Yes, two power transformers can operate in parallel, and this arrangement is widely used in industrial plants, commercial facilities, substations, infrastructure projects, and expanding distribution systems.
Parallel operation can provide additional capacity, improve operating flexibility, and allow one power transformer to be taken out of service while another continues supplying part of the load.
However, connecting two transformers to the same busbar is not simply a matter of matching their kVA ratings and secondary voltages.
If the electrical characteristics of the two power transformers are not compatible, circulating current, unequal load sharing, excessive heating, protection problems, or unexpectedly high short-circuit current may occur.
This becomes especially important when a new power transformer is being added beside an existing unit. A power transformer that is individually suitable for the system may still be unsuitable for parallel operation if its voltage ratio, vector group, impedance, or other characteristics do not match the installed transformer.
The following seven conditions should therefore be checked before two power transformers are operated in parallel.
1. Can Two Power Transformers Operate in Parallel?
Two power transformers can operate in parallel when their primary windings are supplied from the same electrical system and their secondary sides are connected to a common bus under compatible electrical conditions.
In a typical arrangement:
Power Transformer A → Common Busbar ← Power Transformer B
Both transformers then contribute power to the same downstream load.
This arrangement may be used when a project needs:
- More capacity than one transformer can provide
- Future expansion without replacing the original transformer
- Greater operating flexibility
- Maintenance without completely shutting down the facility
- Multiple standardized transformer units instead of one very large unit
- Load sharing between different transformer units
For example, an industrial plant may initially install one 2500kVA power transformer. As production expands, the plant may consider installing another transformer and operating the two units in parallel.
The second transformer should not be selected only by looking for another 2500kVA unit.
Its electrical characteristics must be checked against the existing transformer before the system arrangement is finalized.
The same principle applies whether the project uses oil-immersed transformers or dry-type transformers. Parallel operation depends primarily on electrical compatibility and the surrounding distribution system.
2. Why Are Power Transformers Operated in Parallel?
Parallel power transformer operation is usually driven by capacity, reliability, or system expansion.
Additional capacity
A facility may gradually add production lines, motors, HVAC equipment, pumps, data equipment, chargers, or other electrical loads.
Instead of replacing an existing power transformer with a much larger unit, the project may add another transformer.
This can be practical when the existing transformer still has useful service life and sufficient installation space is available.
Operational flexibility
Two transformers may allow the electrical system to operate in several modes.
For example:
- Transformer A supplies Bus A
- Transformer B supplies Bus B
- A bus coupler normally remains open
- The coupler can be closed under certain operating conditions
- The transformers may temporarily operate in parallel
This type of arrangement is common in facilities where different bus sections need to be transferred or combined.
Maintenance planning
A multi-transformer system may make maintenance easier because one transformer can sometimes be isolated while essential loads are transferred to another source.
However, this does not automatically mean the remaining transformer can carry the entire facility load.
Normal load, emergency load, transformer overload capability, protection settings, and permitted operating modes should all be reviewed separately.
Future expansion
Parallel operation is particularly relevant when a project is constructed in stages.
If the future transformer is expected to operate in parallel with the first power transformer, this should ideally be considered during the original transformer specification.
Matching a future transformer is usually easier when the required voltage ratio, vector group, impedance, tapping arrangement, and other key parameters are already defined.
3. The 7 Conditions for Parallel Power Transformer Operation
The following table gives a practical overview.
| Condition | What Should Be Checked | Why It Matters |
| 1. Voltage ratio | Primary/secondary ratio and effective tap ratio | Prevents voltage difference and circulating current |
| 2. Polarity | Corresponding terminal polarity | Prevents opposing voltages and severe fault conditions |
| 3. Phase sequence | Same three-phase phase order | Ensures corresponding phases are connected correctly |
| 4. Vector group | Same phase displacement and winding connection | Prevents phase-angle mismatch |
| 5. Percentage impedance | Closely matched short-circuit impedance | Supports proper load sharing |
| 6. Impedance angle | Similar X/R relationship | Improves real and reactive load sharing |
| 7. Connection impedance | Similar cables, bus ducts, and path impedance to common bus | Prevents external connections from disturbing load sharing |
Not every parameter has to be mathematically identical in every project. Manufacturing tolerances and differences between transformer ratings have to be considered.
However, these seven items should be reviewed together before parallel operation is approved.
4. Conditions 1–3 Ensure the Transformer Voltages Are Compatible
Condition 1 — Voltage Ratios Must Match Closely
The first requirement is compatible voltage ratio.
Suppose two power transformers are connected to the same primary system.
Transformer A produces:
400V
while Transformer B produces:
410V
If their secondary terminals are connected together, the difference does not disappear.
Instead, the transformers can drive current between each other even when the external load is relatively low.
