Transformer impedance is one of the specifications that can significantly change how an industrial electrical system behaves, even when transformer capacity and voltage ratio remain exactly the same.
Two transformers may both be rated at the same kVA or MVA, have the same primary and secondary voltages, and supply the same type of industrial load. If their percentage impedance values are different, however, the available short-circuit current, voltage drop during heavy loading, motor-starting performance, protection requirements, and parallel load sharing can also be different.
This is why transformer impedance should not be selected by simply asking whether a lower or higher value is better.
For an industrial power transformer, the appropriate impedance needs to balance several parts of the electrical system at the same time. The transformer must limit fault current to a manageable level without creating excessive voltage drop during normal operation or causing unacceptable voltage dips when large motors start.
Quick Answer
Transformer impedance for an industrial power system should be selected by considering transformer capacity, system voltage, available short-circuit level, downstream switchgear ratings, load characteristics, motor starting requirements, voltage regulation, and any planned parallel operation.
A lower percentage impedance generally allows higher short-circuit current but produces less internal voltage drop. A higher impedance helps reduce prospective fault current, but excessive impedance may increase voltage drop and make large motor starting more difficult.
The objective is therefore not to choose the lowest or highest impedance. It is to select an impedance that works with the complete industrial distribution system.
For project-specific power transformers, HONLE reviews impedance together with rated capacity, voltage ratio, vector group, tapping requirements, cooling, load characteristics, installation conditions, and testing requirements before the transformer specification is finalized.
Why Transformer Impedance Matters in an Industrial Plant
Industrial facilities rarely contain only steady resistive loads.
A plant may include induction motors, compressors, pumps, crushers, conveyors, furnaces, welding equipment, variable frequency drives, rectifiers, HVAC systems, production lines, and other loads with very different electrical characteristics.
The transformer therefore has to perform under several operating conditions.
During normal production, it needs to maintain an acceptable secondary voltage while carrying the required load. When a large motor starts, the system may experience several times the motor’s normal operating current for a short period. During a short circuit, the transformer becomes one of the main elements limiting the fault current supplied to the downstream system.
Transformer impedance influences all three conditions.
For this reason, impedance should be considered together with the rest of the electrical design rather than treated as an isolated transformer parameter.
A useful way to see the relationship is:
Transformer impedance → available fault current → switchgear and protection requirements
while at the same time:
Transformer impedance → voltage drop → motor starting and load voltage performance
Improving one side of this relationship can place more pressure on the other. That trade-off is the main reason impedance selection deserves careful attention.
Start With the Short-Circuit Level
One of the first reasons engineers specify transformer impedance is to control the prospective short-circuit current on the secondary side.
For a simplified calculation in which upstream source impedance is ignored, the transformer-limited symmetrical short-circuit current can be approximated as:
Isc ≈ Irated × 100 / Z%
where:
- Isc is the approximate initial symmetrical short-circuit current available from the transformer;
- Irated is the transformer rated current on the side being evaluated;
- Z% is the transformer percentage impedance.
This relationship makes the effect of impedance easy to see.
If two transformers have the same capacity and secondary voltage but one has a lower impedance, the lower-impedance transformer can supply a higher fault current.
An Example of How Impedance Changes Fault Current
Consider a 2500kVA, 10/0.4kV transformer.
The rated secondary current is approximately:
Irated = 2500kVA / (√3 × 0.4kV) ≈ 3608A
If the transformer impedance is 6%:
Isc ≈ 3608A × 100 / 6 ≈ 60.1kA
If the impedance were 8%:
Isc ≈ 3608A × 100 / 8 ≈ 45.1kA
The transformer capacity has not changed. The secondary voltage has not changed. But the theoretical transformer-limited fault current has fallen by about 15kA simply because the impedance changed from 6% to 8%.
This is an important difference when selecting downstream circuit breakers, busbars, switchgear assemblies, cables, and other equipment that must withstand or interrupt fault current.
The values above are intentionally simplified. In an actual industrial network, utility source impedance, upstream transformers, cables, busbars, generators, motors, and other system elements also contribute to the final short-circuit calculation. The transformer impedance should therefore be incorporated into the complete system study rather than used as the only impedance in the calculation.
Low Impedance Can Increase Switchgear Requirements
A very low transformer impedance can appear attractive because it helps reduce internal voltage drop. However, the resulting fault current may become high enough to affect the entire downstream distribution design.
