Core Grounding Box

What did the transfomer manufacturer say on the phone about this?

Just a guess… but I'd say the hole in the cover plate is the very least of your problems! Though perhaps you don't wish to know what wonders lie beneath! Could that hole "in the centre" be a result of arcing inside? And why would that be…?

the hole is not due to arcing.Its in the cover by design.

OK, Just didn't appear to be a "manufactured" hole.

What is leaking - transformer oil? If the hole is in the cover by design (I have my doubts) then oil was not intended to be at that location. If then oil is present at that location, there must be another problem allowing oil to reach the cover and its hole.

I wonder why you are going to plug the hole instead of making a new cover plate? It will be hard to make an effective patch for the hole, but a new plate is easy to make. The other question I have is whether the plate is effectively gasketed. Whatever you do, I believe the use of a seal at this point will work. I remember a lot of the gasket systems used in French and English designs left a lot to be desired. We usually did our own new gasgets based on GE designs to replace them. I am really surprised that this is your only leak.

the hole was there by design.The only point i consider for the hole to be present is that when there is leakage from the bush it should be indicated by overflow from the hole.

I can see that the hole is a vent with a little hat. Now that you know that the bushing is leaking oil, when do you intend to replace it?

First clarification is required from where the oil leak is precisely. Two possibilities exist:

1. Is this coming out from seal around terminal rod and the insulator?

2. Or the leak is from gasket between the flange of insulator and body of Transformer?

In case the leak is from 1 above, Then it could be due to tension of Jumper (which is given a U -kink in the photograph) on terminal rod. You can try loosening the jumper, loosen check nut, tighten the base nut and then lock with check nut. Additionally, can cut a gasket of Teflon (can be done in the Maintenance Dept. workshop by machining out of Teflon rod on a lathe machine) of approx. 3 mm thick, clean the face of bushing, place the Teflon washer, tighten base nut and tighten lock nut. The Washer shall be tight fit on the rod. Compression of gasket shall seal the leak.

If above works, replace present solid link with flexible woven tinned Cu link - so that there is no bending stress on the bushing.

If the leak is from 2 above, can try following technique:

Use toilet soap cake, which shall be softened.

Take a small piece and press over leaky joint. ( I have experienced that soap does not dissolve in oil and blocks the leak for few minuets). I have used this for sealing leaks in small distribution transformers up to 2000kVA.

Immediately clean the oily surface around leaky area thoroughly either with paint thinner, or petrol or CTC (carbon tetra chloride) or whatever solvent permitted in your country by Health Safety Regulations.

Keep quick drying epoxy (5 minuets curing time), apply over the surface to cover soap and extend on all sides on metal to form a lid.

Wait it to cure and if no leak - apply normal curing epoxy over it. If leak persists (sometime it remains but reduces), apply soap again, and repeat above process.

If surface permits, you can cut small pieces of Fibre Glass insulation tape and apply in layers over the surface. This will give lasting effect.

thank you.we have a shut down of the transformer on monday.will check and try to stop leak as you suggested.will let you know

Let us know a little more about the transformer. I assume the bushing in question is below the oil, is that correct? Is it at the top of the unit or the bottom? Is the transformer a conservator type with a tank of oil over the transformer, or does it have a gas space above the oil? (gas oil seal) These facts would allow more recommendations to try on Monday.

the bushing is almost in the center of the transformer tank.Tank is completely filled with oil and the transformer has an oil conservator above the main tank of the transformer

i am attaching another photo showing the location of core grounding box in the transformer.Temporary pipe is to collect the leaking oil in a drum

Probably the earth point is provided external to transformer to monitor insulation of core with respect to ground( the clamping bolts used in core are insulated to not to short the laminations and the core is placed on insulated pads - all this to minimize eddy currents).

Out of interest - you can also check for any current through earth link in energized condition of transformer- using Clamp on ammeter. Normally there shall not be any current unless insulation to earth is weak at one point - resulting in circulating current due to potential difference. To avoid false reading use True 50 HZ RMS clamp on ammeter as there can be misleading results from capacitive currents through insulating due to 3 rd harmonics and its multiple from saturation of core.

In case you find any current - you can check healthiness of core to earth insulation during shutdown on Monday - by removing link from earth and then checking IR value using a Megger. Do not use more than 100V DC. (we do not want to form any tracking path on insulation surface due to sludge by applying higher test voltage. 100V is more than adequate to test condition of such insulation which is not subjected to higher potentials in service. I normally start with a multi-meter which usually has 9V battery, if OK go to 50V and next 100V. I am keeping insulation tester of 0 to 500 V with 50/100/250/500V range and is very useful in testing such cases and old insulations without damaging anything).

Better keep a log of this value (hope there must be some guide lines by the Manufacturer. Also in Factory Test Report there shall be value of this insulation when new)

Hope I am not writing too much. Do not know of your age/experience, myself 65 now- having own business, now a days bit free and so my manpower as no business at hand and able to devote my experience for benefit of others.

you are being of great help.i work in a hydel power station of 3600MW capacity.51 years of age

A core ground test is a useful test, although it's primary use is when the transformer is first received after shipment. I notice that your core ground bushing is near the bottom of the tank. After you test the core for ground (Don't be afraid to use a meggar at 250 or 500, a meggar cannot produce enough energy to cause tracking) I would still recommend fabricating a plate and gasket to cover the bushing after removing the existing cover. If you do anything to disturb the bushing you are more likely to cause the leak to get larger rather than smaller. With a sealed plate over it, the leak will be stopped and when you decide in the future to do another core ground test, you can simply remove the cover with a bucket to catch the oil. Typically on this type of repair, I would tap a hole in the cover near the bottom to put a pipe plug into to more easily drain it if needed.

Please note that the core ground connection should never be disturbed while the transformer is energized!!! I am surprised that no one has mentioned this, but on a transformer of this size the voltage from the bushing to ground could ride up quite high if the ground is not in place. People have been killed by attempting to remove core ground connections on energized transformers.