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Neutral Grounding (Zig-Zag) Transformer

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.

RE: Neutral Grounding (Zig-Zag) Transformer

The 85 ohm figure would seem to imply an associated grounding resistor along with the transformer.  Does this nameplate data include the resistor?  

RE: Neutral Grounding (Zig-Zag) Transformer

(OP)
There is no grounding resistor attached to the zig-zag transformer.

Thanks.

RE: Neutral Grounding (Zig-Zag) Transformer

ProtectionEngineer,

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

The rating gives you the answer, the zero seq impedance is 85 ohms per phase. The other ratings are purely withstand ratings ie the designers sizing for the application.

RE: Neutral Grounding (Zig-Zag) Transformer

As best I can determine, the 85 ohms is a per phase value, so the Zo in your equivalent circuit is actually 1/3 of this, converted to the appropriate per unit quantities.  

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

ProtectionEngineer

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

You might need to reconsider an almost entirely reactive impedance for the grounding.  It is generally considered a bit of a no-no to have reactance grounded systems, due to the problems with power frequency overvoltages under arcing earth fault conditions.  I don't recall all the details, but the mining industry in South Africa had some problems with it in the 1970s.  There were a few papers in the SAIEE journal - people like a Dr Pretorius at the CSIR were involved with it back then, I think.  Eastern Natal Region of Eskom used an NEC / NER combination at 11kV to keep earth fault levels to 300A, and the R componenet was  to ensure that the system was not reactively earthed.

Bung
Life is non-linear...

RE: Neutral Grounding (Zig-Zag) Transformer

I was wrong - you don't need to divide the nameplate impedance by three - just convert the 85 ohms to per unit for Zo.

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

The zero phase current splits into each of the phases by 1/3rd per phase, hence as pu values for currents are phase values then the zero pu impedance is calculated directly from the phase impedance value

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