For example, changing the transformer specification may influence:
- circuit-breaker breaking capacity;
- switchgear short-time withstand current;
- busbar bracing;
- protection settings;
- cable short-circuit withstand;
- arc-flash calculations;
- equipment cost.
This is why transformer and switchgear selection should not be completed independently when the available fault level is close to equipment limits.
Check the Other Side of the Trade-Off: Voltage Drop
If higher impedance reduces short-circuit current, it may seem reasonable to specify as much impedance as possible.
That creates another problem.
Transformer impedance also contributes to voltage regulation. As load current increases, the voltage at the transformer secondary terminals changes because of the transformer’s internal resistance and reactance.
The actual voltage change depends on more than Z% alone. The resistive and reactive components of transformer impedance, load power factor, and operating current all matter. However, in general, a higher impedance gives the system greater potential for voltage variation under heavy loading.
In a plant with relatively stable loads, a moderate increase may not create an operational problem.
In another facility, the same change can be important if the transformer supplies:
- large induction motors;
- compressors;
- crushers;
- large pumps;
- welding loads;
- cyclic production equipment;
- equipment sensitive to voltage dips;
- long low-voltage distribution circuits.
This is why specifying transformer impedance purely to reduce fault current can move the design problem from protection equipment to load performance.
Normal Voltage Regulation Should Be Evaluated at the Expected Load
Industrial transformers do not necessarily operate at one fixed load throughout the day.
A production plant may run at 40% load during one shift and approach full load during peak production. Additional motors may start while other production equipment is already operating.
The voltage regulation assessment should therefore reflect the actual operating cases rather than only nominal transformer capacity.
For example, a project may need to examine:
- normal production load;
- maximum expected simultaneous load;
- starting of the largest motor;
- restart of several loads after an interruption;
- operation with one transformer unavailable in a redundant system.
The same transformer impedance can produce acceptable performance in one operating case and an excessive voltage dip in another.
Large Motor Starting Can Become the Deciding Factor
Large motors are one of the main reasons industrial transformer impedance cannot be selected only from short-circuit calculations.
An induction motor started directly across the line can draw several times its rated current during acceleration. The exact starting current depends on the motor and starting method, but the temporary current increase causes a voltage drop across the source impedance.
The transformer is part of that source impedance.
As transformer impedance increases, the voltage at the motor terminals can fall further during starting if the other system conditions remain unchanged.
An excessive voltage dip can create several problems:
- the motor may develop insufficient accelerating torque;
- acceleration time may increase;
- contactors may drop out;
- control equipment may reset;
- other motors may slow down;
- sensitive loads on the same bus may be disturbed.
For a plant with one or more large motors, transformer impedance should therefore be evaluated against motor-starting requirements before the specification is fixed.
Transformer Capacity Alone Does Not Answer the Motor-Starting Question
It is possible for a transformer to have sufficient kVA capacity for a motor during normal operation while still producing an unacceptable starting voltage dip.
This distinction is especially important for loads such as:
- large water pumps;
- mine ventilation fans;
- compressors;
- crushers;
- mills;
- conveyors;
- process blowers.
Suppose a motor represents a significant percentage of transformer capacity. Increasing transformer impedance to reduce the calculated fault level may cause the motor-starting voltage dip to exceed the plant’s acceptable limit.
Instead of changing transformer capacity immediately, the project should examine the entire starting arrangement.
A soft starter or VFD may reduce starting current compared with direct-on-line starting. A separate transformer may be used for particularly large loads. Starting sequences can sometimes prevent several motors from accelerating at the same time.
The correct solution depends on the actual system.
This is why HONLE normally treats transformer impedance as one part of the power transformer specification rather than selecting it independently from the intended load.
Parallel Transformers Need Closely Coordinated Impedance
Impedance becomes even more important when two or more transformers are intended to operate in parallel.
Parallel operation requires several electrical conditions to be coordinated, including voltage ratio, tap position, polarity, phase sequence, vector group, and impedance characteristics.
If parallel transformers have significantly different percentage impedances, they may not share load in proportion to their rated capacities.
The transformer with the lower effective impedance tends to carry a larger share of the load.
As a result, one unit can approach or exceed its permissible loading while the other still has unused capacity.
Consider Two Transformers With the Same Capacity
Assume two transformers of equal rating operate in parallel.
If their voltage ratios and phase relationships are compatible but one has noticeably lower impedance, the lower-impedance unit will tend to take more current.
The total plant load may still be below the combined nameplate capacity of the two transformers, yet the current distribution between them can be uneven.
