GB2152681A

GB2152681A - Device for detecting current flowing in a conductor

Info

Publication number: GB2152681A
Application number: GB08500901A
Authority: GB
Inventors: Robert Stuart Thorogood
Original assignee: Brush Switchgear Ltd
Current assignee: Brush Switchgear Ltd
Priority date: 1984-01-13
Filing date: 1985-01-14
Publication date: 1985-08-07
Also published as: GB2152681B; GB8500901D0

Abstract

A device for detecting current flowing in a conductor 10, Fig. 1, includes at least one turn of delay- line cable 11 wound in a closed path around the conductor in spaced relation thereto. One end of the inner helical conductor 13, Fig. 2, of the cable is connected to the corresponding end of the outer conductor, and an e.m.f. induced in the inner conductor by current flowing in said first-mentioned conductor 10 is detected 17 at the other end of the cable between the inner and outer conductors. Inner conductor 13 is wound on a non- magnetic core 12. <IMAGE>

Description

SPECIFICATION Device for detecting the current flowing in a conductor This invention relates to a device for detecting the current flowing in a conductor.

Normally, it is possible to detect and measure the current flowing in a conductor directly, for example, by connecting an ammeter in series with the conductor. On other occasions however, direct detection and measurement is not possible owing perhaps to the high magnitude of the current flow or the high voltage applied to the conductor.

One instance where current flow detection and measurement has to be performed directly lies in circuit breakers for electrical distribution switchgear where it is necessary to employ a device which measures the current flowing in a conductor (e.g. a bus bar) of the switchgear. In the event of the current reading an overload condition, the circuit breaker automatially opens to interrupt this current before any damage can be caused.

Conventionally, the current detection device often employs a current transformer of which the said conductor acts as the primary, the current detection being performed by measuring the current or voltage which is induced in a secondary winding by the primary current.

However, such transformers become non-linear when saturated, and also give rise to phase effects which must be corrected. These factors combine to cause inaccuracies in the current detection.

One method of measuring large fluctuating currents in a current-carrying wire involves the use of a Rogowski coil where a Rogowski coil surrounds the current-carrying wire. The terminal ends of the coil are connected to a detector circuit an output voltage of which is proportional to the e.m.f. induced in the Rogowski coil.

In accordance with the broadest aspect of the present invention a delay-line cable is used to form a Rogowski coil.

Also according to the invention, a device for detecting the current flowing in a conductor comprises at least one turn of delay-line cable wound in a closed path around the conductor in spaced relation thereto, one end of the inner helical conductor being connected to the corresponding end of the outer conductor, an e.m.f. induced in the inner conductor by current flowing in said first-mentioned conductor being detected at the other end of the cable between the inner and outer conductors. By connecting the cable in this way, where more than one turn is provided the outer conductor serves as a screen for the signal carried by the inner conductor.

By "delay-line cable" reference is made to a cable composed of an inner helically formed conductor wound onto a non magnetic core and a radially spaced outer conductor wound in even helical manner around the inner conductor.

The turn or turns of the delay-line cable may be spaced from the conductor by air or a solid insulator.

It is preferred that where more than one turn is provided such turns are disposed helically along the length of the conductor rather than in multi-layer relationship. Whether a single or a plurality of turns are provided, it is convenient that a complete turn or number of turns are employed.

Where the cross-sectional width of the inner conductor is small compared with its spacing from said conductor, it can be shown that the current flowing in the conductor is approximately proportional to the time integral of the e.m.f induced in the delay line cable. Accordingly the detector preferably takes the form of a high impedance integrating amplifier.

The invention will now be further described, by way of example only, with reference to the accompanying drawing, in which: Figure 1 is a schematic diagram of a current detector device according to the present invention; and Figure 2 is a side view of a delay cable which forms part of the device shown in Fig.

1, with parts of the delay cable being shown broken away.

Referring first to Fig. 1, there is shown a conductor 10 (e.g. a bus bar) through which a current flows in use. Disposed in a closed path around the conductor 10 is at least one turn of a delay line cable 11, this cable being composed of (see Fig. 2) a non-magnetic core 1 2 around which a wire 1 3 is helically wound. A shield or screen 14 is disposed around the wire and the core and spaced from the wire 1 3 by insulating material. An external covering 1 5 surrounds the shield.

In operation, the delay line cable 11 acts in the manner of an open circuit Rogowshi coil so that the current flowing through the conductor 10 induces an e.m.f. in the wire 13, and a measurement device 1 7 is connected to each end of the wire 1 3 to make measurements on this induced e.m.f.

If the cross-sectional width of the delay line cable 11 (i.e. its width in a plane perpendicular to the direction of extent of the conductor 10) is assumed to be small in comparison with the distance of the cable from the conductor, then it can be shown that the current i flowing in the conductor 10 is related to the e.m.f. e induced in the delay line cable as follows: 1 i-Ye. dt M where M is a constant dependent inter alia on the number of turns of the wire 1 3 in the delay cable, the mean circumferentiai length of each turn of the delay line cable, and the cross-sectional area of the inner conductor (wire 13). Thus, the magnitude of the current flowing in the conductor 10 can be detected by measuring the time integral of the e.m.f.

induced in the delay line cable. Accordingly, the measurement device 1 7 may take the form of a high quality, high stability, high input impedance integrating amplifier.

Theoretically, the above-described relationship between i and e is independent of the position of the conductor relative to the delay line cable 11, and also of the shape of the turn(s) of the inner conductor. However, from a practical viewpoint it is convenient to arrange for the turn(s) of the delay line cable to follow a circular (helical) path which is centred on the conductor 10, as illustrated in Fig. 1.

In a preferred arrangement, the cable 11 is a commercially available delay line cable giving a delay of 60 ns/foot. Such a cable may have a core diameter of about 2.5 mm, and may have 80 turns of the wire 1 3 per cm length of cable. When 10 turns of the cable are wound around the conductor at a radius therefrom of 50mm, it is calculted that in the steady state a peak e.m.f. of 0.0314 volts, will be induced in the cable 11 by an r.m.s.

current of 400 amps flowing through the conductor 10 at a frequency of 50Hz, although this voltage may rise to as much as 1.57 volts when the current is 20KA.

Claims

1. A device for detecting the current flowing in a conductor comprising at least one turn of delay-line cable wound in a closed path around the conductor in spaced relation thereto, one end of the inner helical conductor being connected to the corresponding end of the outer conductor, an e.m.f. induced in the inner conductor by current flowing in said first-mentioned conductor being detected at the other end of the cable between the inner and outer conductors.

2. A device as claimed in claim 1 in which the turn or turns of the delay-line cable around the conductor are spaced from the conductor by air or a solid insulator.

3. A device as claimed in claim 1 or 2 in which, where there are a plurality of turns of delay-line cable, the turns are helically disposed.

4. A device as claimed in any one of claims 1 to 3 in which the or eåch:-.turn of delay-line cable is a complete turn.

5. A device as claimed in any one of claims 1 to 4 in which the detector for detecting the induced e.m.f. comprises a high impedance integrating amplifier

6. A device for detecting the current flowing in a conductor whenever the conductor is wound by one or more turns of delay-line cable.

7. A device for detecting the current flowing in a conductor substantially as herein described with reference to and as illustrated in the accompanying drawings.