GB2030785A - Transformer having a thermally sensitive protective device - Google Patents

Abstract

An oil-filled transformer which includes a current-limiting fuse 46 connected in series with the primary winding also includes a spring-loaded thermally triggered shorting mechanism which at a predetermined temperature shorts out the winding, thus producing an excessive current which causes the fuse to blow, to disconnect the transformer from the power source. As shown, on the melting of a disc 70 a spring-driven plunger is released and engages a contact 72. This completes a conducting path comprising terminal 28, contact 72, end 64 spring 60, conducting sleeve 50', screw 56a, screws 56b and nut 68', thus effecting a path between input terminals 26 and 31, and so short-circuiting the winding which is across terminals 28, 30'. A modified arrangement (Figs. 1, 6, 7,) is also disclosed. <IMAGE>

Description

SPECIFICATION Thermally sensitive protective device for a transformer This invention relates to oil-filled transformers, and in particular to transformers having protective means therein for sectionalizing the transformer from power a source.

It is known in the prior art to utilize currentlimiting fuses in conjunction with transformers for protection from overcurrent. Furthermore, it is known to dispose the current-limiting fuses internal to the transformer within the high-voltage bushing of the transformer. Such an arrangement is disclosed in the specification of U.S. Patent No. 3,659,244. Further, it is known in the prior art to provide transformers which are pad-mounted or pole-mounted distribution transformers having transformer oil disposed therein. The transformer oil generally surrounds the transformer winding and provides a cooling, insulating and/or arc-resisting function. Furthermore, it is known to supply such a transformer with a secondary winding circuit breaker disposed within the oil of the transformer. Such an apparatus is disclosed in the specification of U.S. Patent No.

3,983,454. It would be advantageous if a pad-mounted or pole-mounted distribution transformer could be provided with apparatus for sectionalizing the transformer from the high-voltage line in the event that the transformer oil attained a certain predetermined relatively high temperature. The attainment of the aforementioned temperature would be an indication that a problem existed within the transformer enclosure. The problem may be associated with a relatively high fault current, an overload, or a low energy turn-to-turn fault in the winding. Any of these situations could lead to high oil temperature, and possible destruction of the transformer, thus necessitating protection of some sort. It would be further advantageous if the protective device was relatively simple and inexpensive.

According to the present invention, a transformer comprises an enclosure, first high voltage terminal communicating between the outside and inside of said enclosure, second high voltage terminal communicating between the outside and inside of said enclosure, primary winding means disposed within said enclosure, one terminal of said primary winding means being interconnected with said first high voltage terminal, transformer fluid disposed within said enclosure, fuse member disposed to interconnect said second high voltage terminal and another terminal of said primary winding means, and shorting means disposed electrically between said one terminal and said another terminal of said primary winding means, said shorting means having a movable member which is maintained in an opened position by thermally sensitive material disposed in said fluid until said fluid attains a predetermined temperature in which case said thermally sensitive material changes state to thus allow said movable member to move to a closed position in which said primary winding means is electrically bypassed thereby thus allowing sufficient current to flow through the series combination of said movable member and said fuse means to blow said fuse means thus sectionalizing said transformer primary winding from said first high voltage terminal means.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows an orthogonal view of a pole type distribution transformer partially cut away, with the primary winding, secondary winding and sectionalizing device thereof, shown in schematic form.

Figure 2 shows another embodiment of the invention similar to the apparatus of Fig. 1.

Figure 3 shows a side elevation of a protective device such as shown in Fig. 2, partially broken away and partially shown in schematic or diagrammatic form.

Figure 4 shows a sectional view of the protective device of Fig. 3 at the Section IV-IV.

Figure 5 shows the apparatus of Fig. 3 and Fig. 4 in the shorted disposition.

Figure 6 shows a side elevation of the protective device shown in Fig. 1 partially broken away and partially in schematic form.

Figure 7shows the apparatus of Fig. 6 in a shorted disposition.

Figure 8 shows a double sectionalizing device.

Fig. 1 shows a pole-mounted oil-filled distribution transformer 10 having a cylindrical metallic enclosure 1 2 which is closed off at the top with a lid 1 3. Protruding upward from the lid 1 3 are two transformer bushings 14 and 1 6. Appropriately disposed within the enclosure is a schematically shown transformer assembly 1 7 which may have a magnetically conductive core CO around which is wound a primary winding P and a secondary winding S. The secondary winding S may have three leads or taps A', B' and C'. Conductor B' may comprise a center tap for the secondary winding S. The leads or conductors A', B' and C' are connected to external lowvoltage terminals A, B and C disposed on the outer cylindrical shell of the enclosure 1 2.

