HV transformer - neutral earthing resistor 2

[trbartlett]

I have been told and have no reason to disbelieve that transformers above MV (in my country, 33kV)do not normally have NER's. I have been told that it has something to do with insulation and that it would make the manufacture of a high voltage transformer (i.e. 132/66kV 100Mva)cost prohibitive. I cannot find anything specifically about this on the web and unfortunately I am too curious to just let it go. This has quite a bit to do with a project i am working on at the moment. If someone could fill in the missing detail i would be very greatful.

 

[Marmite]

If you have solid earthing the insulation on the transformer star winding can be graded towards the star point. ie you have progressively less insulation as you approach the star point which makes for a cheaper transformer at the higher voltages say 132kV and above. Balanced against that is the need to keep earth fault currents down to a reasonable level. In my experience at 33kV and 66kV resistance earthing is widespread. At 132kV solid earthing is the norm. The earth fault current would have a major bearing on switchgear rating and on the cable sheath sizes downstream, so if you have a high fault current you need more copper in the sheath/screen.
Regards
Marmite

 

[xxjohnh]

Also reactor grounding is used on 66kV perhaps cheaper than the 66kV resistor.

 

[trbartlett]

Thankyou guys. Next time I visit a transformer manufacturer to witness testing I will take the oportunity to have a closer look at the manufacturing process. I am not sure if this is the proper way to keep a thread going and expand a little on my previous question but here goes...
The client is going to become a customer @ 132kV (previously 66kV) and they are raising questions as to step and touch potential as the TX's in their new substation will have solidly earthed neutrals. They are currently supplied at 66kv, the secondaries at their various substations have NER's. They seem to be comfortable with fault current limiting, so the fact that the new substation will have TX's that have neutrals that are solidly earthed seems to be giving them some reason for concern. I understand the processes etc that take place to ensure earthing at the substation is adequate however, what happens if a fault happens at the downstream substation where the secondary of the 66/(whatever)kv has an NER? Could ther be an issue of step or touch potential there if downstream substation earthing has been engineered for NER configuration? I am thinking that substation earthing is engineered for primary voltage fault current in which case it would make no difference BUT I am new to this so am asking for a little guidence. Thanks in advance.

 

[ScottyUK]

You might find the following useful, courtesy of Central Networks: Primary Substation Earthing Manual and Primary Network Design Manual


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If we learn from our mistakes I'm getting a great education!

 

[Marmite]

Any increase to prospective earth fault current can increase touch and step potentials. Local transformers with earthed neutrals do not normally make major contributions to the site EPR, but they do contribute to the earth fault current for thermal rating purposes. The earthing system may have been designed for a reduced earth fault level and the only way to be safe is to carry out a full earthing study to check whether the existing earthing is sufficient.
Regards
Marmite

 

[odlanor]

trbartlett,
When you change your subestation from 66 to 132kV you have check:
1how much does was the increase of short circuit at switchyard of low level(<66kV)?
2 if you install reactor(more easy to manufacture, no loss)you have to change neutral NBI of your transformer.Your star winding(132kV) should be different NBI graduated.
3-if you do not install reactor, 132kV transmission line should installed with shield wire in all circuit and counterweight cable at beginnig of subestation (6km)you can reduce substantially the potentials at grounding grid subestation.

 

[prc]

Thank you Scotty for those excellent documents.

 

[slavag]

Scotty, thanks!!!!
Great.
Slava

 

[trbartlett]

Many thanks guys for your input. It was very helpful. Cheers.