For new installations, it is therefore preferable to coordinate transformer impedance during the design stage.
For an expansion project where a new transformer will operate in parallel with an existing unit, the existing transformer nameplate and test documentation become especially important.
The new unit may need to match or closely coordinate with parameters such as:
- rated capacity;
- primary and secondary voltage;
- vector group;
- tap range and tap position;
- percentage impedance;
- impedance tolerance;
- frequency;
- connection arrangement.
The impedance magnitude alone is not the entire parallel-operation calculation. The impedance angle and the resistance-to-reactance relationship can also affect load sharing, particularly when transformers of different designs or ratings are combined.
This makes replacement and expansion projects different from selecting a stand-alone transformer for a new substation.
How Should Transformer Impedance Be Selected in Practice?
There is no single percentage impedance that is automatically correct for every industrial power transformer.
The selection process works better when it begins with the electrical system and narrows the transformer specification from there.
1. Confirm Transformer Capacity and Voltage Levels
Rated capacity and voltage ratio establish the transformer’s normal current levels and form the basis for the short-circuit calculation.
A 2500kVA transformer and a 25MVA transformer cannot be evaluated from the impedance percentage alone. The same Z% applied to different transformer ratings can correspond to very different actual current levels and system duties.
The specification should first define:
- rated capacity;
- primary voltage;
- secondary voltage;
- frequency;
- phase;
- intended loading.
2. Calculate the Permissible Short-Circuit Current
The project should determine the maximum fault current that downstream equipment is designed to withstand and interrupt.
This includes the ratings of the switchgear, breakers, busbars, cables, and other distribution equipment.
If the calculated fault current is too high, transformer impedance may be one of the parameters that can be adjusted.
However, it should not be changed without checking voltage performance.
3. Review the Largest and Most Difficult Loads
A load schedule provides much more useful information than total connected kW alone.
Large motors deserve particular attention because their starting current can dominate short-duration voltage behavior.
The project should identify:
- largest motor rating;
- motor starting method;
- expected starting current;
- starting frequency;
- whether motors can start simultaneously;
- acceptable voltage dip;
- sensitive loads connected to the same bus.
Heavy cyclic loads and rapidly changing industrial processes may also need separate evaluation.
4. Check Voltage Regulation Under Normal and Starting Conditions
Once a candidate impedance range has been considered, voltage behavior should be checked at the expected load and power factor.
The calculation should include relevant upstream and downstream system impedances rather than treating the transformer in isolation.
For a motor-heavy industrial facility, both steady-state voltage regulation and transient starting conditions may need to be reviewed.
5. Confirm Parallel-Operation Requirements
If the transformer will operate alone, impedance selection has more flexibility.
If it will operate in parallel with an existing or future transformer, the impedance requirements become more constrained.
For expansion projects, existing transformer test reports and nameplate information should be reviewed before specifying the new unit.
This is particularly important when the existing transformer was supplied years earlier by another manufacturer.
6. Coordinate the Final Value With the Transformer Manufacturer
A transformer purchaser may begin with a target impedance based on the system study. The manufacturer then needs to confirm that the requested value can be achieved together with the required voltage class, capacity, winding arrangement, losses, short-circuit strength, temperature rise, and mechanical design.
At HONLE, impedance is reviewed together with the complete electrical specification rather than treated as a standalone number.
For industrial projects, this can include oil-immersed or dry-type transformers as well as higher-voltage power transformer configurations depending on the system architecture and installation requirements.
Why a Higher or Lower Impedance Is Not Automatically Better
Transformer specifications are sometimes approached as though every parameter has a simple preferred direction: lower losses are better, higher efficiency is better, and therefore lower impedance must also be better.
Impedance does not work that way.
Consider the two extremes.
If Transformer Impedance Is Too Low
A lower impedance can improve voltage stiffness, but it also increases prospective short-circuit current.
That can place greater electrical and mechanical stress on:
- transformer windings during faults;
- low-voltage switchgear;
- busbars;
- circuit breakers;
- cables;
- downstream distribution equipment.
A plant expansion can make this issue more serious. When transformers are added or operated in parallel, the available fault level at a common bus may increase substantially.
If Transformer Impedance Is Too High
Higher impedance can help control fault current but may lead to:
- greater voltage variation under load;
- deeper voltage dips during motor starting;
- reduced motor acceleration performance;
- increased sensitivity to rapid load changes;
- difficulties when matching an existing transformer for parallel operation.
The target is therefore a coordinated value rather than an extreme.
