Improved circuit transformer

Abstract

 A current transformer for use with a long continuous conductor (1) comprises a core (2) having at least two legs (5) extending therefrom to form an air gap (4) through which the continuous conductor may pass. The legs (5) may be plate-like or curved. A secondary winding (3) of the transformer is positioned around the core. The surface area of the legs (5) adjacent the air gap is substantially greater than the cross-sectional area of the core.

Description

[0001] This invention relates to an improved type of current transformer particularly suitable for installation on a long length of conductor without the need to open and reclose the iron core of the current transformer around the conductor.

[0002] In accordance with the invention there is provided a current transformer for use with long continuous conductors comprising a core having at least two legs extending therefrom to form an air gap through which the continuous conductor may pass, the surface area of said legs adjacent said air gap being substantially greater than the cross-sectional area of said core.

[0003] For preference, said legs are substantially plate-like in form in the region of the air gap.

[0004] Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figures 1 and 2 show perspective views of prior art forms of current transformer;.

Figure 3 and 4 show perspective views of different embodiments of the present invention;

Figure 5 shows a perspective view of a further embodiment of the invention;

Figure 6 shows a perspective view of an embodiment of the invention using curved legs.

[0005] The prior art is described by reference to figures 1 and 2. This prior art current transformer fully encloses a continuous conductor 1 with a iron core 2 having a secondary winding 3.

[0006] It is apparent that the current transformer depicted in Figure 1 must either be slipped over a free end of the conductor, thus precluding its use on long continuous conductors, or the iron core 2 must be capable of being opened and then reclosed around the conductor 1. This may be impractical to achieve when installing some types of apparatus which incorporate current transformers.

[0007] Figure 2 depicts a current transformer which has a gap 4 in the iron core to facilitate installation on a conductor. However as discussed below the performance of this type of current transformer is markedly inferior to that depicted in Figure 1.

[0008] The performance of a current transformer depends on the reluctance of the magnetic circuit. It is desirable for this to be as low as possible.

[0009] The reluctance of the iron core in Figure 1 can be expressed by the following simplified formula.





where
R is the magnetic reluctance
K is a constant
ℓ is the mean length of the flux path around the iron core
a is the area of the iron core
µ is the relative permeability of the iron (with respect to air)

[0010] As an example if ℓ = 400mm, a =20 x 20 = 400mm², and µ = 1000 then -

[0011] For the current transformer depicted in Figure 2 the reluctance can similarly be expressed by -

where ℓ_s is the mean length of the flux path in the iron core,
where ℓ_a is the length of the air gap.

[0012] For example if ℓ_s = 360 mm and ℓ_a = 40mm with the other values the same as in the previous example then -

[0013] This value is approximately 100 times the reluctance of the closed iron core with 99% of the reluctance being across the air gap.

[0014] It should be noted that the above calculations are simplified approximations and do not take into account such factors as the spreading of the flux lines within the air gap. However it is sufficient to indicate the marked deterioration of known types of current transformers when a gap of the type is introduced.

[0015] Referring to Figure 3, the current transformer comprises an iron core 2 and a pair of plate-like leg portions 5 extending therefrom to form an air gap 4. Each leg of the iron core is spread into substantially flat plates each having a surface area "A" many times greater than the cross-sectional area of the iron core "a" with the air gap ℓ_a gap between the plates sufficient to permit entry of the conductor 1.

[0016] The benefits of this arrangement can be described by aid of an example.

[0017] The reluctance of the magnetic circuit can be expressed by -

where a is the cross-sectional area of the iron core.

[0018] And A is the surface area of the flat section of the iron on each side of the air gap.

[0019] The other quantities being as previously defined.

[0020] If for example, ℓ_s = 400 mm, ℓ_a = 40mm,
a = 400mm², and A = 120,000mm² (400 x 300) then -

[0021] In this example the reluctance is only 1.3% of that in the previous example of Figure 2 and only 33% more than the example of the closed iron core of Figure 1.

[0022] Obviously the relative values will change with different parameters in the examples. However it can be seen that this invention provides an effective current transformer which is simple to install on a conductor.

[0023] Various versions of this invention are possible, for example a current transformer consisting of a flat iron core formed into a U-shape as depicted in Figure 4. The common requirement is that the surface area of the flat section of iron 5 on each side of the air gap 4 is many times that of the cross-sectional area of the iron core 2.

[0024] A further embodiment of this invention is shown in Figure 5. In this embodiment the secondary winding 3, which consists of one or more turns of conductor with the ends of the conductor connected together to form a continuous path for electric current, is so arranged that it forms the primary circuit of a second current transformer 6.

[0025] This embodiment combines the advantages of the open current transformer as previously discussed with the convenience of winding a large number of turns on a conventional design of current transformer.

[0026] A further embodiment of the invention is shown in Figure 6. In this version the extended parts of the iron core may be curved rather than flat. However, the surface area of the iron core 5 on each side of the air gap 4 is still many times larger than the cross sectional area of the iron core 2. The air gap 4 is substantially constant and of sufficient width to allow passage of the conductor 1.

[0027] There are various applications for this invention, particularly where a current transformer is required to be installed on a high voltage conductor. Possible uses include metering, protection, and devices which obtain power from the conductor such as overhead line warning lights.

[0028] It will be appreciated by those skilled in the art that further embodiments of the invention are possible without departing from the spirit or scope of the invention described.

Claims

1. A current transformer for use with long continuous conductors comprising a core having at least two legs extending therefrom to form an air gap through which the continuous conductor may pass, the surface area of said legs adjacent said air gap being substantially greater than the cross-sectional area of said core.

2. A current transformer according to claim 1 wherein said legs are substantially plate-like in form.

3. A current transformer according to claim 2 wherein said legs have a substantially uniform spacing therebetween forming said air gap.

4. A current transformer according to claim 3 wherein said core and legs are formed from a plate substantially U-shaped in cross section, the base of the U forming the core and the sides of the U the legs.

5. A current transformer according to claim 3 wherein said legs are curved.

6. A current transformer according to anyone of the preceding claims wherein said core is provided with an electrical winding of one or more turns to form the secondary winding of said transformer.

7. A current transformer according to claim 6 wherein said winding comprises at least one continuous winding which forms the primary winding of a further current transformer.

Drawing