This arrangement is not shown in the interconnected state for the purpose of simplicity of illustration. The conductors for the primary winding P of the transformer 1 7 may be interconnected with a thermally sensitive sectionalizing device 1 8. The protective portions of the thermally sensitive sectionalizing device 1 8 may include a current-limiting device 20 and a latchable thermally sensitive shorting device 22. The current-limiting fuse or device 20 may be integrated with the thermally sen sitive shorting device 22 by way of a common combination support and conductive member 24.There may be provided three termi nals-26, 28 and 30-for the thermally sen sitive sectionalizing device 1 8. The first termi nal 26 which is exclusive to the current limiting device or fuse 20 may be intercon nected with a high-voltage line 40 by way of a conductor 32 which is interconnected with a conductive portion of the bushing arrange ment 14. The terminal 28 may be disposed on the common member 24 for the thermally sensitive sectionalizing device 1 8. The termi nal 30 may be interconnected with another high-voltage conductor 42 by way of a line or conductor 34 which is connected to the con ducting part of the bushing arrangement 1 6.

In a preferred embodiment of the invention, line 40 is interconnected with a high-voltage bus, conductor or wire, while line 42 is inter connected with ground thus allowing a substantially high voltage to be developed between the bushings 14 and 16. In other situations the line 42 may be of the higher voltage while the line 40 is interconnected with ground. Alternatively, lines 40 and 42 may be interconnected with relatively higher voltages or relatively lower voltages. In still another situation, terminal or bushing 1 6 may be missing and line 34 may be interconnected with line 42 through a common connection on the cylindrical enclosure 12, for example.

This situation would exist when the line 42 is connected to substantially ground potential. In the preferred embodiment of the invention, a fluid such as transformer oil is disposed within the cylindrical enclosure 1 2 to a level 38, so as to submerge the transformer winding 1 7 and at least the thermally sensitive portion of the thermally sensitive sectionalizing device 18. The transformer oil 38 may perform the traditional functions associated therewith of insulation, cooling and arc suppression. In the embodiment of the invention, depicted in Fig.

1, the primary winding P of the transformer 1 7 is connected between the terminals 28 and 30 of the thermally sensitive sectionalizing device 1 8. Furthermore, the lead 34 is also connected to the terminal 30. The interaction of portions of the thermally sensitive sectionalizing device 1 8 and the winding P of the transformer 1 7 and the oil 36 will be decribed hereinafter with respect to other Figures.

Referring now to Fig. 2, another embodiment 10' of the invention is shown. This embodiment is similar to the embodiment shown in Fig. 1 to the extent that like reference symbols refer to like elements in both embodiments. In this embodiment of the invention, however, the thermally sensitive sectionalizing device 18' differs from the thermally sensitive sectionalizing device 1 8 of Fig.

1. The current-limiting portion 20, its terminal 26, the common conductive portion 24 and the terminal 28 are essentially the same, or may be the same, in both embodiments. However, the latchable shorting device 22' of embodiment 10' differs from the shorting device 22 of embodiment 1 0. In this case, a single terminal 30' is interconnected only with the primary winding P. A new terminal 31 is provided on another portion of the shorting device 22' so that electrical current which normally flows in the primary winding P must traverse a portion of the shorting device 22'.

The operation of the embodiment 10' will be described hereinafter with respect to other drawings.

