Transformer neutral earth cable lug cut off 1

[SA07]

Hi

We have just noticed that the lug is cut off. The transformer is 55 MVA 66kV-11kV YNd11. What might be the cause? We have a second identical transformer. On the 2nd transformer, the cable, lug are good.

Our transformer is in service now. Can we wait for the next shutdown to repair it in about 2 months? Can we work with the cable open? What are the risks? Thks

 

[waross]

Issues will occur when the transformer is energized.
Energization surges may cause transient over-voltages on the secondary.
If the transformer goes off-line for ant reason, it may be well to do repairs before re-energizing.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[esperantes]

What does your utility has to say about this?
Is the 66 k V solidly grounded?
Esperantes

 

[cranky108]

How are your 66kV lines directionalized? Might be a problem.

 

[Kiribanda]

Your transformer is WYE GROUNDED on the 66kV side (YNd11). It looks like you are having in-house generation
and exporting power to the grid. Am I correct?
If that is the case, then, when the neutral earthing is cutoff then your 66kv side does not have any reference now.
That means if there is an earth fault on the 66kV system, the Utility will be tripped off from
their end but your transformer will still be "HANGING" on 66kV system and continue to feed the earth fault.
It will create phase to earth over voltages which may exceed the MCOVs of 66kV side lightning
arresters, push single phase VTs and station service transformers into saturation etc etc.
Therefore, my strong opinion is to take a shutdown ASAP and re-connect the neutral solidly to the earth.

 

[SA07]

Yes we export on the national grid.
We have shutdown the plant and repair the connection.

The tubular busbar is aluminium. The cable & lug below is copper. Can bimetallic corrosion be the cause of the damage?

 

[Kiribanda]

I think yes. As you know both cannot survive together for a long time unless there is a buffer zone between them.
So use Aluminum pipe to 2-HOLE NEMA pad connector and then connect the copper cable lug using tinned copper
lugs and stainless steel bolts.

 

[esperantes]

How old is your power plant? How long had you been operating with the 66 k V ungrounded?I hope you're not relying on eng-tips to help you make decisions for the safety of the plant. You need competent and experienced engineers to manage a power station. Did you discuss the matter with your utility?
Esperantes

 

[3DDave]

Original poster is apparently from the island of Mauritius, so likely some salt air as well.

 

[SA07]

Thanks Kiribanda for your advice

 

[cranky108]

Usually the only time we have that problem is for copper theft. We find this out quickly as we have normal distribution system unbalance, and arcing will happen when the X0 is open.

If it were transmission like this case, we might overlook it. Be careful.