Recently released to the Australasian Industry, the RH Systems model 973 portable SF6 gas testing instrument not only represents the state-of-the-art in such equipment but also has greatly impressed the Australasian SF6 users with its features.

A Collaborative International Design:

Designed under a partnership between RH Systems of New Mexico and MWB in Switzerland, the 973 offers a fully automated SF6 testing approach with the added, and very significant, feature that all gas collected during the testing is stored internally in a pressure vessel for later pump back either to the equipment under test or a holding vessel.

With the 973 one measures the gas pressure, gas humidity, gas ppmV or ppm M, and purity %, all fully automatically and repeatably.

Fully Featured:

The unit has 5 main features in its favour:

1. it tests both SF6 Humidity and Purity
2. it works to an accuracy that is nominally 2-4 times that of its next competitor
3. it holds all tested gas in an internal tank and pumps back to the switchgear afterward.  This is a major feature in its favour, under the Australian Greenhouse Gas Commission and New Zealand Kyoto Protocol gas handling requirements.
4. it is superbly simple to use, being fully automatic and repeatable in its readings, thus minimising any risk of operator error
5. it needs no normal re-calibration or maintenance.

Priced at AUD 47,860 +AGST DDU (at present exchange rates) and with a 2 year warranty, it represents excellent value.

Why Test SF6 Gas?

To guide users in considering such testing, measurement of humidity in SF6 gas is seen as a preventive medicine approach to substation maintenance.  If the humidity content is low, then the possibility of HF and SO2 creation is very low.  If the humidity is high, then those by-products will be created in high enough concentrations to corrode the insulators and result in unwanted arc-over (short-circuiting) in the equipment.  

Here are a couple of sources of unbiased humidity discussion…

The first is an ASTM document describing measurement of humidity in SF6.  It is copyrighted and cost about $35 from ASTM.
D2029-97(2003) Standard Test Methods for Water Vapour Content of Electrical Insulating Gases by Measurement of Dew Point

http://www.astm.org/Standards/D2029.htm

The second point to make is that the purity of the gas has a direct bearing on its effectiveness in the breaker.  If the purity is excellent, there is full assurance on the efficacy of the breaker and no additional testing is needed. Should the tester indicate a poorer level of purity, then one may elect to conduct testing to see what the issue is, or perhaps replace the gas outright.  

Thus, the RH973 carries out both required testing processes to leave the breaker at the end with full confidence of continued insulation quality.

Finally, the routine testing of the breaker alone immediately exposes any pressure drops through leakage and that alone is a major start to maintaining breaker performance!!  It is well known that the pressure gauges on breakers, where fitted, can contribute issues of their own in leakage terms so the use of periodic testing with the likes of the 973 ensures that one may attend to all issues with the one device, quickly, efficiently, repeatability, and normally IN-SERVICE.  

The concept of regular SF6 gas testing with this device is a best practice one that is worthy of a detailed assessment.