A wind farm main transformer does more than raise voltage from the collection system to the grid connection level. Its electrical design has to match the way the wind farm actually generates power, the requirements of the utility network, and the conditions at the substation site.
For example, a project may collect power through a 35kV network and use a power transformer to step the voltage up to 110kV before grid connection. The exact arrangement varies by project, but the design questions are similar: What is the maximum continuous output? How much will generation fluctuate? What reactive power range is expected? What harmonic levels are present? What fault level does the grid impose? And what environmental conditions will the transformer face for the next several decades?
These questions should be clarified before the transformer design is finalized. A nameplate showing the correct MVA and voltage ratio is only the starting point.
Is the Transformer Connecting Individual Turbines or the Whole Wind Farm to the Grid?
This distinction should be clear from the beginning.
A power transformer installed close to an individual wind turbine normally raises generator or converter output to the medium-voltage collection system. The wind farm then combines the output of multiple turbines through collection feeders before sending the power to a main substation transformer.
The main grid-connection transformer works at a different system level. A typical arrangement may look like:
Wind turbines → turbine step-up transformers → 35kV collection system → main power transformer → 110kV grid
Other projects may use different collection and transmission voltages depending on plant capacity, utility requirements, distance to the connection point, and local grid structure.
This distinction also matters when discussing standards. IEC/IEEE 60076-16 specifically addresses transformers connecting wind turbine generators to the wind farm collection system or an adjacent distribution network; its scope does not cover the main transformer connecting multiple turbines to the transmission or distribution network.
For a 35/110kV wind farm main transformer, the project specification therefore needs to be reviewed as a substation power transformer application rather than simply labeling the unit a “wind turbine transformer.”
What Voltage Ratio Does the Grid Connection Actually Require?
The nominal voltage ratio sounds like one of the easiest parameters to specify, but the transformer cannot be designed from “35/110kV” alone.
We also need to confirm:
- Nominal system voltage on both sides
- Highest system voltage
- Required insulation level
- System frequency
- Grounding arrangement
- Neutral requirements
- Vector group
- Tap range
- Tap step
- Whether an OLTC is required
- Expected operating voltage range at the grid connection point
These details determine how the transformer fits into the collection network and the utility system.
A 35/110kV Ratio Does Not Tell the Whole Story
Two projects may both request a 35/110kV transformer but require different tap ranges, neutral arrangements, vector groups, insulation coordination, impedance values, or cooling designs.
That is why the single-line diagram and utility grid-connection requirements are especially useful during technical confirmation. They show how the transformer will operate as part of the complete electrical system rather than as an isolated voltage-conversion device.
OLTC Requirements Should Be Confirmed Early
Wind farm output changes with wind conditions, while grid voltage also varies with network operation.
An on-load tap changer can provide voltage-ratio adjustment while the transformer remains energized, but the required tap range and control philosophy should come from the project specification and grid study rather than being assumed.
The transformer should not be expected to solve every voltage-control problem in the wind farm. Turbine converters, reactive compensation equipment, substation controls, and the transformer tap changer may all participate in voltage regulation.
The important point for manufacturing is to know exactly what operating range the transformer must support.
How Should the Power Transformer Be Rated for Variable Wind Generation?
Wind generation is not a flat industrial load.
A wind farm may operate well below rated output for long periods and then approach full generation when wind conditions are favorable. Output can also change repeatedly over the course of a day.
This does not automatically mean the power transformer needs to be significantly oversized. It means the thermal design should be based on realistic operating conditions rather than only one steady-state number.
IEC 60076-7 addresses loading, operating temperature, and thermal ageing for mineral-oil-immersed power transformers, which is relevant when evaluating how loading patterns and ambient temperature affect transformer operation.
Rated MVA Should Reflect the Real Export Requirement
The transformer rating should be checked against the maximum power that may reach the substation, including the wind farm configuration and operating requirements.
Important information includes:
- Total installed wind turbine capacity
- Maximum expected simultaneous generation
- Required active and reactive power operating range
- Possible future expansion
- Auxiliary loads
- Expected operating profile
- Required overload conditions, if any
Installing a much larger transformer without a system reason is not automatically better. Transformer rating affects losses, cost, dimensions, transport, protection settings, and substation layout.
The goal is to select a rating that provides the required thermal margin without separating the transformer design from the actual wind farm operating profile.
Does Reactive Power Change the Transformer Requirement?
Yes, but the transformer itself should not be treated as the wind farm’s primary reactive power control device.
Modern wind farms may use turbine converters together with capacitor banks, reactors, STATCOMs, or other equipment to satisfy grid-code requirements for voltage and reactive power control.
