Corona forming in 15KV switchgear-WHY?

[HoJoCo]

We have a 15 KV system with insulated bus (powder bed epoxy-60 mils) and G10 glass epoxy supports with silicone rubber inserts. The phase to phase spacing is 5-3/8” and the phase to ground is 3-1/2”. This was installed around Sept./Oct,2001!! We now see white powder forming around the support areas along the full lenght of the bus run and in some locations with the existing GPO3 glastic supports, we have visible tracking!

The application is indoor, clean, dry (filtered and ac). THe load is appox 500amps on the bus.

 

[busbar]

Offhand, the dimensions seem too tight for a 95kVBIL system. 99NEC490 lists ø-ø 7.5 inches and ø-g 5 inches, which can be reduced though appropriate “accepted national standards” testing, but suggest an approximate starting point. The listed dimensions reasonably correlate with IEEE NESC and C37 standards.

 

[RajT]

Very interesting issue. Please let us know whats causing it when you find the answer.

 

[hknair]

Hi,

Well finding your answer will need a glance into the electrochemistry book.But the one word answer that i find is oxidation .Since you are using insulators ,every chance is their for constituting molecules getting dislodged from the parent compound and getting accumulated on the surface.You can give the powdery substance to any nearby lab and can know of the composition very easily.Please comebach with your feedback.

 

[jbartos]

Suggestion: Reference:
1. C.L. Wadhwa "Electrical Power Systems," John Wiley and Sons, 1991, Chapter 4 Corona, for theoretical background and physical spacing calculations and limits. Notice that instead of air dielectricum, the solid dielectricum will have to be considered.

 

[RRaghunath]

I had seen this happen in 33kV switch boards (air insulated) made by Merlin Gerin way back in 1981. The powdery substance was analysed. There used to be that smell characteristic to Corona discharge the moment we enter the switchgear room.

The spouts surrounding the fixed contacts were replaced, space heaters were introduced in each of the panels with thermostats and auto switch on and steps taken to keep the cable trench in the switchgear room dry. This worked and the switchgear has been trouble free otherwise.

The project - Ramagundam Super Thermal Power Project of NTPC in India. Raghunath

 

[ausphil]

Deterioration of the insulation can occur in a number of different ways depending on the conditions.

In dry conditions, ozone formation can physically oxidise the insulation and leave behind the oxide which will appear as a dust or powder.

In moist conditions, oxides of nitrogen (forming nitric acid with the humid air) can literally eat away insulation as well.

The main catalyst is still the low clearances giving a high electric field stress around certain points of the insulation. It sounds like it may well have started around the silicon rubber inserts, which are oxidising, and a silicate powder being left. The tracking will occur once the pathway becomes polluted enough to sustain a burning or corrosion of the insulation. It might be worthwhile doing a PD test on the switchgear (on the busbar mainly) which should confirm the corona presence. Was PD testing done on installation, if so, were the inception and extinction voltages acceptable? if not, this is a good example of why you don't accept the factory test results as being what is there on site. Many authorities have gone away from commissioning PD tests and opt only for an HV test. The PD results provide a valuable reference for new switchgear (especially epoxy gear on which DDF tests can be close to useless).

My concern is the speed at which the deterioration has taken place. We would regard this sort of deterioration as a long term (10-20 years) issue. The fact that you have reached it in 3 years says that the reaction is being driven by detrimental conditions that are way above normal.

 

[HoJoCo]

Thanks for all of the responses! I have had discussions with various manufacturers of insulating materials and I am being told that the silicone inserts are used in a few new manufactured swgr. systems. What may have happened is the silicone (corona pad) was not extended out beyond the insulator support. This abrupt ending of the pad may have caused the corona conditon. We have not gotten back results of evaluating the white powder as of yet and are moving to replace the insulator supports and include corona pads of 1/8" silicone- 2" beyond the supports.

As far as the clearances are concerned, this bus duct is 30+ years old and worked fine when it was installed for many years. It did degrade over time but never as quickly as 15 months! The comments of environment and moisture are good points and will be evaluated.

As for PD testing, none has been done to date. The main switchgear is having the CH PD system installed, but not the overhead bus duct. We will have to look into PD testing of this installation as well.

I will update the status as the project progresses.

 

[GRPatel]

I try to understand your problem. To me this appears to be a common problem when you use insulation system of more than one material. When you use more than one solid insulation material, you expect air to make an intermediate layer of insulation. This air, depending on where it is present, gets stressed like a graded insulation. The gaps being very small and stresses being large, they tend to ionise and you will see behaviour as observed by you, over a long period of time. In a very badly designed system, one can expect even failures leading to intrernal flashovers.
The remedy is to apply a layer of semi-conducting paint on the mating surfaces and bring surfaces to equi-potential. This may be necessary for the gap between the busbar and the support also.
Good luck.

 

[DanDel]

One small addition to everyone else's comments: any sharp edge or point on the metal of the bus will serve as a place for corona to start(similar to the point on a lightning arrestor).

 

[swgrmfg]

HOJOKO:

1. You need to close any air gaps at the inserts or at any overlapping insulation materials or wherever a HV conductor penetrates or touches an insulator. The best material is GE Silicone II, the clear stuff.

You should be able to detect these air gaps by simply hi-poting at 36KV and listening for crackling noise. All noise will go maway when you seal the air gaps.

2. Another thing I would suggest is to ensure that your fluidized bed epoxy insulation is really 'sticking' to the copper bus. You can chisel away at a spare piece to check.

3. The fluidized bed epoxy has a shelf life beyond which it goes bad. You can do a simple test by taking a long insulated piece (36&quotof copper bus, wrapping a piece of aluminum foil in the middle of it, and running a hi-pot test between the copper and the aluminum. If good, you should be able to do about 50ACKV with no problems.

I would go 3 then 2 then 1.

Good luck.

 

[HoJoCo]

'Swgrmfg',

Please Email me directly with your experience in testing insulated bus with this process. Advise millage of insulation that is expected to pass this test. I have been testing these pieces per my supplier at 15 kvac only and did have 4 units fail with pinhole blowouts! Any help on this subject would be appreciated.

HoJoCo
howard.friscia@hvmcorp.com

 

[KenAlmon]

See Powell Electric Technical Brief #59. You can access at

http://www.powellelectric.com/

Pick Technical Briefs on left side frame and look up #59 "Bus Spacings in Metal Enclosed Switchgear