A transformer is part of a power system. The most suitable impedance is the one that allows the transformer, protection system, switchgear, cables, motors, and other loads to operate together within their required limits.
What Should Be Included in the Power Transformer Specification?
Once the system study establishes the required impedance, it should be included clearly in the transformer technical specification.
A typical industrial power transformer specification may define:
| Parameter | Project Information |
| Rated capacity | For example, 2500kVA, 10MVA or project-specific |
| Primary voltage | According to incoming network |
| Secondary voltage | According to plant distribution system |
| Frequency | 50Hz or 60Hz |
| Phase | Single-phase or three-phase |
| Vector group | According to system design |
| Impedance | Required Z% and applicable tolerance |
| Tap range | Off-circuit or on-load arrangement as required |
| Cooling | According to transformer type and duty |
| Installation | Indoor or outdoor |
| Ambient temperature | Project-specific |
| Altitude | Project-specific |
| Major loads | Motors, VFDs, rectifiers, furnaces, etc. |
| Parallel operation | Yes / No and existing transformer data |
| Applicable standard | According to project requirement |
| Testing | Routine and additional tests as specified |
For a new industrial plant, a single-line diagram and load schedule can provide useful context alongside this data.
For a replacement or parallel transformer, information from the existing transformer becomes equally important.
HONLE manufactures oil-immersed and dry-type transformers for industrial and power-distribution applications, including project-specific power transformer solutions for different voltage and capacity requirements. When impedance is specified, we review it together with the winding design, short-circuit strength, losses, cooling, insulation, connection arrangement, and other confirmed project requirements before production.
Selecting Impedance as Part of the Complete Industrial Power System
Transformer impedance may appear as only one percentage value on a technical datasheet, but its effects extend well beyond the transformer itself.
Lower impedance can support stronger voltage performance while increasing available short-circuit current. Higher impedance can reduce fault current while making voltage regulation and motor starting more demanding. Parallel operation introduces another requirement because impedance affects how transformers share load.
For this reason, industrial transformer impedance should be selected from the complete system design.
The process should connect:
power transformer rating → impedance → fault level → switchgear → voltage regulation → motor starting → protection → operating arrangement
When these factors are considered together, impedance becomes a useful system-design tool rather than simply another value to copy from a previous transformer specification.
For HONLE industrial power transformer projects, the required impedance can be reviewed together with capacity, voltage ratio, vector group, load characteristics, tapping, cooling, installation environment, testing requirements, and any planned parallel operation. This allows the transformer configuration to reflect the electrical system it will actually serve.
FAQ
What is a good transformer impedance for an industrial power system?
There is no single impedance value that is suitable for every industrial system. The required value depends on transformer rating, voltage level, available fault current, downstream equipment ratings, load characteristics, motor starting requirements, voltage regulation, and parallel-operation requirements.
Does higher transformer impedance reduce short-circuit current?
Generally, yes. For the same transformer capacity and voltage, increasing percentage impedance reduces the fault current that the transformer can supply. Actual system fault current also depends on upstream source impedance and other elements in the network.
Does lower transformer impedance improve voltage regulation?
Lower impedance generally reduces the voltage drop associated with transformer internal impedance, but actual voltage regulation also depends on the resistive and reactive components of impedance, load current, and power factor. Lower impedance also increases prospective short-circuit current, so it cannot be selected from voltage regulation alone.
Why is transformer impedance important for large motors?
Large motors can draw high current during starting. This current produces a voltage drop across the system impedance, including the transformer impedance. If the source is relatively weak or transformer impedance is high, the resulting voltage dip can affect motor acceleration and other loads connected to the same bus.
Can transformers with different impedance values operate in parallel?
Small differences may be manageable within the applicable design limits, but significant impedance differences can cause unequal load sharing. Voltage ratio, vector group, tap position, capacity, percentage impedance, impedance characteristics, and other parallel-operation conditions should all be reviewed before transformers are connected in parallel.
Does a larger power transformer always have higher impedance?
Not necessarily. Transformer impedance depends on the electrical and mechanical design as well as project requirements. Capacity alone does not determine the required percentage impedance. The specified value should be coordinated with the short-circuit level, voltage performance, transformer design, and applicable project requirements.
Can HONLE manufacture a power transformer with project-specific impedance?
Yes. HONLE supplies oil-immersed and dry-type transformers for industrial power distribution and can review project-specific impedance requirements together with rated capacity, voltage ratio, vector group, cooling, tap arrangement, installation conditions, accessories, testing, and other technical specifications.