GB2245704A

GB2245704A - Current sensor

Info

Publication number: GB2245704A
Application number: GB9014761A
Authority: GB
Inventors: Alan Rogers
Original assignee: GEC Alsthom Ltd
Current assignee: Alstom UK Ltd
Priority date: 1990-07-03
Filing date: 1990-07-03
Publication date: 1992-01-08
Anticipated expiration: 2010-07-03
Also published as: GB2245704B; GB9014761D0

Abstract

Apparatus for measuring a current in a conductor, comprising an optical source feeding a high birefringent optical fibre positioned adjacent to a portion of the conductor, means to apply a field produced by the current in the conductor to the fibre in a periodic manner so as to couple with the light signal therein, and means to monitor the signal when output from the fibre. One manner of applying the periodic field to the optical fibre is to provide an incomplete shield between the fibre and the conductor. An alternative method is to provide a portion of the conductor configured so as to provide spaced conductor portions each disposed at an angle to the fibre axis. <IMAGE>

Description

Current Sensor The present invention relates to apparatus which can be used to determine the value of an electric current in a conductor.

In particular, the present invention relates to a sensor which utilises Faraday rotation of a signal in a birefringent optical fibre. Faraday rotation occurs when polarised light in an optical fibre is affected by a longitudinal field such as the magnetic field produced by the passage of a current through a conductor. The effect is to cause the input polarisation of light of in a fibre to differ from the output polarisation from the fibre, such that the magnitude of the field can be determined.

Traditionally, electric current is measured with a current transformer which makes use of magnetic induction, or the change in magnetic field, to measure the current.

However, these devices tend to be large, expensive and of bandwidth which limits the range of applications where such devices are useful or cost effective. A number of optical-fibre current sensing devices has previously been proposed in which the optical fibre acts as both the current transducing element and the information transmission medium.

The optical fibre is an electrically and chemically passive device and is therefore free from electro-magnetic interference, electrical short circuit hazards, and is suitable for long haul use in inaccessible and hostile environments such as inside a sub-station or a power station.

Such optical-fibre sensors have used low birefringent fibres, which wrap around the current-carryinging conductor.

However, the accuracy of such sensors is reduced by unintentional perturbation of the fibre. Both temperature fluctuations and asymmetrical pressure imposed on the fibre will lead to noise on the output signal, constituting a major drawback in low birefringence versions of the devices.

The present idea has arisen in an attempt to provide an optical-fibre current sensor which obviates or mitigates these problems.

In accordance with the present invention, there is provided apparatus for measuring a current in a conductor, comprising an optical source feeding a high birefringent optical fibre positioned adjacent to a portion of the conductor, means to apply a field produced by the current in the conductor to the fibre in a periodic manner so as to couple resonantly with the light signal therein, and means to monitor the signal when output from the fibre.

One manner of applying the periodic field to the optical fibre is to provide an incomplete shield between the fibre and the conductor. An alternative method is to provide a portion of the conductor configured so as to provide spaced conductor portions each disposed at an angle to the fibre axis. The periodic field is applied in order that resonant coupling should take place. Resonance occurs when the spatial period of the magnetic field is equal to the birefringence beat length of the fibre.

Where a shielding arrangement is used, the shield preferably comprises a plurality of substantially regularlyor irregularly- spaced shield portions around the fibre. The shield portions can comprise spaced ferrite rings around the fibre or spaced bands of conductive paint on the fibre.

Alternatively, the shield can comprise a shield body having a plurality of substantially regularly spaced slots therein.

Where the configured conductor arrangement is used, the spacing of the conductor portions can also be substantially regular.

In both cases, the spacing is typically close to the birefringence beat length of the fibre. Within one unit, the spacing can be varied such that the range of spacings corresponds to the range of beat lengths encountered in normal use due to temperature variation. Where the conductor portions or the slots are regularly spaced, the path of the fibre can be non-linear in order to provide a suitable range of spatial period to account for temperature variation.

The source is conveniently arranged to provide a plurality of different wave lengths at around the beat length of the fibre. This can be achieved by either sweeping or jittering a narrow band source, or alternatively by providing a wide band source.

The present invention will now be described by way of example, with reference to the accompanying drawings, in which:- Figure 1 shows a diagrammatic representation of an apparatus according to all aspects of the present invention; Figures 2 to 4 show alternative manners of providing a periodic field on the fibre; and Figure 5 shows the variation of coupling efficiency against current.