This is known as circulating current.
The larger the voltage difference and the lower the loop impedance between the transformers, the greater this circulating current may become.
It creates transformer current without supplying useful power to the load.
This can cause:
- Additional winding heating
- Higher copper losses
- Reduced available capacity
- Unequal current between transformers
- Protection complications
For this reason, nominal primary and secondary voltages alone are not enough.
The actual effective transformer ratio at the operating tap position should be checked.
For a power transformer equipped with an off-circuit tap changer, both transformers should normally be placed at compatible tap positions before being paralleled.
For transformers using OLTC systems, coordinated voltage regulation becomes even more important. If one transformer raises its effective secondary voltage while the other remains at a lower ratio, circulating reactive current can increase between the two units.
Condition 2 — Transformer Polarity Must Be Correct
Transformer polarity describes the instantaneous voltage relationship between corresponding winding terminals.
For two single-phase transformers, incorrect polarity can effectively connect their secondary voltages against each other.
The resulting condition may resemble a secondary short circuit rather than normal parallel operation.
In three-phase power transformers, polarity is closely related to winding connections, terminal identification, phase displacement, and vector group.
Before a new transformer is connected to an existing bus, terminal correspondence and phasing should therefore be verified rather than assumed from voltage ratings alone.
Condition 3 — Phase Sequence Must Be the Same
Three-phase power transformers must have the same phase sequence at the parallel connection point.
If one transformer presents the sequence:
A-B-C
while the other is effectively connected:
A-C-B
corresponding phases cannot simply be tied together.
Even when measured line-to-line voltage values appear correct, the phase relationship is not.
Phase sequence is normally confirmed as part of commissioning and phasing checks before the bus-tie or incomer breaker is closed.
This is especially important in retrofit projects where:
- Existing cable markings may be unclear
- Old transformer drawings are incomplete
- Cable routes have been modified
- Switchgear has been replaced
- Primary or secondary connections have been rearranged
The nameplate is important, but the actual installed system should also be verified.
5. Conditions 4–5 Determine Phase Compatibility and Load Sharing
Condition 4 — Vector Groups Must Be Compatible
For three-phase power transformers, vector group is one of the most important conditions for parallel operation.
A vector group identifies the winding connection and the phase displacement between the high-voltage and low-voltage sides.
Typical examples include:
- Dyn11
- Dyn1
- Yyn0
- YNd11
Two transformers may have exactly the same:
- Rated power
- Primary voltage
- Secondary voltage
- Frequency
and still be unsuitable for direct parallel operation because their phase displacement is different.
For example, a Dyn11 transformer and a Dyn1 transformer do not produce secondary voltages with the same phase-angle relationship.
Connecting incompatible vector groups to the same secondary bus can create severe current.
For straightforward parallel operation, using the same vector group is normally the preferred arrangement.
In existing installations, the vector group shown on the installed power transformer nameplate should therefore be confirmed before specifying the new transformer.
This becomes particularly important when replacing older equipment because a modern replacement transformer may otherwise be designed with a commonly used vector group that does not match the original unit.
Condition 5 — Percentage Impedance Must Be Closely Matched
Matching voltage and phase relationship allows two power transformers to be connected electrically.
Matching impedance determines whether they can share the load properly.
Transformer percentage impedance represents the internal impedance of the transformer relative to its rated values.
If two parallel transformers have significantly different percentage impedances, the unit with the lower impedance tends to take a larger share of the load.
Consider two transformers with similar ratings.
If:
Transformer A has lower %Z and Transformer B has higher %Z
Transformer A may carry significantly more current.
The total facility load could still appear to be below the combined transformer capacity while Transformer A approaches or exceeds its rated current.
This is why simply adding the two nameplate ratings can be misleading.
For example:
Transformer A = 2000kVA
Transformer B = 2000kVA
It may appear that the system has 4000kVA of usable capacity.
But if their impedances cause unequal load division, one transformer may reach its allowable loading before the combined load reaches 4000kVA.
Percentage impedance also affects short-circuit current.
When two power transformers operate in parallel on a common bus, both may contribute current to a downstream fault.
Therefore, impedance selection should consider both:
Load sharing and Short-circuit performance rather than treating these as separate topics.
Condition 6 — The Impedance Angle Should Be Similar
Percentage impedance tells us the magnitude of transformer impedance, but transformers also have resistance and reactance components.
These are often considered through their X/R relationship.
Two power transformers may have similar percentage impedance while having different resistance-to-reactance characteristics.
This can affect how real power and reactive power are shared.
One unit may take a disproportionate amount of reactive current even when total current appears reasonably balanced.
The result can include:
- Unequal kVA loading
- Different power factors between the transformers
- Additional heating
- Reduced usable combined capacity
This is one reason parallel transformer assessment should not stop after comparing a single nameplate impedance percentage.
