a drive unit for driving the mobile contact. This allows a very compact design of small diameters. The connecting means can also be at least partially the first shield which may in an advantageous embodiment extend from the first contact of the first switch to the first contact of the second switch. If the ratio between the capacitances of second and first capacitor is less than 0,5, preferably less than 0,1 and especially less than 0,05, then the total voltage to be switched is substan-tially equally shared by the two switches. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The invention is described in the following with reference to Figs. 1 to 3 showing in exemplary, non-limiting way Fig. 1 a schematic drawing of an electrical switchgear according to an embodiment of the invention, Fig. 2 a schematic drawing of the capacitors formed according to the invention and Fig. 3 an electric circuit diagram of the electrical switchgear. DETAIL DESCRIPTION OF THE INVENTION The inventive electrical switchgear 1, e.g. a circuit breaker, is shown in Fig. 1 and comprises an enclosure 5 into which two switches 2, 3 are arranged. The two switches 2, 3 are connected in series between two terminals T1 (e.g. high potential) and T2 (e.g. ground) by a connecting means 4. In order to perform a switching operation (open or close) a mobile contact 6 (indicated by the double arrow in Fig. 1) of both switches 2,3 is moved simultaneously by means of a drive unit acting also as connecting means 4 for mechanically and electrically connecting the two switches 2,3. The drive unit 4 is arranged between the switches 2, 3 and may comprise a number of levers and a driving rod 8 mechanically connecting the drive unit 4 to a driving mechanism 9, in this example located outside the enclosure 5, as shown in Fig. 1. The drive unit 4 can be driven by a suitable driving mechanism 9, like e.g. a well-known spring mechanism, hydraulic mechanism or motor drive. The driving rod 8 itself may be of insulating material. The drive unit 4 is mechanically connected to a mobile contact 6 of each switch 2,3, thus driving the mobile contacts 6. A second contact 7 of each switch 2,3 is either fixed or could also be moveable to form a double acting circuit breaker. But basically, any other suitable drive unit or any other arrangement of one or more drive units could be employed as well, it would e.g. be possible that both contacts are moveable contacts and/or that each switch has its own drive unit. To allow an electrical connection between the terminals T1 and T2 of the switchgear 1, the second contact 7 of the first switch 2 is connected to terminal T1 e.g. the high voltage terminal. In closed position the first 6 and second contacts 7 of switches 2, 3 are in contact and the first contact 6 of the first switch 2 is electrically connected to the connecting means 4, in this example the drive unit, which is again electrically connected to the first contact 6 of the second switch 3 and hence, via second contact 7 of the second switch 3 also to terminal T2, e.g. the grounded terminal. In open position of the switches 2, 3 the contacts 6, 7 are separated and the electrical connection is interrupted. The switches 2,3 must have sufficient dielectric strength (i.e. the ability to withstand the maximum nominal voltage of the switchgear 1 without electric breakthrough) in order to prevent arcing between the two contacts 6, 7 in open position. In order to increase the dielectric strength of the switches 2, 3 or to allow a more compact design of the switchgear 1, the enclosure 5 could also be filled with insulating gas, e.g. like SF6. In conventional circuit breakers capacitors are often connected in parallel to the contacts of the switch which further increases the dielectric strength of the switch, as is well-known. The following is described with reference to only one of the switches 2,3 of the switchgear 1 because of the symetrical arrangement of the switches 2 and 3. The first contact 6 is partially surrounded by a first shield 10. The first shield 10 is made of electrical conductive material and is electrically connected to the first contact 6 and hence also to the connecting means 4 (in this example the drive unit). Consequently, first shield 10 has the same electrical potential as first contact 6. An electrical conductive second shield 11 is arranged in the enclosure 5 such that it is electrically connected to the second contact 7, thus having the same electrical potential as second contact 7, and that it is at least partially surrounding the first contact 6 and the first shield 10. The second shield 11 may also surround at least partially the connecting means 4, here the drive unit, as indicated in Fig. 1. But it would also be possible that the first shield 10 itself is at least partially the connecting means 4, e.g. by providing only one shield 10 which extends from the first contact 6 of the first switch 2 to the first contact 6 of the second switch 3. In this case the electrical connection between the two switches 2, 3 is at least partially formed by the shield 10. Due to the arrangement of the shields 10,11, additional capacitors are formed as is schematically shown in Fig. 2. Between the first (in this example mobile) contact 6 and the second (in this example fixed) contact 7 the natural capacitor C1 is formed between the two open contacts 6, 7. Between first shield 10 and second shield 11a shield capacitor C1" is formed and between second shield 11 and connecting means 4, e.g. the drive unit, a capacitor C1" is formed. Since these three capacitors are connected in parallel, the capacitors can be combined to a first capacitor C1= C1 + C1 + C1'". Therefore, the natural capacitance of the switch 2 is increased and hence also the dielectric strength of the open switch 2. The longer the shields 10, 11 become, the greater the capacitance of capacitor C1' will be. The more