Neutral Grounding (Zig-Zag) Transformer
Neutral Grounding (Zig-Zag) Transformer
(OP)
Hi,
Can anyone help me to calculate the per-unit value for this neutral grounding (Zig-Zag) transformer?
Nameplate data: Rated 800 kVA, Zeo sequence Impedance 85 ohms per phase, H0 Current: 1000A for 10Sec, 30A Continuous.
Voltage 34500/19919V.
I need to know the per-unit value so that I can caluculate the phase-ground fault level.
Thanks.
Can anyone help me to calculate the per-unit value for this neutral grounding (Zig-Zag) transformer?
Nameplate data: Rated 800 kVA, Zeo sequence Impedance 85 ohms per phase, H0 Current: 1000A for 10Sec, 30A Continuous.
Voltage 34500/19919V.
I need to know the per-unit value so that I can caluculate the phase-ground fault level.
Thanks.






RE: Neutral Grounding (Zig-Zag) Transformer
RE: Neutral Grounding (Zig-Zag) Transformer
Thanks.
RE: Neutral Grounding (Zig-Zag) Transformer
Let me do some checking. I thought I knew the answer, but I've found conflicting information.
Basically, if you know the impedance in ohms, you just convert to pu using standard pu conversion equation.
But I'm a little unclear if the 85 ohms is each phase individually to ground, in which case the effective Zo is one-third of that value, or if that is the equivalent reactance through all three phases. And I can't get either approach to match up with the 1000A rating you found on the nameplate.
Maybe someone else has the answer, but I will do a little more checking.
RE: Neutral Grounding (Zig-Zag) Transformer
RE: Neutral Grounding (Zig-Zag) Transformer
But this is still something of a guess. I'm still trying to get a more definitive answer.
No one buys these things anymore, so there isn't much data available.
RE: Neutral Grounding (Zig-Zag) Transformer
Here is the Z p.u. calculation(based on the transformer KVA rating):
Z p.u. = Z actual / Z base
Z actual = 85 Ohms
Z base = ((19919)^2)/(800 x 10^3)
Z p.u. = 85 / 500
Z p.u. = .170 = (Zero Sequence Impedance p.u.)
RE: Neutral Grounding (Zig-Zag) Transformer
Bung
Life is non-linear...
RE: Neutral Grounding (Zig-Zag) Transformer
I checked this with an expert and although I don't quite understand yet why this is, I believe this to be the case.
Sorry for the confusion - but thanks for the question - I learned something.
RE: Neutral Grounding (Zig-Zag) Transformer