CURRENT TRANSFORMERS

Abstract: High voltage current transformers of the type having a U-shaped primary conductor (21) and a multi-turn secondary conductor (27) associated with the primary so that current can be induced in the secondary consequent on current flowing through the primary. The secondary conductor (27) is normally situated near the bridging portion (25) of the arms of the primary conductor (21) and the primary and secondary are electrically insulated from each other by insulation (23) provided therebetween. The current transformers of the invention are characterized by having the termination (31) of each arm of the primary conductor (21) supported by a respective hollow insulator (33) through which the primary passes.

Description: CURRENT TRANSFORMERS FIELD OF THE INVENTION The present invention relates to current transformers and relates particularly but not exclusively to very high voltage current transformers such as those used at electrical switch yards for monitoring high voltage current flow from the switch yards. DESCRIPTION OF THE PRIOR ART A current transformer usually comprises a primary without any coil turns or one with a small number of coil turns, and a multi-turn secondary suitably associated with said primary so that current can be induced in the secondary consequent on current flowing through the primary. In high voltage current transformers used at switch yards, the primary is a straight line conductor bent into a generally U-shaped configuration and termed in this art, "a hair pin". The secondary comprises toroids which are fitted over the arms of the primary near the bridging portion thereof; Because the secondary is connected with measuring equipment which is subject to user contact, the secondary is maintained at approximately earth potential. Typically the secondary is encased in an earthed box. Accordingly, the terminations for the primary are at the ends of the arms remote from the bridging portion and these must be physically supported and insulated from the earthed box. In the prior art, the arms of the primary may be inclined towards one another so that the spacing at the free ends is closer than the spacing at the bridging portion. Typically, this allows the toroids to be mounted on each of the arms near the bridging portion whilst maintaining a relatively small distance between the free ends where the electrical terminations are provided. A single insulator such as of porcelain is fitted over both of the arms so as to physically hold and support the free ends of the primary where the terminations are made. The cost of producing this insulator has been relatively high. The high cost has been consequent on the fact that the insulator is expensive to make being of a relatively large diameter compared to that of the toroids. Additionally, the insulator has an unnecessarily high volume and as it is filled with an electrically insulating oil and the oil is expensive, the cost has been unnecessarily high. STATEMENT OF THE INVENTION The present invention has been devised to attempt to overcome or reduce one or more of the problems of the prior art. Therefore, according to a first broad aspect of the present invention there may be provided a current transformer comprising a primary for, in use, allowing a primary current to serially flow therethrough to a load, said primary having a secondary for, in use, producing a secondary current proportional to the primary current, said primary being, in use, at a substantially higher potential than the potential of said secondary, said primary and said secondary being electrically insulated from each other by insulation therebetween, there being terminations provided for said primary, each of said terminations being supported by a respective insulator through which said primary passes. It is particularly preferred that the primary be a single conductor which has a generally U-shaped conf guration and that the secondary is situated near the bridging portion of the arms thereof. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention can be more clearly ascertained a preferred embodiment will now be described with reference to the accompanying drawings, wherein:- Figure 1 is a view of a prior art high voltage current transformer; Figure 2a is a view of a preferred current transformer according to the present invention; Figure 2b is an end view of the current transformer shown in Figure 2a; Figure 3 is a top perspective view of the current transformer shown in Figures 2a and 2b; Figure 4 is a transverse cross-sectional view taken along line 4-4 of Figure 1; and Figure 5 is a transverse cross-sectional view taken along line 5-5 of Figure 2a. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring firstly to Figure 1 which shows a prior art current transformer, it can be seen that it contains a primary 1 which has a generally U-shaped or "hair pin" conf guration. The primary 1 comprises an electrical conductor of approximately constant cross-section. A graded insulation 3 is wound over the primary 1 so as to provide a plurality of stepped layers of graded insulation 3 which has the maximum number of layers at the bottom 5 of the primary 1. One or more secondary windings 7 in the form of toroids are situated at the bottom 5 of the primary 1 where the graded ■ insulation 3 has its maximum thickness. The secondaries 7 are placed within a tank 9 which is maintained at earth potential. The secondaries 7 are also maintained at earth potential or approximately so. The graded insulation 3 has sufficient insulation characteristics at the bottom 5 of the primary 1 to electrically isolate the high voltage of the primary 1 from the secondaries 7. Terminations for the secondaries 7 have not been shown, but these are terminated at the tank 9. Terminations for the primary 1 are shown by numeral 11. The terminations 11 are held at a sufficient distance from the tank 9 by locating with an insulator 13 such as of porcelain which completely surrounds the arm portions of the primary 1. The insulator 13 is fitted to the top of the tank 9 and secured thereto by means not shown. The insulator 13 is of circular cross-section and is usually frusto-conical configuration. The insulator 13 has a top housing 15 attached thereto and- an oil expansion means 17 such as a membrane is fitted therein. In use, the tank 9 and the insulator 13 are filled with an electrical insulating oil or other fluids or other insulating mediums. A high voltage line such as 362kv line is attached to the respective terminals 