Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F38/00—Adaptations of transformers or inductances for specific applications or functions
  • H01F38/20—Instruments transformers
  • H01F38/22—Instruments transformers for single phase ac
  • H01F38/28—Current transformers
  • H01F38/30—Constructions

Definitions

• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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;
• Figure 5 is a transverse cross-sectional view taken along line 5-5 of Figure 2a.
• FIG. 1 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the primary 21 may be of a low number of turns.
• other forms of insulation such as gas may be used.
• other expansion means can be used.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Transformers For Measuring Instruments (AREA)

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• 1986
  • 1986-12-09 EP EP19870900012 patent/EP0285604A4/de not_active Withdrawn
  • 1986-12-09 BR BR8607231A patent/BR8607231A/pt unknown
  • 1986-12-09 WO PCT/AU1986/000377 patent/WO1987004002A1/en not_active Application Discontinuation
  • 1986-12-19 CN CN198686108680A patent/CN86108680A/zh active Pending

Patent Citations (3)

Non-Patent Citations (3)

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