GB2205963A

GB2205963A - Measuring current

Info

Publication number: GB2205963A
Application number: GB08813151A
Authority: GB
Inventors: John Stanley Stewart
Original assignee: Brush Switchgear Ltd
Current assignee: Brush Switchgear Ltd
Priority date: 1987-06-06
Filing date: 1988-06-03
Publication date: 1988-12-21
Also published as: GB8713314D0; GB8813151D0

Abstract

To measure the current flowing through power line (26), a signal induced in a toroid (24) is voltage to frequency converted (30) to form a signal which is applied to a light emitting diode (32). A light signal is transmitted along a fibre optic cable (34) to a detector (36) which drives a frequency to voltage converter (38). The output of the converter is then amplified (40) and applied to an indicator (24). A second toroid (46) is used to provide power for the voltage to frequency converter (30). The system may be incorporated in a gas-blast interrupter (Fig. 1 not shown). <IMAGE>

Description

Title: Improvements relating to current transformers The present invention relates to current transformers and particularly to the metering of current flowing through the conductors of high voltage circuit breakers.

Conventional current transformers have a primary winding which is connected in a primary conductor of.the power line and a secondary winding connected to a metering device. The current induced in the secondary winding is proportional to the current flowing in the primary winding and therefore in the primary conductor and the metering device can therefore be calibrated to indicate the actual current flowing in the primary conductor. Conventional current transformers are extremely bulky and expensive devices. In each case the primary winding has to be capable of carrying high currents, typically as high as 4000 amps and capable of withstanding high short circuit current of 25kA to 40kA for several seconds.In order to provide the high voltage primary conductors with electrical insulation to earth potential the primary windings normally use paper impregnated with oil as a dielectric and the whole of the transformer is normally enclosed within a fabricated steel tank, porcelain and a spun steel conservator, the enclosure being hermetically sealed. Alternative means of providing this electrical insulation use SF6 gas and are equally bulky and expensive.

Such current transformers are used with high voltage circuit breakers, and in normal usage on 3-phase power lines a total of up to six such current transformers may be used, two per phase with a respective one on each side of the circuit breaker. This clearly renders circuit breaker installations large, expensive and cumbersome with large areas having to be allocated to the installations.

The present invention enables these conventional current transformers to be replaced by very simple devices which are considerably less expensive and far less bulky, and can be incorporated in circuit'breakers.

The present invention seeks to provide an improved apparatus for metering current flowing through a power line.

Accordingly, the present invention provides an apparatus for metering current flowing through a power line comprising a winding in the form of a toroid surrounding said power line, first means for generating an optical signal representative of an electrical signal induced in said winding, and detector means remote from said first means for detecting said optical signal and generating an electrical signal in dependence thereon wherein said electrical signal is representative of the current flowing through said power line.

The present invention also provides a gas-blast type inerrutr incorporating an appparatus as described in the immediately preceding paragraph.

In a preferred form of the present invention the first mentioned means includes a voltage to frequency converter for converting the voltage induced in said secondary winding into an alternating frequency signal. This is converted into an optical signal which is transmitted to a remote receiver which converts the signal into an alternating frequency electrical signal. This in turn is converted to a voltage by a frequency to voltage converter and applied to a visual indicator calibrated to indicate the current flowing through the power line.

Alternatively, the signal can be applied to protective relays serving to protect the power line or other apparatus connected to the power line.

An apparatus according to the present invention provides a simple and relatively inexpensive way of monitoring the current flowing through a high voltage power line, the device occupying considerably less space than conventional current transformers.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partial section through a gas-blast type interrupter incorporating a preferred form of apparatus according to the present invention; Figure 2 is a diagrammatic representation of a stand-alone form of the apparatus incorporated in the gas-blast type interrupter of Figure 1; and Figure 3 is a schematic diagram of the circuit of the apparatus in Figures 1 and 2.