Referring now Figure 1, the arrangement shown therein comprises a Spectra Physics model 1051 helium neon laser with an output wavelength of 633 nm. This provides a coherent signal to a high birefringent fibre, with wavelength measurement via a Bentham Model 300 monochromator (slit width of 0.37 mm giving 1 nm resolution). The fibre is an Andrew Corporation high birefringent fibre having an extinction ratio of 36 dB, a core of 1 micron by 2 microns and a beat length Lb of 3.8 mm. The fibre leads the signal to a periodic arrangement A via a Corman Instruments 158 WM25 1/4 wave plate, a Carl Lambrecht MGT 25A10 Glan Thompson Polariser, a x40 Kyowa Strain free objective lens and a mode stripper.

The periodic arrangement comprises a perspex base through which the fibre is directed. 44 conductors in the form of 1.5 mm brass rods are embedded in the base and are parallel and spaced at regularly intervals of 3.88 mm. The conductors lie substantially at right angles to the line of the fibre but the angle can be varied by rotation of the arrangement with respect to the fibre. The conductors are joined in series and arranged to operate at up to 10 amps RMS on 50 Hz A/C supply and have a resistance of 0.6 ohms.After passing through the periodic structure A, the fibre passes another mode stripper, a x20 objective lens and in to a final polariser/analyser which compares the output rotation with the input rotation and provides an appropriate electrical signal enabling the calculation of the current from 0 = VNI wherein 0 = the angle of rotation relative to the input N = the number of conductor passes and'I = current.

Figures 2 to 4 show alternatives to the periodic structure A. In Figure 2, the fibre has annular ferrite beads 10 spaced apart thereon to provide a periodic shielding pattern when the fibre is laid alongside a conductor (not shown). The field between the beads is shown by the arrows.

In Figure 3, the shielding is achieved by bands of conductive paint 12, which gives the periodic variation in the H field as shown in the graph below.

Figure 4 shows a further alternative in which slots 14 are provided in bus bar 16 and the fibre F is led past the slots substantially at right angles thereto.

Coupling can be affected by temperature variation causing changes in the length of the fibre and hence in the beat length Lb. This may be overcome in several ways. The laser signal source can be swept or jittered to give various wavelengths of signal which in turn will give various beat lengths. Alternatively a broad bandwidth source such as a superluminescent diode could be used. A further alternative is to provide a range of conductor spacings or to make the fibre path curve passed the conductors in a predetermined manner.

It will be appreciated that variations can be made while remaining within the scope of the invention.

Claims

1. Apparatus for measuring a current in a conductor, comprising an optical source feeding a high birefringent optical fibre positioned adjacent to a portion of the conductor, means to apply a field produced by the current in the conductor to the fibre in a periodic manner so as to couple with the light signal therein, and means to monitor the signal when output from the fibre.

2. Apparatus as claimed in claim 1, wherein the periodic field is applied to the optical fibre by means of an incomplete shield between the fibre and the conductor.

3. Apparatus as claimed in claim 1, wherein a portion of the conductor is configured so as to provide spaced conductor portions each disposed at an angle to the fibre axis to provide said periodic field.

4. Apparatus as claimed in claim 2, wherein the shield comprises a plurality of substantially regularly spaced shield portions around the fibre.

5. Apparatus as claimed in claim 4, wherein the shield portions comprise spaced ferrite rings around the fibre or spaced bands of conductive paint on the fibre.

6. Apparatus as claimed in claim 2, wherein the shield comprises a shield body having a plurality of substantially regularly spaced slots therein.

7. Apparatus as claimed in claim 3, wherein the spacings of the conductor portions are substantially regular.

8. Apparatus as claimed in claim 3 or 4, wherein a range of spacings is provided corresponding to the range of beat lengths encountered in use due to temperature variation.

9. Apparatus as claimed in claim 8, wherein the conductor portions, or shield portions are regularly spaced and the path of the fibre is non-linear with respect thereto in order to provide a range of spacings.

10. Apparatus as claimed in any preceding claim, wherein the source is arranged to provide a plurality of different wavelengths to correspond to the range of beat lengths in the fibre.

11. Apparatus as claimed in claim 10, wherein the plurality of wavelengths is achieved by sweeping or jittering a narrow band source, or by providing a wide band source.

12. Apparatus which is substantially as herein described in relation to Figure 1-5 of the accompanying drawings.