Transformer Short-Circuit Impedance (Uk%): 4%, 6%, 8% Selection Guide

Transformer short-circuit impedance, commonly expressed as Uk%, is an important parameter in transformer selection. It affects short-circuit current, voltage drop, voltage regulation and downstream switchgear requirements.

A lower impedance generally results in higher short-circuit current and lower voltage drop. A higher impedance limits short-circuit current but may increase voltage drop.

Therefore, transformer impedance should be selected according to the transformer rating, system short-circuit level, load characteristics and project requirements.

1. What Is Transformer Short-Circuit Impedance (Uk%)?

Transformer short-circuit impedance is the impedance of a transformer measured during a short-circuit test and expressed as a percentage of rated voltage.

For example, a transformer with 6% impedance requires approximately 6% of its rated voltage during the short-circuit test to produce rated current.

The basic relationship is:

Lower Uk% → Higher short-circuit current → Lower voltage drop

Higher Uk% → Lower short-circuit current → Higher voltage drop

This makes Uk% an important parameter when coordinating transformers with switchgear and other electrical equipment.

2. How Does Transformer Impedance Affect Short-Circuit Current?

For a simplified calculation of the transformer-limited three-phase short-circuit current:

Isc ≈ Ir × 100 / Uk%

Where:

· Isc = approximate short-circuit current

· Ir = rated transformer current

· Uk% = short-circuit impedance

Example: 1000kVA, 10/0.4kV Transformer

The rated LV current is approximately 1443A.

The example shows that increasing impedance significantly reduces the transformer-limited short-circuit current.

Actual system fault current also depends on the upstream grid, cables, busbars and other network impedances. Final switchgear selection should therefore be based on a complete short-circuit study.

3. Transformer Impedance 4% vs 6% vs 8%

4% Impedance

4% impedance provides relatively low voltage drop and can be considered when voltage regulation is an important requirement.

However, the higher prospective fault current must be checked against the rating of downstream switchgear and protection equipment.

6% Impedance

6% impedance provides a practical balance between voltage regulation and short-circuit current limitation.

It is commonly considered for conventional industrial, commercial, residential and distribution transformer applications, subject to project specifications.

8% Impedance

8% impedance provides stronger limitation of transformer-limited short-circuit current.

It may be considered for systems with high available short-circuit levels, provided that the resulting voltage drop is acceptable for the connected loads.

4. Typical Transformer Impedance by Capacity

Transformer impedance depends on capacity, voltage level, transformer design and project requirements.

The following values are general engineering references rather than universal requirements:

The final Uk% should always be confirmed according to applicable standards, utility requirements and project specifications.

5. Transformer Impedance and Voltage Drop

Transformer impedance affects both fault current and voltage regulation.

A lower impedance generally produces lower voltage drop under load, while a higher impedance provides stronger fault-current limitation.

Therefore:

Lower impedance = better voltage regulation but higher fault current

Higher impedance = lower fault current but greater voltage drop

For voltage-sensitive loads, the voltage drop should be checked before selecting a higher impedance transformer.

6. How to Select the Right Transformer Impedance?

When selecting transformer impedance, consider:

· System short-circuit level 

· Required voltage regulation 

· Transformer rated capacity and voltage 

· Load characteristics 

· Switchgear breaking capacity 

· Parallel transformer operation 

· Customer or utility specifications 

Transformer impedance should not be selected according to capacity alone. The complete electrical system should be considered.

7. Common Transformer Impedance Selection Mistakes

Selecting Impedance Only by Transformer Capacity

Capacity provides a useful reference, but system fault level, voltage drop and project requirements must also be considered.

Choosing Low Impedance Without Checking Fault Current

Lower impedance can increase short-circuit current and may require higher-rated switchgear.

Choosing High Impedance Without Checking Voltage Drop

Higher impedance reduces fault current but can increase voltage drop under full-load conditions.

Ignoring Project Specifications

If a tender, consultant or utility specifies a particular Uk% value, it should be confirmed with the transformer manufacturer before production.

8. Quick Selection Guide

Consider 4% impedance when:

· Low voltage drop is important

· The system fault current is acceptable

· Good voltage regulation is required

Consider 6% impedance when:

· A balanced solution is required

· The transformer is used for general distribution

· Project specifications allow this value

Consider 8% impedance when:

· The system has a high available short-circuit level

· Stronger fault-current limitation is required

· The additional voltage drop is acceptable

9. Frequently Asked Questions

1.What is transformer impedance?

Transformer impedance is the internal impedance of a transformer, normally expressed as a percentage of rated voltage as Uk%. It affects short-circuit current and voltage drop.

2.What does 6% transformer impedance mean?

It means approximately 6% of the rated voltage is required during the short-circuit impedance test to produce rated current.

3.Is 4% or 6% impedance better?

Neither is universally better. 4% generally provides lower voltage drop but higher fault current, while 6% provides a more balanced performance.

4.Does higher transformer impedance reduce short-circuit current?

Yes. For the same transformer rating, a higher Uk% generally results in lower transformer-limited short-circuit current.

5.Does transformer impedance affect voltage drop?

Yes. Higher impedance generally results in greater voltage drop under load.

6.Can transformer impedance be customized?

Yes. A specific Uk% can be considered according to the transformer design, capacity, voltage level, applicable standards and project requirements.

10. Conclusion

Transformer short-circuit impedance (Uk%) is an important parameter for transformer and power system design.

The basic relationship is:

Lower impedance → Higher short-circuit current → Lower voltage drop

Higher impedance → Lower short-circuit current → Higher voltage drop

The appropriate impedance should be selected according to the system short-circuit level, load characteristics, voltage regulation, switchgear requirements and project specifications.

Taili Electric can provide dry-type and oil-immersed transformers with technical parameters matched to specific project requirements. If you have a required Uk%, transformer capacity, voltage ratio and application, our engineering team can help evaluate the appropriate transformer solution.