the second shield 11 extends also over the connecting means 4, the greater the capacitance of capacitor C1' will be. Since a compact design of the switchgear 1 is desired it is advantageous to arrange first and second shield 10,11 as close together as possible, whereat the minimum distance is basically defined by the maximum voltage of the switchgear 1 and the media inside the enclosure 5 (e.g. SF6) which acts as insulator for the capacitors C1 and C2. Furthermore, a second capacitor C2 is formed between the grounded enclosure 5 and the connecting means 4, e.g. the drive unit, which has the same electrical potential as the first contacts 6 of the switches 2, 3. The capacitance of capacitor C2 is the smaller, the more the second shield 11 extends over connecting means 4 and the shorter the connecting means 4 is. The resulting potential between the two switches 2,3 can easily be derived from the equivalent circuit diagram of the electrical switchgear 1 shown in Fig. 3. The closed switches are not shown in Fig. 3. Employing basic physical relationships, the middle voltage UM (i.e.the voltage between the first contacts 6 and terminal T2) can be found as Um=C1/(C2+2C1)-U, with U being the voltage between the terminals T1 and T2. From this equation it can gathered that the middle voltage UM is approximately U/2 if C1»C2. Therefore, it can be achieved that the total voltage to be switched is substantailly equally shared by the two switches 2,3 connected in series by making the capacitance of capacitor C1 as big as possible and of capacitor C2 as small as possible. In an example the geometry of the switches 2,3 and the shields 10,11 (e.g. length, distance) can be chosen so that the capacitance of the first capacitor C1 is 250pF and the capacitance of the second capcitor C2 to the earthed enclosure 5 is 20pF. This would lead to a middle voltage UM=0.48.U which means that both switches 2, 3 would have about the same voltage to switch. Generally, C2 should be less than 0,5-C1 preferably less than 0,1 -C1 especially less than 0,05-C1, to achieve a good voltage distribution. From the above it can be followed, that it is advantageous to make the second shield 11 as long as possible and especially to extend shield 11 also over the connecting means 4 since this would increase C1" and C1" (and hence also C1) and would decrease C2. WE CLAIM 1. An electrical switchgear with two switches (2,3) arranged in the switchgear enclosure (5) and electrically connected in series whereat each of the switches (2,3) comprises a first and second contact (6,7), at least one of the first and second contact (6,7) of each switch 5 (2,3) being a mobile contact (6), the first contacts (6) of the two switches (2,3) are mechanically and electrically connected by means of a connecting means (4), the first contact (6) of a switch (2,3) is at least partly surrounded by a first electrical conductive shield (10) and the second contact (7) of the switch is at least partially surrounded by a second electrical conductive shield (11), characterized in that the second shield (11) is electrically 10 connected to the second contact (7) and is at least partially surrounding the first shield (10) which is electrically connected to the first contact (6) so that a shield capacitor (C1") is formed between the first and second shield (10,11), in that the second shield (11) is at least partially surrounding the connecting means (4) so that a further capacitor (C1'") is formed between the second shield (11) and the connecting means (4) and in that a second 15 capacitor (C2) is formed between the, preferably grounded, enclosure (5) of the switchgear (1) and the connecting means (4). 2. An electrical switchgear as claimed in claim 1, wherein the connecting means (4) is at least partially formed by a drive unit for driving the mobile contact (6). 3. An electrical switchgear as claimed in one of claims 1 or 2, wherein the 20 connecting means (4) is at least partially formed by the first shield (10). 4. An electrical switchgear as claimed in claim 3, wherein the first shield extends from the first contact (6) of the first switch (2) to the first contact (6) of the second switch (3). 5. An electrical switchgear as claimed in one of claims 1 or 4, wherein the 25 ratio between the capacitance of the second capacitor (C2) and the capacitance of the first capacitor (C1) is less than 0,5, preferably less than 0,1 and especially less than 0,05. ABSTRACT TITLE : "A COMPACT DOUBLE-CHAMBER SWITCHGEAR WITH IMPROVED DIELECTRIC STRENGTH AND VOLTAGE DISTRIBUTION FOR HIGH VOLTAGE APPLICATIONS" The invention relates to an electrical switchgear with two switches (2, 3) arranged in the switchgear enclosure (5) and electrically connected in series whereat each of the switches (2,3) comprises a first and second contact (6, 7), at least one of the first and second contact (6,7) of each switch 5 (2, 3) being a mobile contact (6), the first contacts (6) of the two switches (2, 3) are mechanically and electrically connected by means of a connecting means (4), the first contact (6) of a switch (2, 3) is at least partly surrounded by a first electrical conductive shield (10) and the second contact (7) of the switch is at least partially surrounded by a second electrical conductive shield (11), the second shield (11) is electrically 10 connected to the second contact (7) and is at least partially surrounding the first shield (10) which is electrically connected to the first contact (6) so that a shield capacitor (C1") is formed between the first and second shield (10,11), in that the second shield (11) is at least partially surrounding the connecting means (4) so that a further capacitor (C1"') is formed between the second shield (11) and the connecting means (4) and in that a second 15 capacitor (C2) is formed between the, preferably grounded, enclosure (5) of the switchgear (1) and the connecting means (4).