11 so that current can flow serially through the primary 1 to a load. In this manner, the current flowing in the primary 1 will induce a current in the secondary 7 proportional to that in the primary 1. The cost of the prior art transformer shown in Figure 1 is unnecessarily high. A major contribution to the high cost is that the insulator 13 is difficult to make. This is because it is very large, i.e. being approximately three metres in height and approximately 70 cm in diameter. Furthermore the volume of oil which is needed to fill the current transformer is larger than necessary because the oil has to fill the space within the insulator 13 which surrounds the graded insulation 3 on the primary 1. This space is quite considerable, in the region between the arms of the primary 1 as will be explained later. Referring now to Figures 2a and 2b and 3, there is shown a preferred embodiment of the present invention, suitable for use as a 362kv current transformer. The preferred current transformer has a primary 21. The primary 21 is of generally U-shaped "hair pin" configuration similar to that in the prior art. The primary 21 comprises a single conductor of approximately constant cross-section throughout its length. A graded insulation 23 is provided on the primary 21. The graded insulation 23 is of the same construction as in the prior art having a multi-layered arrangement which increases in the number of layers towards the bottom 25 of the primary 21. The insulation of the primary 21 may however take other forms, including for example epoxy impregnated, cast epoxy or other insulation systems. The graded insulation is preferably applied in a continuous and unbroken manner. Secondary windings 27 are provided over the primary 21 at the bottom 25. The secondaries 27 are toroids which are, in use, held at earth^potential or approximately so. The secondaries 27 are held in a tank 29 which is maintained at earth potential. Terminations 31 for the primary 21 are provided at the top end of the primary 21 and are spaced and held in that position by two insulators 33 such as of porcelain. The insulators 33 are .cylindrical in nature and are fixed to the top of the tank 29 so that one arm of the primary 21 s held within one of the insulators 33 and the other arm of the primary 21 is held within the other of the insulators 33. Top housings 15 are provided on each of the insulators 33 and these, in turn, contain a respective oil expansion means 37 such as a membrane. Electrical insulating medium such as oil is placed within the tank 29 and arranged so that it will fill the insulators 33 and position itself underneath the expansion means 37 as shown. A secondary terminal box 39 is provided on the tank 29. A spacer bar 41 is provided between the upper portions of the insulators 33 so as to inhib t the arms of the primary 21 from moving apart under the influence of heavy currents flowing through the primary conductor 31. By referring to Figures 4 and 5, it can be seen that considerable savings in oil can be made with the present invention. If Figure 4 is viewed it can be seen there are large areas designated by numerals 43 between the graded insulation 3 of the arms of the primary and the internal surfaces of the walls of the insulator 13. Accordingly, this considerable volume of oil is unnecessary and by observing Figure 5 it can be seen that it is now dispensed with, as the internal diameter of the insulators 33 is only marginally larger than the external diameter of the graded insulation 23. Accordingly, the use of a large volume of oil as in the prior art is now avoided. Most importantly the diameter of the insulators 33 is now considerably smaller than in the prior art where the insulator 13 had to have an internal diameter at least sufficient to take in both arms of the primary 1 with the graded insulation 3 thereon. Because the diameter is reduced, there can be a considerable saving in the manufacturing cost of the insulators 33. Furthermore, it is noted that the insulators 33 are parallel cylinders and not tapered cylinders as in examples of the prior art. Accordingly, insulators 33 can be manufactured less expensively than the prior art insulators 13. Additionally, the insulators 33 can be made in small lengths and arranged to stack one of top of the other. In this way a plurality of small length insulators 33 can be produced to provide the required electrical insulation for a particular voltage current transformer without the need to make special insulators 13 for specific voltage current transformers as in the prior art. Here, one simply adds the required number of individual insulators on top of each other to provide the required spacing. It should be appreciated that the width of the transformer as a whole is now considerably reduced over that of the prior art construction shown in Figure 1. This, in turn, means that the current transformers take up less space in a switch yard than in the past. Further, it is possible to easily mechanize application of the graded insulation in a continuous and unbroken manner, due to the fact that the arms of the primary 21 can be made parallel and can have a relatively large radius at the bottom, as the spacing apart of the legs of the primary conductor does not have to be maintained as small as possible as in the prior art where both arms have to be received within the insulator 13. If the arms of the primary 21 are spaced a considerable distance apart in the present invention, then the only extra penalty is the volume of oil which is in the earth tank 29. With the construction shown it is possible to pre-assemble the secondary 27 under the cover plate 45 and then simply fit it over the primary 21 after the primary 21 has been insulated and assembled in tank 29. Preassembly of secondary cores 27- on the underside of tank cover plate 45 is facilitated by the parallel orientation of the arms of the primary 21. Modifications may be made to the invention as would be apparent to persons skilled in the electrical transformer arts. For example, the primary 21 may be of a low number of turns. Instead of using insulating oil within tank 29 and insulators 33, other forms of insulation such as gas may be used. Additionally instead of using a membrane 37 for expansion control of the insulating medium, other expansion means can be used. These and other modifications may be made without departing from the ambit of the invention, the nature of which is to be determined from a foregoing description and the appended claims.