Referring now to Figs. 3 and 4, the construction and operation of the thermally sensitive sectionalizing device 18' of embodiment 10' shown in Fig. 2 is set forth in detail. In particular, the thermally sensitive device 1 8 includes the current-limiting section or fuse 20 on the left, as viewed in Fig. 3. The shorting device 22' is shown on the right in Fig. 3. The leftmost terminal 26 for the current-limiting device or fuse 20 is interconnected with a fuse ferrule 26a. Fuse ferrule 26a in turn is supported on a fuse barrel 44 which is supported at its other end by the common terminal 24. Disposed between the ferrule 26a and the common terminal 24 or, said in another way, the terminals 26 and 28, is a current-limiting fuse element 46.The latter element is disposed within a pulverulent arc-quenching material 48 and is protected or enclosed by the insulating tubular member 44. On the right of Fig. 3, the shorting portion 22' may comprise a tubular cylindrical insulating member 52' which is threaded into an appropriate tapped opening 54 in the common terminal 24. An elongated electrically conducting tubular member 50' may be threaded or otherwise secured to the internal portion of the member 52' and may extend to the right where it is threaded or otherwise secured to an internal portion of a cylindrical insulating member 56 at the region 57. The previously described terminal 31 may be disposed on an arcuate curvilinear ccnductive pad 31a which is disposed upon the end member 56 by way of a complementary threaded screw or bolt 56b. In a like manner, terminal 30' may be disposed upon a curvilinear arcuate electrically conductive pad 30'a which in turn is secured to the cylindrical end member 56 by way of another complementary threaded member or bolt 56b. On the right portion of the insulating tube 50' is diposed an inwardly protruding lip member 58a. The lip member 58a provides a seat for the right end of a compression spring 60, the left end of which abuts against an appropriately sized circular flange in a conductive metal end piece 64. The conductive metal end piece 64 is secured to an insulating rod 62' which has a bolt or flange member 68' threaded or other wise secured to the other end thereof. Disposed against the right portion of the end member 56 may be a washer-like member 70'.Member 70' may comprise thermally and electrically sensitive material which has the characteristic of melting of otherwise deteriorating at or near a predetermined temperature, and which may have the characteristic of melting or otherwise deteriorating under the influence of the heat of the electrical current which is conducted thereby. Member 70' in the preferred embodiment of the invention has a central opening or hole 71 through which the rod 62' protrudes, but which is not sufficiently large to allow the flange or nut member 68' to fit therethrough. The nut member 68', therefore, is trapped or latched against the outside of the member 70' on the right, as viewed in Fig. 3 so as to maintain the spaced flanged member 64 in a fixed position relative thereto.This maintains or latches the compression spring 60 in a compressed state between the flange member 64 and the lip member 58a. There is also provided a flexible terminal member 72 which is interconnected with the common terminal 24 so that electrical contact may be made between the terminal member 24 by way of the flexible terminal 72 with the conductive end cap 64 in appropriate circumstances which will be described in more detail hereinafter with respect to Fig. 5. In the preferred embodiment of the invention, the main path for the electrical current 11 during a nonshorting state is through terminal 26, ferrule 26a, the main fuse element 46, the common terminal member 24 and the terminal 28 to one side of the winding P (not shown in Fig. 3).

Thence, current 11 flows to the other side of the winding P and the terminal 30'. From that point the current 11 flows through the pad member 30'a and into the thermally sensitive conductive material 70' and thence through the other pad 1 a to the terminal 31. In the preferred embodiment of the invention, the tubular member 50' is made of electrically conducting material and the end members 51' and 56 are made of electrically insulating material. Threaded screw or bolt 56a communicates with the pad member 31a and the conducting tubular memver 50'.

Referring now to Fig. 5, in conjunction with Figs. 3 and 4, the disposition of the sectionalizer 1 8 is shown when the thermally sensitive shorting portion 22' is in a shorting state. In this case, it is presumed that the recent condition of the electrical current 11 or the immediate temperature is such that the temperature sensitive member 70' has been melted, thus unlatching the member 68' and allowing the insulating shorting rod 62' to move to the left under the influence of the relaxing compression spring 60, so that the flexible terminal 72 and the conducting end flange 64 interconnect electrically.In this situation, the current (now identified as 12) flows through a low impedance path including the common terminal 24, the flexible terminal 72, the end piece 64, the spring 60, the electrically conductive screw or screws 56a, a portion of the pad 30a, the electrically conductive screw 56b, the electrically conductive flange member 68', the other electrically conductive screw 56b, the pad 31 a and the terminal 31. The latter path is chosen so as to have significantly less impedance than the primary winding P (not shown). This essentially short-circuits the primary winding P, thus causing the current 12 to increase appreciably, thus forcing the current-limiting fuse section 46 to blow or fuse.

The net result of this is to sectionalize the transformer from the high-voltage line. It will be noted that the flexible terminal member 74 provides electrical continuity in the manner previously described even though the arrangement of the flange or bolt member 68' on the insulating rod 62 relative to the disposition of the conductive members 56b may vary slightly from one device to another. An important feature associated with the embodiment outlined in Figs. 3, 4 and 5 is the fact that the thermally sensitive material 70' also conducts the primary current of an operating transformer.