Full Text

FIELD OF THE INVENTION
The present invention relates to an electrical switchgear with two switches
arranged in the switchgear enclosure and electrically connected in series whereat
each of the switches comprises a first and second contact, at least one of the
first and second contact of each switch being a mobile contact, the first contacts
of the two switches are mechanically and electrically connected by means of a
connecting means, the first contact of a switch is at least partially surrounded by
a first electrical conductive shield and the second contact of the switch is at least
partially surrounded by a second electrical conductive shield.
BACKGROUND OF THE INVENTION
Electrical switchgear, e.g. a circuit breaker, must in general provide good
dielectric strength in open position in order to avoid breakthrough by arcing
between the separated contacts or between a contact and a grounded part of
the switchgear, like the grounded switchgear enclosure. To improve the dielectric
strength capacitors are often arranged in parallel between the contacts of the
switchgear. Due to the required capacitances which make the capacitor big and
heavy such switchgear requires a lot of space. For very high voltage applications,
e.g. >500kV, two circuit breaker are connected in series for switching such high
voltages, i.e. the voltage to be switched needs to be shared by the two switches.
For such double chamber circuit breaker each circuit breaker is provided with a
capacitor connected in parallel between the contacts of each switch for
improving dielectric strength. Such a double chamber circuit breaker is shown in
US 3 786 216 A. Some arrangements of prior art show either capacitors made by
solid isulators integrated into single-chamber circuit breaker (allowing transitory
voltage to be reduced particularly when short-line fault occur) and into two-
chamber circuit-breaker (allowing to share the voltage equally by the chambers)

or shields, e.g. made by metallic sheets, around the chambers for dielectric
purposes.
Examples of such switchgears are given in US 5 728 989 A or EP 335 338 A2.
US 3 953 693 A shows a vacuum switch with integrated capacitor shields. Such
vacuum
switches can be used in series using the integrated capacitors to assure proper
voltage
distribution between the switches. The integrated capacitors are also effective as
shields and serve as a labyrinth to shield against diffusions of arc products. To
this end a number of shields are arrangend labyrinth-like to form a labyrinth
passage which effectively intersects arc particels which are generated on
separation of the contacts. To form a labyrinth a great number of such shields
are required which leads to a costly design with great dimensions, especially
diameters. Each switch is arranged in its own enclosure of insulating material.
It is also known from prior art, e.g. from US 3 541 284 A, to employ a capacitor
made of two tubular, concentric and partly overlapping shields in parallel to an
electrical single-chamber switch to increase the inherent capacitance of the
single-chamber switch, and consequently also its dielectric strength.

