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Earthing mat design with YnYn transformer

Cameron Hart

Hello,

I'm struggling to get my head around the following:

I'm conducting earthing system design for a secondary substation with a 132/22/22kV YnYnYn transformer connected via overhead line to the upstream primary substation.

If an earth fault occurs directly onto the local earth mat, the impedance to the local 132kV neutral earth will logically be much less than the ground return path to the upstream primary substation's transformer 132kV neutral earth, hence ground return current will be negligible. Is EPR even a concern in this case?

Therefore the only situation I can imagine where the earth mat design will need consideration for EPR is if the earth fault appears in the cables connecting the substations, meaning ground current could flow to either neutral earth point via each of their earth mats?

However, my design software (ELEK Safegrid V8) appears to only provide fault cases where the earth current is injected directly to the local earth mat, and there is no option to attach a neutral-earth point (and no NER). 

Any help would be greatly appreciated.

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  • +1
    Cameron Hart said:
    132/22/22kV YnYnYn

    Yes this is pedantic but do you mean YNynyn? Or is it deliberately different?

    Cameron Hart said:
    the local 132kV neutral earth will logically be much less than the ground return path

    This rather depends on the specifics of the site, ie the relative impedances of the cable vs the earth grid. The current into the soil won’t be zero, though it may be a small fraction of the total earth fault current. There are calculations that allow you to estimate the share taking into account line geometry etc in either ENA S34 and ENA TS 41-24 (I normally end up with both open so can’t remember which now I’m not at desk) (and EN50522 for non-UK installations or those happy with other-than-DNO-spec solutions)

    Cameron Hart said:
    Is EPR even a concern in this case?

    1% of 132kV is still a lot so yes in principle, it should be checked and shown to be okay

  • 0 in reply to Jam

    Thanks for the response Jam. Yes it's YNynyn.

    It seems the main issue is that I can't add a connection from the local earth mat to the local transformer neutral / star-point.

    As you say, even a small ground return current to the upstream transformer neutral could create a significant EPR. But this will still be a lower (and easier to mitigate with mat design) than 100% ground return to the upstream transformer, which is what the software will surely model without the local neutral-earth being implemented. 

    Do you agree? Thanks and regards

  • 0 in reply to Cameron Hart

    I'm not an earthing consultant (but I do review their reports frequently) and I am not familiar with the software you mentioned previously but if it is not possible to model with returns to a remote substation through both the soil and directly wired conductors, that sounds like a fairly serious (and basic) limitation, granted it doesn't come up on all sites. Looking briefly at the product, it does seem that what they call "above ground earthing" is an extra... I'm not sure what that entails but perhaps you need the next price bracket up?

    If it's YNynyn, are the neutrals interconnected and solidly connected with a neutral / earth conductor back to the source?

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  • 0 in reply to Cameron Hart

    I'm not an earthing consultant (but I do review their reports frequently) and I am not familiar with the software you mentioned previously but if it is not possible to model with returns to a remote substation through both the soil and directly wired conductors, that sounds like a fairly serious (and basic) limitation, granted it doesn't come up on all sites. Looking briefly at the product, it does seem that what they call "above ground earthing" is an extra... I'm not sure what that entails but perhaps you need the next price bracket up?

    If it's YNynyn, are the neutrals interconnected and solidly connected with a neutral / earth conductor back to the source?

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