Transformer Impedance Voltage (%Uk) – What It Means and How to Choose

Impedance voltage is one of the few transformer parameters that a purchaser genuinely chooses rather than accepts. It appears on the rating plate as a percentage, and that single number determines the fault current the installation must withstand, how much the output voltage sags under load, and whether the unit can be paralleled with an existing one.

It is also the parameter most often specified by copying a previous project without asking whether the value still fits.

The Definition

Impedance voltage is the voltage that must be applied to one winding, with the other short-circuited, to drive rated current through the transformer. It is expressed as a percentage of rated voltage.

A transformer with 4 % impedance requires 4 % of rated voltage — 16 V on a 400 V winding — to circulate full rated current when the other side is short-circuited. This is measured directly as a routine test on every unit.

Where It Comes From

Impedance is not a component added to the transformer. It is the combined effect of two things inherent in the construction:

• Winding resistance — the resistive component, responsible for load losses. Small in comparison to the reactive part in most designs.
• Leakage reactance — the dominant component. Arises from magnetic flux that links one winding but not the other, determined mainly by the physical separation between HV and LV windings and by the winding geometry.

Because leakage reactance depends on geometry, impedance is set at the design stage by how the windings are arranged. A designer increases impedance by moving the windings further apart or changing their proportions — which also increases size, material and cost.

Standard Values

IEC 60076-5 and EN 50588-1 establish standard impedance values for distribution transformers:

Effect on Fault Level

This is the consequence that most often governs the choice. Short-circuit current at the transformer terminals is inversely proportional to impedance:

The switchgear implication is direct. Moving from 4 % to 6 % on a 1000 kVA unit reduces the fault level from 36 kA to 24 kA — which can be the difference between a standard LV panel and a considerably more expensive one.

Effect on Voltage Regulation

Higher impedance means a larger voltage drop between no load and full load. The approximate regulation is:

For practical purposes, regulation at full load approaches the impedance percentage when the load power factor is low, and is considerably less at unity power factor.

Where voltage stability at the load matters — sensitive electronics, long feeders already carrying voltage drop, installations near the lower limit of the permitted supply range — lower impedance is preferable.

Effect on Motor Starting

Direct-on-line motor starting draws six to eight times full load current for a few seconds. That inrush passes through the transformer impedance and produces a voltage dip across the whole installation.

Where a large motor starts against a relatively small transformer, this dip can be severe enough to drop out contactors, trip undervoltage protection, or stall the motor itself. In such installations the impedance selection may be governed by motor starting rather than by fault level.

Effect on Parallel Operation

Transformers in parallel share load in inverse proportion to their impedances. Two units of the same rating but different impedance will not share equally — the one with lower impedance takes more.

IEC guidance is that paralleled transformers should have impedances within 10 % of each other. Beyond that, one unit reaches its rating while the other still has spare capacity, and the total usable output is less than the sum of the ratings. The calculation and worked examples are in Parallel Operation of Transformers.

Effect on Losses and Cost

Higher impedance requires greater separation between windings, which means a larger core window, more core material, longer mean turn length and more conductor.

The trade-off is genuine and there is no universally correct answer. What matters is that the choice is made deliberately rather than inherited from a previous specification.

When to Depart from the Standard Value

Specify higher impedance where:

• Calculated fault level exceeds the rating of existing or preferred switchgear
• Several transformers will operate in parallel and the combined fault level is the constraint
• The installation is close to a strong network with a high upstream fault level
• Existing switchgear is being retained in a retrofit

Specify lower impedance where:

• Voltage regulation at the load is critical
• Large motors start direct-on-line against a modest transformer rating
• The supply voltage is already near the lower end of the permitted range
• Reactive power consumption is penalised by the tariff

Match an existing value where:

• The new unit will be paralleled with an installed transformer
• A utility standard or project specification states the required value
• The transformer is a like-for-like replacement in an installation whose protection settings are already established

Practical Notes

• Impedance is measured on the principal tap. On a transformer with a wide tap range the value at the extreme taps differs, and the test report should state the values across the range where this matters.
• The percentage refers to the transformer's own rated power. A 6 % impedance on a 630 kVA unit and a 6 % impedance on a 1600 kVA unit are different absolute impedances entirely.
• Where the specification does not state an impedance, the manufacturer supplies the standard value for that rating. If a particular figure is needed, it must appear in the enquiry — changing it after the design is fixed means redesigning the transformer.
• Protection coordination studies should use the measured value from the test report, not the nominal specification, once the unit is delivered.