Learnings

Outcomes

The different factors investigated so far that could impact by-product formation in C4F7N gas mixtures suggest that no unexpected products are formed during in-service conditions but there is further work to do.

Recommendations for further work

None at this stage

Lessons Learnt

In designing a test set up for HVAC testing of gas mixtures where the effect of pressure is being considered as one of the variables, it was identified that conventional sensors respond too slowly.  An integrated electronics piezo-electric (IEPE) sensor is being trialled in combination with another method.  Thermal sensors are also slow and an optical technique was employed.

A commercial gas sensor for C4F7N was used for by-products but has challenges because some potential by-products are not detectable.  GC-MS was therefore chosen as the best option for identifying them.  The commercial detector is better than GC-MS at separating carbon monoxide from other gases (oxygen and nitrogen) and enabling it to be quantified.

Repetitive breakdown testing (400 shots) under LI and HVAC conditions in small gas volumes shows evidence of breakdown of the C4F7N gas mixture.  CF4 is the first product identified and then C2N2.  The -CN group is lost readily and combines to produce C2N2, but there are not many other -CN containing products and it may recombine to reform C4F7N.  C4F10 is more clearly identified in larger gas testing experiments.

Increasing the moisture content in the breakdown tests has a number of impacts. For example, it reduces the dielectric strength and breakdown voltage and may affect the recombination process of the gas mixture, however it does not appear to change the type of by-products present.  Adding atmospheric air affected the concentration of by-products but not the compounds produced.  Increasing pressure had no effect on by-products either.

Dissemination 

No formal dissemination events as results are still being collected.  All three GB transmission owners are invited to join project update meetings so that the work proceeds collaboratively and all results are received at the same time.