OBJECT OF THE INVENTION
Hence, it is an object of the present invention to provide a compact double-
chamber switchgear for high voltage applications with improved dielectric
strength and good voltage distribution between the two serially connected
switches of the switchgear.
SUMMARY OF THE INVENTION
This object is achieved by arranging the first and second shield such that a shield
capacitor is formed between the first and second shield, by arranging the second
shield that partially surrounds the connecting means so that a further capacitor is
formed between the second shield and the connecting means and in that a
second capacitor is formed between the, preferably grounded, enclosure of the
switchgear and a connecting means.
Such an arrangement increases the dielectric strength of the electrical switchgear
signifi-cantly by increasing the natural capacitor between the open contacts of
the switch thus reducing the rise of breakthrough and discharges when the
switchgear is in open position.
Since no bulky capacitors are required to improve the dielectric strength such a
switchgear can be of compact design and reduced overall dimensions, espescially
of reduced enclosure diameter. This means that the switchgear requires less
space which is especially advantageous. Furthermore, since the costs of the
shields are small compared to classical capacitors, such a switchgear is also
cheaper than conventional ones. The large surface of the shields act also as
radiative surface which increases the thermal capability of the switchgear and
which is also advantageous for temperature rise tests.

The dielectric strength of the switchgear is further increased, if the second shield
is at least partially surrounding the connecting means so that a further capacitor
is formed between the second shield and the connecting means. The further
capacitor is parallel to the shield capacitor and the natural capacitance of the
switch and increases consequently directly the capacitance of the switch further.
Indeed, according to the example described below, the fact that the second
shield (11) is at least partially surrounding the connecting means (4) so that a
further capacitor (Ci'") is formed between the second shield (11) and the
connecting means (4) is very relevant for the invention, because this increases
capacitor CI (being CI +C1" + CI'"), and decreases capacitor C2, and thus
improves voltage distribution between the two switching units, while the voltage
ratio is C1/(C2+2C1) and thus its value tends towards 1/2.
An especially compact design can be achieved when the connecting means is at
least partially a drive unit for driving the mobile contact. This allows a very
compact design of small diameters. The connecting means can also be at least
partially the first shield which may in an advantageous embodiment extend from
the first contact of the first switch to the first contact of the second switch.

If the ratio between the capacitances of second and first capacitor is less than
0,5, preferably less than 0,1 and especially less than 0,05, then the total voltage
to be switched is substan-tially equally shared by the two switches.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention is described in the following with reference to Figs. 1 to 3 showing
in exemplary, non-limiting way
Fig. 1 a schematic drawing of an electrical switchgear according to an
embodiment of the invention,
Fig. 2 a schematic drawing of the capacitors formed according to the invention
and
Fig. 3 an electric circuit diagram of the electrical switchgear.
DETAIL DESCRIPTION OF THE INVENTION
The inventive electrical switchgear 1, e.g. a circuit breaker, is shown in Fig. 1
and comprises an enclosure 5 into which two switches 2, 3 are arranged. The
two switches 2, 3 are connected in series between two terminals T1 (e.g. high
potential) and T2 (e.g. ground) by a connecting means 4. In order to perform a
switching operation (open or close) a mobile contact 6 (indicated by the double
arrow in Fig. 1) of both switches 2,3 is moved simultaneously by means of a
drive unit acting also as connecting means 4 for mechanically and electrically
connecting the two switches 2,3. The drive unit 4 is arranged between the
switches 2, 3 and may comprise a number of levers and a driving rod 8
mechanically connecting the drive unit 4 to a driving mechanism 9, in this
example located outside the enclosure 5, as shown in Fig. 1. The drive unit 4 can
be driven by a suitable driving mechanism 9, like e.g. a well-known spring
mechanism, hydraulic mechanism or motor drive.

