US 3611034 A
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United States Patent Inventors John J. Astlelord, Jr. Sharon; William J. Willis, Sharpsville, both of Pa. Appl. No. 887,706 Filed Dec. 23, 1969 Patented Oct. 5,1971 Assignee Westinghouse Electric Corp.
ELMI'IRICAI. TRANSFORMER 7 (Tlulmu, 2 Drawing FIRM.
317/40 R, 317/46, 337/95 Int. Cl "02h 7/04 Field of Search 317/14, 15,
 References Cited UNITED STATES PATENTS 3,152,287 10/1964 Edmunds 337/38 X 3,398,323 8/1968 Anderson 317/15 Primary Examiner-James D. Trammell Attorneys-A. T. Stratton, Donald R. Lackey and F. E.
Browder ABSTRACT: A transformer having a casing, a liquid dielectric in the casing, and an electrical winding in the casing having at least two circuits protected by a circuit breaker. The circuit breaker has contacts in each protected circuit, with the contacls in each circuit being electrically connected to a different pair of parallel-connected, like thermal responsive elements. Only one thermal-responsive element of each pair is operative to trip the circuit breaker.
BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates in general to electrical transformers, and more specifically to electrical transformers of the distribution type which include a circuit breaker.
2. Description of the Prior Art Electrical distribution transformers often include a circuit breaker within their casing, for protecting the transformer against predetermined overload conditions in the secondary or load circuit. The circuit breaker hascontacts connected in each secondary circuit of the transformer to be protected, with a thermally responsive element, such as a bimetal or trimetal, being located and connected to be responsive to both the temperature of the insulating fluid disposed in the casing, and the magnitude of the current flowing in the protected circuit.
The circuit breakers used in distribution transformers are made up of single-pole assemblies, assembled and interlocked in gangs of two or three, for single or three-phase operation, respectively. Each pole assembly has a molded, insulating case or housing, with the poles being available in different frame sizes and electrical ranges to accommodate the different kv.-a. ratings of distribution transformers. Since the cost per breaker pole increases with its electrical rating, and since the physical size of the thermally responsive element is the first factor which necessitates going to a larger frame breaker, rather than the current capability of the contacts, various arrangements have been used in the prior art to subject the thermally responsive element to only a predetermined portion of the total secondary current, while connecting the contacts of the circuit breaker to interrupt the total secondary current, instead of only the portion flowing through the thermally responsive element. This reduces the size of the thermally responsive element required, and enables smaller and less costly circuit breaker pole assemblies to be assembled to protect a transformer.
All of the prior art arrangements for reducing the amount of secondary current flowing through the thermally responsive element, while interrupting the total secondary current with the circuit breaker contacts, have certain disadvantages. For example, providing a conductor in shunt with a thermally responsive element is, in general, undesirable, as the differences in the electrical impedances of the two paths provide a large unbalance in the current division. Adding a resistor of predetermined value to the shunt circuit about the thermally responsive element, to match the impedance of the thermally responsive element, is costly, as the resistor required is neither a standard breaker nor a standard transformer element, and it adds to the assembly cost of the transformer as special mounting and insulating hardware is required.
Providing a two-part or split winding for each section of the transformer secondary, with both parts being connected to the breaker contacts, and only one part going through a thermally responsive element, such as disclosed in US. Pat. No. 2,597,185, which is assigned to the same assignee as the present application, reduces the current unbalance, but has the disadvantage of the additional cost of providing split windings.
Therefore, it would be desirable to be able to controllably and predictably reduce the amount of secondary current flowing through a thermally responsive element of a circuit breaker associated with a distribution transformer, while interrupting the total secondary current with the breaker contacts, without resorting to split winding sections and/or shunt resistors which are nonstandard to both circuit breakers and transformers, requiring special mounting and insulating hardware.
SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved transformer having a circuit breaker disposed within its casing, which for each secondary circuit to be protected includes a pair of parallel-connected, like thermal responsive elements, and breaker contact means. The breaker contact means is connected in series with a pair of parallel-connected thermally responsive elements in a secondary circuit to be protected.
The pair of thermally responsive elements for each circuit to be protected is provided by utilizing two circuit breaker pole assemblies, each having a thermally responsive element, but only one of which has breakercontacts and associated operating mechanism. Thus, one pole assembly is a conventional or active" pole, and the other is a dummyflpole, with the dummy pole supporting the thermally responsive element, insulating the element and enabling it to be mounted in a conventional manner. Since the same thermally responsive element is used in each branch of the parallel circuit, the current divides in a substantially equal manner without resorting to split secondary winding sections, allowing thermally responsive elements of one-half the normal rating to be used. Further, the dummy and active pole assemblies are assembled using standard circuit breaker items, enabling them to be ganged together in a conventional manner and mounted without special hardware and mounting procedures.
BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:
FIG. 1 is a schematic diagram of a transformer and circuit breaker arrangement constructed according to the teachings of the invention; and
FIG. 2 is a partially schematic and partially perspective view of a transformer and circuit breaker arrangement constructed according to the teachings of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, there is shown a transformer 10, having a casing shown generally at 12, which is filled to a predetermined level with a fluid insulating and cooling dielectric, such as mineral oil. Transformer [0 includes a core-coil assembly 14 disposed within casing 12 and immersed in the liquid dielectric, with the core-coil assembly 14 having primary and secondary windings l6 and 18, respectively, disposed in inductive relation with a magnetic core 20. The secondary winding 18 has first and second sections 22 and 24, respectively, which are connected serially at junction 26.
The primary winding 16 is connected to an external source of alternating potential (not shown) through insulating primary bushings 28 and 30, and the secondary winding 14 is connected to a load circuit (not shown) through a circuit breaker, shown generally at 40, and insulating secondary bushings 32, 34 and 36. The common connection or junction 26 of the first and second secondary winding portions 22 and 24 is connected via conductor 38 to secondary insulating bushing 34, which is a ground terminal, and the other ends of the first and second winding portions are connected through the circuit breaker 40 to insulating bushings 32 and 36, respectively.
Each of the secondary circuits to be protected, such as two as illustrated in this embodiment, includes contact means of a pair or set of contacts from circuit breaker 40, such as one of the contact pairs 42 and 44. One end of the first secondary winding portion 22 is connected to one of the contacts of pair 42, via conductor 46, such as the stationary contact of the pair, and one end of the second secondary winding portion 24 is connected to one of the contacts of pair 44 via conductor 48, such as the stationary contact of the pair.
The remaining contact of each pair is connected to a secondary insulating bushing through a thermally responsive element, such as a bimetal, which bimetal is operative to effect the actuation of the circuit breaker 40. More specifically, the remaining contact of pair 42, such as the movable contact, is connected to one end of a thermal responsive element 50, such as the movable end of the element, and the other end, such as the fixed end of the element, is connected to insulating bushing 32 via conductor 52. In like manner, the remaining contact of pair 44, such as the movable contact, is connected to one end of a thermally responsive element 54, and the other end is connected to insulating bushing 36 via conductor 56.
Up to this point, the described construction requires two standard circuit breaker pole assemblies, shown generally at 60 and 62, which pole assemblies are interlocked mechanically to operate together. The poles may be used with, or without magnetic trips (not shown), as desired. The thermally responsive elements 50 and 54 of breaker 40 are disposed in the insulating fluid of the transformer and are thermally actuated load-responsive elements for initiating the operation of circuit breaker 40 to disconnect the transformer secondary winding 18 from an external load circuit upon predetermined overload conditions in the secondary circuit. The operation of either thermal responsive element 50 or 54 will trip both poles 60 and 62 of the breaker 40, through trip means and the mechanical interlock. The thermally responsive elements are responsive to the temperature of the transformer cooling fluid, and also to the magnitude of the current flowing in the transformer secondary circuit.
However, with the arrangement described to this point, the total secondary current will flow through the thermally responsive elements. For example, if the transformer 10 is rated 100 kv.-a., the thermally responsive elements 50 and 54 must also each be rated 100 kv.-a.
This invention teaches how the ratings of the thermally responsive elements may be reduced by one-half, dividing the current flow into two substantially equal portions in each circuit to be protected, with a thermally responsive element being disposed to be responsive to only the current in one of the portions. Thus, the rating of the thermally responsive elements 50 and 54 may be reduced by one-half, which reduces thephysical size of the thermally responsive element and enables smaller and less costly circuit breaker pole assemblies to be utilized. Further, this result is accomplished without splitting each secondary winding section into two parts, and without introducing nonstandard parts with costly insulating and mounting hardware.
