Gas blast, puffer type circuit breaker with improved nozzle

A gas blast circuit breaker comprises an insulation nozzle for blowing extinguishing gas to an arc generated between a stationary contact and a movable contact. The nozzle has a throat section into and out of which one of the two contacts is movable and a divergent section provided downstream of the throat section. A slanting surface for increasing a reflectivity of energy intensity of the arc is formed on the divergent section of the nozzle. The nozzle is formed of a fluoroplastic material and boron nitride powder of not more than 15 vol. % is added as a filler.

BACKGROUND OF THE INVENTION 
This invention relates to a gas blast circuit breaker and more particularly 
to a gas blast circuit breaker provided with an insulation nozzle disposed 
in the vicinity of an arc generating section so as to blast extinguishing 
gas, such as SF.sub.6 gas, to an arc generated between a movable contact 
and a stationary contact when large electric current is interrupted. 
Recently, with an increasing consumption of electric power, electric 
devices have been required to operate under high voltage and large 
electric current. In a gas blast circuit breaker, which is a final 
protective device for an electric power system, it is necessary to provide 
an insulation nozzle capable of withstanding high voltages. 
To meet this requirement, a new nozzle construction has been proposed 
differing from a conventional nozzle with the new nozzle construction 
being achieved by advanced techniques of analysis such as a gas flow 
analysis. 
In such a nozzle construction disclosed, for example, in Japanese Patent 
Unexamined Publication No. 60-218722 corresponding U.S. Pat. No. 
4,667,072, a high-pressure gas region space is formed at a downstream side 
of a throat section of the nozzle by a normally-slanting surface which 
extends along the direction of flow of an extinguishing gas and a 
reversely-slanting surface intersecting this normally-slanting surface, 
and a region near a distal end portion of a stationary contact constitutes 
the high-pressure gas region until the stationary contact passes through 
this space position, thereby making it possible to enhance voltage 
performance. 
One method of enhancing the internal arc resistance of the nozzle has been 
proposed, for example, in Japanese Patent Unexamined Publication No. 
57-210507, in which 20% by volume of boron nitride (BN) is mixed as a 
filler in a fluoroplastic material of the nozzle. 
With respect to the nozzle disclosed in the above-mentioned Japanese 
Publication 60-218722, it has been experimentally determined, as described 
in the specification thereof, that, the shape of the reversely-slanting 
surface and the diameter of the throat section greatly influence the 
dielectric interrupting performance. 
On the other hand, in this type of nozzle, in order to enhance the internal 
arc resistance, it is necessary that boron nitride should be mixed in the 
nozzle material, as disclosed in the above-mentioned Japanese Publication 
57-210507. In this case, however, it is not considered how much the energy 
lines of the arc intrude into the nozzle, and there exists a portion on 
the surface of the nozzle where the absorption of the arc energy is 
increased. This results in a drawback that the surface consumption by the 
arc is increased, and the above-mentioned nozzle construction suffers from 
the problems that the shape and size of the reversely-slanting surface are 
changed by the consumption with the surface result being that the intended 
performance can not be achieved after large electric current is 
interrupted many times. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a gas blast circuit breaker 
having a nozzle construction which is capable of withstanding high 
voltages and is free from the lowering of its performance due to a 
consumption deformation even after the interruption of a large electric 
current. 
In order to achieve the above object, the present invention provides a gas 
blast circuit breaker comprising an insulation nozzle for blowing 
extinguishing gas to an arc generated between a stationary contact with 
and a movable contact, the nozzle having a throat section into and out of 
which one of the two contacts is movable. A divergent section is provided 
down-stream of the throat section, and a slanting surface for increasing a 
reflectivity of energy intensity of the arc is formed on the divergent 
section of the nozzle. The nozzle is formed by adding not more than 15 
vol. % of boron nitride powder as a filler to a fluoroplastic material. 
When the movable contact moves away from the stationary contact, the energy 
lines readily radiated from the arc generated between these two contacts 
are decreased in an amount of intrusion of these energy lines into the 
nozzle by the slanting surface provided downstream of the throat section 
of the nozzle. As a result, an amount of boron nitride to be added can be 
reduced, and even in this case, the internal arc resistance of generally 
the same level as conventionally achieved can be maintained. Further, with 
the reduced amount of boron nitride, the surface deformation due to the 
consumption of the nozzle can be restrained, and therefore the same 
performance as obtained with a new nozzle can be achieved even after large 
electric current is interrupted many times.