The driving rod 8 itself may be of insulating material. The drive unit 4 is
mechanically connected to a mobile contact 6 of each switch 2,3, thus driving the
mobile contacts 6. A second contact 7 of each switch 2,3 is either fixed or could
also be moveable to form a double acting circuit breaker. But basically, any other
suitable drive unit or any other arrangement of one or more drive units could be
employed as well, it would e.g. be possible that both contacts are moveable
contacts and/or that each switch has its own drive unit.
To allow an electrical connection between the terminals T1 and T2 of the
switchgear 1, the second contact 7 of the first switch 2 is connected to terminal
T1 e.g. the high voltage terminal. In closed position the first 6 and second
contacts 7 of switches 2, 3 are in contact and the first contact 6 of the first
switch 2 is electrically connected to the connecting means 4, in this example the
drive unit, which is again electrically connected to the first contact 6 of the
second switch 3 and hence, via second contact 7 of the second switch 3 also to
terminal T2, e.g. the grounded terminal. In open position of the switches 2, 3 the
contacts 6, 7 are separated and the electrical connection is interrupted.
The switches 2,3 must have sufficient dielectric strength (i.e. the ability to
withstand the maximum nominal voltage of the switchgear 1 without electric
breakthrough) in order to

prevent arcing between the two contacts 6, 7 in open position. In order to
increase the dielectric strength of the switches 2, 3 or to allow a more compact
design of the switchgear 1, the enclosure 5 could also be filled with insulating
gas, e.g. like SF6. In conventional circuit breakers capacitors are often connected
in parallel to the contacts of the switch which further increases the dielectric
strength of the switch, as is well-known.
The following is described with reference to only one of the switches 2,3 of the
switchgear 1 because of the symetrical arrangement of the switches 2 and 3.
The first contact 6 is partially surrounded by a first shield 10. The first shield 10
is made of electrical conductive material and is electrically connected to the first
contact 6 and hence also to the connecting means 4 (in this example the drive
unit). Consequently, first shield 10 has the same electrical potential as first
contact 6. An electrical conductive second shield 11 is arranged in the enclosure
5 such that it is electrically connected to the second contact 7, thus having the
same electrical potential as second contact 7, and that it is at least partially
surrounding the first contact 6 and the first shield 10. The second shield 11 may
also surround at least partially the connecting means 4, here the drive unit, as
indicated in Fig. 1.
But it would also be possible that the first shield 10 itself is at least partially the
connecting means 4, e.g. by providing only one shield 10 which extends from the
first contact 6 of the first switch 2 to the first contact 6 of the second switch 3.
In this case the electrical connection between the two switches 2, 3 is at least
partially formed by the shield 10.

Due to the arrangement of the shields 10,11, additional capacitors are formed as
is schematically shown in Fig. 2. Between the first (in this example mobile)
contact 6 and the second (in this example fixed) contact 7 the natural capacitor
C1 is formed between the two open contacts 6, 7. Between first shield 10 and
second shield 11a shield capacitor C1" is formed and between second shield 11
and connecting means 4, e.g. the drive unit, a capacitor C1" is formed. Since
these three capacitors are connected in parallel, the capacitors can be combined
to a first capacitor C1= C1 + C1 + C1'". Therefore, the natural capacitance of
the switch 2 is increased and hence also the dielectric strength of the open
switch 2. The longer the shields 10, 11 become, the greater the capacitance of
capacitor C1' will be. The more the second shield 11 extends also over the
connecting means 4, the greater the capacitance of capacitor C1' will be. Since a
compact design of the switchgear 1 is desired it is advantageous to arrange first
and second shield 10,11 as close together as possible, whereat the minimum
distance is basically defined by the maximum voltage of the switchgear 1 and the
media inside the enclosure 5 (e.g. SF6) which acts as insulator for the capacitors
C1 and C2.
Furthermore, a second capacitor C2 is formed between the grounded enclosure 5
and the connecting means 4, e.g. the drive unit, which has the same electrical
potential as the first contacts 6 of the switches 2, 3. The capacitance of capacitor
C2 is the smaller, the more the

