US 3842226 A
In a circuit breaker of a gas blast type an improved arc interrupting scheme is provided by means of a wind tunnel type double-throat orifice or nozzle which maintains supersonic flow at high overall discharge side to reservoir pressure ratio. The high pressure generated by an arc, which ordinarily causes the clogging phenomenon, is utilized for initiating a supersonic flow and a rapic arc transfer to a longer gap. Thus, the clogging phenomenon is prevented, or at least minimized, and by the rapid transfer of the arc into a longer path length the effective contact opening time is shortened.
Description (OCR text may contain errors)
1Jnited States Patent 1191 Yoon 1 Oct. 15, 1974 CIRCUIT INTERRUPTER USING A 3,390.240 6/1968 Circle et a1. 200/148 A DOUBLE'THROAT NOZZLE FOREIGN PATENTS OR APPLICATIONS lnvemori K YOOH, 5806 5th Ave, 1,123,140 6/1956 France 200/148 R P1ttsburgh,Pa. 15232 1,120,545 12/1961 Germany 200/148 R  Filed: Dec. 27, 1972 Primary ExaminerRobert S. Macon  PP N05 319,078 Attorney, Agent, or FirmW. A. Elchik; W. R. Crout Related U.S. Application Data  Continuation of Ser, No. 9,374, Feb. 6, 1970, ABSTRACT abandoned In a circuit breaker of a gas blast type an improved arc interrupting scheme is provided by means of a wind  U.S. Cl 200/148 R, 200/148 E tunnel type double throat orifice or nozzle which  1111. C1. H011] 33/70 maintains Supersonic flow at high overall discharge  held of Search 200/148 R1 148 148 side to reservoir pressure ratio. The high pressure gen- 200/148 148 148 148 E erated by an are, which ordinarily causes the clogging phenomenon, is utilized for initiating a supersonic  References cued flow and a rapic arc transfer to a longer gap. Thus, the
U ITED TATE PA ENT clogging phenomenon is prevented, or at least mini- 2,367,934 1/1945 Flurscheim 200/148 R mized, and y the rapid transfer of the are into 8 2,399,412 4/1946 Webb 200/148 R longer path length the effective contact opening time 2,481,996 9/1949 Grunewald et al 200/148 C is shortened. 3,133,176 5/1964 Schneider 200/148 B 3,270,173 8/1966 Barkan 200/148 B 8 Claims, 6 Drawing Figures STORAGE RESERVOIR L' COMPRESSOR PATEmmnm 1 51514 SHEET 10! 4 mwhJE LII/ w mommwmazou PATENTEU 151974 3,842.226
SHE 2 0f 4 FIG.4
CIRCUIT INTERRUPTER USING A DOUBLE-THROAT NOZZLE This is a continuation, of application Ser. No. 9,374 filed Feb. 6, 1970, now abandoned.
BACKGROUND OF THE INVENTION This invention relates, generally, to circuit breakers and, more particularly, to circuit interrupters for circuit breakers of the compressed-gas type in which a flow of an interrupting medium is directed through and around an arc to aid in interrupting the are.
A circuit interrupter disclosed in a copending application Ser. No. 7,074 filed Jan. 30, 1970, by W. V. Bratkowski and W. H. Fischer, and assigned to the Westinghouse Electric Corp. is provided with a flow system comprising a supersonic nozzle and an arc chute constructed to take advantage of high speed flow to remove ionized gases quickly and of directed expansion which stretches the are core and length to interrupt the are.
An object of this invention is to provide an improved arc interrupting scheme by utilizing a double-throat orifice or nozzle to maintain supersonic flow at high overall discharge side to reservoir pressure ratio of the interrupting medium.
Another object of the invention is to utilize the high pressure generated by an arc to initiate a supersonic flow and a rapid arc transfer to a longer gap, thereby minimizing the clogging effect heretofore produced during high current interruption.