Referring now to Fig. 6, the sectionalizing device 1 8 associated with embodiment 10 as shown in Fig. 1 is depicted again in greater detail. In this embodiment of the invention, as in the last, the current-limiting section 20 comprises a tubular barrel of electrically insulating material 44 disposed between a ferrule 26a on the left, as shown in Fig. 6, and the common ferrule 24 on the right, as shown in Fig. 6. As was the case previously, the terminals 26, 28 are interconnected with the ferrules 26a and 24, respectively. Furthermore, disposed within the enclosure of the insulating member 44 is a current-limiting fuse ribbon 46 which may be surrounded by pulverulent arc-quenching material 48.In this embodiment of the invention, the shorting section 22 comprises an electrical insulating tubular cylindrical member 52 which is threaded into the common terminal 24 at the complementary threaded opening 54. The insulating member 52 has threaded into the internal portion of the other end thereof an electrically conductive member 50 which is hollow and cylindrical. Electrically conductive member 50 has a lip portion 58 at the right end thereof against which rests or resides a thermally sensitive latching member 70 having a central opening therein through which a conductive shorting rod 62a protrudes. The conductive shorting rod 62a has a circumferentially disposed flange member 64 which is utilized to compress a compression spring 60 against a portion of the lip 58 when an end flange 68 of the rod 62a is latched or trapped against the latch member 70.There is a shorting portion 60 or a terminal portion 64a at the left end of the shorting rod 62a which is adapted to make electrical contact with a portion of the common terminal 24 for shorting out the primary winding P (not shown) which is interconnected between the terminals 28 and 30. In a normal operating condition, the path of electrical current 13 is through the terminal 26, the ferrule 26a, the fuse ribbon 46, the common terminal 24, the terminal 28, the primary winding P (not shown), the terminal 30 and the conductor 34. It will be noted in this case that the thermally sensitive material 70, although in a disposition to be effected by environmental heat, does not conduct the current 13.

Referring now to Fig. 7, the apparatus 18 is shown in a shorting disposition. In this case the end member 64a abuts against a portion of the terminal 24. Current 14 proceeds as is shown in Fig. 6 to the common terminal 24.

At that point current 14 is diverted through the relatively low impedance of the left portion of the conducting shorting rod 62a, and thence through the circular flange 64 to the now-relaxed compression spring 60, and thence to the terminal 30, and from there to the line or lead 34.

With the respect to the embodiments shown in all of the Figures, it is expected that at least that portion of the sectionalizing device associated with the thermally sensitive material 70 is disposed within the transformer oil so that as the transformer oil becomes hot due to an abnormal condition such as interwinding short-circuiting, overload or the like, the thermally sensitive material will melt, thus unlatching the shorting apparatus, thus shortcircuiting the relatively high impedance of the primary winding P, thus deliberately overloading the current-limiting fuse ribbon 46, thus blowing that fuse ribbon 46 and sectionalizing the windings of the transformer from the highvoltage line or lines.In one embodiment of the invention, as best shown in Fig. 3, the electrical current is conducted by the thermally sensitive latching member 70' so that its heat may be sufficient to melt the latching member even if the heat of the transformer oil is not sufficient to do so. In experiments, the thermally sensitive material has been chosen so as to melt at 150"C in one situation, or at 293"F in another situation. Neither of these temperatures is limiting.

Referring now to Fig. 8, another embodiment of the invention is shown. The sectionalizing apparatus 18" of Fig. 8, is similar to the sectionalizing apparatus 18' of Fig. 2 except that a second current-limiting fuse portion 20a is added to the right thereof so that under appropriate circumstances both ends of the primary winding P may be isolated from the lines 32 and 34 so that the entire winding P is free of contact with either the highvoltage lead 40 or the high-voltage lead 42, assuming that both are considered to be at high-voltage potentials. In this case, the right end of the winding P as shown in Fig. 8, is connected to a terminal 33a which may be part of a second common terminal 24a which interconnects the right portion of the shorting device 22 with a second or right currentlimiting portion 20a.

It is to be understood with respect to the embodiments of this invention that the orientation of the devices 1 8 and 18', for example, shown in the Figures, may be reverse so that the current-limiting portion is on the right and the shorting portion is on the left under appropriate voltage circumstances for the conductors 40 and 42. It is also to be understood that the primary winding P may have the higher or the lower of the voltages relative to the secondary winding impressed thereacross in appropriate circumstances, depending upon the function of the transformer. It is also to be understood that the center tap B' may be eliminated in some embodiments of the invention.It is also to be understood that the sectionalizing members 18, 18' and 18", for example, may be appropriately supported within the enclosure 12, either on a portion of the transformer 1 7 or otherwise. It is also to be understood that the teachings with respect to the pole-mounted transformers 10 and 10', for example, apply equally to pad-mounted transformers, or to any transformer in which it is desirous to sectionalize in the event that the heat within the transformer, for example, is determined to be excessive. It is also to be understood that the transformer is not limited to single phases but may be utilized for multiphase or polyphase transformers. It is also to be understood that an access bore hole may be placed in the enclosure 1 2 for servicing the temperature sensitive device.Alternatively, at least a portion of the temperature sensitive device may be disposed in a bushing.