second shield 11 extends over connecting means 4 and the shorter the
connecting means 4 is.
The resulting potential between the two switches 2,3 can easily be derived from
the equivalent circuit diagram of the electrical switchgear 1 shown in Fig. 3. The
closed switches are not shown in Fig. 3. Employing basic physical relationships,
the middle voltage UM (i.e.the voltage between the first contacts 6 and terminal
T2) can be found as Um=C1/(C2+2C1)-U, with U being the voltage between the
terminals T1 and T2. From this equation it can gathered that the middle voltage
UM is approximately U/2 if C1»C2. Therefore, it can be achieved that the total
voltage to be switched is substantailly equally shared by the two switches 2,3
connected in series by making the capacitance of capacitor C1 as big as possible
and of capacitor C2 as small as possible.
In an example the geometry of the switches 2,3 and the shields 10,11 (e.g.
length, distance) can be chosen so that the capacitance of the first capacitor C1
is 250pF and the capacitance of the second capcitor C2 to the earthed enclosure
5 is 20pF. This would lead to a middle voltage UM=0.48.U which means that
both switches 2, 3 would have about the same voltage to switch.
Generally, C2 should be less than 0,5-C1 preferably less than 0,1 -C1 especially
less than 0,05-C1, to achieve a good voltage distribution.
From the above it can be followed, that it is advantageous to make the second
shield 11 as long as possible and especially to extend shield 11 also over the
connecting means 4 since this would increase C1" and C1" (and hence also C1)
and would decrease C2.

WE CLAIM
1. An electrical switchgear with two switches (2,3) arranged in the
switchgear enclosure (5) and electrically connected in series whereat each
of the switches (2,3) comprises a first and second contact (6,7), at least
one of the first and second contact (6,7) of each switch 5 (2,3) being a
mobile contact (6), the first contacts (6) of the two switches (2,3) are
mechanically and electrically connected by means of a connecting means
(4), the first contact (6) of a switch (2,3) is at least partly surrounded by
a first electrical conductive shield (10) and the second contact (7) of the
switch is at least partially surrounded by a second electrical conductive
shield (11), characterized in that the second shield (11) is electrically 10
connected to the second contact (7) and is at least partially surrounding
the first shield (10) which is electrically connected to the first contact (6)
so that a shield capacitor (C1") is formed between the first and second
shield (10,11), in that the second shield (11) is at least partially
surrounding the connecting means (4) so that a further capacitor (C1'") is
formed between the second shield (11) and the connecting means (4) and
in that a second 15 capacitor (C2) is formed between the, preferably
grounded, enclosure (5) of the switchgear (1) and the connecting means
(4).
2. An electrical switchgear as claimed in claim 1, wherein the connecting
means (4) is at least partially formed by a drive unit for driving the mobile
contact (6).
3. An electrical switchgear as claimed in one of claims 1 or 2, wherein the 20
connecting means (4) is at least partially formed by the first shield (10).

4. An electrical switchgear as claimed in claim 3, wherein the first shield
extends from the first contact (6) of the first switch (2) to the first contact
(6) of the second switch (3).
5. An electrical switchgear as claimed in one of claims 1 or 4, wherein the 25
ratio between the capacitance of the second capacitor (C2) and the
capacitance of the first capacitor (C1) is less than 0,5, preferably less than
0,1 and especially less than 0,05.

ABSTRACT

TITLE : "A COMPACT DOUBLE-CHAMBER SWITCHGEAR WITH
IMPROVED DIELECTRIC STRENGTH AND VOLTAGE DISTRIBUTION FOR
HIGH VOLTAGE APPLICATIONS"
The invention relates to an electrical switchgear with two switches (2, 3)
arranged in the switchgear enclosure (5) and electrically connected in series
whereat each of the switches (2,3) comprises a first and second contact (6, 7),
at least one of the first and second contact (6,7) of each switch 5 (2, 3) being a
mobile contact (6), the first contacts (6) of the two switches (2, 3) are
mechanically and electrically connected by means of a connecting means (4), the
first contact (6) of a switch (2, 3) is at least partly surrounded by a first electrical
conductive shield (10) and the second contact (7) of the switch is at least
partially surrounded by a second electrical conductive shield (11), the second
shield (11) is electrically 10 connected to the second contact (7) and is at least
partially surrounding the first shield (10) which is electrically connected to the
first contact (6) so that a shield capacitor (C1") is formed between the first and
second shield (10,11), in that the second shield (11) is at least partially
surrounding the connecting means (4) so that a further capacitor (C1"') is
formed between the second shield (11) and the connecting means (4) and in
that a second 15 capacitor (C2) is formed between the, preferably grounded,
enclosure (5) of the switchgear (1) and the connecting means (4).

A COMPACT DOUBLE-CHAMBER SWITCHGEAR WITH IMPROVED DIELECTRIC STRENGTH AND VOLTAGE SISTRIBUTION FOR HIGH VOLTAGE APPLICATIONS

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