Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Polyphase
Patent
1990-11-13
1995-04-18
Karlsen, Ernest F.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Polyphase
324127, 324512, 364483, G01R 1900, G01R 120, G01R 3102
Patent
active
054081769
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to the monitoring of electrical currents flowing in conductors, and more particularly, to apparatus for monitoring a high voltage switchyard and for the detection of fault conditions therein.
BACKGROUND ART
The monitoring of high voltage electrical switchyards for revenue metering, instrumentation control, bus and line protection are generally carried out using current transformers. The current transformers are electrically connected to the high voltage conductors and therefore, are themselves high voltage devices which can and do develop faults resulting in damage to the switchyard.
Research into ways of eliminating the use of current transformers in the monitoring of high voltage currents has resulted in a number of alternatives being proposed. However, all of the proposed methods so far have the common feature requiring some form of attachment to the high voltage system being monitored. Most of the methods proposed are based on a transducer which uses the Faraday effect to monitor magnetic fields in a closed magnetic path around the conductor. However, due to the closeness of the coil to the conductor being monitored, the transducer is substantially at the same potential as the conductor.
DISCLOSURE OF INVENTION
The present invention relates to the use of magnetic field detectors which are physically and electrically isolated from the network under surveillance, and as the detectors are located a safe working distance from the high voltage conductors, the entire monitoring and protection system can be maintained at substantially earth potential.
Therefore, according to one aspect of the invention there is provided apparatus for monitoring currents flowing through a cable, said apparatus comprising: distance away therefrom to remain at substantially earth potential, said detector being electrically shielded by a discontinuous shield to isolate the detector from the effects of an electric field generated by the current flowing through the cable and able to be influenced by a magnetic field generated by the current flowing through the cable; of the magnetic field detected; detector and at predetermined instances of time, measuring the value of the analogue signal and converting that value to a digital signal; and instances of time and for receiving and processing the digital signal to produce a final signal representative of the current flowing through the conductor. monitoring electrical currents flowing through conductors in a poly-faced network having junctions, comprising: generated by the electric currents flowing through the conductors, each conductor being electrically isolated from the conductors and shielded from electrical fields, wherein each detector produces an analogue signal representative of the magnetic field detected; station including a controller for receiving the analogue signal and at predetermined instances of time, measuring the value of the analogue signal and producing a second signal representative of the measured value of the analogue signal; and processing the second signals to calculate the currents flowing through the conductors of the network.
Preferably, the detectors are air cored epoxy potted coils with a discontinuous electrostatic shielding.
Preferably, each remote station converts the analogue signal to a digital signal which is sampled at the predetermined instances in time by a sample and hold circuit.
Preferably, the controller is a microprocessor.
Preferably, the second signal is relayed to the control centre as a current loop signal terminating at a varistor termination board within the control centre, the control centre also includes optoisolation means for optically isolating the second signal from the CPU.
Alternatively, the second signal may be relayed to the central centre by optical fibre.
BRIEF DESCRIPTION OF DRAWINGS
Notwithstanding any other forms that may fall within its scope, one preferred form of the invention will now be described, by way of example only, with reference to the accompanying drawings in
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Pettinga, J. A. J. and Siersma, J., "A Polyphase 500kA Current Measuring System with Rogowski Coils," IEEE Proc. 130B:360-363 (1983).
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