Project snapshot
| Client | [utility EPC contractor] |
| Region | [country / region] |
| Scope | 11 kV and 33 kV prefabricated compact substations |
| Quantity | [N] units, delivered in [N] batches |
| Transformer rating | [N] kVA per unit |
| Applicable standard | IEC 62271-202 |
| Delivery window | [quarter / year] |
The challenge
A utility distribution programme is not one substation repeated – it is the same substation built correctly [N] times, in [N] different places, by different crews, often inside a fixed outage window. A conventional civil-work substation multiplies the risk on every site: foundations are poured on site, switchgear and transformers arrive separately, cabling and termination happen outdoors, and every commissioning step depends on local labour and weather. When one site slips, the energisation date for that feeder slips with it.
Our client – an EPC contractor delivering for a regional utility – needed to compress that per-site workload without giving up the utility’s acceptance criteria. Three constraints shaped the specification:
- Site time was the scarce resource. Each location had a short access window, and civil work on site was the least predictable part of the schedule.
- Every unit had to be identical. The utility’s maintenance teams would stock spares and train crews against one design, so unit 1 and unit [N] had to match electrically and mechanically.
- Ambient conditions were not temperate. [Describe: high ambient temperature, humidity, coastal salt fog, altitude, or pollution class] – all of which change what the enclosure can actually deliver.
Our solution
Moving the work from the site into the factory
Each unit was delivered as a complete, factory-assembled substation: MV switchgear, distribution transformer, LV switchboard, internal interconnections, earthing, metering and protection were installed, wired and tested before shipment. Compartments are physically separated – MV, transformer and LV – so a fault in one does not propagate into the next. On site the remaining work was reduced to setting the unit on its foundation, terminating the incoming and outgoing cables, and connecting the earth grid.
The footprint saving is not incidental. A compact substation typically occupies one fifth to one tenth of the area of an equivalent civil substation, which is what allows it to be placed close to the load centre instead of on the edge of the network – shorter LV feeders, lower losses, better voltage at the far end.
Selecting the enclosure class for the actual climate
This is the parameter most often skipped, and it is the one that decides whether the transformer can actually deliver its nameplate rating. IEC 62271-202 defines rated power as a three-parameter value – capacity, total losses and temperature-rise limit – and defines six enclosure classes (5, 10, 15, 20, 25, 30 K) that express the permissible temperature difference between the transformer inside the enclosure and the ambient air outside it. A Class 10 enclosure holds that difference to 10 K; a Class 30 enclosure permits 30 K.
The correct class follows from the annual average ambient temperature at the site and the expected load factor (the standard provides selection curves in Annex D). In a hot, humid climate the difference between a Class 20 and a Class 30 enclosure is not a paperwork detail – it is the difference between a unit that carries its rated load in summer and one that has to be derated for years. For this programme we specified Class [10 or 20], with ventilation sized accordingly.
Type-tested safety, verified rather than claimed
Because these units stand in publicly accessible areas, the enclosure had to be proven, not described. The design meets:
- Protection degree – minimum IP23D on the enclosure, with IP54 available on the MV and LV compartments where dust or driving rain is expected.
- Mechanical impact – IK10 (20 J), so doors and covers resist vandalism and accidental impact.
- Internal arc classification – IAC-AB, verified by type test, covering both operator protection (IAC-A) and public protection (IAC-B). Test current and duration were specified as [N] kA / [N] s.
- Earthing – a main earthing conductor capable of carrying the rated short-time withstand current, with a copper cross-section of at least 30 mm², and all metallic enclosure parts bonded to it.
For coastal or industrial sites, corrosion protection was raised to ISO 12944 class C4-H (hot-dip galvanised steel base with polyester powder coating), which is the difference between a 5-year and a 20-year enclosure life in salt-laden air.
Batch consistency across all [N] units
With a multi-site rollout, the hard part is not making one good substation – it is making [N] identical ones. We held one drawing set, one bill of materials and one approved component list across the whole programme, and every unit went through the same routine test sequence before release. The client’s inspectors witnessed Factory Acceptance Tests (FAT) on [first-article / sampled] units before batch release, so deviations were caught in the factory rather than at the foundation.
Technical specification
| Rated HV voltage | 11 kV / 33 kV |
| Rated LV voltage | 0.4 kV |
| Rated transformer capacity | [N] kVA per unit (typical range 315–2 500 kVA) |
| Rated frequency | [50 / 60] Hz |
| Transformer type | [S20 oil-immersed / SCB13 cast resin] |
| Vector group | Dyn11 |
| Tap range | Off-circuit ±2 × 2.5% (on-load optional) |
| Short-circuit impedance | 6% (≤1 600 kVA) / 8% (2 000–2 500 kVA) |
| HV insulation level, 11 kV class | Power frequency 42 kV / BIL 75 kV |
| HV insulation level, 33 kV class | Power frequency 95 kV / BIL 185 kV (40.5 kV equipment) |
| Short-time withstand current | [20 / 25] kA · [N] s |
| Internal arc classification | IAC-AB, [N] kA / [N] s |
| Enclosure protection | IP23D (transformer compartment) · IP54 (MV/LV compartments) |
| Mechanical impact | IK10 (20 J) |
| Enclosure class | [10 / 20] K |
| Ambient temperature | −25 °C to +45 °C |
| Relative humidity | ≤95% (non-condensing) |
| Altitude | ≤1 000 m (plateau type to 4 000 m) |
| Corrosion protection | ISO 12944 C4-H (coastal / industrial) |
| Sound level | ≤50 dB |
| Applicable standard | IEC 62271-202 |
Testing and verification
Every design was qualified by type test to IEC 62271-202, and every unit shipped against the same routine test sequence.
Type tests covered dielectric performance (lightning impulse and power-frequency withstand on HV interconnections), the continuous-current temperature-rise test, short-time and peak withstand current on the main and earthing circuits, verification of the protection degree, mechanical load and impact (IK10), and – because these units are publicly accessible – the internal arc test to Annex A for the declared IAC classification.
The 2022 edition of the standard makes the temperature-rise test considerably more explicit. For oil-immersed transformers the preferred method supplies the HV and LV sides independently; for dry-type transformers the test is now a three-step sequence – open-circuit, short-circuit, then combined load – because a dry-type winding reaches its hottest spot under conditions that a single-power-source test does not reproduce. Where a site is exposed to strong sun, Annex G adds a solar-radiation assessment, since >1.09 kW/m² of irradiance measurably raises the internal temperature rise.
Routine tests were performed on every single unit, and the client witnessed FAT on [first-article / sampled] units before each batch was released for shipment.
Result
[N] units were delivered across [N] batches. Site work per location was reduced to foundation setting, cable termination and earthing, which removed civil construction from the critical path. All units were energised against the utility’s standard acceptance procedure, and because the design, drawings and component list were held constant, the utility’s maintenance teams received one spare-parts list and one training package for the whole fleet.
[Add one quantified outcome if available – e.g. site installation hours per unit, schedule days saved, or first-time energisation rate. If you do not have a verified number, delete this paragraph rather than estimating.]
Frequently asked questions
What is the difference between a compact substation and a conventional civil substation?
A civil substation is built on site: foundations, a building, equipment delivered separately and terminated outdoors. A prefabricated compact substation integrates MV switchgear, transformer and LV switchboard in one factory-assembled enclosure, so assembly, wiring, testing and commissioning happen under factory conditions. On site only the foundation, cable terminations and earthing remain. The typical footprint is one fifth to one tenth of the civil equivalent.
Why does the enclosure class matter as much as the transformer rating?
Because a transformer inside an enclosure cannot dissipate heat as freely as one in open air. IEC 62271-202 quantifies this with enclosure classes from 5 K to 30 K, expressing the permitted temperature difference between inside and outside. The same 630 kVA transformer will deliver materially different continuous output in a Class 10 enclosure than in a Class 30 one, and the correct choice depends on the site’s annual average ambient temperature and load factor. In hot climates, choosing too high a class forces permanent derating.
Should the transformer inside be oil-immersed or cast resin?
It depends on where the unit stands. Oil-immersed units are more compact and cost-effective for outdoor utility duty and tolerate higher overload; cast resin dry-type units contain no flammable liquid and are preferred where fire risk, indoor installation or water-source protection governs. Both are available in the same enclosure platform. See our comparison guide for the full selection logic.
What should a utility EPC require from the manufacturer beyond the hardware?
Three things. First, type-test evidence to IEC 62271-202 for the exact configuration supplied – not a generic certificate. Second, a FAT programme you can witness, with hold points before batch release. Third, drawing and component consistency across every unit in the programme, so spares, training and maintenance procedures stay valid for the whole fleet rather than per site.
Planning a similar programme?
We supply YBW series prefabricated compact substations from 50 kVA to 2 500 kVA, with type-tested enclosures to IEC 62271-202 and FAT witnessing available. Send us your single-line diagram and site conditions and we will return a configuration and delivery schedule.