Insulation gas filled circuit breaker

An insulation gas filled circuit breaker in which a circuit breaking portion including two separably opposing contacts and a capacitor connected in parallel between the two contacts in the circuit breaking portion are disposed in a closed metal container filled with gas having an insulating property. A shield is provided at both the side of the capacitor facing the circuit breaking portion and the side of the capacitor facing the closed metal container for relaxing the electric field concentration near the capacitor. The top end of the shield is positioned so as to extend from the contacting face between the capacitor and an electrode pressing the capacitor toward the capacitor by a predetermined distance, whereby the dielectric strength of the capacitor connected in parallel with the circuit breaking portion is increased and a highly reliable and compact insulation gas filled circuit breaker of a high voltage and a large capacity use is provided.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an insulation gas filled circuit breaker 
(hereinafter called a gas circuit breaker) and, in particular, to a 
structure of a gas circuit breaker in which an impedance element such as a 
capacitor is connected between electrodes in the circuit breaking portion 
thereof for suppressing a recovery voltage increase which is caused 
immediately after a current interruption. 
2. Description of Related Art 
A gas circuit breaker includes a circuit breaking portion consisting of a 
stationary contact and a movable contact accommodated in a metal container 
filled with insulation gas. Further, a capacitor for suppressing the 
recovery voltage is electrically connected in parallel between the 
stationary and movable contacts in the circuit breaking portion and is 
disposed in the separating direction of the movable contact. In a 
conventional gas circuit breaker of this type, for example as disposed in 
JP(U)-A-58-41949(1983), a shield for a circuit breaking portion is 
provided for reducing electric field intensity at the circuit breaking 
portion so as to reduce the electric field intensity, in particular, at 
the top ends of the two contacts, and the positional relationship between 
the circuit breaking portion and the capacitor is determined so as to 
further reduce the electric field intensity at the circuit breaking 
portion. Thereby the circuit breaking performance as well as the 
dielectric strength between the electrodes of the gas circuit breaker are 
improved. 
At present, networking of an electric power transmission system has 
advanced and the need for an interruptable current required for a circuit 
breaker is increasing. Accordingly, the capacitance of the capacitor 
required for connection between the electrodes has increased. Further, in 
association with voltage level increases in the electric power 
transmission system the dielectric strength required for the capacitor 
also has increased. 
In the above exemplified conventional gas circuit breaker, the dielectric 
strength of the capacitor was poor, because the electric field intensity 
near the capacitor was not taken into account. Therefore, in order to 
achieve a required dielectric strength. The number of serially connected 
capacitor elements was increased to prolong the insulation distance. As a 
result the entire size of the gas circuit breaker was enlarged. Further, 
through the increase of the serially connected capacitor elements, the 
resultant electrostatic capacitance thereof was decreased. Therefore in 
order to achieve a required electrostatic capacitance, the number of 
parallel connected capacitors was increased which also caused the size 
enlargement of the entire gas circuit breaker and the cost increase 
thereof. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a high voltage and compact 
gas circuit breaker having a large capacity through an improvement of the 
dielectric strength of a capacitor connected thereto. 
For achieving the above object, in a gas circuit breaker according to the 
present invention, a shield for relaxing the electric field concentration 
near the capacitor is respectively provided at both sides of the 
capacitor. At the side facing a metal container and at the side facing a 
circuit breaking portion thereof, the shield is disposed in such a manner 
that the top end of the shield extends from a contact face between the 
capacitor end and a pressing electrode for the capacitor by a 
predetermined distance toward the side of the capacitor, and the 
predetermined distance is selected from 0.5 to three times the distance 
from the side face of the capacitor to the top end of the shield. 
According to the present invention, distortion of the electric field near 
the capacitor is reduced and the dielectric strength of the capacitor is 
increased. Thereby, the number of serially connected capacitor elements is 
reduced and as well, the electrostatic capacitance of the capacitor can be 
increased. Accordingly, the number of parallel connected capacitors is 
decreased and a low cost and compact gas circuit breaker is realized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinbelow, embodiments of the present invention are explained with 
reference to the drawings. 
