Cermax Hygrometers and The National Grid Company (November 1999)

This information was written by:
Brooks Randall Marketing Communications
Triangle Business Park, Aylesbury, Bucks, HP22 5BL.
Tel: UK 01296 614222. International 44 1296 614222
Fax: UK 01296 615222. International 44 1296 615222
Contact: Julie Mercer

CERMAX MONITORS SF6 QUALITY IN
NATIONAL GRID TRANSMISSION OPERATIONS

The National Grid Company, which owns and operates the high voltage transmission system throughout England and Wales, is using Michell Instruments' Cermax hygrometers to test moisture content in SF6.

During electrical power distribution at transmission voltages, power switching or load current interruption is a problem which results in arcing between two contact points. The insulation properties of SF6 are therefore utilised to pressurise such switchgear enclosures, effectively preventing arc formation. The ingress of moisture into this process, however, can result in electrical discharge which causes the SF6 to decompose into by-products such as hydrolisable fluorides or, in the presence of high levels of moisture, into highly corrosive HF, which can accelerate switch contact corrosion to the point of physical breakdown.

The accurate monitoring of moisture content in the SF6 is therefore extremely important if the gas is to be maintained in good condition, ie, with a moisture content of <10ppm(V) equivalent to a dewpoint of<-60ºC at atmospheric pressure.

In order to carry out this monitoring effectively, the National Grid needed hygrometers which could work to high specifications and pressures up to 10bar to ensure effective monitoring of national transmission systems, while retaining traceability to ensure compliance to the company's ISO9002 standards.

Consequently three Cermax hygrometers were selected, as the instruments offer a high accuracy of ±1ºC dewpoint between -60ºC and +20ºC, and ±2ºC dewpoint in the range -100ºC to -60ºC, with a resolution of 0.1ºC dewpoint across the entire range. Furthermore, traceability is provided as the hygrometers are calibrated in Michell Instrument's NAMAS accredited laboratory.

The instruments are used to make spot measurements of the SF6 gas using drawn off samples which can then be either recycled or vented to the atmosphere. Operators are therefore notified of any changes in moisture content at an early stage, avoiding decomposition problems with the result that the likelihood of arcing is minimised.

Michell's Cermax portable hygrometer is lightweight, weighing only 3kg and is enclosed in a strong moulded polyurethane case with protection to IP66. Furthermore, each unit has its own data logging facility capable of storing up to 10,000 measurement points for dewpoint, temperature and pressure, as well as recording start time, measurement intervals and test duration. Information can then be downloaded to a PC utilising the Windows operating system, or a similar data transfer program, for further analysis.

A spokesman for the National Grid Company comments, "We chose to use Cermax because it met our strict operating requirements while remaining cost effective. The hygrometers have now been in use for a year and we are pleased with their operation."

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