For a straightforward project using two transformers of the same design, similar ratings and the same manufacturer, impedance characteristics can often be coordinated more easily.
Retrofit projects can be more complicated because the existing transformer may have been designed decades earlier using different winding dimensions, conductor arrangements, materials, or design practices.
Condition 7 — Connection Impedance to the Common Bus Should Be Similar
The transformers themselves are only part of the parallel circuit.
The electrical path between each transformer and the common busbar also has impedance.
This includes:
- Secondary cables
- Bus ducts
- Busbars
- Connection length
- Conductor cross-sectional area
- Number of parallel cables
- Conductor material
- Joint resistance
Imagine two identical power transformers.
Transformer A connects to the main switchboard through a short bus duct.
Transformer B is installed farther away and connects through a much longer cable route.
Even though the transformer impedances are identical, the total impedance from each source to the common bus is no longer identical.
This can influence load sharing.
For this reason, transformer matching should be reviewed together with the actual single-line diagram and physical connection arrangement.
Moving the second transformer farther away or changing its cable size is not merely a mechanical layout decision.
It can change the electrical behavior of the parallel system.
7. What Else Should Be Checked Before Two Power Transformers Are Paralleled?
Meeting the seven compatibility conditions does not automatically mean the entire system is ready for parallel operation.
Several surrounding system conditions also need attention.
Different kVA Ratings Are Not Automatically Prohibited
Two power transformers do not always need exactly the same rated capacity.
A 2000kVA and a 2500kVA transformer may potentially operate together if the electrical design supports appropriate load sharing.
However, the transformer impedance values must be considered relative to their ratings.
Large differences in transformer capacity can make satisfactory load sharing more difficult.
For this reason, replacing an existing transformer with a larger unit should not be based only on:
“The voltage is the same, so the new transformer can run with the old one.”
The complete transformer data should be compared.
Parallel Transformers Can Increase Short-Circuit Current
This is one of the most important system-level consequences.
With one transformer supplying a busbar, the transformer limits the available fault current according to its capacity and impedance.
If a second power transformer is connected in parallel, both units may contribute to a fault on the common bus.
Prospective short-circuit current can therefore increase substantially.
The project should recheck:
- Main breaker breaking capacity
- Incomer breaker ratings
- Bus-coupler rating
- Busbar short-time withstand rating
- Feeder breaker capacity
- Cable withstand capability
- Protection settings
A transformer expansion should not create a fault level that exceeds the existing switchgear rating.
Protection Coordination May Need to Change
Adding another power transformer changes the network.
Protection settings that were suitable for a single transformer may no longer be appropriate after parallel operation is introduced.
The review may include:
- Overcurrent protection
- Earth-fault protection
- Transformer differential protection
- Restricted earth fault protection
- Busbar protection
- Incomer and bus-coupler interlocking
- Selectivity between upstream and downstream devices
The exact protection scheme depends on the transformer rating and system architecture.
OLTC-Controlled Transformers Need Coordinated Regulation
When two transformers with on-load tap changers operate in parallel, voltage control requires special attention.
If Transformer A moves to one effective ratio and Transformer B moves to another, the mismatch can produce circulating current.
Parallel OLTC arrangements may therefore use coordinated control methods such as:
- Master-follower control
- Circulating-current control
- Other project-specific parallel regulation logic
The appropriate method depends on transformer ratings, tap steps, system arrangement, and control requirements.
Neutral and Earthing Arrangements Should Be Reviewed
Parallel transformers can also affect zero-sequence current paths and earth-fault behavior.
Neutral earthing should therefore be considered together with:
- Transformer vector group
- Neutral grounding method
- Earthing resistor or reactor, where used
- Protection scheme
- Number of transformer neutrals connected simultaneously
This is a system-design question rather than simply a transformer nameplate question.
8. Matching a New HONLE Power Transformer With an Existing Unit
Parallel operation is particularly common in expansion and replacement projects.
A customer may already have one transformer operating successfully and want to add a second power transformer rather than replace the entire installation.
In this situation, HONLE can review the new transformer specification together with the available information from the existing unit.
Useful information may include:
- Existing transformer rated capacity
- Primary and secondary voltage
- Frequency
- Vector group
- Percentage impedance
- Tap range and tap positions
- Load losses, where available
- Existing transformer nameplate
- Previous test report, where available
- Single-line diagram
- Secondary cable or bus-duct arrangement
- Main switchboard ratings
- Expected operating mode
- Normal and maximum load
- Future expansion requirement
These details help us compare the new power transformer with the existing unit more accurately.
If some historical information is unavailable, the project can still be discussed using the data that is available. Nameplate photographs, drawings, test records, and the site single-line diagram can often provide useful starting information.
