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I am looking at the results for an earth grid study for a 50Hz substation.

  • Primary Voltage (Vp) = 11,000 V
  • Secondary Voltage (Vs) = 415 V
  • Secondary Fault Current (If)= 24 kA
  • Secondary Grid Current (Ig)= 2.4 kA
  • Ground Resistance (Rg) = 1.7 Ω
  • Calculated Ground Potential Rise (GPR) = 4080 V

I understand that GPR = Ig x Rg, but what is the rationship between Vp/Vs and GPR?

If 4000+ V is available at the earth grid, wouldn't current flow back up the faulted conductor to the source?

Is it related to different reference points?

I would appreciate it if anyone could help me get my head around this one.

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  • \$\begingroup\$ @ocrdu : I notice you've been "editing" lots of zombie posts from years past. I'm wondering why... Similarly, the "community" robot resurrects stale posts from long ago. Many if not most of these have valid answers but just were never accepted. If the original poster didn't accept the answer 3 years ago I doubt it will ever get accepted. Can you (or anyone, a moderator perhaps) explain the benefit of bringing these old posts back from the dead - seems like a lot of clutter, and just encourages people to waste time concocting an answer to deaf ears. Thanks. \$\endgroup\$
    – td127
    Dec 5, 2021 at 18:29
  • \$\begingroup\$ @td127: I only edit the ones already resurrected by the community bot, I don't resurrect any myself. If you don't want to see these posts resurrected, talk to whomever controls the bot. \$\endgroup\$
    – ocrdu
    Dec 5, 2021 at 18:34
  • \$\begingroup\$ OK thanks, that answers half the mystery! \$\endgroup\$
    – td127
    Dec 5, 2021 at 18:36
  • \$\begingroup\$ @ocrdu One thing you might consider is that when we see that "Community" has raised an old question it is easy to ignore, but when it looks like you have modified a question then we can't tell that a zombie has come back. Your modifications, while entirely correct, seem to be mostly cosmetic. Maybe it's better to let sleeping zombies sleep? (I agree they should be deleted, though.) \$\endgroup\$ Dec 5, 2021 at 21:10

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This looks like the specifications for a 1 MVA transformer (2.4kA*415V) with a Zo of 10% so the Short Circuit current If = 2.4k/10%

Vp/Vs=25.1 and secondary , if shorted to earth ground with If =24kA max and a GRP rise of 4kV max until the fault is tripped. Vp would also be attenuated while current rises 10x on the primary as the surge goes from ground to grid source.

When tripped there will also be some expected transient voltage. (V=LdI/dt) But I'm guessing lightning arrestors may clamp this.

schematic

simulate this circuit – Schematic created using CircuitLab

I am thinking of a grid with only 2 sources with a local fault and remote feeders.

There is a better way to show this. But 1st cut above.

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  • \$\begingroup\$ Hi Sunnyskyguy, thanks for the response but are you able to address the questions I raised? How does current flow from the fault into the ground if the voltage in the ground is higher than the source of the fault? \$\endgroup\$ Jan 8, 2019 at 11:32

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