No-load loss of transformer and reference to the highest rated power 2

[Krustabas]

Hello.

I’d like to ask if anyone knows why no-load loss of multi-winding power transformers is not referenced to the winding with the highest rated power as it is done with no-load current?

In IEC 60076-1 it is mentioned that no-load current, load-loss, and short circuit impedance can be referred to the highest/lowest rated power. At the same time nothing is said about no-load loss.

Have a look at the following example (form ABB handbook about testing of power transformers):
Power: 146,5 / 146,5 / 40 MVA
Voltage: 250 / 165 / 24 kV
Current: 338 / 513 / 962 A
Measured values during no load test: Uo = 24 KV / Io = 4,55 A / Po = 98,98 kW.
I understand how and why no-load current is expressed in % and referenced to the highest rated power resulting in Io = 0,129 %, but I do not understand why Po is left at 98,98 kW?

The same can be observed in Electron CAPE software where parameter “Magnetizing branch MVA” influences only no-load current.

Am I missing something?

Thank you.

 

[prc]

No-load loss depends only on rated voltage and frequency.It will be there whether the load is zero or full rating. No-load current is expressed as a % of full load current so that one need not mention the reference circuit.

 

[mgtrp]

Magnetising power (not current - which will change based on voltage ratio) should be more or less the same from each voltage source, as all three windings are being energised regardless. Under load, everything will change!

 

[waross]

Apples and oranges.
Transformer full load capabilities are expressed in VA, KVA or MVA.
No load losses are expressed in Watts.

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

 

[Krustabas]

Actually I still have some doubts about this. Just to be clear – the problem is with multi-winding transformers with different rated powers.

It is clear that no-load current can be referenced to the highest rated power (therefore it is not enough to know only the % value of the no-load current - I have to know how it is obtained as well). It is confirmed by IEC standard, manuals of manufacturers, and numerous examples.

For instance, in the example given in my first post the no-load current can be presented in different ways:
a) Io = 4,55 A at 24 kV and 40 MVA (need to know base kV and base MVA);
b) Io = 0,662 A at 165 kV and 40 MVA (need to know base kV and base MVA);
c) Io = 0,473 % at 40 MVA (need to know base MVA);
d) Io = 0,129 % at 146,5 MVA (need to know base MVA);
etc.

The magnetizing current remains practically constant and so does the no-load loss. These quantities are closely related and I can't figure out why only one of them is refered to different base value.

 

[Krustabas]

Thanks to all respondents.

I mislead myself with wrong interpretation of:
a) the sentence: "...for a multi-winding transformer this percentage is referred to the winding with the highest rated power..."
b) and numerical example that looked like this: (Io/Irated)*(S3/S1).

Current is not referred to the highest rated power but to the winding of the highest rated power. No-load current is simply referred to different voltage and no-load loss stays the same.