HONLE manufactures dry-type, oil-immersed, distribution, and project-specific power transformers for industrial, utility, commercial, renewable-energy, and infrastructure applications.
For a transformer intended for parallel operation, parameters such as voltage ratio, vector group, impedance, tap arrangement, cooling method, insulation requirements, terminal configuration, and testing requirements can be reviewed as part of the overall specification.
The important point is to define the parallel-operating requirement early.
Designing a new transformer specifically to match an existing unit is usually more practical than completing production first and only later discovering that the two transformers do not share load correctly.
9. Conclusion
Two power transformers can operate in parallel, but equal kVA and equal nominal voltage are not enough to guarantee successful operation.
The seven key conditions are:
- Closely matched voltage ratio
- Correct polarity
- Same phase sequence
- Compatible vector group
- Closely matched percentage impedance
- Similar impedance angle
- Compatible connection impedance to the common bus
Among these, voltage ratio, phase relationship, and transformer impedance deserve particular attention.
Voltage mismatch can create circulating current. Phase-angle mismatch can make direct connection impossible. Impedance mismatch can cause one power transformer to carry more load than the other.
The surrounding electrical system also matters.
Parallel operation can increase short-circuit current, change protection requirements, affect neutral and earthing arrangements, and require coordinated OLTC control.
For a new project, these requirements can be included when the transformers are specified.
For an expansion project involving an existing transformer, comparing the existing nameplate data, test information, connection arrangement, and single-line diagram with the proposed new power transformer can help identify compatibility issues before manufacturing and commissioning.
Frequently Asked Questions
Can two transformers with different kVA ratings operate in parallel?
Yes. Different kVA ratings do not automatically prevent parallel operation. However, voltage ratio, vector group, impedance, and expected load sharing must still be compatible.
Can two power transformers with different impedance values operate in parallel?
They can in some cases, but large impedance differences can cause uneven load sharing. The transformer with lower impedance usually carries more load.
Do parallel transformers need the same vector group?
For normal parallel operation, the vector groups should be compatible and provide the same phase displacement. In practice, using the same vector group is the simplest and safest arrangement.
Do parallel transformers need the same voltage ratio?
Their effective voltage ratios should match closely. A noticeable secondary voltage difference can create circulating current even when the external load is low.
Can an old transformer operate in parallel with a new transformer?
Yes, if their electrical characteristics are compatible. The existing transformer’s nameplate, impedance, vector group, tap range, voltage ratio, and available test data should be checked before selecting the new unit.
Can dry-type and oil-immersed transformers operate in parallel?
Potentially, yes. The cooling method itself does not determine whether transformers can operate in parallel; electrical compatibility and system design are more important.
Can transformers with different tap positions operate in parallel?
This is generally undesirable because different tap positions can create different secondary voltages and circulating current. Their effective voltage ratios should be coordinated before parallel operation.
Does connecting two transformers in parallel double the available capacity?
Not always. Actual usable capacity depends on transformer impedance, load sharing, cooling conditions, connection impedance, and system limits.
Will parallel transformers increase short-circuit current?
Yes. Both transformers can contribute current to a downstream fault, so the available short-circuit current at the common bus may increase significantly.
One transformer may become overloaded while the other remains underused. This can increase heating, losses, and the risk of exceeding the transformer’s rated current.
Can two transformers from different manufacturers operate in parallel?
Yes. Manufacturer brand is not the deciding factor. The important issue is whether their voltage ratio, vector group, impedance characteristics, phase sequence, and other electrical parameters are compatible.
What information is useful when adding a new transformer in parallel with an existing one?
The existing transformer’s nameplate, rated capacity, voltage ratio, vector group, impedance, tap range, test report, and system single-line diagram are especially useful. These details help HONLE match the new power transformer more accurately.
Information to send when adding a transformer to an existing installation
When requesting an additional transformer for a site with an existing unit, provide the existing nameplate and test information together with the intended operating arrangement. A model name or capacity alone does not establish that two units can operate together.
| Existing installation information | What to include in the enquiry |
|---|---|
| Electrical identification | Rated capacity, primary and secondary voltages, phases, frequency, vector group, and tap information. |
| Available test records | Impedance and other relevant values from the actual unit’s documents. |
| System arrangement | A single-line diagram, protection information, intended operating mode, and current load profile. |
| Site constraints | Location, cooling and ventilation conditions, dimensions, and commissioning responsibilities. |
For candidate equipment, consult the SC(B) dry-type transformer and S13-M.RL oil-immersed transformer pages. These listings are starting points for review, not confirmation of parallel compatibility.
Request a review of the proposed transformer specification and have the final operating arrangement assessed by the project’s responsible electrical engineer.