Transformer impedance is an important parameter in transformer design and power system operation. It affects short-circuit current, voltage regulation, protection equipment selection, and transformer parallel operation.
What Is Transformer Impedance?
Transformer impedance, commonly expressed as a percentage (%Z), represents the internal impedance of a transformer under specified test conditions.
During a short-circuit test, one winding is short-circuited while voltage is applied to the other winding until rated current flows. The voltage required for this condition, expressed as a percentage of rated voltage, corresponds to the transformer's percentage impedance.
For example, a transformer with 6% impedance requires approximately 6% of its rated voltage to produce rated current during the short-circuit test.
Transformer impedance mainly consists of winding resistance and leakage reactance.
How Is Transformer Impedance Calculated?
The percentage impedance can be expressed as:
%Z = (Short-Circuit Voltage / Rated Voltage) × 100%
For example, if the rated voltage is 10 kV and the short-circuit voltage is 600 V:
%Z = 600 / 10,000 × 100% = 6%
Therefore, the transformer has approximately 6% impedance.
Why Is Transformer Impedance Important?
1. Short-Circuit Current
Transformer impedance directly affects the available short-circuit current.
Under simplified conditions where other system impedances are neglected:
Isc ≈ Irated / Zpu
For example:
4% impedance → approximately 25 × rated current
6% impedance → approximately 16.7 × rated current
8% impedance → approximately 12.5 × rated current
In an actual power system, fault current also depends on the utility source, cables, upstream transformers, and other system impedances.
2. Voltage Regulation
Transformer impedance contributes to voltage drop when the transformer is under load.
In general, a higher impedance tends to produce greater voltage drop, while a lower impedance tends to reduce impedance-related voltage drop.
However, actual voltage regulation also depends on the resistance/reactance components and load power factor.
3. Protection Equipment
The transformer impedance is an important input for short-circuit calculations.
The calculated fault current helps engineers select suitable ratings for:
Circuit breakers
Switchgear
Busbars
Fuses
Protection relays
The equipment must have appropriate interrupting and short-circuit withstand capabilities for the calculated fault level.
Is Higher Transformer Impedance Better?
Not necessarily.
A lower impedance can help reduce impedance-related voltage drop, but it generally allows a higher short-circuit current.
A higher impedance can help limit the transformer's contribution to fault current, but it may result in greater voltage drop under load.
Therefore, the appropriate transformer impedance should be selected according to the complete electrical system rather than simply choosing a higher or lower value.
Typical Transformer Impedance
Many distribution transformers use impedance values in the approximate range of 4%–8%, although the actual value varies according to transformer capacity, voltage level, design, and project requirements.
The required impedance should be confirmed through system calculations and the project technical specification.
Transformer Impedance and Parallel Operation
Transformer impedance is also important when multiple transformers operate in parallel.
Engineers generally need to check:
Voltage ratio
Vector group
Percentage impedance
Impedance angle
Rated capacity
Tap settings
Significant differences in impedance can result in unequal load sharing between parallel transformers.
How to Choose the Right Transformer Impedance?
When selecting transformer impedance, consider:
1. Short-circuit level
Check the expected fault current at the transformer and downstream equipment.
2. Voltage regulation
Evaluate voltage drop under the actual load and power factor.
3. Protection equipment
Verify that circuit breakers, switchgear, and busbars have suitable short-circuit ratings.
4. Parallel operation
Check impedance compatibility when transformers are intended to operate in parallel.
5. Project requirements
Follow the requirements specified by the utility, consultant, EPC contractor, or project specification.
Conclusion
Transformer impedance is an important parameter affecting short-circuit current, voltage regulation, protection system design, and parallel transformer operation.
A suitable impedance value should be determined based on the transformer capacity, system voltage, load characteristics, short-circuit level, and project requirements.
For custom dry-type and oil-immersed transformers, impedance can be considered during the design stage according to the specific electrical system and technical specification.