The apparatus taught with respect to the embodiments of this invention have many advantages. One advantage lies in the fact that sectionalization may take place in response to the heating of the transformer oil in advance of the occurrence of a more serious problem, which in turn may be related to an overload in the transformer windings, or an interwinding short circuit. Another advantage lies in the fact that all of the aforementioned may be done internal to the transformer enclosure without necessitating any separate communication between the outside and inside thereof. Another advantage lies in the fact that the choice of the thermally sensitive material for the restraining disc 70 or 70', for example, may be such as to provide a sectionalizing effect at almost any preset temperature level for the transformer oil up to the flash point of that oil. Another advantage lies in the fact that the sectionalizing device may be relatively small. The shorting portion 22 and 22', for example, may be 3" in length by 1" in diameter, and its cost relative to the current-limiting portion may be no more than two thirds. Another advantage lies in the fact that the actual interrupting device need only be a "backup" type of current-limiting fuse rather than the generally larger and more expensive "general purpose" type. Another advantage lies in the fact that in the event of a highcurrent fault, the current-limiting fuse will operate independent of the device operation, thus providing further versatility in the protective aspects of the apparatus.

Claims (8)

1. A transformer comprising an enclosure, first high voltage terminal communicating between the outside and inside of said enclosure, second high voltage terminal communicating between the outside and inside of said enclosure, primary winding means disposed within said enclosure, one terminal of said primary winding means being interconnected with said first high voltage terminal, transformer fluid disposed within said enclosure, fuse member disposed to interconnect said second high voltage terminal and another terminal of said primary winding means, and shorting means disposed electrically between said one terminal and said another terminal of said primary winding means, said shorting means having a movable member which is maintained in an opened position by thermally sensitive material disposed in said fluid until said fluid attains a predetermined temperature in which case said thermally sensitive material changes state to thus allow said movable member to move to a closed position in which said primary winding means is electrically bypassed thereby thus allowing sufficient current to flow through the series combination of said movable member and said fuse means to blow said fuse means thus sectionalizing said transformer primary winding from said first high voltage terminal means.

2. A transformer as claimed in claim 1 wherein said thermally sensitive material is disposed to additionally conduct the current of said primary winding means so that said ther mally sensitive material may also change state when said current exceeds a predetermined value which is related to the temperature necessary to cause said thermally sensitive material to change state to thus sectionalize said transformer primary winding.

3. A transformer as claimed in claim 1 or 2 wherein said fluid comprises transformer oil.

4. A transformer as claimed in any one of claims 1 to 3 wherein said fuse means com prises a current limiting fuse.

5. A transformer as claimed in any one of claims 1 to 4 wherein said thermally sensitive material comprises fusible material. said ther mally sensitive material comprises fusible material.

6. A transformer, as claimed in any one of claims 1 to 5 in which a ground terminal is disposed on said enclosure and communicating between the inside and outside thereof, the high voltage terminal disposed on said enclosure and communicating between the inside and outside thereof, the the high voltage primary winding means is disposed within said enclosure in a bath of transformer oil and one terminal thereof being connected to said ground terminal, the fuse member being disposed between said high voltage terminal and another terminal of said high voltage primary winding means, and a spring loaded movable grounding rod disposed in a charged disposition between said one terminal and said another terminal by a thermally sensitive latch until the temperature of such oil exceeds a predetermined value in which case said thermally sensitive latch melts thus freeing said grounding rod to short circuit said primary winding means and thus conduct sufficient current through said current limiting fuse to blow said fuse and sectionalize said high voltage primary winding from said high voltage terminal means.

7. A transformer as claimed in claim 6 wherein said thermally sensitive latch comprises an additional fuse member connected in series between said ground terminal and said one terminal to conduct current flowing in said primary winding means until said current reaches a value sufficient to blow said additional fuse member to thus cause said movable grounding rod to move to a disposition to cause said current limiting fuse member to blow to thus sectionalize said primary winding means from said high voltage terminal member.

8. A transformer, constructed and adapted for use, substantially as hereinbefore described and illustrated with reference to the accompanying drawings.