Transformer impedance voltage selection is one of the most consequential decisions an electrical engineer or procurement manager makes during specification. The percentage impedance (uk%) governs short-circuit current, voltage regulation, paralleling behaviour and even upstream protection settings. Choose too low and switchgear may be destroyed by fault current; choose too high and motors stall, losses rise and the utility may reject the design. This guide explains how to select uk% correctly, with worked calculations, standards references and a real FIJ TECH factory case study.
What Is Transformer Impedance Voltage (uk%)?
Impedance voltage is the percentage of rated voltage that must be applied to one winding, with the other short-circuited, to circulate rated current. It combines resistance and leakage reactance into a single figure that defines the transformer’s internal electrical “stiffness”.
For a 10kV distribution transformer, uk% is typically 4%. For larger 35kV power transformers it may range from 6% to 12% depending on capacity and application. IEC 60076-1 and IEEE C57.12.00 define the measurement method and permissible tolerances, which are normally ±7.5% of the declared value, or ±10% for the first of a pair when specified.
Every unit supplied by FIJ TECH is routine-tested for impedance at the factory, and the measured value is recorded on the test certificate alongside no-load loss and load loss data.
Why Impedance Voltage Selection Matters
Impedance voltage directly controls the prospective short-circuit current (Isc). A lower uk% produces a higher fault current, which stresses breakers, busbars, cables and CTs. A higher uk% limits fault current but increases voltage drop under load and reduces the transformer’s ability to ride through motor starting surges.
It also affects:
- Voltage regulation: Higher uk% means larger voltage sag from no-load to full-load.
- Paralleling: IEC 60076-1 requires paralleled transformers to have impedance within ±10% of each other to prevent circulating current.
- Loss evaluation: Impedance is linked to leakage reactance and winding geometry, which influence load loss and temperature rise.
- Protection coordination: Fuse and relay curves must be coordinated with the actual fault level, not an assumed one.
How to Select the Right Impedance Voltage
Selection is a balance between fault current limitation and voltage regulation. The standard approach is to start from the switchgear breaking capacity, then verify voltage drop at the most critical load.
Step 1 – Match uk% to Switchgear Breaking Capacity
Calculate the maximum fault current the transformer can deliver, then confirm it is below the rated breaking capacity of the downstream breaker or fuse. If the fault current exceeds the rating, increase uk% or add series reactance.
Step 2 – Verify Voltage Drop Under Load
Voltage drop must remain within acceptable limits for the connected load, especially motors. A common design rule is to keep full-load voltage drop below 5% at motor terminals.
Step 3 – Confirm Paralleling and Standard Compliance
If the transformer will run in parallel, impedance must match existing units within ±10%. Confirm the design against IEC 60076, GB 1094, IEEE C57.12.00, and where applicable EU EcoDesign Tier 2 or DOE 2016 efficiency requirements.
Typical Impedance Ranges by Transformer Type
| Transformer Type | Voltage Level | Typical uk% | Typical Application |
|---|---|---|---|
| Distribution transformer | 6–11kV | 4–5% | Industrial and commercial distribution |
| Distribution transformer | 20–33kV | 5–7% | Utility distribution networks |
| Power transformer | 35kV and above | 7–12% | Substation and transmission step-down |
| Dry type transformer | 10–35kV | 4–8% | Indoor, fire-sensitive locations |
These figures are indicative. FIJ TECH builds oil-immersed and dry-type units from 30kVA to 2500kVA as standard, with custom designs up to 31500kVA, at 50Hz or 60Hz, with ONAN, ONAF, AN or AF cooling. See the full product catalog for capacity and voltage options.
Voltage Drop & Short-Circuit Impedance Check
This worked example shows how to verify a 10kV distribution transformer selection before ordering switchgear.
1. Verify transformer impedance voltage (uk%) matches switchgear breaking capacity. If the calculated fault current exceeds the breaker rating, increase uk% or specify a higher-rated breaker.
2. Typical 10kV distribution transformer: uk = 4%. For a 2000kVA unit at 400V secondary, the rated secondary current is approximately 2887A. The prospective short-circuit current is roughly rated current divided by uk: 2887 / 0.04 ≈ 72kA. This must be below the switchgear breaking capacity.
3. Calculate voltage drop under load: ΔU = β × uk × (cosφ + sinφ × tanθ), where β is the load factor, cosφ is the load power factor and tanθ is the ratio of reactance to resistance of the transformer (X/R). For a typical distribution transformer with cosφ = 0.85 and tanθ ≈ 0.1, at full load (β = 1): ΔU = 1 × 0.04 × (0.85 + 0.527 × 0.1) ≈ 0.036, or 3.6%.
4. Ensure voltage drop at full load stays below 5% at motor terminals. The 3.6% transformer drop leaves headroom for cable and motor starting drops. If the total exceeds 5%, consider a lower uk% or larger cable.
This check should be repeated for the actual load profile and motor starting current. FIJ TECH provides impedance and loss data on request to support this calculation.
Factory Case Study: 11kV S13 Oil Immersed Distribution Transformer for a Southeast Asia Textile Mill
A textile mill in Vietnam operated with ageing S9 transformers that suffered frequent voltage fluctuation and high no-load loss. The mill needed a reliable, energy-efficient replacement that could handle continuous motor loads without disrupting production.
FIJ TECH supplied an oil immersed distribution transformer model S13-M-2000/11, rated 2000kVA, 11kV/0.4kV. The S13 design reduced no-load loss by approximately 30% compared with the old S9 units. Impedance voltage was specified at 4.5% to balance fault current limitation with voltage regulation for the mill’s motor loads.
The transformer was manufactured to IEC 60076 and GB 1094, with ISO 9001:2015 quality management. Delivery was completed within 35 days. According to the customer, “After upgrading to S13 transformers, our monthly energy cost dropped by approximately 18%. The delivery was completed within 35 days.”
This project illustrates how correct impedance voltage selection, combined with an energy-efficient core, delivers both technical and financial returns.
Product Spotlight: FIJ TECH Transformers for Global Projects
FIJ TECH manufactures oil-immersed and dry-type transformers for export markets, with certifications including ISO 9001:2015, IEC 60076, IEC 60076-11, IEEE C57.12.00, GB 1094, GB/T 17467, CE and VDE where applicable.
- 35kV Power Transformer Manufacturer (Oil Transformer) – suited to utility and industrial step-down applications.
- ZGS Combined Substation – compact prefabricated solution for distribution networks.
- Dry type transformers – for indoor, fire-sensitive or environmentally restricted sites.
Every unit is routine-tested for impedance, no-load loss, load loss and dielectric strength before shipment. Custom impedance values can be specified to match existing switchgear and paralleling requirements.
Key Takeaways for Specification Teams
- Select uk% based on switchgear breaking capacity first, then verify voltage drop.
- Keep full-load voltage drop below 5% at motor terminals.
- Match impedance within ±10% when paralleling transformers.
- Confirm compliance with IEC 60076, IEEE C57.12.00, GB 1094 and applicable efficiency standards.
- Request factory test data for impedance and losses before finalising protection settings.
Correct transformer impedance voltage selection protects your switchgear, stabilises voltage and avoids costly rework. To discuss your project requirements or request a quotation, contact FIJ TECH with your voltage level, capacity, frequency and impedance target. Our engineering team will confirm the optimal uk% and provide a full technical proposal.
Related reading: transformer impedance, oil immersed transformer.