How Transformer Impedance Affects Fault Level and Parallel Operation

Transformer impedance is the bridge between a power transformer’s nameplate and the current a switchboard may see during a fault. It also determines how two transformers divide load when they share a bus. The calculations below are screening tools; a protection, arc-flash or equipment-duty study must include the complete network.

Read %Z correctly

Percent impedance (%Z) is the voltage, expressed as a percentage of rated voltage, needed to circulate rated current with the other winding shorted during the impedance test. In per-unit form, 5%Z is 0.05 pu. Lower impedance generally means better voltage regulation, but it also permits more short-circuit current. The value must be read with rated kVA, voltage, frequency, vector group and tap position.

Estimate a first-pass fault level

For a balanced three-phase, transformer-only approximation:

text
I_fault ≈ I_rated / Z_pu
S_sc ≈ S_transformer / Z_pu

Example (illustrative, not a Shenheng rating): a 1,000 kVA, 11 kV transformer with 5%Z has rated current of about 52.5 A on the 11 kV side. Dividing by 0.05 gives roughly 1.05 kA, or about 20 MVA fault level at the terminals before source, cable and motor impedance are added.

Do not convert this estimate directly into a breaker rating. The actual study adds utility fault level, generator/motor contribution, grounding method, X/R and the worst operating configuration.

What changes when transformers run in parallel?

Before closing a tie, verify all of the following:

1. Same phase sequence, polarity and vector group (or an explicitly engineered compatible connection).
2. Nearly equal secondary voltage ratio at the selected tap; otherwise circulating current can flow at no load.
3. Compatible rated voltages, frequency and insulation levels.
4. Comparable percent impedance and X/R. Units with different impedance do not share load equally.
5. Protection, neutral/grounding and short-circuit duties remain acceptable for every switching state.

For two units on a common bus, an initial sharing estimate is inversely proportional to their leakage impedances:

text
I1 / I2 ≈ Z2 / Z1

If equal-kVA units have 5%Z and 6%Z, the 5%Z unit tends to carry about 6/5 (1.2) times the current of the 6%Z unit, before tap, temperature and circulating-current effects. Treat this as a study starting point, not a guaranteed field result.

Data request checklist

Ask the OEM or utility for:

• rated power, primary/secondary voltage, frequency and connection symbol;
• guaranteed %Z and X/R, with tolerance and test standard;
• tap range, tap location and ratio at the proposed operating tap;
• grounding/neutral arrangement and zero-sequence information where relevant;
• upstream short-circuit level, cable impedances and motor/generator contributions;
• intended parallel combinations, load profile and tie-breaker states;
• routine/type-test evidence and the latest nameplate or test report.

Specify the transformer package

Shenheng Power’s transformer product family is the appropriate next path when you need to request these fields for a dry-type or oil-immersed power transformer. The category link does not establish a particular model’s %Z, fault duty or parallel compatibility. Include your one-line diagram, study voltage base, utility fault level and required tap/grounding details so the supplier can return a checkable datasheet.

References

• Transformer impedance and equivalent-circuit explanation
• Schneider Electric short-circuit current calculation guidance
• ABB technical note on parallel operation
• CIGRE technical-brochure library for system studies
• Shenheng Power transformer product family