Specifically, each thermally responsive element 50 and 54 has a thermally responsive element of like rating connected in shunt therewith, using the same length and size of electrical leads for the shunt element as used to connect elements 50 and 54 into the protected circuits. Thermally responsive element 50 has a thermally responsive element 70 of like rating connected in shunt therewith, providing a parallel-connected pair of thermally responsive elements between contact pair 42 and secondary terminal 32, with each path of the parallel circuit having similar leads of like cross section and length, and similarly rated thermally responsive elements, i.e., bimetals or trimetals, to provide a substantially equal division of current through each branch of the parallel circuit, eliminating the necessity of split secondary winding sections. In like manner, thermally responsive element 54 has a thermally responsive element 72 of like rating connected in shunt therewith, providing a parallel-connected pair of thermally responsive elements between contact pair 44 and secondary terminal 36, which also provides a substantially equal division in the two branches of the parallel circuit.
Since the thermally responsive elements used to shunt the thermally responsive elements actively associated with contact pairs 42 and 44 are similar, they are standard, readily available items which are stocked along with the connecting leads of the proper size and length. Further, the use of like thermally responsive elements to reduce by one-half the current flow through the active thermally responsive elements 50 and 54 enables certain economies in both the assembly of the additional thermally responsive elements, and in the mounting of them in the transformer, to be realized, as breaker pole assemblies may be assembled which include only a thermally responsive element, eliminating the breaker contact pair and operating mechanism. The molded insulating case or housing of the additional breaker pole assemblies, shown generally at and 82, may be ganged with pole assemblies 60 and 62, i.e., their housings are disposed in aligned, contacting, side-by-side relation, thus assembling and mounting the additional thermally responsive elements in a conventional manner, which eliminates special mounting and insulating hardware, as well as special mounting procedures. The breaker pole housing supports and insulates the thermally responsive element, and yet the housing without the associated contact pair and operating mechanism has a relatively low manufacturing cost due to the high production of this item.
The advantages of using thermally responsive elements of like rating to shunt thermally responsive elements which actuate the operating mechanism of the circuit breaker may be more readily appreciated by examining FIG. 2, which is a partially schematic, partially perspective view of the transformer 10 shown in FIG. I. Like reference numerals in FIGS. 1 and 2 refer to like components.
Specifically, FIG. 2 illustrates circuit breaker 40 in an exploded perspective view, with pole assemblies 80 and 82 being of similar construction, having thermally responsive elements, such as bimetals 70 and 72 mounted in insulating housings 92 and 94, respectively. The remaining pole assemblies 60 and 62 are of like construction, having insulating housings 96 and 98, respectively, which are of the same frame size as housings 92 and 94 of poles 80 and 82, respectively, enabling them to all be ganged together for mounting within the transformer. In addition to thermally responsive elements, such as bimetals 50 and 54, breaker poles 60 and 62 each have contact pairs 42 and 44, respectively, and an operating or trip mechanism. For example, pole 60 has an operating mechanism which includes a main breaker latch 100, latch spring 102, a catch 104 on the bimetal 50 for engaging latch 100, a trip arm I06, a contact arm 108 which carries the movable contact of pair 42, a toggle link 110, a toggle spring 112, a contact opening spring 114 and a handle (not shown) interlocked with handle 116 of pole 62, via connecting pin 118. The operating mechanism of pole 60, is shown merely for purposes of example, as any suitable breaker-operating mechanism may be used. Since the operation of the breaker mechanism, for closing and opening the set of contacts, is well known in the art, such as disclosed in US. Pat. No. 2,686,242, which is assigned to the same assignee as the present application, the details of the breaker operation will not be described.
The stationary contact of pair 42 of pole 60 is connected to a flexible lead 120, which is connected to the secondary winding 18, and the circuit proceeds through contact pair 42 to a parallel circuit which includes its own associated bimetal 50, via leads 122, and then via similar leads, shown generally at 124, to bimetal 70 of adjacent pole 80. Bimetals 50 and 70 include flexible leads I26 and 128, respectively, which are connected together and to the secondary insulating bushing 32.
The stationary contact of pair 44 of pole 62 is connected to a flexible lead 130, which is connected to secondary winding 18, and the circuit proceeds through the contact pair 44 to a parallel circuit which includes its own associated bimetal 54, via leads 132 and also via similar leads, shown generally at 134, to bimetal 72 of adjacent pole 82. Bimetals 54 and 72 include flexible leads 136 and 138, respectively, which are connected together and to the secondary insulating bushing 36.
Poles 80, 60, 62 and 82 are shown arranged in the preferred order, as the two active poles may be most conveniently mechanically interlocked if mounted adjacent one another. but any other order may be used wherein the bimetals to be connected in parallel are adjacent one another, as the dummy poles do not have handles which would interfere with an interlocking pin between the two active poles.
