What is the impedance percentage of the described 750-kVA transformer?

What is the impedance percentage of the described 750-kVA transformer?

• A. 3.0%
• B. 5.75%
• C. 7.5%
• D. 10.0%

Answer: (B)

Impedance percentage shows how much voltage is needed to push the transformer's rated current when the secondary is shorted, reflecting leakage reactance and resistance in per-unit terms. You find it with a short-circuit test: apply voltage to the primary with the secondary shorted until the current reaches the rated value, then express the applied voltage as a percentage of the rated primary voltage. For this 750-kVA transformer, a value of 5.75% means that about 5.75% of the rated primary voltage is enough to carry the full load current with the secondary shorted. In practical terms, the short-circuit current from the source would be roughly the rated current divided by 0.0575, i.e., about 17 times the rated current, if you applied the full rated voltage. This level of impedance is typical for a unit of this size, and it directly influences voltage regulation under load as well as fault current capability. A different impedance percentage would change those behaviors, which is why the 5.75% figure is the correct match for the described transformer.

Impedance percentage shows how much voltage is needed to push the transformer's rated current when the secondary is shorted, reflecting leakage reactance and resistance in per-unit terms. You find it with a short-circuit test: apply voltage to the primary with the secondary shorted until the current reaches the rated value, then express the applied voltage as a percentage of the rated primary voltage. For this 750-kVA transformer, a value of 5.75% means that about 5.75% of the rated primary voltage is enough to carry the full load current with the secondary shorted. In practical terms, the short-circuit current from the source would be roughly the rated current divided by 0.0575, i.e., about 17 times the rated current, if you applied the full rated voltage. This level of impedance is typical for a unit of this size, and it directly influences voltage regulation under load as well as fault current capability. A different impedance percentage would change those behaviors, which is why the 5.75% figure is the correct match for the described transformer.