A further object of the invention is to improve the interrupting performance of a circuit interrupter by utilizing the shock wave generated during an interrupting operation to maintain supersonic flow of the interrupting medium at high discharge side to reservoir pressure ratio.
Other objects of the invention will be explained fully hereinafter or will be apparent to those skilled in the art.
SUMMARY OF THE INVENTION In accordance with the invention, an arc is drawn when a movable contact is separated from a ring contact at the first throat of a double-throat orifice or nozzle. The inlet of the first throat is connected to a reservoir containing an interrupting medium under pressure. The heat of the are causes the gas pressure in the region of the first throat to rise rapidly. Because of the pressure difference between this region and the region between the two throats, a supersonic flow is initiated which causes the arc to transfer from the ring contact to a stationary contact disposed at the discharge side of the second throat and electrically connected to the ring contact. Once the supersonic flow is initiated it can be maintained in the region between the throats until the ratio between the reservoir pressure and the discharge pressure approaches one, thereby minimizing the clogging effect which might otherwise be caused by a shock wave generated during an interrupting operation.
BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the nature and objects of the invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a diagrammatic view of a circuit interrupter embodying features of the invention;
FIG. 2 is a curve showing the relations between Area Ratio, Pressure Ratio and Mach Number in adiabatic air flow through a nozzle;
FIGS. 3a, b, c and d are graphical views showing the effect of pressure ratio on flow in a Laval Nozzle;
FIG. 4 is a diagrammatic view of a Laval Nozzle and a diffuser;
FIG. 5 is a diagrammatic view of a double-throat nozzle; and
FIG. 6 is a curve showing the relation between pressure ratio and test section Mach Number for the nozzle shown in FIG. 5.
DESCRIPTION OF PREFERRED EMBODIMENT In certain circuit breakers of the gas-flow or gas-blast type, at the time of opening a moving contact travels through the distance of the length of an orifice, usually toward the discharge side. In such a flow scheme, as the contacts are separated an arc is drawn and, depending on the magnitude of the current, either the positive column fills up the most part of the space within the orifice or the surrounding gas of the arc is heated and the pressure within the orifice is greatly increased. Until the moving contact is fully open, the limited openings at both ends of the orifice cannot drain the hot gas fast enough. As a result, until the current is sufficiently decreased, within a half cycle or in the magnitude of R-M-S value, the flow of cold gas is prevented. This is known as the clogging phenomenon. In this situation the hot gas may flow out in limited quantity to both reservoir and discharge side, but the benefit of a flow of cold gas is nil. Thus, the current interrupting limit is reached when this occurs. Furthermore, because of the residue of hot gas in the orifice and because of an inherent nature of a flow through such a nozzle even without the clogging phenomenon, the dielectric recovery in such a flow scheme is poor.
Assuming that a cold flow situation exists near current zero, such a flow is governed by the laws of one dimensional steady state isentropic flow through a Laval Nozzle, and the essential flow quantities are related as shown by the flow ing guation s:
Where wer more, these relations are plotted (qualitatively) in FIG. 3 with respect to the position in the orifice at different values of the discharge side pressure P,.;.
For a subsonic flow, as stated previously, A, A*. Also, for a given P /P say P corresponding to point a in FIG. 3b (P /P is close to l), A*/A is uniquely determined in the upper part of the curve in FIG. 2. A is the cross section at the discharge end of the orifice, and with this fixed, A* can be obtained. Then other points are all determined from values of A*/A along the orifice in the upper part of the curve. Since A, is the minimum cross section in a given orifice, the curve in FIG. 2 starts from P/P l and comes to a minimum pressure ratio point, say point t, and then goes back toward the starting point and ends at a point corresponding to P /P This is the reason for the minimum pressure ratio and the maximum Mach number always on the throat for subsonic flow (FIG. 3).
