I'm told that a 1% transformer costs more than a 5% transformer of the same power rating. I understand that the 5% transformer has higher output impedance, which seems to imply more windings and thinner wire. Is this the only construction difference between a 1% and 5% transformer of the same power rating? Or are there more considerations?
Electronic – What are the construction differences between a 1% transformer and a 5% transformer
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Best Answer
From ABB's Transformer Handbook, 3e:
The 'short circuit impedance' mentioned above is the transformer's percent impedance. The above quote says that the transformer's impedance can be varied by changing the construction of the core.
Note that a transformers' impedance is mostly inductive "leakage reactance", i.e. magnetic impedance. Therefore, the difference between a 1% transformer and a 5% transformer is mostly to do with the design of the transformer's magnetic core. The 1% transformer would require much more iron core than the 5% transformer, and would be physically larger to match, which explains the higher cost.
From J&P Transformer Book, 12e:
The conductor resistance (i.e. copper winding resistance) is typically small, 1/10th of the total impedance or less. The guideline given in AS3851 is that power transformers of less than 10 MVA may be considered to have X/R = 10.