FIG. 1 is an embodiment of a gas circuit breaker according to the present 
invention. 
A gas circuit breaker has a structure wherein a circuit breaking portion 
consisting of a stationary contact 1 and a movable contact 2 is disposed 
in a closed cylindrical metal container 19 substantially at the center 
thereof as shown in FIG. 1. In the metal container 19, SF6 gas having 
excellent arc extinguishing and electrical insulating properties is 
filled. The stationary contact 1 and the movables contact 2 are 
respectively secured to conductors 4a and 4b and electrically connected 
.thereto, and the conductors 4a and 4b are fixedly connected by an 
inter-electrode insulator supporting member 3. The movable contact 2 is 
designed to receive an activating force therefor from the outside of the 
metal container 19 via an insulated operation rod not shown, and performs 
a making and breaking operation. The inter-electrode insulator supporting 
member 3 is provided with a plurality of cylindrical holes into which 
respective inter-electrode capacitors 7 each constituted by a plurality of 
capacitor elements are accommodated and the respective interelectrode 
capacitors 7 are connected in parallel with the circuit breaking portion 
via respective capacitor pressing electrodes 8a and 8b contacting the ends 
of the respective capacitors 7. 
FIG. 2 is a cross sectional view seen from line II--II of the gas circuit 
breaker as shown in FIG. 1, wherein the number of parallel capacitors 7 is 
selected to be 10. 
According to the present embodiment of the gas circuit breaker having the 
above explained components, outer shields 5a and 5b and inner shields 6a 
and 6b are provided for relaxing electrical field concentration near the 
capacitors 7. With these measures the dielectric strength of the 
capacitors 7 is improved, the number of serially connected capacitor 
elements in respective capacitors 7 is reduced as well the number of 
parallel capacitors 7 so that the size of the gas circuit breaker is also 
reduced. Further, with the provision of shields 9a and 9b for the circuit 
breaking portion separate from the shields 5a, 5b, 6a and 6b, the electric 
field intensity near the stationary contact 1 and the movable contact 2 is 
reduced and the interelectrode dielectric strength after circuit 
interruption, namely after completing separation of the movable contact 2 
from the stationary contact 1, is improved. Thereby, because of the 
shortening of the inter-electrode distance, the size of the circuit 
breaking portion is reduced as well. Because of the shortening of the 
stroke of the movable contact 2, the size of the operating mechanism 
therefor is also reduced. 
Hereinbelow, the principle of how the dielectric strength is improved in 
the gas circuit breaker according to the present embodiment as indicated 
above is explained in greater detail. 
FIG. 3 is an enlarged cross sectional view of a portion of the capacitor 7. 
The shields 5a and 6a for the capacitor 7 are arranged so as to extend 
toward the side of the capacitor 7 beyond capacitor pressing electrode 8a 
and the distance Ls between the top or inner ends of the shields 5a and 6a 
and the capacitor pressing electrode 8a is selected to be about twice as 
long as the distance Lg between the top or inner ends of the shields 5a 
and 6a and the side face of the capacitor 7. Thereby, the electric field 
distortion near the capacitor 7 is reduced and the dielectric strength of 
the capacitor 7 is improved. 
FIG. 4 is a graph showing exemplary relationships between the ratio of 
Ls/Lg and the dielectric strength of the capacitor 7. The graph shows that 
when the ratio of Ls/Lg is about 2 the dielectric strength is maximized, 
in that the dielectric strength at that ratio is about 1.5 times of those 
when Ls=0. Further, in the range from 1 to 3 of the ratio of Ls/Lg, the 
dielectric strength reduces merely about less than 3% of those when the 
ratio of Ls/Lg is 2. However, when the ratio of Ls/Lg deviates from the 
above indicated range, the dielectric strength reduces to an unacceptable 
level. 
The reason why the dielectric strength near the capacitor 7 is increased by 
the proper selection of the distance Ls is that the electric field 
distortion near the capacitor 7 is reduced thereby. 