DESCRIPTION OF THE EMBODIMENTS 
As shown in FIG. 1, a movable contact 2 is disposed in opposed relation to 
a stationary contact 1, and is movable into and out of contact with the 
stationary contact 1. A drive shaft 3 is connected to the movable contact 
2, and a fixed piston 4 slidably supports the drive shaft 3. A movable 
cylinder 5 is mounted on the drive shaft 3 and encloses the fixed piston 
4. A cylinder chamber 6 is defined by the fixed piston 4 and the movable 
cylinder 5. An opening 7 is formed through one end wall of the movable 
cylinder 5 disposed adjacent to the movable contact 2. A nozzle 8 is 
mounted on the movable cylinder 5, and this nozzle 8 serves to blow 
extinguishing gas, discharged from the cylinder chamber 6 through the 
opening 7, to an arc 9 generated between the contacts 1 and 2. The nozzle 
8 includes a throat section 10 which fits on the stationary contact 1 with 
a slight gap therebetween upon movement of the movable cylinder 5, a first 
slanting surface 11 disposed downstream of the throat section 10 and 
extending along the direction of flow of the extinguishing gas so as to 
increase the reflectivity of the energy intensity of the arc, a second 
slanting surface 12 intersecting the first slanting surface 11, and a 
divergent section 13 extending from the second slanting surface 12. In 
order that the nozzle 8 can have insulating properties, the nozzle 8 is 
composed of a fluoroplastic material, and boron nitride (BN) is added to 
this fluoroplastic material as later described. 
Next, the condition of reflection of an energy line 14 of the arc 9 by the 
first and second slanting surfaces 11 and 12 will be described with 
reference to FIG. 2. In FIG. 2, assuming that the angle between the first 
slanting surface 11 and the centerline (axis) of the nozzle 8 is 6, an 
energy line 14 from the arc 9 becomes an energy line 14A directed into the 
nozzle 8 and an energy line 14B obtained as a result of reflection by the 
first slanting surface 11. A reflectivity Io of the energy line intensity 
at this time is generally expressed by the following equations: 
##EQU1## 
where .epsilon.1 represents the dielectric constant of the gas, .epsilon.2 
represents the dielectric constant of the nozzle and k is a optical 
constant. 
From the equations (1) and (2), in FIG. 3 is shown a relative value I 
(P.U.) of the reflectivity Io of the arc energy line intensity with 
respect to the angle .theta. of the slanting surface when the reflectivity 
at .theta.=0 is equal to 1. The characteristics of the reflectivity of the 
energy line intensity shown in FIG. 3 are obtained when an amount of the 
boron nitride is 0%. The reflectivity of the arc energy line intensity 
obtained, for example, with the angle .theta. of 40.degree. is twice as 
large as that obtained when the angle .theta. is equal to zero, and 
therefore with respect to the same arc energy line, the intensity of the 
energy line incident into the nozzle can be halved because the total arc 
energy is constant. Preferably, based on the characteristics curve shown 
in FIG. 3, the angle .theta. of the slanting surface should preferably be 
in a range of between 25.degree. and 45.degree.. When the angle .theta. of 
the slanting surface is 25.degree., the reflectivity of the energy line 
intensity is 1.4 times greater, as can be seen from FIG. 3. Therefore, by 
increasing the reflectivity of the energy line intensity 1.4 times in this 
manner, there can be obtained the effect equal to or greater than the 
effect that the incident energy line into the nozzle 8 is decreased by one 
grade with respect to the rated interrupting, current, for example, when 
the rating is decreased from 50 KA to 40 KA, this is represented by 
50/40=1.3 times on the contrary, if the some energy line is maintained, 
the arc energy line must be increase, for example, from 40 kA to 50 kA. 
Therefore, there can be provided an ample margin of the performance for an 
internal arc resistance of the nozzle. On the other hand, from the 
viewpoint of the reflectivity of the energy line intensity, it is 
preferable that the angle .theta. of the slanting surface is larger. 
However, if the angle .theta. is too large, a vortex flow of the gas is 
produced in a space defined by the first and second slanting surfaces 11 
and 12, and the gas density is decreased, and the withstanding voltage 
characteristics is decreased. Therefore, it has been determined from the 
gas flow analysis that the maximum angle .theta. the slanting surface 
should not be greater than 45.degree.. 
Next, reference is now made to the relation between the reflectivity of the 
arc energy line intensity and the amount of addition of the boron nitride. 