The disclosed transformer and circuit breaker arrangement 10, has a distinct advantage over using four active poles ar ranged as illustrated in FIG. 2, beyond the obvious cost savings realized by eliminating the active elements of two of the poles. For example, if four active poles were to be used, the differences in the contact resistances from pole to pole would be such that substantially equal current division would no longer be obtainable, making it necessary to resort to split secondary winding sections, adding still further to the manufacturing cost of the apparatus. Further, four active poles would be difficult to coordinate, as the mechanical load on the trip latch would be double that experienced when using the teachings of the invention. Latch spring 102 may not be sufficient to trip four poles, and if a stronger spring is resorted to it would change the thermal sensitivity of breaker 40, as the friction between the bimetal catch 104 and latch 100 would be greater in each of the poles.
While the invention has been described relative to a singlephase transfonner having two secondary circuits to be protected, it will be obvious from the foregoing disclosure that a three-phase transfonner having three secondary circuits may be protected in a similar manner, as each secondary circuit to be protected includes a pair of parallel-connected, like thermally responsive elements connected in series with contact means, with only one of the thermally responsive elements in each circuit to be protected being operative to trip the circuit breaker.
In summary, there has been disclosed a new and improved transformer and circuit breaker arrangement which enables thermally responsive elements to be used having one-half the current rating of the transformer circuit to be protected, without resorting to split secondary winding sections and/or resistive shunt elements which are not standard to either the circuit breaker or the transformer, and which must be specially insulated and mounted with nonstandard hardware. The disclosed arrangement utilizes only standard circuit breaker and transformer components already used in circuit breaker protected transfonners, simplifying the stocking requirements and the assembly and the mounting of the circuit breaker within the transformer, and it obtains a substantially equal division of current between an active thermally responsive element and a circuit in shunt therewith, in each secondary circuit of the transformer to be protected.
Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative, and not in a limiting sense.
We claim as our invention: 1. A transformer comprising: a casing, a circuit breaker disposed in said casing, electrical winding means disposed in said casing, having at least one circuit to be protected by said circuit breaker,
said circuit breaker including a pair of parallel-connected, like thermally responsive elements, connected in series with contact means, in each of the circuits to be protected, and means for tripping the circuit breaker,
only one of the thermally responsive elements in each circuit to be protected being operative to actuate said means for tripping said circuit breaker.
2. The transformer of claim 1 wherein the electrical winding means is a single-phase secondary winding having first and second circuits to be protected.
3. The transformer of claim 2 wherein the circuit breaker includes first, second, third and fourth pole assemblies, each including an insulating housing and a thermally responsive element, and with only the second and third poles having contact means.
4. The transformer of claim 3 wherein the insulating housings of first, second, third and fourth pole assemblies are disposed in side-by-side relation, res ctivel and including means interlocking the second and t ird p0 e assemblies, to open both contact means when either is tripped by its associated thermally responsive element and the means for tripping the circuit breaker.
5. The transformer of claim 3 wherein the thermally responsive elements of the first and second pole assemblies are connected in parallel, and the thermally responsive elements of the third and fourth pole assemblies are connected in parallel, with the parallel-connected thermally responsive elements of the first and second poles being connected in series with the contact means of the second pole assembly, in the first circuit of the secondary winding, and the parallel-connected thermally responsive elements of the third and fourth pole assemblies are connected in series with the contact means of the third pole assembly, in the second circuit of the secondary winding.
6. A transformer comprising:
electrical winding means disposed in said casing; said electrical winding means having at least two electrical circuits to be protected;
a circuit breaker disposed in said casing; said circuit breaker including first and second pole assemblies for each electrical circuit of said winding means to be protected; said first and second pole assemblies each having an insulating housing and a thermally responsive element; only the first pole assembly for each electrical circuit to be protected including a set of electrical contacts, means for opening and closing the set of electrical contacts, and trip means disposed to open the set of electrical contacts in response to predetermined movement of its associated thermally responsive element;
and means connecting the thermally responsive elements of the first and second pole assemblies, and the set of electrical contacts of the first pole assembly, in each circuit to be protected, with the thermally responsive elements being in series with the set of electrical contacts, and in parallel with one another.
7. The transformer of claim 6 wherein all of the pole assemblies for the electrical circuits to be protected have their insulating housings disposed in aligned side-by-side relation, and including interlock means disposed to open the set of electrical contacts of all of the first-pole assemblies, upon the opening of any of the set of electrical contacts.