Zero-sequence impedance vs size of transformer 1

[CuriousElectron]

Greetings,
Am I correct in thinking that if zero-sequence impedance value goes up, so does the dimensions on the transformer? More loops of copper around the core would increase the size of the unit.
How does that correlate with the kVA rating of the transformer? Is it a direct relationship between the impedance and the kVA of the unit?

Thanks,
EE

 

[waross]

Ohmic value goes up.
P.U. value depends on the transformer design and is not directly related to the size of the transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[argotier]

Impedance might have some influence on the transformer dimensions, but there is no simple relation to follow.

Now for kVA size, waross is right, but usually with increasing kVA it corresponds increasing values of impedance.

For IEC 60076-5 there is a table for minimum recommended impedances vs kVA (as kVA increases so does the minimum impedance).

And for ANSI/IEEE C57.12.10 there is a similar table but listed for different HV BIL in kV (increasing values of impedances with increasing BIL).

There is no law relating the two, in either case. It's common practice for limiting the short-circuit currents in the grid.

 

[waross]

Sometimes transformers are re-rated.
When a transformer is re-rated the Ohmic impedance stays the same but the PU impedance is now calculated on a different base KVA and does change.
Example:
Two 50 KVA transformers are to be connected in parallel for emergency service.
Transformer #1 has a PU impedance of 5%
Transformer #2 has a PU impedance of 2.5%
If transformer #2 is derated to 25 KVA, the Ohmic impedance stays the same, but the PU impedance on the new KVA base becomes 25 KVA.
The transformers will operate in parallel and the maximum loading will be 75 KVA.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

The zero sequence impedance of a transformer can be estimated as a fraction of the positive sequence impedance. A practical rule of thumb is a follow:
[ul]
[li]Delta connection :.................................X₀ = (80%-100%).X₁[/li]
[li]Non-delta w/3 leg core (Ex YG/Y):........X₀ = (30% - 60%).X₁[/li][/ul]

For power transformers in the MVA size and for distribution transformers up to 500 kVA, the general trend observed is that the impedance goes up as the transformer size increases. However, for unit substation transformer 500 kVA and larger the tendency is that the overall transformer impedance Z% will remain unchanged or reduced with the transformer kVA rating.

We hope the table below helps to provide an illustration of the impedance correlation with the transformer capacity in kVA for a different type of design.

>>>>

 

[prc]

1) First let us consider positive sequence impedance. There is no law limiting percentage impedance for transformers. Some loco transformers may have 40% impedance for a 6 MVA unit and <1 % for a 200 MVA special transformer used in a Power Lab where short circuit testing is done. But for normal transformers used in service, % impedance is limited to a band for each kVA rating due to geometrical dimensions of core and coil. For large units even though a high % impedance may be required from dimensions, it is brought down to avoid too much voltage regulation.Similarly 120 years back distribution transformers were provided with 1-2 % impedance to limit voltage drop. But the protective benefit of impedance ( to limit short circuit current) was understood later and nowadays minimum 3-5 % is used for distribution transformers in various parts of the world.For the same kVA rating, higher the impedance,unit becomes a copper machine. Lower impedance means a core machine.
2) Zero sequence impedance value depends on the connection of the excited winding, type of core used and certain connections on the positive sequence impedance. For example for a YNy transformer, Z0 from Yn side will be 50 % for 3 limbed core and 10,000 % for a 5 limbed core. For a YNyn unit, it will be 0.8-0.9 of Z for 3 limbed core and same as Z for a 5 limbed core.Please see Table 1 of IEC 60076-8 for Z0 for all options in transformers