When an amount boron nitride added to the nozzle is increased, the 
dielectric constant of the nozzle is increased, On the other hand, as is 
clear from equations (1) and (2), a square root of the dielectric constant 
of a substance is proportional to the index of refraction of the 
substance. This means that in the case of the same incident angle of the 
arc energy line, the greater the dielectric constant of the substance is, 
that is, the greater the amount of boron nitride added, the greater 
refraction the arc energy line penetrates into the substance. FIG. 4 
illustrates a relationship of the amount of boron nitride (BN) added and 
the reflectivity of the arc energy line intensity with respect to the 
angle .theta. is the slanting surface of the nozzle. The reflectivity in 
the ordinate axis of FIG. 4 is expressed as the relative value obtained 
when the reflectivity at the angle (FIG. 3) of 0.degree. is "1". As is 
clear from FIG. 4, when the amount of addition of the boron nitride is up 
to about 10 vol. %, the reflectivity at each angle shown in FIG. 3 is 
maintained, even when the angle .theta. of the slanting surface 11 is in 
the range of between 25.degree. and 45.degree.. When the amount of added 
boron nitride is 15 vol. %, the reflectivity is slightly decreased, but an 
effect similar to the effect that the rated interrupting current is 
decreased by one grade can be maintained. However, when the amount of 
boron nitride added is 20 vol. %, the reflectivity at each angle of the 
slanting surface is decreased, and the effect similar to the effect that 
the rated interrupting current is decreased by one grade cannot be 
maintained. In other words, by keeping the amount of boron nitride added 
to not more than 15 vol. %, the reflectivity at each angle of the slanting 
surface can be maintained. 
To determine, the amount of surface consumption of the nozzle cylindrical 
test pieces were prepared, and an arc of 10 kAp was ignited in each test 
piece at a frequency of 0.5 cycle (60 Hz), and the nozzle consumption 
amount W (P.U./kA.S) at the electrode gap of 10 mm was measured. The 
results thereof are shown in FIG. 5. As is clear from FIG. 5, when the 
amount of boron nitride added is not more than 15 vol. %, there is no 
large difference in the consumption amount. However, particularly, the 
consumption amount at 20 vol. % of boron nitride is greatly different from 
the consumption amount at 15 vol. %. Incidentally, even at 0 vol. % of the 
boron nitride, the consumption amount is increased, and this is due to the 
formation of voids in the interior of the nozzle and a partial peeling at 
the surface, because the internal arc resistance of the nozzle is not 
provided. 
In view of the above consumption amount, it is preferred that the amount of 
boron nitride added should be in the range of between 5 vol. % and 15 vol. 
%. 
With the above construction, by providing the first and second slanting 
surfaces 11 and 12 downstream of the throat section 10 of the nozzle, the 
extinguishing gas can be always applied to the surface of that portion of 
the stationary contact subjected to an increased electric field, and the 
transient withstanding voltage after the current interruption can be 
maintained. And besides, by suitably determining the angles of the first 
and second slanting surfaces and the amount of boron nitride, the internal 
arc resistance of the nozzle can be enhanced, and the consumption amount 
can be restrained. As a result, a gas blast circuit breaker is provided 
which enables the interruption of small capacitive current after a 
frequent interruption of a large current. 
The above-mentioned embodiment of the invention has been described without 
particularly distinguishing between the angles .theta.1 and .theta.2 of 
the end portions of the first and second slanting surfaces 11 and 12 as 
shown in FIG. 6. However, the effects can be expected even if only one of 
the angles .theta.l and .theta.2 is set to the above range of the present 
invention. Namely, if the internal arc resistance is increased at the 
first slanting surface 11 or the second slanting surface 12, the 
dielectric interrupting performance is enhanced at the surface thereof. 
Further, by such setting, the degree of freedom of setting of the angles 
.theta.l and .theta.2 of the slanting surfaces is increased, and the 
angle-setting for controlling the flow of gas to the stationary contact 
can be easily done. 
In the present invention, as shown in FIG. 6, a plurality of pairs of first 
and second slanting surface 11, 12 can be provided. In this case, the 
angle-setting is done in the same manner as described above. 
Since the amount of incidence of the arc energy line is larger at the 
throat section 10 of the nozzle 8 than at the slanting surfaces in the 
present invention, the amount of boron nitride added at the throat section 
10 can be 20 vol. % to increase the internal arc resistance at the throat 
section 10 so as to restrain the surface deformation due to the 
consumption. 
According to the present invention, by suitably determining the angle of 
the slanting surface disposed downstream of the throat section of the 
nozzle, as well as the amount of boron nitride, there can be provided a 
nozzle capable of withstanding high voltages which is free from 
deformation of its surface configuration which would be caused by the 
consumption after a frequent interruption of large electric current.