A compact substation takes the three things a distribution point needs – medium-voltage switching, a transformer, and low-voltage distribution – and puts them in one enclosure that is assembled and tested before it reaches your site. You terminate cables on the outside. Everything on the inside has already been wired, interlocked and checked.
What Goes Inside the Enclosure
The arrangement follows the single-line diagram, so no two projects come out quite the same. A typical build has three compartments that are separated from each other:
- Medium-voltage compartment – incoming and outgoing switching with protection and metering. Ring main units, circuit breakers or load break switches, depending on what the diagram calls for.
- Transformer compartment – a distribution transformer rated to the load, with ventilation or forced cooling worked out against the enclosure and the ambient range.
- Low-voltage compartment – outgoing feeders, protection, metering, and the control and interlocking that ties the three sections together.
Cable terminations, earthing, internal lighting, space heaters and any anti-condensation provision are settled on the drawing rather than improvised on site. That drawing is the document your engineer signs off before anything is built.
How It Differs From a Built-Up Substation
| Compact substation | Site-built substation | |
|---|---|---|
| Where it is assembled | In the factory, tested as one unit | Piece by piece on the site |
| Site work | Foundation, cable termination, commissioning | Fencing, building, equipment installation, wiring |
| Time on site | Days | Weeks to months |
| Design flexibility | Set at the drawing stage, then fixed | Can be changed late, at a cost |
| Who coordinates | One supplier for the assembly | You coordinate several trades |
The trade-off is real. A compact substation has to be right on the drawing, because changing the arrangement after the enclosure is built is expensive. A site-built substation absorbs late changes more easily and suits projects where the load is still moving.
Where the Ratings Come From
Capacity, voltage class, short-circuit rating, cooling and enclosure rating are all derived from your project, not picked from a stock list. In practice that means we work from:
- The load list, with diversity applied, which sets the transformer rating.
- The system voltage and fault level at the point of connection, which sets the switchgear ratings.
- Altitude and ambient temperature range, which derate the transformer and decide whether you need forced cooling.
- Indoor or outdoor placement and the required ingress protection, which sets the enclosure specification.
If you only know the load in kVA and the voltages, that is enough to start. The rest can be worked out together.
Which Standard Applies
The applicable standard and its clause set are agreed against your project specification before production begins. Where a project names IEC 62271-202 for the prefabricated substation assembly, or IEC 60076 for the transformer inside it, we work to those clauses and issue the routine test report with the unit. If your specification is silent on the standard, that gets settled in writing first rather than assumed. You can read more about how we handle this on the quality control page.
What Testing Happens Before It Ships
Testing is done on the finished assembly, not on individual components in isolation. Wiring is checked against the diagram, protection and interlocking are function-tested, the transformer is tested to its own routine test schedule, and the enclosure is checked for degree of protection. The reports travel with the equipment. The full sequence is described on the testing page.
When a Compact Substation Is the Wrong Answer
It is worth saying where this does not fit. Very large ratings, or layouts that have to change as the project develops, are usually better served by separate equipment in a building. Sites with difficult access may not take a single large enclosure even where the rating is fine. And projects that need significant civil work anyway – a new building, a new switchroom – often lose the speed advantage that makes a compact unit worth buying.
Related reading: our MV switchgear selection guide.
Frequently Asked Questions
What is the difference between a compact substation and a prefabricated substation?
The two terms overlap and suppliers use them differently. Compact usually describes the enclosure and its footprint. Prefabricated describes how it is built and tested off site. Most units on the market are both. What matters is what is inside and how it was tested, which is what the drawing and the test report tell you.
How long does installation take?
Site work is the foundation, cable termination and commissioning, which is measured in days rather than weeks. The schedule is set by your civil work and cable routes more than by the equipment itself.
Can a compact substation be installed outdoors?
Yes, and most are. The enclosure rating, ventilation, anti-condensation heating and corrosion protection are specified against your site conditions. Coastal or heavily polluted sites need more than a standard finish, and that is a drawing-stage decision.
What rating should I choose?
It comes from your load list with diversity applied, not from a round number that looks safe. Oversizing a transformer costs you in no-load losses and capital for nothing. Send the load list and we will size it with you.
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.