What should be the percentage of impedance voltage for 1000kva and 1250kva , 11kv / 433 volts Transformer, what is the effect on transformer if it changes below and above the standards ?
Normally, as per minimum requirements of supply authorities, %Z of a transformer for 11kv designs is 5%. with IS tolerence of 10% +/- permissible.
Normally, 11kv 1000 kva / 12 50 kva transformers shall be designed with %Z of between 4.5% to 5.5%.
More the %Z of transformer, more Copper used for winding, increasing cost of the unit. But short circuit levels will reduce, mechanical damages to windings during short circuit shall also reduce. However, cost increases significantly with increase in %Z.
Lower %Z means economical designs. But short circuit fault levels shall increase tremendously, damaging the winding & core.
For a transformer, the 'Percentage Impedance Voltage' and 'Percentage Impedance (%Z)' mean the same thing.
The high value of %Z helps to reduce short circuit current but it causes more voltage dip for motor starting and more voltage regulation (% change of voltage variation) from no load to full load.
The effect of the low value is the other way around.
- Short the secondary side of the transformer with a current measuring device (Ammeter)
- Apply low voltage in primary side and increase the voltage so that the secondary current is the rated secondary current of the transformer. Measure the primary voltage (V1).
- Divide the V1 by the rated primary voltage of the transformer and multiply by 100. This value is the percentage impedance of the transformer.
-MS
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Impedance Voltage for a Transformer
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