Two transformers can have the same rated power and voltage but behave very differently in the same system. One of the main reasons is short-circuit impedance.

This parameter affects prospective fault current, voltage regulation, protection coordination, motor starting and parallel load sharing. It also influences the rating of switchgear, busbars, cables and other equipment around the transformer.

A lower or higher value can both be reasonable, depending on the system. Problems usually arise when the impedance is selected without reference to the short-circuit study, load profile and protection design.

Short-circuit impedance is usually expressed as a percentage. During the test, one winding is short-circuited and a reduced voltage is applied to the other winding until rated current flows. The applied voltage, expressed as a percentage of rated voltage, is the transformer’s impedance value.

For example, a transformer with 5% impedance requires approximately 5% of rated voltage to produce rated current under short-circuit test conditions.

Ignoring upstream system impedance, the initial symmetrical fault current at the transformer terminals can be estimated as:

Isc ≈ Irated ÷ Zpu

A 5% impedance corresponds to approximately 20 times rated current. An 8% impedance corresponds to approximately 12.5 times rated current. Actual system fault current will also depend on source impedance, conductor impedance, grounding method and system X/R ratio.

Lower impedance generally improves voltage regulation. The voltage drop across the transformer is smaller under load, which can help with motor starting, large load pickup and applications that are sensitive to voltage variation.

The tradeoff is higher prospective fault current. Circuit breakers may need greater interrupting capacity, while busbars, cables and switchgear must withstand higher short-circuit duty.

Inside the transformer, a through-fault produces strong electromagnetic forces on the windings. These forces rise rapidly with fault current and can stress winding conductors, spacers, leads and clamping structures. Repeated external faults may increase the risk of winding displacement or mechanical deformation if the transformer and protection system are poorly coordinated.

Low impedance can also increase project cost outside the transformer. Higher fault-rated switchgear, stronger bus systems and additional current-limiting measures may cost more than the transformer design itself saves.

Higher impedance limits fault current. This can reduce the interrupting duty placed on circuit breakers and lower the short-circuit stress on downstream equipment.

The main concern is voltage drop. Under heavy load, motor starting or sudden load pickup, a high-impedance transformer may produce a larger voltage dip. This can affect motor acceleration, contactor operation, process equipment and other voltage-sensitive loads.

High impedance can also reduce fault current to a level that makes protection coordination more difficult. For remote or lower-level faults, relay and fuse settings may require closer review to maintain adequate sensitivity and clearing time.

A higher impedance value does not automatically mean higher transformer losses or lower reliability. The result depends on winding geometry, leakage flux control, conductor arrangement and thermal design. However, an unusually high custom impedance may require additional design work and can increase stray loss or local heating if leakage flux is not properly managed.

There is no simple rule that says a higher or lower impedance transformer will always cost more. Reaching a project-specific impedance may require changes to winding dimensions, conductor arrangement, insulation clearances, magnetic shielding and structural support.

A value outside the manufacturer’s normal design range often requires additional engineering and production control. Tight impedance tolerances can also increase design, material and testing costs.

The broader system cost may be more significant:

Low impedance may require higher fault-rated breakers, switchgear and bus systems.

High impedance may require additional voltage support, different motor-starting arrangements or changes to conductor sizing and protection settings.

For procurement teams, the lowest transformer purchase price may therefore lead to a higher total installed cost if the selected impedance shifts additional requirements to the rest of the system.

Parallel transformers need compatible voltage ratios, vector groups, tap positions and impedance characteristics. When two transformers have different percentage impedances, they will not share load equally.

For two transformers with equal MVA ratings, the unit with lower impedance carries more current. If one transformer has 5% impedance and the other has 7%, the 5% unit will take a larger share of the total load and may reach its rated capacity first.

Impedance magnitude alone is not enough for a detailed parallel-operation study. The X/R ratio, voltage ratio, tap setting and winding connection also affect current sharing and circulating current.

Short-circuit impedance is an important system parameter, but it cannot be used alone to judge transformer reliability.

For low-impedance designs, winding mechanical strength and short-circuit withstand capability require careful attention. For high-impedance designs, voltage regulation, leakage flux, local heating and protection sensitivity need closer review.

Insulation design, conductor temperature, winding clamping, manufacturing consistency, routine testing and protection clearing time all contribute to long-term performance.

The appropriate impedance is the value that works with the complete electrical system and can be produced, tested and maintained within the approved design.

• Rated MVA and the capacity base used for the impedance value

• Rated voltage, frequency and applicable winding pair

• Tap position used as the impedance reference

• Reference temperature for the guaranteed and tested value

• Target impedance or acceptable impedance range

• Applicable tolerance under the governing standard or project specification

• X/R ratio if required for short-circuit or protection studies

• Available utility fault level and upstream source impedance

• Motor-starting, large load pickup and voltage regulation requirements

• Parallel-operation requirements with existing or future transformers

• Factory test report and final measured impedance

For multi-winding transformers, the RFQ should state which winding pair each impedance value applies to. A single percentage value may be insufficient when several voltage outputs or tertiary windings are involved.

There is no universal “best" short-circuit impedance. A lower value improves voltage regulation but increases fault current. A higher value limits fault current but creates more voltage drop and may complicate protection or load starting.

The final value should come from the system short-circuit study, voltage regulation analysis, protection coordination and operating plan. Procurement teams can then compare transformer quotations on the basis of total system impact rather than transformer price alone.

WINLEY supports distribution and power transformer projects across liquid-filled, pad-mounted, substation and dry-type applications. We offers UL/cUL certified transformer products under UL File E536138. Depending on product type, configuration and target market, transformers can also be designed to applicable ANSI/IEEE, CSA, U.S. DOE efficiency and NEMA requirements.

Learn more about WINLEY transformer solutions:
https://www.winley-electric.com/