From the transformer side, what matters is the resulting current, voltage range, power factor, tap requirement, and operating condition.
If the plant needs to export or absorb substantial reactive power while also transferring active power, transformer loading should be evaluated using apparent power rather than MW alone.
For example, specifying only the wind farm’s active power output can leave an important part of the electrical requirement undefined.
Confirm the Required Power Factor or MVAr Range
Before finalizing the design, useful project information includes the required power-factor range at the point of connection or the expected MVAr operating range.
This allows the transformer rating and current levels to be checked against realistic operating conditions.
It also helps avoid a common problem: selecting the transformer according to the wind farm MW rating and only considering reactive power requirements later.
How Much Harmonic Content Will Reach the Transformer?
Wind turbines using power electronic converters can introduce harmonic currents into the electrical system. The actual harmonic spectrum depends on turbine technology, converter design, collection system configuration, filters, operating conditions, and other equipment connected to the network.
For the power transformer manufacturer, the useful information is not simply a statement that “the wind farm has harmonics.”
We need the harmonic data from the electrical study when harmonic loading is significant.
Harmonics Can Increase Transformer Losses and Heating
Harmonic currents can increase winding and stray losses and may therefore affect temperature rise.
The design review should consider the actual harmonic spectrum and expected current rather than applying a generic allowance to every wind project.
If the harmonic study identifies meaningful harmonic loading, it can influence decisions involving winding design, conductor arrangement, thermal margin, loss evaluation, and temperature-rise performance.
This is another reason why the transformer specification should follow the completed or sufficiently developed electrical studies rather than being based only on MVA and voltage.
What Short-Circuit Level Should the Transformer Be Designed Around?
The short-circuit level at the grid connection point is one of the most important pieces of information for a substation transformer.
During an external fault, very high currents can create both thermal stress and strong mechanical forces inside the transformer windings.
IEC 60076-5 specifically addresses the ability of power transformers to withstand the thermal and dynamic effects of external short circuits.
Transformer Impedance and Grid Fault Level Need to Be Considered Together
Transformer percentage impedance affects fault current and voltage performance.
A lower impedance can result in higher downstream short-circuit current, while a higher impedance limits fault current but can also influence voltage regulation and system performance.
There is therefore no single impedance value that is automatically correct for every wind farm.
The project engineer should provide the required impedance or the information needed to coordinate it with:
- Grid short-circuit level
- Collection network
- Switchgear ratings
- Protection settings
- Parallel transformers, if used
- Voltage regulation requirements
When two or more transformers may operate in parallel, their voltage ratio, impedance, tap arrangement, vector group, and other relevant characteristics also need to be coordinated.
Which Parameters Should Be Confirmed Before Manufacturing?
For a wind farm grid-connection transformer, a concise technical schedule is more useful than a general request for a “35/110kV wind power transformer.”
Key Information for a Wind Farm Main Power Transformer
| Item | Information to Confirm | Why It Matters |
| Rated power | MVA | Defines continuous loading requirement |
| Voltage ratio | e.g. 35/110kV | Matches collection and grid voltage |
| Frequency | 50Hz or 60Hz | Determines electrical design |
| Vector group | Project requirement | Affects phase relationship and grounding |
| Impedance | Specified % value | Affects fault current and voltage performance |
| Tap changer | DETC or OLTC | Determines voltage adjustment capability |
| Tap range | Required range and steps | Matches grid voltage variation |
| Harmonics | Harmonic spectrum if relevant | Supports thermal and loss evaluation |
| Reactive power | PF or MVAr range | Affects transformer current and MVA loading |
| Short-circuit level | Grid fault data | Required for fault withstand review |
| Cooling | ONAN/ONAF or project requirement | Determines thermal performance |
| Site conditions | Temperature, altitude, pollution, etc. | Affects insulation and cooling design |
| Standards | IEC/IEEE/local utility specification | Defines design and testing requirements |
This information should ideally be confirmed together with the single-line diagram, technical specification, load-flow study, short-circuit study, and relevant grid-connection requirements.
What Outdoor Conditions Can Change the Transformer Design?
Wind farms are often built where wind resources are strong rather than where transformer operating conditions are convenient.
That may mean remote plains, coastal areas, mountains, deserts, cold regions, or high-altitude sites.
The transformer specification therefore needs to include the real site conditions.
High Ambient Temperature Changes the Thermal Margin
Cooling performance depends partly on the surrounding air temperature.
If the project operates in an unusually hot climate, the specified ambient conditions should be provided during design rather than after production.
IEC 60076-2 covers temperature-rise requirements and test methods for liquid-immersed transformers, while IEC 60076-7 provides guidance related to operating temperature and thermal ageing.
