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UHF coupler design
External couplers are designed to fit over a dielectric aperture in the chamber, such as an inspection window or the exposed edge of a barrier. They are mounted permanently when used as part of an installed PDM, or in an easily removed version when used with a DMS portable UHF monitor.
External couplers are designed using broadband antenna techniques, and when used on an aperture of the recommended size are as sensitive as internal couplers. Should only a smaller aperture be available, an internal pre-amplifier powered through the coaxial cable connection may be used to boost the signal.
DMS couplers mounted on exposed barriers have efficient screening that minimises the effect of any radiated interference from radar or communication systems.
DMS protector on an internal coupler
DMS external coupler on a window
Coupler design
DMS designs, manufactures and calibrates UHF couplers for all applications. The two main types used on GIS are internal and external couplers.
Internal couplers are mounted in the GIS during its assembly, or may be retrofitted later. They are fitted inside the hatch cover plates, and normally take the form of aluminium discs insulated from the GIS enclosure. Internal couplers are preferred for continuous monitoring, because they are shielded from external interference. More than 4000 internal couplers of DMS design are currently in operation, and because of their advanced electrical design and robust construction they are giving excellent performance with very high reliability.
An internal coupler readily picks up the transient overvoltages caused by disconnector and other switching operations. It is therefore fitted with a DMS transient protector, which removes the transients and completely protects the sensitive electronics of the measuring system.
For new GIS the internal couplers are usually manufactured and fitted by the switchgear manufacturer, but for retrofit applications DMS can supply them directly to the user.
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Diagnost ic Monitor ing Systems Ltd
Issued Dec 2004. Copyright © Diagnostic Monitoring Systems Litd.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in a form or by means electronic, mechanical, photocopying,recording or otherwise, without the prior permission in writing of Diagnostic Monitoring Systems Ltd.
Diagnostic Monitoring Systems Ltd has made every effort to ensure the accuracy of this material, but assumes no liability for the consequences incurred directly or indirectlyof any errors, omissions or discrepancies it may contain. The company pursues a policy of product development, and this specification may change without notice.
Coupler calibration
The partial discharge caused by a defect in a GIS is a very fast-fronted current pulse, which excites the chamber into resonances at frequencies up to and exceeding 1GHz. The amplitude of these UHF waves is proportional to the charge in the pulse, although as the waves propagate along the chambers they are attenuated by reflections from discontinuities such as barriers and junctions. Nevertheless, with calibration their amplitude is a useful, although approximate, measure of the charge in the PD.
The purpose of the UHF coupler is to allow the UHF electric field to be measured, and it needs to be calibrated to ensure that it meets the specified performance level.
The sensitivity of a coupler is its voltage output (v) divided by the UHF electric field in the chamber (V/mm). The sensitivity is expressed in mm, known as the effective height of the coupler. The sensitivity is frequency dependent, and is measured over a frequency range of, typically, 200-2000MHz.
UHF couplers are calibrated in a cell, shown in the figure, where the coupler under test is mounted along one of the faces. A very fast-fronted (< 100ps) pulse is injected at the narrow end, and as it propagates along the cell it sweeps over the coupler. The output voltage of the coupler is recorded, and processed to give the sensitivity of the coupler as a function of frequency.
External coupler and its calibration curve
UHF coupler calibration cell
