A transformer substation is a distribution point built around a transformer: something arrives at a medium voltage, gets stepped down, and leaves as several low-voltage feeders. That description covers an enormous range of installations, from a single unit in a factory yard to a multi-transformer substation feeding a site. This guide covers how to specify one.
Start With the Load, Not the Transformer
The transformer rating follows from the diversified load and the way it behaves. Continuous base load, intermittent peaks, motor starting, and how much of it runs at once. A load list with a stated diversity factor is the input. Guessing a round number tends to produce either nuisance tripping or an oversized transformer that costs money standing still.
Choose an Arrangement
| Arrangement | What it is | Suits |
|---|---|---|
| Compact substation | Switchgear, transformer and LV board in one tested enclosure | Fixed layouts, tight programmes, constrained sites |
| Separate equipment in a switchroom | Each item installed and wired on site | Large installations, designs still developing, easier extension |
| Unit substation | Transformer close-coupled to an LV switchboard | Industrial plants with distribution at utilisation voltage |
| Pad-mounted unit | Sealed ground-mounted transformer with integrated switching | Utility and commercial distribution, outdoor |
Most projects fall into the first two. The test is whether the design is settled and whether the site programme is tight. Details of the compact route are on the transformer substation page.
Specify the Transformer
- Rating, from the diversified load with the margin recorded.
- Primary and secondary voltage, matched to the supply and to what the load needs.
- Vector group and winding configuration, which affects parallel operation and harmonic behaviour.
- Cooling, selected against the load profile and the enclosure.
- Tap range, chosen against how much the supply voltage actually varies.
- Impedance, which sets fault level and voltage regulation.
Our dry type and cast resin pages cover the choices for indoor installations; oil-immersed designs remain the usual answer outdoors.
Specify the Switchgear
On the medium-voltage side, rated voltage, normal current and short-circuit withstand are the three that matter, plus internal arc classification where the specification calls for it. On the low-voltage side, the outgoing feeder schedule is the work: how many ways, what size, what protection, and what spare provision you want. See the MV switchgear page for arrangements.
Site Conditions Decide the Enclosure
Altitude, ambient temperature range, humidity, salt, dust, indoor or outdoor, and the required ingress protection. These drive ventilation, whether you need anti-condensation heating, creepage distances and finish specification. They are far cheaper to handle at design stage than after delivery.
Protection, Earthing and Interlocking
Protection is set to the single-line diagram with coordination checked, not copied from a default schedule. The earthing arrangement is a system decision made early because it sets fault current magnitude and protection sensitivity. Interlocking between switching devices, earthing switches and access doors is function-tested on the completed assembly.
Standards and Factory Tests
The applicable standard and clause set are agreed against your project specification before production. Where a project names IEC 60076 for the transformer or IEC 62271-202 for a prefabricated assembly, we work to those clauses. Routine tests are carried out on the finished equipment and the reports ship with it. See testing for the sequence and quality control for how the specification is held.
What to Send With an Enquiry
- Single-line diagram.
- Load list with diversity stated.
- Supply voltage, frequency and fault level at the point of connection.
- Site conditions and indoor or outdoor placement.
- Any utility, consultant or client specification.
The switching side of the design is covered in more detail in our MV switchgear selection guide, which covers insulation medium, fixed versus withdrawable, and protection and control.
Frequently Asked Questions
What size transformer do I need?
It comes from the diversified load with a stated margin, checked against peak and motor starting conditions rather than the average. An oversized transformer costs capital and no-load losses; an undersized one trips. Send the load list and the margin can be agreed with you.
Dry type or oil immersed?
Dry type and cast resin are the usual choice indoors and where fire risk or oil containment is a concern – commercial buildings, hospitals, data centres. Oil immersed remains common outdoors and at higher ratings, where cost and cooling favour it.
How much spare capacity should I allow?
Enough for known growth, and no more than that. Record the figure you used so the next engineer knows what the margin was for. Large speculative margin is paid for twice.
Which standard applies?
It depends on your project specification and, in many cases, on the utility. The standard and clause set are agreed in writing before production rather than assumed, and routine test reports are issued against it.
Get a Drawing and a Price
Send the single-line diagram, the load list and the site conditions – altitude, ambient range, indoor or outdoor, enclosure rating. We come back with a proposed arrangement, the standard set we would work to, and a quotation. Email info@xsdfftransformer.com or call +8615867897761.