Earthing mat design with YnYn transformer

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

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

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?

Hi Cameron,

I've been looking into your query about the earthing system design for your secondary substation with a 132/22/22kV YnYnYn transformer. I understand that your design software (ELEK Safegrid V8) has some limitations, so here are a few points to consider:

1. Earth Fault on Local Earth Mat: If an earth fault occurs directly onto the local earth mat, the impedance to the local 132kV neutral earth will be much lower than the ground return path to the upstream primary substation's transformer 132kV neutral earth. This means the ground return current will be negligible, and EPR might not be a major concern in this specific case.

2. Earth Fault in Connecting Cables: If an earth fault occurs in the cables connecting the substations, ground current could flow to either neutral earth point via each of their earth mats. In this situation, EPR could be a concern, as fault current might take a path through the ground, causing a rise in ground potential.

3. Software Limitation: The limitation of ELEK Safegrid V8 in not being able to model returns to a remote substation through both the soil and directly wired conductors can be an issue. You might want to look into alternative software or methods that allow for more comprehensive modeling of fault scenarios, including the ability to attach a neutral-earth point.

Additional Resources:

• ENA TS 41-24: This standard provides calculations that estimate the share of current, taking into account line geometry, and could be useful for your analysis.

• ETAP Software: ETAP offers comprehensive features for analyzing and designing grounding and earthing systems. It integrates with short-circuit studies and single-line diagrams, allowing for more accurate modeling.

I hope this helps clarify your concerns and provides a direction for your design work. If you need further assistance, feel free to reach out!

Best regards, 

Gary R Jones