Transformers
Distribution transformers
Transformers that step the medium voltage of a distribution network down to the low voltage a building, a plant or a process uses.
At a glance
- Ratings on this site
- 100 to 2500 kVAother ratings on request
- Primary voltage
- commonly 6.6 to 33 kVset by the network at the connection point
- Secondary voltage
- 400 V, 415 V or 433 V480 V on 60 Hz systems
- Construction
- liquid immersed or dry type
- Standards
- IEC 60076-1, IEC 60076-11 for dry typeIEEE C57.12.00 on North American specifications
A distribution transformer is the last voltage step before the loads, from a medium voltage network to the low voltage a building or process uses. The ratings step in a standard series so that manufacturers build a known set of designs and a buyer can compare like with like. A rating between two standard values usually costs more and takes longer for no technical benefit, so the practical decision is which standard value the load sits under.
The rated current on the low voltage side follows directly from the rating and the secondary voltage. It is usually that current, rather than the kVA figure, that sizes the low voltage incomer, the busbar and the cables, which is why every rating page on this site states it at the common secondary voltages.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Rating | 100 to 2500 kVA in the standard series | The load, its growth and the starting of the largest motor |
| Primary voltage | 6.6, 10, 11, 13.8, 20, 22 or 33 kV | The network at the connection point |
| Secondary voltage | 400, 415 or 433 V at 50 Hz; 480 V at 60 Hz | The installation and the market |
| Vector group | Commonly Dyn11 or Dyn5 | The earthing arrangement and any parallel operation |
| Tapping | Off circuit taps on the primary | The network voltage range at the connection point |
| Construction | Mineral oil, ester, or cast resin dry type | Placement, fire strategy and liquid containment |
| Conductor | Copper or aluminium windings | Specification preference and manufacturer design |
| Losses | At or below the limit of the destination market | Regulation, and the owner's view of lifetime running cost |
Typical options are the ones buyers most often specify. They are not a list of what is available, which is confirmed per enquiry.
Ratings
| Rating | Full load current at 400 V | Full load current at 433 V |
|---|---|---|
| 100 kVA | 144 A | 133 A |
| 160 kVA | 231 A | 213 A |
| 200 kVA | 289 A | 267 A |
| 250 kVA | 361 A | 333 A |
| 315 kVA | 455 A | 420 A |
| 400 kVA | 577 A | 533 A |
| 500 kVA | 722 A | 667 A |
| 630 kVA | 909 A | 840 A |
| 800 kVA | 1155 A | 1067 A |
| 1000 kVA | 1443 A | 1333 A |
| 1250 kVA | 1804 A | 1667 A |
| 1600 kVA | 2309 A | 2133 A |
| 2000 kVA | 2887 A | 2667 A |
| 2500 kVA | 3608 A | 3333 A |
Rated current at 400 V, three phase. Open a rating for voltage configurations, loss limits, documents and what we need to quote.
Applicable standards
| Standard | What it covers |
|---|---|
| IEC 60076-1 | Power transformers, general requirements |
| IEC 60076-11 | Dry type transformers with highest voltage for equipment up to 72.5 kV |
| Regulation (EU) No 548/2014, as amended | Maximum losses for transformers placed on the EU market, and still applied in Great Britain |
| EN 50708-1-1 and EN 50708-2-1 | European additional requirements for medium power transformers, which replaced EN 50588-1 in 2020 |
| IEEE C57.12.00 | General requirements for liquid immersed distribution, power and regulating transformers, used on North American specifications |
The standard that applies is set by the specification and the destination market. Editions change, so the edition cited in a specification is confirmed in the quotation.
Documents normally requested
| Document | What it evidences | When it is normally provided |
|---|---|---|
| Technical datasheet | Rating, voltages, losses, impedance, dimensions and weight of the unit offered | With the quotation |
| Outline drawing | Footprint, lifting points and cable connection positions | With the quotation or at order |
| Routine test report | The individual unit's tests to its standard | With the delivered unit |
| Type test report | The design has passed the type tests of its standard | On request, where a type test exists for the design |
| Rating plate details | The declared values carried on the unit itself | With the delivered unit |
| Declaration of conformity | Conformity with the legislation of the destination market | Where the destination market requires one |
| Operating and maintenance manual | Installation, operation and maintenance instructions | With the delivered unit |
Which documents apply depends on the destination market, the standard and the manufacturer. Documents are confirmed in the quotation for the specific unit offered.
What we need to quote accurately
| 01 | Rated power in kVA, or the connected load and the largest motor if the rating is not fixed yet |
| 02 | Primary and secondary voltage, and the tapping range if the network operator sets one |
| 03 | Phase, frequency and the vector group, or the earthing arrangement it has to suit |
| 04 | Construction: liquid immersed or dry type, and whether oil is acceptable where the unit will stand |
| 05 | Indoor or outdoor, ambient temperature, altitude and any corrosive or dusty exposure |
| 06 | The standard and the loss requirement the destination market applies |
| 07 | Quantity, destination and the date the equipment has to be on site |
| 08 | Whether surplus or refurbished units are acceptable, or new only |
Delivery and logistics
Distribution equipment is heavy, and a delivery date is only real once the site is ready for it. The dimensions and weight of the unit offered decide the vehicle, the lifting equipment and the route to the foundation, and they come from the manufacturer's drawing rather than from the rating.
Liquid immersed transformers are usually shipped filled, which affects transport rules and the handling on arrival. Anything crossing a border also carries customs documentation, and where the destination applies its own conformity or energy efficiency rules the paperwork has to match that market rather than the country of manufacture.
Lead times vary widely with rating, specification and manufacturer loading. We state a delivery basis in each quotation for the specific unit offered and do not publish a general lead time we cannot hold.
Common questions
How much headroom should a rating carry?
Enough for the growth that is actually planned and for the starting behaviour of the largest motor, and no more. An oversized unit carries higher no load losses for its whole life, and those are paid every hour whether the load is there or not.
Is a 33 kV unit covered by the same loss limits?
Not directly. The EU loss tables apply where one winding is at most 24 kV Um. A 36 kV Um primary, which is what a 33 kV system uses, is corrected under a separate table in the regulation, and the applicable limit is confirmed for the specific configuration.
Related
Request a quote for this equipment
Send the rating, the voltage and the destination, or submit the specification, drawing or nameplate. Anything missing is raised before it reaches a supplier.