Corona Discharge in 22 KV HT Panel

Who is the certified engineer in charge?

Is opening the cover of a live CT housing even allowed?

If there is a discharge you don't want it to go bang when you open the door do you?

I am not an EE but it sounds pretty much like you are walking the line.

People getting killed that way!

Jus saaiin

Dear Ideasmith,

It was obvious that cover was not opened when panel is charged. I didn't bothered to mention safety precautions taken while inspection. I am just sharing the observations to get experiences of experts.

Please suggest if there is some solution to my problem. Don't panic, I am taking all safety precautions including shutdown before opening cover and arc flash suit during inspection.

From your original post (OP): "On opening cover, we found sparks between CT housing and nearby busbar" - this certainly sounds as if the cover was opened with the box live. You should word your posts more clearly if you don't want people getting the wrong idea.

My point exactly!

Well, may be my English is no so good to clarify everything.

Let me clear the issue. We took shutdown of panel after listening buzzing sound. After removing cover we restored power. We found that there was continuous arcing (like domestic gas lighter sparks) between CT housing (epoxy) and nearby bus bar connected to the same CT about 5-8 mm away. Problem increased after removing and cleaning bus bar and CT and refitting them.

I think I have cleared issue. Please suggest. (sparking occurs at no load condition as breaker is OFF)

Given the high breakdown voltage of epoxy, and assuming that the surface of the epoxy is clean and dry (so that there is no surface tracking), it suggests to me that the CT encapsulation has cracks (possibly on the microscopic scale). How old is the CT?

My first step would be to obtain a spare, and try changing it. If it doesn't fix it, at least you have a spare for future backup.

You say:

1. Shut down power to panel
2. Open panel access
3. Restore power

While it seems you did not open the panel, it still was open while under power.
You best course of action to engage with a certified engineer to review the design and performs all the testing.Good luck!

How can observations be made if covers aren't removed? How about hot work? Some things can't be de energised.

Nowadays there should be camera systems that can do this.

Hot work on a 22KV HT panel? Somebody who is doing this kind of work should know what to do and not asking in the internet about it.

I still hope there is a Electrical Engineer in this plant who knows what to do.

You said you tried HT tape and that didn't help.....

How did you clear the issue?

We used to deal with this problem by heating the area with a hot air dryer, making sure that we did not overheat anything, just removing any adsorbed water and brushing off any dust accumulation due to the high voltage.

Then we painted the insulator with Corona Dope, a product made for this use in the old TV business, which uses a similar voltage. Old TVs will accumulate dust and fine cracks abd gaps = snap snap snap

if you pick images or video in the search you will see more application info.

it is caused by dust deposits, fine cracks and adsorbed water

The snap-snap-snap in a TV is not supplied by a transformer with a fault current of, maybe, 8 kA. I would agree with John DG.

Thanks for coming back.

I agree on importance of space heaters and their location. I know of a couple of instances of serious shorts. A single heater can evaporate moisture in one location which settles into the coolest spot elsewhere. One incident was in a bus duct, which are often too dispersed for one heater to cover. Also, too little ventilation can be a problem and one must be particularly careful where the equipment is off or lightly loaded for long periods.

I have replied for possible action to be taken for permanent solution to eliminate corona discharge.

For your immediate solution you take shut down and test the CT for its healthiness and compare with commissioning test result and arrange for spare CT immediately.Monitor frequently and you must arrange for thermal scanning.

The problem is the termination. It has broken down and the stress cones are going down to earth, (tracking on the insulation of the core), strip down the termination, and clean it thoroughly of all carbon traces, check the core earths at the stress cones, check the earthing points and ensure the spacing between cores is not less than 30mm on each core, no cores should touch each other above the stress cones. Check for scortching on the CT inner face, If scortched or burn, remove the CT and replace it with a new one. The termination is tracking. Putting tape around the area will do nothing for you. The buzzing is corona and the smell is burning epoxy and ozone. The termination will fail quickly.

This is a very serious and hazardous condition because of the Ozone build-up concentrations inside the cabinet. When I was the Engineer in charge of the University of PR, Mayagüez Campus, I designed a new ( 8-Cubicle) 4.16KV Switchgear. It was manufactured and put in service. Shortly after, we found all the equipment back doors (30"Wx7'H-Ga.11 S/S doors) busted open and all the handle latches had been bent by the previous night explosion. This was a high humidity area and we almost had rain activity every day. After an exhaustive investigation, we concluded (and the equipment manufacturer concurred) that we had a primary arcing from the bus bars to nearby steel members which as the ozone concentration increased caused secondary and multiple corona discharge instances which led to critical ozone concentration and a large magnitude explosion which forced all back doors open. Since the bus-bar clearances were built to code standards for that voltage, it was concluded that because of the humidity conditions, the recommended manufacturing code standards for bus-to-bus and bus-to-ground clearances were not adequate for this location. The manufacturer( FPE) concluded that additional insulation measures had to be considered and proceeded to install Benelex sleeving and plates at all busbars. This corrected the arcing condition and we lived happily ever after. Benelex is a proprietary fiberglass and resin insulator specially designed and marketed plate insulator which is now commonly used for inter-phase isolation of MV switching units. I recommend you identify the main corona discharge occurrence location and insulate all busbars and, to be on the safe side, the metal panels next to the busbars. This should do the trick. Be sure you buy the correct thickness Benelex plate for your 22KV operating voltage or higher. As I said before, if allowed to persist, this condition is highly dangerous and might lead to hazardous concentrations of Ozone plasma explosive atmosphere. Once a primary arc is stablished(because of the Ozone concentration), secondary arcing will follow and multiple corona instances will generate throughout the equipment enclosure.

Before you ever re-energize that circuit it would be wise to ensure that the CT circuit is closed and that the CT Burden was not exceeded. The symptoms you describe are exactly what occurs when someone/something opens the secondary circuit even briefly. The damage is instantaneous due to the high voltages produced as the CT tries to push its secondary current through an infinite impedance.

You are all being overly harsh on the OP for things he did not mention in his request, and not answering his actual question satisfactorily.

Maybe he did or didn't follow appropriate safety precautions when investigating his problem, but in my business (which the above is a typical example) we would have done the same as he states (with appropriate PPE, minimum approach distances, safeguards, etc.). You can't really investigate corona & tracking fully without some such actions.

As to the OP's problem, once corona tracking begins, you usually can't clean it away. Any insulating materials that have been dmaaged by the tracking will need to be replaced. Often, the problem begins around sharp edges or changes in topology (such as a busbar penetrating an insulating barrier). CTs are also a problem area, if the insuation is not kept clean and the molding or encapsulation is not smooth and rounded.

Putting busbar tape over the tracked areas, even after cleaning, will not eradicate it. That insulation needs to be fully replaced. As another poster did say, immediately get a replacement CT (and maybe also an extra for spare) and exchange the faulty one. Corona tracking only gets worse until finally it causes a path to another phase or to ground, at which point you find yourself with a true fault.

If you need help, hire a competent firm that does high voltage electrical maintenance and repair. They will have the skills, knowledge, and equipment to do the job right.