FIG. 5 shows a structure near a capacitor pressing electrode 8a in an 
example of conventional gas circuit breakers. The position of the top end 
of the shield 5a with respect to the capacitor 7 is selected substantially 
the same as that of the top end of the capacitor pressing electrode 8a. In 
this instance an equipotential line 12 creeps in between the shield 5a and 
the capacitor pressing electrode 8a and increases an electric field 
distortion at the corner of the capacitor pressing electrode 8a which 
causes reduction of dielectric strength of the capacitor 7. Further, 
although the shield 9a for the circuit breaking portion is provided, no 
shield 6a is provided near the inner side of the capacitor 7 and the 
electric field distortion at the corner of the capacitor pressing 
electrode 8a is further increased. 
Contrary thereto, in the present embodiment an equipotential line 11 
intersects perpendicularly with the side face of the capacitor 7 as shown 
in FIG. 3. Thereby, the electric field distortion at the top end of the 
capacitor pressing electrode 8a is reduced and the dielectric strength of 
the capacitor 7 is improved. However, if the distance Ls is excessively 
increased, the resultant electric field is distorted in the opposite 
direction as in FIG. 5, and the dielectric strength to the contrary 
reduces as will be understood from FIG. 4. 
The above explained fact is applicable to a case wherein, instead of 
inserting the capacitors 7 into the inter-electrode supporting insulator 
member 3, the respective capacitors 7 are inserted into corresponding 
separate insulator cylinders and are arranged in parallel between the 
electrodes in the circuit breaking portion. Normally, the thickness of the 
these insulator cylinders is thinner than that of the inter-electrode 
supporting insulator member 3 which contributes to reduce the distance Lg. 
Accordingly, proportional thereto, a proper distance of Ls is likely 
shortened while maintaining the above explained relationship with regard 
to the ratio of Ls/Lg. Further, the above explained fact is also 
applicable when impedance elements other than the capacitors are disposed 
within the gas circuit breaker. 
In the embodiment as shown in FIGS. 1 through 3, before inserting the 
capacitor 7 into the inter-electrode supporting insulator member 3, the 
capacitor 7 is at first inserted into an insulator cylinder 10 having a 
lower dielectric constant than that of the interelectrode supporting 
insulator member 3. With this measure, and the electric field intensity at 
the side face of the capacitor 7 is reduced and the dielectric strength of 
the capacitor 7 is improved. 
FIG. 6 shows a relationship between electric field intensity at an electric 
field concentrating portion of the capacitor 7 and the thickness of the 
insulator cylinder 10. In FIG. 6, the electric field is represented by a 
relative value with respect to an electric field value when the thickness 
of the insulator cylinder 10 is sufficiently thick, the thicker is the 
thickness of the insulator cylinder 10, the more relaxed is the electric 
field concentration, and when the thickness of the insulator cylinder 10 
is selected to be more than 0.1 mm, the electric field concentration is 
sufficiently relaxed. Accordingly, with the thickness of the insulator 
cylinder 10 of more than 0.1 mm the dielectric strength of the capacitor 7 
is increased. 
However, if the thickness of the insulator cylinder 10 is increased more 
than 10 mm, the electric field concentration reduction effect does not 
further increase and further the diameter of the holes to be formed in the 
inter-electrode supporting insulator member 3 for accommodating the 
respective capacitors 7 excessively increses which reduces the mechanical 
strength of the inter-electrode supporting insulator member 3. Therefore 
the thickness of the insulator cylinder 10 is selected to be less than 10 
mm. 
The inter-electrode supporting insulator member 3 is mostly 
injection-molded by such as alumina filled epoxy resin having a dielectric 
constant of 5.about.8. Accordingly, as dielectric materials having a lower 
dielectric constant for the insulator cylinder 10, fluorocarbon resin 
materials such as polymers of ethylene tetrafluoride including ones having 
a dielectric constant less than 2.5 can be used. 
In the FIG. 3 embodiment, the diameter of the capacitor pressing electrodes 
8a and 8b is larger than the inner diameter of the insulator cylinder 10. 
With this measure when inserting the capacitors 7 into the inter-electrode 
supporting insulator member 3, the insulator cylinder 10 can be pressed in 
by the capacitor pressing electrodes 8a and 8b. Thereby and the insulator 
cylinder 10 is reliably arranged along the side face of the capacitor 7. 
In this instance, the length of the insulator cylinder 10 is selected less 
than that of the capacitor 7. 
Further, when the diameter of the capacitor pressing electrodes 8a and 8b 
is smaller than the outer diameter of the insulator cylinder 10, the 
diameter of the holes to be formed in the inter-electrode supporting 
insulator member 3 for receiving the capacitors 7 can be determined 
substantially the same as the outer diameter of the insulator cylinder 10. 
Thereby the displacement of the capacitors 10 in the holes is suppressed 
and mechanical damage thereof is prevented which improves the reliability 
of the capacitors 7 inserted therein. 
With the above explained measures, the dielectric strength of the 
capacitors 7 can be increased. Thereby the number of serially connected 
capacitor elements which constitute respective capacitors 7 can be reduced 
and the length of the respective capacitors 7 may be shortened. Further, 
with the reduction of serially connected capacitor elements, the resultant 
capacitance of the respective capacitors 7 is increased, whereby a 
required electrostatic capacitance for a gas circuit breaker can be easily 
obtained and a down sizing and reliability of a high voltage and large 
capacity gas circuit breaker is achieved. Still further, with the 
shortening of the capacitor length, the length of the inter-electrode 
supporting insulator member 3 is also shortened whereby the mechanical 
strength and reliability of the inter-electrode supporting insulator 
member 3 are improved and the production cost thereof is also reduced. 
When the dielectric strength of the capacitors 7 is maintained with the 
thus constituted shields 5a, 5b, 6a and 6b for the capacitor use, the 
shields 9a and 9b for the circuit breaking portion do not have any 
influence on the dielectric strength of the capacitors 7. Accordingly, the 
shields 9a and 9b for the circuit breaking portion can be designed with 
regard to their location and configuration primarily for the purpose of 
reducing electric field intensity at the circuit breaking portion. 
FIGS. 7 and 8 are cross sectional views of other embodiments according to 
the present invention in which the present invention is applied to a gas 
circuit breaker including make resistance contacts 20. FIG. 7 is an 
embodiment in which the shield 9a for the circuit breaking portion is 
connected to the outer shield 5a for the capacitor 7. FIG. 8 is another 
embodiment in which the shield 9a for the circuit breaking portion is 
provided independent from the outer shield 5a for the capacitor 7. The 
shield 9a for the circuit breaking portion as shown in FIG. 8 can be, for 
example, easily produced by combining two metal pipes. 
With these constitutions, the electric field intensity at the stationary 
contact 1 and the movable contact 2 is reduced and the dielectric strength 
between electrodes in the circuit breaking portion is increased, whereby 
the inter-electrode dielectric strength after the current interruption is 
improved. As a result, with the shortening of the inter-electrode 
distance, the size of the circuit breaking portion is reduced as well as 
with the shortening of the stroke of the movable contact 2. The size of 
the operating mechanism therefor is reduced. 
With the above constitution, when effecting the coordination of insulation 
by maintaining the dielectric strength of the capacitors 7 and that 
between the electrodes in the circuit breaking portion at a same level, 
the capacitors 7 assume a length in a range from more than 2 and less than 
3 times of the distance between the electrodes in the circuit breaking 
portion. 
According to the present invention as explained above, the amount of 
capacitors can be reduced which are connected in parallel with the circuit 
breaking unit while maintaining a desired dielectric strength for a gas 
circuit breaker. Further the size of the circuit breaking portion can be 
also reduced. Whereby a highly reliable and compact gas circuit breaker 
for a high voltage and a large capacity use is realized.