An auxiliary transformer supplies everything inside a substation or plant that is not the main power path: protection and control supplies, cooling pumps and fans, tap changer drives, lighting, heating, battery chargers, and the small power loads that keep a site operable. It is a small item on the bill of materials and one of the first things that stops the installation working when it is specified wrongly.
100 kVA – 2.5 MVA Dry or Oil IEC 60076
| Parameter | Typical range |
|---|---|
| Capacity | 100 kVA – 2.5 MVA |
| Primary voltage | 6.3 / 11 / 33 / 35 kV |
| Secondary voltage | 400 / 415 V, or 690 V |
| Type | Epoxy cast resin dry-type, or oil-immersed |
| Vector group | Dyn11, Yyn0 |
| Cooling | AN, AF, ONAN |
| Standards | IEC 60076, IEC 60076-11 for dry-type |
| Delivery | 30 days from drawing approval |
Where Auxiliary Transformers Are Used
In a transmission or distribution substation the station service transformer is usually fed from the tertiary winding of the main transformer or from a dedicated MV feeder, and it carries everything from SCADA and protection panels to the OLTC motor drive and the fire suppression system. In a power plant the unit auxiliary transformer serves the same function at larger ratings, feeding pump motors, fans, and the plant’s own distribution boards. In a renewable or BESS installation the auxiliary supply covers inverters, cooling, monitoring and the control building.
Dry-Type or Oil-Immersed
Most indoor station service duties use epoxy cast resin dry-type units because they can sit in the control building without a separate oil containment or fire separation. Where the auxiliary transformer is installed outdoors alongside the main unit, an oil-immersed construction is usually preferred for its lower cost at the same rating and its tolerance of weather. We build both, and the choice usually comes down to where the unit physically goes rather than to any electrical advantage.
See dry-type transformers for indoor installations and oil-immersed units for outdoor duties.
Sizing an Auxiliary Transformer
The load list is the starting point, and the mistake to avoid is sizing against the sum of connected load. Auxiliary loads are intermittent: a cooling pump runs on a thermostat, a tap changer drive runs for seconds, a battery charger tapers as the bank recovers. Applying a diversity factor to the connected load, then checking the worst-case simultaneous switching scenario, gives a rating that is neither oversized nor marginal. Motor starting is the other check, because a directly started pump can pull several times its rated current and the voltage dip at the auxiliary board has to stay within what the protection and control equipment will tolerate.
Technical Highlights
- Cast resin or oil-immersed construction to suit location
- Dyn11 or Yyn0 vector group with earthing to specification
- Screened winding option for improved transient behaviour
- Enclosure options to IP23 and above for outdoor units
Application Scenarios
- Substation station services and control building supplies
- Power plant unit and station auxiliary boards
- Renewable and BESS sites for inverters and cooling
- Industrial plant auxiliary distribution
Key Features
- Routine tests per IEC 60076 with English reports
- Terminal arrangement matched to the existing board
- Off-circuit taps for site voltage correction
- Temperature monitoring and protection devices fitted
Optional Configurations
- Electrostatic screen between windings
- Forced air cooling for a second rating step
- Degree of protection upgraded for coastal sites
- Matching an existing unit for like-for-like replacement
Replacing an Existing Auxiliary Transformer
Replacement is the most common reason buyers contact us about this product. Send the rating plate and a photograph of the terminations and we will match capacity, ratio, vector group and terminal layout, including the base dimensions where the unit has to drop onto an existing plinth. Where the original drawings are lost, we work from the rating plate and measurements taken on site.
Related Pages
- Power transformers — main transmission and distribution units
- Step-down transformers — 33-132kV to 11kV
- Compact substations — complete packaged substations
- Switchgear and control — LV boards and protection panels
- Build a quote list for multiple units
Frequently Asked Questions
What size auxiliary transformer do I need?
It depends on the load list rather than on the main transformer rating. Add up the auxiliary loads, apply a diversity factor for intermittent duty, then check motor starting voltage dip. Send the list and we will size it with you.
Dry-type or oil-immersed for a substation?
Indoor installations normally use epoxy cast resin dry-type because no oil containment or fire separation is needed. Outdoor units sitting beside the main transformer are usually oil-immersed. The deciding factor is location.
Can you match an existing unit for replacement?
Yes. Send the rating plate, the terminations, and the base dimensions if the unit drops onto an existing plinth, and we will match capacity, ratio, vector group and layout.
Do you supply the LV board as well?
We manufacture switchgear alongside our transformers, so the auxiliary transformer and its LV distribution can be specified and tested together.
What is the delivery time?
30 days from drawing approval for standard ratings. Replacement units built to match an existing specification are quoted once we have confirmed the details.