Cooling arrangement, radiator design, temperature rise, loading requirements, and accessory selection can then be reviewed against the actual site conditions.
High Altitude Affects External Insulation and Cooling
Altitude should also be stated clearly.
Reduced air density at higher elevations can affect both external insulation performance and air cooling. A transformer intended for a high-altitude wind farm should therefore be specified for the actual installation elevation rather than assuming normal service conditions.
Coastal Wind Farms Need More Attention to Corrosion
For coastal or high-humidity sites, corrosion protection becomes more important.
Salt contamination, moisture, wind-driven rain, and long outdoor exposure can affect tanks, radiators, control cabinets, terminal structures, fasteners, and other external components.
The project should state the environmental and corrosion requirements so that surface treatment, coating systems, material selection, sealing, and accessories can be specified accordingly.
Cold climates may introduce different concerns, including low-temperature oil performance, sealing materials, ice, snow, and accessory suitability.
There is no single “outdoor wind farm design” that covers every site.
Should Transformer Losses Matter When Wind Output Is Variable?
Yes.
A wind farm transformer does not spend its entire service life at full load, so both no-load and load losses matter.
No-load loss exists whenever the transformer is energized. Load loss rises with current and becomes more important as generation increases.
The relative importance of each depends on the wind farm’s operating profile.
For a transformer that remains energized continuously while generation frequently falls below rated output, focusing only on full-load efficiency does not provide a complete picture of long-term energy losses.
When project specifications include guaranteed loss values or capitalization formulas, these should be confirmed before design because they can affect core material, conductor selection, dimensions, manufacturing cost, and lifetime operating cost.
Which Tests Should Be Agreed Before the Transformer Is Built?
Testing requirements should not be left until the transformer is ready for shipment.
IEC 60076-1 provides the general framework for power transformer ratings, requirements, and testing. The exact inspection and test plan should then follow the applicable standard, purchaser specification, voltage class, and utility requirements.
Depending on the project, the agreed scope may include routine tests and specified type or special tests.
For wind farm substations, it is useful to confirm early:
- Applicable IEC, IEEE, or local standard
- Routine test requirements
- Temperature-rise test requirements
- Dielectric test requirements
- Loss and impedance tolerances
- Noise requirements
- Oil tests
- Accessory functional checks
- Factory acceptance test procedures
- Witness inspection requirements
- Test reports and final documentation
If a utility, EPC contractor, or project owner needs to witness particular tests, that should be included in the production and delivery schedule from the beginning.
Grid Studies Should Come Before the Final Power Transformer Specification
A wind farm main transformer should not be ordered from a short description such as:
40MVA, 35/110kV, outdoor oil-immersed transformer.
That gives us a starting point, but not enough information to finalize the electrical and mechanical design.
A stronger specification connects the transformer directly to the wind farm’s grid studies and site conditions.
Before manufacturing begins, we want the voltage ratio, MVA rating, impedance, vector group, tap range, reactive power conditions, harmonic data, fault level, insulation requirements, ambient conditions, altitude, cooling arrangement, accessories, standards, and testing requirements to be clear.
This is especially important for wind farms because the transformer sits between a variable generation system and a utility network that may impose specific voltage, reactive power, fault, and protection requirements.
For HONLE, our role as a power transformer manufacturer is to turn those system requirements into a transformer specification that can be manufactured, tested, installed, and operated as intended. For projects using 35kV collection systems and 110kV grid connections, the discussion should therefore begin with the electrical system rather than simply selecting a transformer from a standard rating list.
FAQ
Can a 35kV power transformer be used for wind farm grid connection?
It depends on the wind farm network. 35kV is commonly used as a collection or substation voltage in some projects, while the main transformer may then step the voltage up to 110kV or another grid level. The final voltage arrangement must follow the project and utility requirements.
Does a wind farm power transformer need an OLTC?
Not every project requires one. An OLTC is selected when on-load voltage adjustment is required by the system design, grid connection requirements, or voltage-control strategy.
Do wind turbine harmonics require a special transformer?
Not automatically. The harmonic spectrum and magnitude should first be evaluated. If harmonic loading is significant, its effect on transformer losses, heating, and design margin should be considered.
Why is short-circuit level important when specifying a power transformer?
The grid fault level and transformer impedance influence the current that may flow during faults. The transformer also has to withstand the resulting thermal and mechanical stresses.
What information should be sent to a power transformer manufacturer for a wind farm project?
The most useful documents include the single-line diagram, transformer data sheet, grid-connection requirements, rated MVA and voltage, impedance requirement, tap range, short-circuit data, harmonic information, reactive power range, site conditions, applicable standards, and required test scope.