Burnt 34.5 KV Cables

Sorry but things don't tally here.

Why have 30MVA of transformers for 4MVA of load?
Why use 34.5KV? The equipment costs wouldn't warrant it.

As to the termination failures, look to the cable contractors and kick arse.

Thanks for your reply.

As mentioned in the report, it's 3 MVA transformer not 30 MVA.

Thanks for giving an outline of your system, it helps a lot! I suppose your 34.5/0.38 kV transfos have 34.5 kV terminals in open air, since you are using Raychem. It is possible the seals are fine but the air gap is breaking down or dirt on the bushing insulator is starting conduction causing arc which damages seal. Some places have conductive dust or salt in the air which coats insulators or reduces air gap insulation, for example, 150 KV class insulators have to be used for 100 kV line. Since you have not mentioned any frequent intended switching at 34.5 kV to cause voltage surges or any event at the same time as the failures which could be a cause, this is my first suggestion. In places where there is no rain to regularly wash insulators, this can be a problem. On the other hand, you have not mentioned any arc damage to metalwork or bushings and maybe the damage is all in the Raychem seal, which could be bad seal installation - your plant is "just built". If experienced personnel have replaced the seals mentioned and you never get another fault at the same transformer and phase again the problems will be solved in time, but if this is the case it may be better to replace all the seals as soon as possible before the plant is in full operation. In any case, you have reason to question the contractor as to why these faults have occurred so soon after commissioning and task him with investigating the cause and removing it at his cost.

Many thanks for your comments and suggestions. I would like to send images / pictures of the burnt cable so the explanation will be more clear, but technically couldn't manage to attach or insert them in this report.

Please send me your e-mail so I can send the images of the burnt cables. My e-mail address is mi7f@hotmail.com

I asked CR4 Admin how to post pictures and they gave me the following :- http://cr4.globalspec.com/blogentry/12466/How-to-Post-Pictures But try to make the picture "jpeg" or .png, bitmap (.bmp) makes huge files from photos, which make CR4 over-filled. I wish you success with your problems.

It would appear as others have said a large mismatch between generator and load. It would be worth checking the power factor maybe there is a high current node at the connections?

Not overcurrent. Only one gen-set is running, it is 2MW, say it is 3MVA. Every transfo is 3 MVA, and every substation has two transfo in parallel. How can a 3MVA transformer be overloaded by a 3 MVA generator?? With so many transformers, the power factor may be low and lagging, but only the generator might find trouble from that

Thanks for your reply,

The load on the generator as of now is 1250 KW, 0.9 pf lag. As mentioned in the report, we are now in a test commissioning mode and we are testing each equipment before final commissioning. It's good for me that these problems are happening now not later after final start up.

Power factor and load are good. If you had 30% load, I would have written that "sooting up" or "coking" of engine may be a problem in long-time operation. Regards

OK, we will do so. Regards.

Bending radius of cables should have been maintained at all stages also there is a possibility that the terminals of transformers were dirty/greasy and the workers did not clean before applying the cable contact.

Has the cables been subjected to HI-pot test after installing the termination kits.

Please check the air gaps between arcing horns across the transformer bushings, also check the settings of over current relay elements.

You did not mention if you have used 3 core or single core cables, in case of parallel cables the lengths of any parallel run should be symmetric otherwise unbalanced load sharing happens and causes stress.

For outdoor and indoor terminations Raychem advises different types of kits.

Many thanks for your reply.

Bending radius were maintained. Yes, it was possible that the terminals got dirty, but the contractor confirmed that the terminals were cleaned before applying the cable contact.

HI-pot test was applied to the cables after installing the termination kits, and the results were acceptable.

We will check the air gaps between arcing horns across the transformer bushings, and also check the settings of over current relay elements.

We have used single core parallel cables.

We will contact Raychem and inform them about the kit we used so they will confirm if we used the correct type or not.

I do not know if this is the best answer to resolve this, years ago, I would work with burnt power transfer switches on 8-12kw generators and we discovered that the slightest resistance would build until failed connections opened. We started coating ALL or repaired terminations with a silver oxide compound from a company called AMP.

The silver poweder compound would cover the wire/cable and the new terminal on a surface micron level coating..... however it worked we stoped our terminal end failers.

You can trend the terminal with a FLIR camera to check for heat resulting from poor conections just to insure you are on the right track. Good Luck!

Dear MILF, improper cable termination causes the contact surface area reduces and increase the resistance to current path,.

Yes agree. We requested the cable manufacturer will take this burnt cable for testing and inspection. Later on, they will inspect one of the "good" cable termination. Regards