GB2221763A

GB2221763A - Current sensing

Info

Publication number: GB2221763A
Application number: GB8909553A
Authority: GB
Inventors: Andrew Michel
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1988-08-12
Filing date: 1989-04-26
Publication date: 1990-02-14
Also published as: GB8819163D0; GB8909553D0

Abstract

A current sensing apparatus comprising a current transformer (ET) coupled to a convertor (O) for producing from the electrical input light signals which are fed by one or more optical fibres (F) to a monitoring device M responsive to said signals, e.g. for current limiting. Signals from the current transformer (CT) are converted (VF) to frequency form prior to the optical conversion (O). A further current transformer (62) provides energy for operating the converter (VF). The output of the monitor (M) may operate a circuit breaker. <IMAGE>

Description

Current Transformer This invention relates to current transformers and to equipment incorporating such transformers.

Current transformers are normally operated close to earth potential, and accordingly need to be adequately insulated from the current carrying conductors around which they are fitted, but this is not always convenient and can be expensive in systems operating at very high voltages.

An object of the invention is to provide a way of avoiding this disadvantage.

According to the invention a current transformer is coupled to a convertor for producing from the electrical input light signals which are fed by one or more optical fibres to a monitoring device responsive to said signals.

The current transformer and convertor can, in such a case, be maintained at the line potential, the need for a high degree of insulation between the current carrying conductor and the current transformer being thereby avoided.

The convertor may be energised by a voltage derived by means of a second current transformer associated with the current carrying conductor, and in some cases it may be arranged to be quiescent when the current in the conductor is zero or below a predetermined value, and to become operative to generate said light signals only when the current reaches this value.

The monitoring device may, for example, be associated with switchgear arranged to cut off the electrical supply to the current carrying conductor automatically in the event of the current through it exceeding a predetermined safe value.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawing.

In the drawing there is shown a high voltage supply line HV having associated with it a current transformer CT connected to a solid state voltage to frequency convertor VF.

The power for operating the latter is derived from an a.c.

to d.c. convertor AD energised from a second current transformer C2 associated with the supply line HV. The output from the voltage to frequency convertor is fed to a further convertor 0 which is arranged to convert the electrical input signals into light signals which are fed, by means of an optical fibre cable F, to a remotely positioned monitoring device M of any convenient form. The current transformers CT, C2 and the associated units AD, VF and 0 are maintained at or around the supply line voltage, and the need for a high degree of insulation between the line and the current transformers is thereby avoided. An electrical screen may be placed around these units as shown in part at S. The monitoring device M operates at around earth potential but is spaced remotely from the remainder of the units and is insulated from them by the insulating properties of the optical cable.

The electronic circuitry within the units AD, VF and 0 is arranged to be quiescent when no, or only a small, current flows in the high voltage supply line HV, but when the current builds up to a predetermined value at which monitoring is required, the circuitry becomes operative to cause optical signals to be generated, and fed to the monitoring device M.

The monitoring device M is preferably arranged to generate an output signal which is fed to the operating circuit of a circuit breaker or other form of switchgear (not shown) so as to actuate the latter automatically and de-energise supply line HV, in the event that the current through the supply line reaches a predetermined value indicative of fault conditions.

Claims

1. Current transforming apparatus comprising a current transformer coupled to a convertor for producing from the electrical input light signals which are fed by one or more optical fibres to a monitoring device responsive to said signals.

2. Apparatus according to Claim 1, comprising a second current transformer connected to energise the convertor, both current transformers being adapted to be associated with the same current-carrying conductor.

3. Apparatus according to Claim 1 or 2, in which the convertor is arranged to be quiescent when the current in the conductor is zero or below a predetermined value, and to become operative to generate said light signals only when the current reaches this value.

4. Apparatus according to any preceding claim, associated with switchgear arranged to cut off the electrical supply to the current carrying conductor automatically in the event of the current through it exceeding a predetermined safe value.

5. Current transforming apparatus substantially as described herein with reference to the accompanying drawing.