However, when the flow velocity reaches the sonic velocity at the throat, 21* A,, and the downstream pressure ratio and the Mach number will follow either the upper part or the lower part of the curve in FIG. 2 only at fixed values for P /P corresponding to points c and j in FIG. 3, respectively. Therefore, with a given orifice and values of P corresponding to points between and j in FIG. 3, in some portion of the orifice after the throat the flow velocity is supersonic and only by some compression can the pressure return to a fixed value of P,;. This is the reason why normal shock waves are produced in a Laval nozzle at improper values of P /P The positions of such shock waves are indicated, qualitatively, by S in b and c, and figuratively in d of FIG. 3.
This points out the general misunderstanding that an undisturbed supersonic flow velocity may be obtained if the value P /P is less than a certain value, say P /P 0.528 for air. This value has to be very low for an undisturbed supersonic flow as can be seen in FIG. 3. Such formation of shock waves is a handicap in practical breakers using Laval nozzles.
In practical breakers, both reservoir and discharge capacities are limited. Therefore, when a flow of gas once begins the reservoir pressure decreases and the discharge side pressure increases rapidly. This means that values of P /P become larger and larger toward the final value of P /P I. If this happens, as can be seen in FIG. 3, the position of the normal shock wave moves toward the throat, expanding the compressed regions from the discharge end toward the throat. It is already known that the compressed region by shock waves lowers the breakdown strength of the interrupting medium. In addition to the problem of clogging, this expansion of the compressed region toward the throat is deleterious to the interrupting performance.
As far as the normal shock problem is concerned, the situation can be improved by means of a diffuser, as shown in FIG. 4. This is just an extension of a Laval nozzle with a properly expanding cross section. Consider a situation marked byfin FIG. 3. If P /P P,/P,, is maintained, the normal shock will hang'on at the discharge end of the orifice and the entire orifice length from the throat on will have a supersonic flow. Now, if
a diffuser is added as shown in FIG. 4, for the same situ-.
ation an operation at still higher pressure ratio (discharge side to reservoir pressure ratio) is possible since the subsonic flow downstream of the normal shock may be decelerated isentropically to the stagnation pressure P,, which is the pressure of the discharge tank if it is sufficiently large. The pressure ratio required then is the ratio of the stagnation pressure across a normal shock wave in terms of the nozzle section Mach numberMs However, the effect of a diffuser can be improved further by introducing a second throat before a diffuser as shown in FIG. 5. Assuming that there is a supersonic flow in the test section between the throats, this supersonic flow can be compressed isentropically, at least in theory, to sonic condition at the second throat and then it could be decelerated further subsonically in the diffuser. In an'ideal case, no shock wave will occur and the pressure will be completely recovered, giving P P In theory at least, this means that if a supersonic flow is once established, a shock free supersonic flow can be maintained in the test section until the overall discharge side to the reservoir side pressure ratio be comes one. However, some lower pressure ratio (discharge side to reservoir) is required to initiate the flow, which is governed by the relation shown inFIG. 3 between the reservoir and the test section. Once the flow is initiated, especially when the pressure ratio is such (FIG. 3) that there is a normal shock before the second throat, the continuity requirement demands 2 1 01/ o2 (2 o/ o (4) This can be obtained for a given M from Eq. 3, and a sample curveof A, vs. M is shown in FIG. 6. This is the minimum area ratio to accommodate the shock in the test section, for the desired Mach number M With this minimum area for the second throat, and with larger ones, the shock wave is able to jump from the test section to the throat. This is called swallowing the shock. The test section is then fully supersonic, but unfortunately so is the second throat and part of the diffuser.
In terms of the flow in circuit breakers, as the discharge side pressure builds up closely to the value of the reservoir pressure, the supersonic velocity is maintained in the test section. Furthermore, the chances for clogging are minimized, if not eliminated entirely, not only in the first throat, but also in the second throat by the swallowing action.
