Gas blast circuit breaker

Abstract: A gas blast circuit breaker comprises an insulation nozzle (8) for blowing extinguishing gas to an arc (9) generated between a stationary contact (1) and a movable contact (2). The nozzle (8) has a throat section (10) into and out of which one of the two contacts (1, 2) is movable and a divergent section (13) provided downstream of the throat section (10). A slanting surface (11) for increasing a reflectivity of energy intensity of the arc (9) is formed on the divergent section (13) of the nozzle (8). The nozzle (8) is formed of a fluoroplastic material and boron nitride powder not more than 15 vol.% added as a filler.

Description: This invention relates to a gas blast circuit breaker, and more particularly to a gas blast circuit breaker provided with an insulation nozzle of high withstand voltage characteristics which is disposed in the vicinity of an arc generating section so as to blast extinguishing gas, such as SF₆ 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, an insulation nozzle which decides the interrupting performance of the gas blast circuit breaker has been strongly required to have high withstand voltage characteristics. To meet this requirement, there has been proposed a new nozzle construction which is different from a conventional one and has been recently 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 (which corresponds to U.S.P. 4,667,072 and EP-A-0 135 158), 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 the withstand voltage performance. On this prior art the preamble of the appended claim is based. 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 born 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 made clear through experiments and analyses, 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 result that the intended performance can not be achieved after large electric current is interrupted many times. It is an object of this invention to provide a gas blast circuit breaker having a nozzle construction which has high withstand voltage characteristics and is free from the lowering of its performance due to a consumption deformation even after the interruption of large electric current. SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a gas blast circuit breaker according to the appended claim. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a vertical cross-sectional view of a gas blast circuit breaker; Fig. 2 is an enlarged, cross-sectional view of slanting surfaces of a nozzle of the embodiment shown in Fig. 1; Fig. 3 is a characteristics diagram showing the relation between the angle of the slanting surface of the nozzle of the gas blast circuit breaker of the present invention and the reflectivity of intensity of an arc energy line; Fig. 4 is a characteristics diagram showing the relation between the amount of addition of boron nitride and the reflectivity of intensity of the arc energy line; Fig. 5 is a characteristics diagram showing the relation between the amount of addition of boron nitride and the amount of consumption of the nozzle; and Fig. 6 is an enlarged cross-sectional view showing slanting surfaces in a gas blast circuit breaker. DESCRIPTION OF THE EMBODIMENTS Embodiments of the present invention will now be described with reference to the drawings. Fig. 1 shows one embodiment of a gas blast circuit breaker of the present invention. 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 contact 2, 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 now 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 ϑ, 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 formulas (1) and (2): where ε1 represents the dielectric constant of the gas, and ε2 represents the dielectric constant of the nozzle. From the above formulas (1) and (2), in Fig. 3 shown is a relative value I (P.U.) of the reflectivity Io of the arc energy line intensity with respect to the angle ϑ of the slanting surface when the reflectivity at ϑ=0 is made to be 1. The characteristics of the reflectivity of the energy line intensity shown in Fig. 3 are obtained when an amount of addition of the boron nitride is 0%. The reflectivity of the arc energy line intensity obtained, for example, with the angle ϑ of 40° is twice larger than that obtained with the angle ϑ of zero, and therefore with respect to the same arc energy line, the intensity of the energy line incident into the nozzle can be made half because total arc energy is constant. Preferably, based on the characteristics curve shown in Fig. 3, the angle ϑ of the slanting surface should be set to a range of between 25° and 45°. The reason for this setting will be explained. When the angle ϑ of the slanting surface is 25°, the reflectivity of the energy line intensity is 1.4 times larger, as can be seen from Fig. 3. Therefore, by increasing the reflectivity of the energy line intensity 1.4 times larger 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 keeped, the arc energy line must be increased (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 ϑ of the slanting surface is larger. However, if the angle ϑ 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 withstand voltage is decreased. Therefore, it has been decided from the gas flow analysis that the maximum angle ϑ the slanting surface should be not more than 45°. In view of the foregoing, the angle ϑ of the slanting surface is in the range of between 25° and 45°. 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 of addition of the boron nitride to the nozzle is increased, the dielectric constant of the nozzle is increased. On the other hand, as is clear from the above formulas (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 larger the amount of addition of the boron nitride is), the greater refraction the arc energy line penetrates into the substance. A relationship of the amount of addition of the boron nitride (BN) and the reflectivity of the arc energy line intensity with respect to the angle ϑ of the slanting surface of the nozzle of shown in Fig. 4. 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° 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 ϑ of the slanting surface 11 is in the range of between 25° and 45°. When the amount of addition 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 addition 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 addition of the boron nitride to not more than 15 vol.%, the reflectivity at each angle of the slanting surface can be maintained. Then, the amount of surface consumption of the nozzle will be explained. 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 addition of the boron nitride is not more than 15 vol.%, there is no large difference in the consumption amount. However, particularly, the consumption amount at 20 vol.% 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, the amount of addition of the boron nitride is 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 withstand 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 addition of the boron nitride, the internal arc resistance of the nozzle can be enhanced, and the consumption amount can be restrained. As a result, there can be provided the gas blast circuit breaker which enables the interruption of small capacitive current after a frequent interruption of large current. The above-mentioned embodiment of the invention has been described without particularly distinguishing between the angles ϑ1 and ϑ2 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 ϑ1 and ϑ2 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 ϑ1 and ϑ2 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 surfaces can be provided. In this case, the angle-setting is done in the same manner as described above. 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 addition of the boron nitride, there can be provided the nozzle of the high withstand voltage characteristics which is free from deformation of its surface configuration which would be caused by the consumption after a frequent interruption of large electric current.

Claims: A gas circuit breaker comprising an insulation nozzle (8) for blowing extinguishing gas to an arc generated between a stationary contact (1) and a movable contact (2), said nozzle (8) having a throat section (10) into and out of which one of said two contacts (1, 2) is movable, a divergent section (13) provided downstream of said throat section (10) and at least one slanting section provided intermediate said throat section (10) and said divergent section (13), said slanting section including a first slanting surface (11) extending in the flow direction of said extinguishing gas and a second slanting surface (12) intersecting said first slanting surface (11), and said nozzle (8) being formed of a fluoroplastic material impregnated with a filler material of boron nitride powder, whereby at least one of the angle (ϑ1) between said first slanting surface (11) and the centerline of said nozzle (8) and the angle (ϑ2) between said second slanting surface (12) and the centerline of said nozzle (8) is in the range of 25° to 45°, characterized in that the amount of boron nitride is in the range of 5 vol. % to 15 vol. %.

Inventors: ENDO FUMIHIRO, HASHIMOTO AKIRA, HIRASAWA KUNIO, KUROSAWA YUKIO, SATOH MINORI, TSUKUSHI MASANORI, YAMAGIWA TOKIO, ENDO, FUMIHIRO, HASHIMOTO, AKIRA, HIRASAWA, KUNIO, KUROSAWA, YUKIO, SATOH, MINORI, TSUKUSHI, MASANORI, YAMAGIWA, TOKIO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H01H33
IPC codes: H01H33