Referring to the drawings, Figure 1 is a diagramatic representation shown partially in section of a gas-blast type interrupter 10 incorporating a preferred form of apparatus 12 according to the present invention. The gas-blast type interrupter 10 has two coaxial insulating enclosures 14, 16 secured together by flanges 18. The upper insulating enclosure 14 incorporates axially aligned fixed electrode 20 and movable electrode contained in a tubular gas flow guide 22. A power line (not shown in Figure 1) enters the interrupter at the upper end of the insulating enclosure 14 and connects with the fixed electrode 20. The power line exits the interrupter at the lower end of the enclosure 14. The gas-blast type interrupter shown in Figure 1 is of well known construction and is not described further here.

Figure 3 is a schematic diagram of the circuit of the apparatus 12 in the interrupter 10 and, as can be seen, has a secondary transformer winding in the form of a toroid 24. The toroid 24 is located at or adjacent the lower end of enclosure 14 and surrounds the power line 26.

The power line 26 serves as a primary winding although, as can be seen from the drawing, the power line 26 is not in fact wound and passes through the toroid 24 as a straight conductor. Current flowing in the power line 26 induces a current in the winding of the toroid 24 which in turn develops a voltage across an output resistance 28. This voltage is converted into an alternating frequency signal by a voltage to a frequency converter 30, the signal from which is applied to a transducer 32 such as a light emitting diode.

The transducer 32 provides a light signal which is transmitted along a fibre optic cable 34 to a detector 36, conveniently a photocell. This provides an electrical signal corresponding to the electrical signal applied to the transducer 32, which is in turn converted to a voltage by a frequency to voltage converter 38. The resulting voltage is amplified in an amplifier 40 and applied to a device 42 which provides a visual indication of the amplitude of the applied voltage. The device 42 can be in the form of a moving coil meter or liquid crystal dispaly or the like and is calibrated to indicate the size of the current flowing through the power line 26.

Since the interior of the insulating enclosure 16 is filled with pressurized gas the device 42 is located externally of this, the electrical connections being made through, for example, a sealed terminal block.

Power to for example, the frequency to voltage convertor 38 is provided by a power supply 44 which derives power from current induced in a further toroid 46 around the power line 26.

The apparatus 12 can also, of course, be used a stand-alone apparatus and an example of this is shown in Figure 2.

It will be appreciated that the described and illustrated apparatus is considerably simplified over the current transformers hitherto used and in addition allows the power line current to be metered at a preferred location remote from the power line, if desired.

In normal use the device 10 simply replaces a conventional current transformer. Since the device can employ low power electronics and opto electrics, it is of simplified design, avoiding the necessity for bulky and expensive components such as the primary winding of conventional current transformers.

The device can be used on its own (Figure 2) or in combination with outdoor circuit breakers in the same way as conventional current transformers but without the problems associated therewith and with considerable savings in both cost and size of installation.

Claims

CLAIMS:

1. Apparatus for metering current flowing through a power line comprising a winding in the form of a toroid surrounding said power line, first means for generating an optical signal representative of an electrical signal induced in said winding, and detector means remote from said first means for detecting said optical signal and generating an electrical signal in dependence thereon wherein said electrical signal is representative of the current flowing through said power line.

2. Apparatus as claimed in claim 1 wherein said first means includes a voltage to frequency converter for converting the voltage induced in said secondary winding into an alternating frequency signal.

3. Apparatus as claimed in Claim 2 further comprising transducer means for converting said alternating frequency signal into an optical signal.

4. Apparatus as claimed in Claim 2 or 3 wherein said detector means includes transducer means for converting said optical signal into an alternating frequency electrical signal.

5. Apparatus as claimed in Claim 4 wherein said detector means further comprises a frequency to voltage converter coupled to said transducer means.

6. Apparatus as claimed in any preceeding claim further comprising means for transmitting said optical signal to said detector means.

7. Apparatus as claimed in Claim 6 wherein said transmitting means comprises a fibre optic means.

8. Apparatus for metering current flowing through a power line substantially as hereinbefore described with reference to the accompanying drawings.

9. A gas-blast type interrupter incorporating an apparatus as claimed in any of the preceeding claims.

10. A gas-blast type interrupter substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.