The circuit interrupter shown in FIG. 1 utilizes a double-throat orifice or nozzle of the type shown in FIG. 5. The structure shown comprises a movable contact 1 which engages a stationary ring contact 2 when in the closed position, a stationary arc transfer contact 3 spaced from the ring contact, a high pressure reservoir 4, a discharge side volume 5, a double-throat orifice or nozzle 10 having throats T1 and T2 spaced longitudinally of the nozzle, an initial arcing region 6, a supersonic flow section 7 and a diffusing region 8, contact fingers 9 which slidably engage the movable contact 1, a storage reservoir 11, a compressor 12, a filter l3 and a valve 14 disposed between the storage reservoir 11 and the reservoir 4. Power conductors L1 and L2 are connected to the contact fingers 9 and the arc transfer contact 3, respectively. The contacts 2 and 3 are connected through a conductor 15.
The discharge volume 5 contains an interrupting medium, such as air or sulfur-hexafluoride, SF, gas at a relatively low pressure. The storage reservoir 11 contains the interrupting medium at a relatively high pressure, the pressure being maintained by the compressor 12 which draws gas from the volume 5 through the filter 13. During an interrupting operation, gas is admitted through the valve 14 to the reservoir 4 from which it flows through the nozzle into the discharge volume 5. The movable contact 1 and the valve 14 may be actuated in a manner well known in the circuit breaker art. The valve 14 should be opened a short time before the contact 1 disengages the contact 2.
The area 14 of the throat T may be made larger than the area A,* of the throat T to accommodate the initial flow. As the moving contact 1 separates from the ring contact 2 at the first throat T an arc is drawn in the region 6 and the gas pressure in this region rises rapidly. Then, because of the pressure difference between this region and the region 7, a flow is initiated according to the relation shown in FIG. 3. The higher the pressure ratio of 6 to 7 becomes the higher the Mach number M becomes. Thus, the pressure rise in 6 can initiate a supersonic flow in this scheme, whereas in a Laval nozzle it may cause the clogging phenomenon. Then, by this supersonic flow, the arc is rapidly transferred to the stationary arc transfer contact 3. Thus, the effective contact opening time can be reduced.
From the double-throat theory as discussed hereinbefore, once a supersonic flow is initiated it can be maintained until A P /P approaches 1 (FIG. 6). If the gas pressure generated by the arc is low, then the reservoir pressure will function to start the supersonic flow initially. If the gas pressure generated by the arc is high, then the reservoir pressure will take over as the pressure generated by the arc becomes lower near current zero. Of course, these are continuous processes. The dielectric recovery between contacts 1 and 3 is rapid because of the supersonic flow in region 7.
A specific example could be made of the case involving a circuit breaker capable of interrupting 60,000 to 70,000 amperes at a voltage of 138 KV utilizing sulfurhexalfluoride (SP gas. The upstream reservoir pressure within the chamber 4 would be, for example, 240 p.s.i. gauge, whereas the downstream reservoir pressure within the chamber 5 would be, for example 60 p.s.i. gauge. The movable contact makes a gas seal at the ring sleeve contact 2 in the closed position. During opening operation, the inrushing gas through the first orifice T1 would be at the sonic velocity because the pressure ratio is less than 0.58 required for supersonic flow, and supersonic velocity in the downstream. Such supersonic flow will carry the arc terminal to the downstream catcher electrode 3 to result in an arc of considerable length extending through the two nozzles T1 and T2. For example, to interrupt 60 to 70 KA at 138 KV, the nozzle diameter at T1 is 2 inches, the middle section diameter is about 3.5 inches, and the second nozzle diameter T2 is about 3 inches. The total length between the upstream and downstream arc catcher is about 7 inches and the time of interruption would be two cycles.
Background Information can be found in ELE- MENTS OF GAS DYNAMICS" by H. W. Lipmann and A. Roshko, published by John Wiley & Sons, Inc. copyrighted 1957 and also THE DYNAMICS AND THERMODYNAMICS OF COMPRESSIBLE FLUID FLOW, Volumes 1 and 2, by Ascher H. Shapiro, published by Ronald Press Co., New York, copyrighted 1953 and 1954.
