Patent Publication Number: US-7589295-B2

Title: Electrical switchgear

Description:
BACKGROUND OF THE INVENTION 
   FIELD OF THE INVENTION 
   The present invention is pertaining 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. 
   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. &gt;500 kV, 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 U.S. Pat. No. 3,786,216 A. Some arrangements of prior art show either capacitors made by solid isolators integrated into single-chamber circuit breaker (allowing transitory voltage to be reduced particularity 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 U.S. Pat. No. 5,728,989 A or EP 335 338 A2. U.S. Pat. No. 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 arranged labyrinth-like to form a labyrinth passage which effectively intersects arc particles 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 U.S. Pat. No. 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. 
   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. 
   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 significantly by increasing the natural capacitor between the open contacts of the switch thus reducing the risk 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, especially 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. 
   SUMMARY OF THE INVENTION 
   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 (C 1 ′″) is formed between the second shield ( 11 ) and the connecting means ( 4 ) is very relevant for the invention, because this increases capacitor C 1  (being C 1 ′+C 1 ″+C 1 ′″), and decreases capacitor C 2 , and thus improves voltage distribution between the two switching units, while the voltage ratio is C 1 /(C 2 +2C 1 ) and thus its value tends towards ½. 
   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 substantially equally shared by the two switches. 
   The invention is described in the following with reference to  FIGS. 1 to 3  showing in exemplary, non-limiting way 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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 T 1  (e.g. high potential) and T 2  (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 T 1  and T 2  of the switchgear  1 , the second contact  7  of the first switch  2  is connected to terminal T 1 , 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 T 2 , 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 SF 6 . 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 symmetrical 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 C 1 ′ is formed between the two open contacts  6 ,  7 . Between first shield  10  and second shield  11  a shield capacitor C 1 ″ is formed and between second shield  11  and connecting means  4 , e.g. the drive unit, a capacitor C 1 ′″ is formed. Since these three capacitors are connected in parallel, the capacitors can be combined to a first capacitor C 1 =C 1 ′+C 1 ″+C 1 ′″. 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 C 1 ″ will be. The more the second shield  11  extends also over the connecting means  4 , the greater the capacitance of capacitor C 1 ′″ 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. SF 6 ) which acts as insulator for the capacitors C 1  and C 2 . 
   Furthermore, a second capacitor C 2  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 C 2  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 U M  (i.e. the voltage between the first contacts  6  and terminal T 2 ) can be found as U M =C 1 /(C 2 +2C 1 )·U, with U being the voltage between the terminals T 1  and T 2 . From this equation it can gathered that the middle voltage U M  is approximately U/2 if C 1 &gt;&gt;C 2 . Therefore, it can be achieved that the total voltage to be switched is substantially equally shared by the two switches  2 ,  3  connected in series by making the capacitance of capacitor C 1  as big as possible and of capacitor C 2  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 C 1  is 250 pF and the capacitance of the second capacitor C 2  to the earthed enclosure  5  is 20 pF. This would lead to a middle voltage U M =0.48·U which means that both switches  2 ,  3  would have about the same voltage to switch. 
   Generally, C 2  should be less than 0.5·C 1 , preferably less than 0.1·C 1 , especially less than 0.05·C 1 , 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 C 1 ″ and C 1 ′″ (and hence also C 1 ) and would decrease C 2 .