Transformer test report - zero sequence values

[chuckd83]

On a 138/25 kV, Dyn1 transformer test report, I'm given
Test = LV - HV open
MVA = 43.0
%Z0 = 8.11%
Ohms = 0.393

Is there a way to calculate the zero sequence X/R from this?

Edit: I think the %Z0 and ohms are saying the same thing.
Zbase = (25/sqrt(3))^2 / 43 = 4.845 ohms
Zpu = 0.393 / 4.845 = 0.0811

I thought the 0.393 may be the R, but I guess not.

 

[JJ_Roy]

The ohms are probably percent (0.393%), so your X/R would be = 20.6

What's your Z1%??? A check would be Z0% is approx. 90% of Z1% but others may disagree.

 

[waross]

Using your numbers:
Z = 4.845 Ohm
R = 0.393 Ohms
4.845^2 = 23.474
0.393^2 = 0.15445
23.474 - 0.15445 = 23.32
X = Root 23.32 = 4.829 Ohms
X/R = 4.829 Ohm/0.393 Ohm = 12.29

 

[dpc]

With a delta-grounded wye connection, the Zo is generally approximated at 0.85 of the positive sequence impedance, at least in this part of the world. If you need something more precise, you need to contact the transformer supplier. In the ANSI world, it's not a standard factory test.

 

[davidbeach]

A bit closer than Dave's 85% is R0 = R1 and X0 = 0.85 * X1. But that's for generic core form 3-phase transformer with three legs. More legs (but unusual on a DY), shell form, or a bank of three single phase units result in X0 = X1. Some designs and material selections can also push that 0.85 a bit higher. But then you'll never actually have that impedance during a real fault; it's the value at one very specific set of conditions and movement away from those conditions changes the actual impedance.

We've been using R0 = R1 and X0 = 0.85 * X1 for many years and haven't found a specific need to change it. Sometimes we get faults that suggest we should have gone lower than 0.85 and we get faults that suggest that 0.85 is too low. But no clear pattern that suggests anything other than that most impedance values are calculated with too many significant digits.

 

[prc]

1) When we have test report of Z0 measurement, why we go for assumptions?
2) Zo=8.11 %
3) To me 0.393 seems R0 in %. With 0.393 ohms, I2 R loss will be very high, much more than typical copper loss of such a transformer.
4) So X0= Square root( 8.11 square-0.393 square) = 8.1 %
5) X/R = 8.1/0.393 = 20.8

 

[dpc]

prc,

In my experience in the US, it's rare to find Z0 on a transformer test report, except for very large transformers.

 

[System Protection]

On a 138/25 kV, Dyn1 transformer test report, I'm given
Test = LV - HV open
MVA = 43.0
%Z0 = 8.11%
Ohms = 0.393

Is there a way to calculate the zero sequence X/R from this?

Edit: I think the %Z0 and ohms are saying the same thing.
Zbase = (25/sqrt(3))^2 / 43 = 4.845 ohms
Zpu = 0.393 / 4.845 = 0.0811

I thought the 0.393 may be the R, but I guess not.

Is that MVA for a 3 phase transformer? Or do you have three single phase transformers?

 

[prc]

prc,

In my experience in the US, it's rare to find Z0 on a transformer test report, except for very large transformers.

Agree, dpc. When it is not measured, best way to judge Z0 is IEC 60076-8 where all possible connections are covered with approximate Z0 for each case.