From the foregoing description it is apparent that the invention provides an improved arc interrupting scheme in which a double-throat nozzle is utilized to maintain a supersonic flow at high discharge side to reservoir pressure ratio. The high pressure generated by an are, which ordinarily causes the clogging phenomenon, is utilized to initiate a supersonic flow and a rapid arc transfer to a longer gap. Thus, the clogging phenomenon is minimized and the effective contact opening time is shortened by the rapid transfer of the are into a longer path length. By the supersonic flow along a longer path, both the arc time constant and the dielectric recovery, and consequently the interrupting performance are improved.
I claim as my invention:
1. A gaseous-type circuit-interrupter comprising a unitary nozzle structure having a pair of spaced first and second throat-portions, each of said throatportions having a convergent-divergent nozzle configuration, said first and second throat-portions being connected by an interrupting passage having substantially a constant-area cross-section, the inner surface of the interrupting section between said two throat-portions being smoothly surfaced, said unitary nozzle structure having an entrance portion upstream of the first throatportion, means for establishing an are within the entrance portion of the first throat-portion comprising a stationary contact disposed adjacent the constriction of the first throat-portion and a movable contact movable during the opening operation upstream away from said first throat-portion, means providing a relatively highpressure region within said entrance portion and a relatively low-pressure region in the downstream exhausting portion beyond the second throat-portion of the unitary nozzle structure, means including the pressure established in conjunction with said arcing for establishing a supersonic flow of gas within said interrupting passage portion within the unitary nozzle structure between the first and second throat-portions so that a substantial axial length of the arc is subjected to supersonic-gas flow, means including a second stationary contact disposed downstream of the second throatportion for transferring said arcing from the entranceportion of the first throat-portion axially through the two spaced throat-portions and extending downstream into the downstream portion beyond the second throatportion, whereby the flow of gas through the first throat-portion will be from subsonic to supersonic flow and the flow is supersonic within said interrupting passage portion between the two spaced-throat-portions, and the gaseous flow is from supersonic to subsonic through the second throat portion, and the gas pressure increasing as the gas passes through the second throatportion for rapid arc interruption.
2. The compressed-gas circuit-interrupter of claim 1, wherein the cross-sectional area of the second throatportion is greater than the cross-sectional area of the first throat-portion.
3. The combination according to claim 1, wherein the second contact located downstream of the second throat-portion comprises a stationary rod-shaped contact located along the axis of the unitary nozzle structure.
4. The combination according to claim 1. wherein sulfur-hexafluoride (SF gas is utilized as the arcextinguishing medium.
5. A compressed-gas circuit-interrupter including means at least partially of insulation defining a doublethroated nozzle having an upstream entrance-portion, an intervening interrupting portion, extending between the two spaced throats within the double-throated nozzle and a downstream exhausting portion located downstream of the second throat, a stationary electrode disposed in the constricted portion of the first throat and cooperable with a movable electrode, means for moving the movable electrode during the opening operation in an upstream direction away from the first throat-portion to thereby open up the first throatportion of the double-throated nozzle and additionally establish arcing between the movable electrode and the stationary electrode upstream of the first throatportion, means for causing a blast of high-pressure gas to flow in a downstream direction through the first and second spaced throats in sequence, means providing transfer-contact means in said downstream exhausting portion downstream of the second throat-portion including a second stationary contact disposed downstream of the second throat-portion, means providing a supersonic flow of said gas in said intervening interrupting portion extending between the two spaced throats within the double-throated nozzle so that generally an appreciable portion of the arc length is subjected to supersonic gas flow, whereby quick are interruption is achieved.
6. The compressed-gas circuit-interrupter of claim 5, in which the stationary electrode is a ring-shaped electrode disposed in the constricted portion of the first throat.
7. The compressed-gas circuit-interrupter of claim 5, in which the transfer-contact means is constituted by a rod-shaped stationary electrode situated in the downstream exhausting portion and generally axially related to the two throat portions.
8. The combination according to claim 5, wherein the cross-sectional area of the second throat is larger than the cross-sectional area of the first throat-portion. k l