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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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    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

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  • 0

    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

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