Transformer Percent Impedance
Transformer percent impedance is the internal opposition to AC current flow, expressed as the percentage of rated voltage needed to drive rated current when the secondary is short-circuited under specified test conditions. It is a key parameter for fault current, voltage drop, breaker/fuse sizing, and parallel-transformer matching in power systems.
On the shop floor, percent impedance is used during equipment selection and electrical coordination studies to predict short-circuit current a transformer can supply to downstream gear. It affects protective device application: lower impedance yields higher fault current, requiring different overcurrent protection and verifying equipment ratings. Canadian procurement documents often specify impedance limits, e.g., not greater than 6% for low-voltage transformers or 5% to 7% for medium-voltage. Impedance is a manufacturer's guaranteed value with tolerances, such as ±15% per CSA standards, and is critical for fault-current calculations and parallel operation.
- Ignoring impedance when calculating fault duty can lead to undersized breakers, fuses, or switchgear due to higher short-circuit current from lower-impedance transformers.
- Specifying the wrong impedance range disrupts voltage regulation, coordination, or parallel operation, violating project specifications or utility criteria.
- Assuming nameplate impedance equals actual performance without checking tolerances and test conditions distorts coordination studies and compliance checks.
What is the core test definition of percent impedance?
It is the percentage of rated voltage required to circulate rated current through a transformer winding with the other winding short-circuited, under specified test conditions.
Why does lower impedance increase hazard severity?
Because lower internal opposition allows the transformer to supply more secondary fault current, increasing arc-flash and equipment duty concerns.
Why is impedance important for overcurrent protection selection?
Protection settings and device interrupting ratings must be coordinated with the transformer's available fault current; impedance is a main input to that calculation.