Impedance (%Z)
Total opposition to current flow in a transformer as a percentage, determining fault current and voltage regulation.
Transformer impedance (%Z) is total opposition to current, expressed as % of rated voltage to circulate rated current with one winding shorted. Determines fault current, voltage regulation, and parallel compatibility.
Components: resistance (R%) and reactance (X%). For large transformers, X >> R. Typical: distribution 4-6%, power 8-15%.
Parallel operation requires matching impedance (within 10%). X/R ratio affects fault asymmetry.
QDTB designs impedance to meet project requirements. See selection guide for impedance guidance.
Related Products
Related Terms
Short-Circuit Impedance (%Zsc)
Transformer impedance measured with one winding short-circuited, determining fault current and regulation.
Voltage Regulation
The change in secondary voltage from no-load to full-load as a percentage, indicating voltage stability.
Fault Current
The extremely high current during a short circuit that transformers must withstand mechanically.
Leakage Reactance
Reactance from magnetic flux not linking both windings, a major component of transformer impedance.
More Terms Starting with “I”
Inrush Current
A high transient current when a transformer is first energized, caused by core saturation, reaching 10-30x rated.
Insulation Class (A/E/B/F/H)
Standardized temperature ratings for transformer insulation materials defining maximum operating temperatures.
IEC 60076
The IEC standard series defining requirements for power transformers, the global reference standard.
IEEE C57
The IEEE standard series for transformers, the primary reference for the North American market.
More in “Performance Parameters”
Core Loss
Power dissipated in the transformer magnetic core due to hysteresis and eddy currents, also called iron loss or no-load loss.
Copper Loss
Power lost as heat in transformer windings due to electrical resistance, proportional to the square of load current.
Efficiency Class (IE1-IE4)
International classification for transformer energy efficiency, IE1 (standard) to IE4 (premium) with progressively lower losses.
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