Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Partial discharge & breakdown of polymeric materials

Status
Not open for further replies.

ScottyUK

Electrical
May 21, 2003
12,915
I've just had a fun-packed evening investigating a fault on a generator excitation transformer. In the course of the investigation we removed the HV (15.5kV) cable box cover and found a real mess.

The HV supply from the generator terminals is via 3x single-core aluminium, XLPE insulated, wire screen, aluminium armoured, PVC sheathed cable.

The lugged connection onto each of the three porcelain phase bushings of the transformer had been wrapped in a self-amalgamating rubber tape over the metal parts. The reason for the tape wrapping, and those who authorised its use, is not known at present.

There appears to have been a chemical reaction taking place within the cable box which has evolved some highly corrosive vapour or gas which has attacked the steel cable box interior, the brass glands, and the copper earthing bar. The steel in locations where the liquid had breached the paint coating was in a severely corroded state. There was also a significant volume of liquid - I am reluctant to say water - over all internal surfaces of the cable box and the cables, yet all gaskets and gland washers appear intact. It is almost as though the liquid appeared within the box and condensed on every internal surface. We treated the liquid as a chemical release, took samples, then flushed the whole lot with water into the bund before investigating further.

The rubber tape - which normally becomes a homogeneous hard rubbery mass over time - has deteriorated into something with the consistency of chewing gum. The Raychem heatshrink termination - which had been made off correctly so far as can be determined - shows signs of major PD activity with large-scale treeing visible and some surface burning and material loss also being evident in the area where the overlap to the rubber tape occurred.

Has anyone seen anything like this before? Also, can anyone suggest what the liquid might be? I think it is likely to be some sort of acid - sulphuric or nitric - although I can't account for its presence unless it is a byproduct of the chemical breakdown. I believe the rubber tape is a natural rubber (latex) based material which is (was?) presumably vulcanised, hence my suspicion of sulphuric acid. Any other comments about any part of what I have described are very welcome too.



----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!
 
Replies continue below

Recommended for you

Sounds scary. I once found a brown liquid on the floor in a factory where electron beam sterilization was being done. The people in the factory didn't know what it was. All they could say was: "Watch out! That's nasty stuff!".

It turned out to be nitric acid. The electron beam had cracked the gas molecules into free atoms which combined to create the nearest stable compound - which happened to be nitric acid - which then condensed on the floor (with a plastic carpet). There was probably also a lot of acid on the walls. But they had probably absorbed it.

I think that strong electric fields can cause the same cracking (the ozone created by PD is such a thing, isn't it?) and that the liquid you found could be a similar nasty thing. Hope you didn't hurt yourself.

Gunnar Englund
 
In an electric arc or coronoa, nitrogen in the air reacts with oxygen to form various nitrogen oxides and acids when moisture is present.
 
Thanks for the replies.

Alehman, any idea what happens to the solid materials? I assume the polymers are broken down into smaller molecules or individual atoms by the electric field in the same way. PD is something I'm aware of, and can recognise in the more advanced stages, but I'm no expert. Keen to learn.


Skogs, no one was hurt. We realised something other than water was present by the extent of corrosion and the black colour of some steelwork, and the slightly acrid smell. The closest comparison I can make would be the smell in a battery room after the battery has been accepting a heavy charge. It wasn't the usual ozone smell, although the amount of other contaminants present might have confused my nose. We ensured no one made contact with the stuff, took samples, then drenched the whole cable box in utility water until the liquid and loose contaminants were flushed away into the contaminated water drains. The more persistant deposits on the bushings and metalwork were dislodged with a disposable cloth while wearing gloves, then the box was flushed again.

I'll post the results of the analysis if the lab can get anything conclusive.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!
 
We have also experienced a few incidents where the HS termination failed inside a seal up cable box (at RMU). We suspect the heavy condensation which leads to treeing problem over a long period. There is no failure that I know of after removing cable gland (but installing cable support externally) for cable box 'breathing' purpose. Perhaps it is just a coincidence ??
 
I believe what you are refering to as "treeing" is actually tracking. "Treeing" is a microscopic breakdown usually within polyethleyne (sp?) cable insulation. Tracking is discharge over the surface possibly contaminated or moist.

It sounds like you have moisture egress. I would seek the source, seal it up, clean up all the putty like tape and put new heat shrink terminations on according to instruction.

Old pot heads were made up using the tape like you describe, and some new methods use tape as filler. It seems like someone tried to jury rig something using old with new, but 15kV is not the place to be uncertain and I would go with a new termination with good instructions.
 
Tracking involves the breakdown or oxidation of the surface involved or contamination thereon. Plastic or rubber surfaces may carbonize. Ceramics typically fail due to conduction of contamination on the surfaces.
 
Re the moisture. I would suspect that the junction box was "Breathing" when the ambient temperature and/or atmospheric pressure changed. I have seen this many times. Most often in locations such as the control cabinets of automatic car washes or pole moumted control boxs for underground sewage lift pumps. Once we had a problem with the primary connection junction box on a 1000 KVA transformer. It seems that if a box is ALMOST sealed, then moisture vapor can get in, but condensed water can't get out. My solution has always been to drill one or more 1/8 inch drain holes to let the water out. I have done this many times and it has always been successful.
The chemical effects of the breakdown are out of my field, but this may help explain the presence of the original water.
yours
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor