A prefabricated substation is a factory-built enclosure that contains medium-voltage switchgear, a distribution transformer and a low-voltage panel, delivered ready to connect on site. Buying one well means choosing between the European and American layouts, then specifying each of the three main parts and the enclosure itself. This guide explains what to compare when sourcing box-type or compact substations from China.
What a prefabricated substation is
Suppliers use several names for the same family of products: prefabricated substation, box-type substation, compact substation, kiosk substation and package substation. All of them replace a brick-built substation room with a steel or concrete enclosure assembled and tested in the factory.
The relevant IEC standard for the complete unit is IEC 62271-202, which covers high-voltage/low-voltage prefabricated substations. The parts inside follow their own standards: IEC 62271-200 for MV switchgear, IEC 60076 for the transformer and IEC 61439 for the LV assembly.
European-type vs American-type
The two main layouts differ in how the switchgear and transformer are combined.
European-type units have separate compartments for MV switchgear, transformer and LV panel inside one enclosure. The MV side is usually a ring main unit or compact switchgear. The transformer can be oil-immersed or dry-type. This layout is larger but flexible, easy to maintain and easy to expand.
American-type units, often called pad-mounted or combined transformers, put the transformer core and coil, load-break switch and fuses in the same oil tank. MV fusing typically combines a bayonet-type expulsion fuse with a current-limiting backup fuse. Units are built for loop feed or radial feed. The result is compact and lower in cost, but the MV switching and protection options are limited, and a fault inside the tank can mean replacing the whole unit.
Chinese catalogues often use a YB prefix for European-type units and ZGS for American-type combined transformers, but check the technical data rather than relying on the model code.
| Factor | European-type | American-type |
|---|---|---|
| Layout | Separate MV, transformer and LV compartments | Transformer, switch and fuses share one oil tank |
| MV equipment | RMU or switchgear with load-break switches, fuses or circuit breakers | Oil-immersed load-break switch with bayonet and backup fuses |
| Transformer | Oil-immersed or dry-type | Oil-immersed only |
| Size and weight | Larger | Smaller footprint |
| Protection and automation | Wide choice, including relays and remote control | Limited |
| Maintenance and repair | Components can be replaced individually | Faults often affect the whole unit |
| Typical use | Urban networks, industrial sites, commercial complexes, renewable plants | Residential estates, street networks, loop distribution |
| Relative cost | Higher | Lower |
What is inside: the three main parts
MV switchgear or RMU
Specify rated voltage, rated current, short-time withstand current and duration, and the insulation medium (air, SF6 or solid insulation). List the functional units, for example two cable feeders plus one transformer protection unit with fuses or a circuit breaker. If you need remote operation, state motorised mechanisms and the communication protocol. Ask for the internal arc classification (IAC) under IEC 62271-200. For ranked suppliers, see high-voltage switchgear.
Transformer
Specify rated power in kVA, primary and secondary voltage, frequency, vector group (Dyn11 is common), impedance, cooling, tap changer range and guaranteed no-load and load losses. State oil-immersed or dry-type. Confirm the routine and type tests under IEC 60076. See ranked transformer manufacturers.
LV panel
Specify the incomer (usually an air circuit breaker or moulded case circuit breaker), the number and rating of outgoing feeders, busbar rating and short-circuit rating, metering, and any capacitor bank for power factor correction. The panel should be designed and verified to IEC 61439. See circuit breakers and power capacitors.
The enclosure matters
The enclosure is not just a box. It sets ventilation, safety and service life.
- Material: galvanised or coated steel, stainless steel, aluminium alloy, composite or concrete. Match it to the climate, especially coastal or desert sites.
- Degree of protection: the IP rating for each compartment.
- Enclosure class: under IEC 62271-202, the class indicates how much the enclosure raises the transformer temperature compared with open installation. A lower class means better ventilation and less derating.
- Internal arc classification: shows the unit has been tested to protect people nearby if an arc fault occurs.
- Access: door arrangement, locks, interlocks, lighting and cable entry.
- Site conditions: altitude, ambient temperature, solar radiation, wind and seismic requirements.
Specification checklist
- Layout: European-type or American-type.
- Standards: IEC 62271-202 for the unit, plus IEC 62271-200, IEC 60076 and IEC 61439 for the parts.
- System data: MV voltage, frequency, earthing system, short-circuit level at site.
- MV switchgear: type, functional units, ratings, insulation medium, IAC, motorisation.
- Transformer: kVA, voltages, vector group, impedance, losses, cooling, oil or dry-type.
- LV panel: incomer, feeders, busbar rating, metering, capacitors, surge protection.
- Enclosure: material, IP rating, enclosure class, colour, dimensions, lifting points.
- Tests: routine tests on each part and the assembled unit, type-test reports, factory acceptance test.
- Documents: single-line diagram, layout drawings, foundation drawing, manuals.
- Shipping: transport dimensions, whether the transformer ships installed, Incoterms, warranty.
Add surge arresters on the MV side if your network or utility requires them.
What to check with a Chinese supplier
A prefabricated substation is an assembly. Many factories build the enclosure and LV panel themselves and buy in the RMU or transformer. That is normal, but you need to know which parts are bought in and from whom.
- Credit code and scope: check the 18-character Unified Social Credit Code and confirm the business scope includes the products quoted.
- Factory vs trader: ask for a video tour of the enclosure shop, assembly area and test bay.
- Type-test reports: for the complete prefabricated substation under IEC 62271-202, not only for the parts. Check the laboratory, date and the tested configuration.
- Component sources: the make and model of the RMU, transformer, breakers and meters, with their own test reports.
- Certificates: ISO 9001, CCC where relevant in China, and the approvals your utility requires.
Our scores rate data completeness, not quality; read the methodology, then see the ranked list of prefabricated substation manufacturers.
Frequently asked questions
What is the difference between a box-type substation and a compact substation?
In most supplier catalogues they mean the same product: a factory-built enclosure with MV switchgear, a transformer and an LV panel. Always check the layout and contents rather than the name.
Is European-type or American-type better?
Neither is better in general. European-type suits sites that need flexible protection, automation and easy maintenance; American-type suits residential loop networks where a small, lower-cost unit is enough.
Can a prefabricated substation use a dry-type transformer?
Yes, in a European-type layout. American-type units use an oil-immersed transformer because the switch and fuses sit in the same oil tank.
Which tests should the complete unit have?
Ask for routine tests on the assembled substation and a type-test report for the design under IEC 62271-202. Each main part should also have its own routine tests and type-test reports.