Claims: CLAIMS : 1. A current transformer comprising a primary for, in use, allowing a primary current to serially flow therethrough to a load, said primary having a secondary for, in use, producing a secondary current proportional to the primary current, said primary being, in use, at a substantially higher potential than the potential of said secondary, said primary and said secondary being electrically insulated from each other by insulation therebetween, there being terminations provided for said primary, each of said terminations being supported by a respective insulator through which said primary passes. 2. A current transformer according to Claim 1, wherein said primary is a single conductor having a generally U-shaped configuration with two arms extending to said primary terminations, and wherein said secondary is situated adjacent a bridging portion of said arms. 3. A current transformer according to Claim 1 or Claim 2, wherein said insulation between the primary and the secondary is a graded insulation applied in a continuous unbroken manner. 4. A current transformer according to Claim 2 or Claim 3, wherein said two primary arms are substantially parallel. 5. A current transformer according to anyone of the preceding claims, wherein each respective insulator is of substantially uniform transverse cross-section throughout its length. 6. A current transformer according to Claim 5, wherein said insulators have a substantially circular transverse cross-section. 7. A current transformer according to Claim 5 or Claim 6, wherein each respective insulator comprises a plurality of insulators fitted end to end and wherein said primary passes through each of said insulators. 8. A current transformer according to anyone of the preceding claims wherein said secondary is housed within a casing. 9. A current transformer according to Claim 8, wherein said casing is substantially at earth potential. 10. A current transformer according to Claim 8 or Claim 9 , wherein said insulators are mounted on said casing. 11. A current transformer according to anyone of Claims 8 to 10, wherein said casing is a tank having a cover on which said insulators are mounted. 12. A current transformer according to Claim 11, wherein said secondary comprises one or more toroidal windings fitted onto said primary so that the primary extends through the central opening in each toroid. 13. A current transformer according to Claim 12, wherein said secondary is mounted on the underside of said tank cover. WO 87/04002 AMENDED CLAIMS PCT/AU86/00377 [received by the International Bureau on 6 May 1987 (06.05.87); original claims 1-13 replaced by new claims 1-12 (2 pages)] CLAIMS : 1. A dedicated current transformer comprising a primary for, in use, allowing a primary current to serially flow therethrough to a load, said primary having a secondary for, in use, producing a secondary current proportional to the primary current, said primary being, in use, at a substantially higher potential than the potential of said secondary, said secondary being at or substantially at ground potential, said primary and said secondary being electrically insulated from each other by insulation therebetween, there being terminations provided for said primary, each of said " terminations- being supported by a respective insulator through which said primary passes. 2. A current transformer according to Claim 1, wherein said primary is a single conductor having a generally ϋ-shaped configuration with two arms extending to said primary termina¬ tions, and wherein said secondary is .situated adjacent a ~ bridging portion of .said arms. _. ' --- " - _ ; 3. A current transformer according to Claim 1 or Claim 2, wherein said insulation between the primary and the secondary is a graded insulation applied in a continuous unbroken manner. 4. A current transformer according to Claim 2 or Claim 3, wherein said two primary arms are substantially parallel. 5. A current trans ormer according to any one of the preceding claims, wherein each respective insulator is of substantially uniform transverse cross-section throughout its length. 6. A current transformer according to Claim 5, wherein said insulators have a substantially circular transverse cross-section. 7. A current transformer according to Claim 5 or Claim 6, wherein each respective insulator comprises a plurality of insulators fitted end to end and wherein said primary passes through each of said insulators. 8. A current transformer according to any one of the preceding claims wherein said secondary is housed within a casing. 9. A current transformer according to Claim 8 , wherein said insulators are mounted on said casing. 10. A current transformer according to Claim 8 or Claim 9, wherein said casing is a tank having a cover on which said insulators are mounted. 11. A current transformer according to Claim 10, wherein said secondary comprises one or more toroidal windings fitted onto said primary so that the primary extends through the central opening in each toroid. 12. A current transformer according to Claim 11, wherein " said secondary is mounted on the underside of said tank cover. STATEMENT UNDER ARTICLE 19 The claims have been replaced with new claims 1 through 12. New claim 1 is a combination of former claims 1 and 9. It also includes a limitation in defining that the current transformer is a dedicated current transformer. Thus, new claim 1 now specifically recites that the current transformer is a dedicated current transformer, and that the secondary fs at or substantially at ground potential . The effect of the amendment is to make it clear that the invention is applicable only to a particular class of dedicated current transformers, i.e., those which are known in the art as suitable for operating with extremely high voltages, i.e., in the order of about 500 k.v.

Inventors: LEE ANTHONY, LEE, ANTHONY
Applicants: TYREE ELECTRICAL, TYREE ELECTRICAL COMPANY PTY. LIMITED
Application year: 1986
Main classification: H01F40
IPC codes: H01F38