Substations
Secondary substations
The point where a distribution network steps medium voltage down to the low voltage that buildings and sites use, whatever physical form it takes.
- Role
- medium voltage to low voltagethe last transformation before the consumer
- Forms
- prefabricated, walk in, package, in buildingset by the site
- Ownership
- network operator or customerdecides whose standard applies
- Standards
- IEC 62271-202 when prefabricatedIEC 61936-1 for the installation
Secondary substation names a role in the network. It is the node where medium voltage is transformed to low voltage and distributed to the consumers around it. It can be a prefabricated compact station, a walk in station, a package substation or a room in a building.
Who will own and operate it matters as much as its form. A station the network operator adopts is built to that operator's standard. The operator's specification then decides the switchgear, the transformer and often the enclosure. A customer owned station follows the customer's specification within the operator's connection conditions.
In Great Britain the building of a new secondary substation is contestable work. Ofgem's rules let an Independent Connection Provider build it, while non contestable work stays with the network operator, and an ICP cannot operate or maintain what it builds. ICPs are accredited under the National Electricity Registration Scheme, which LRQA runs for the network operators. UK Power Networks, for example, adopts an ICP built asset only when it has approved the equipment for use, a NERS accredited ICP has installed it to its satisfaction and it matches an accepted design submission.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Ownership | Network operator or customer | Whether the operator will adopt the station, which decides whose standard applies |
| Physical form | Prefabricated, walk in, package or in building | Space, access and operating practice |
| Transformer | The ratings the operator's standard allows | The demand of the area the node supplies, and its growth |
| Medium voltage | Ring main unit, or panels at larger nodes | Where the node sits in the ring, and whether it is switched remotely |
| Low voltage | A board or feeder pillars with fused ways | How many low voltage circuits leave the node |
| Earthing | Earthing arrangement and electrode design | Soil conditions, the earth fault current and local rules |
Applicable standards
| Standard | What it covers |
|---|---|
| IEC 62271-202 | Applies when the node is a prefabricated station |
| IEC 61936-1 | Power installations exceeding 1 kV AC, for the design and erection of the installation. It does not cover the design of prefabricated, type tested switchgear or prefabricated substations |
| IEC 62271-200 and IEC 61439-2 | The switchgear and the low voltage board, whatever form the node takes |
In a specification, each standard covers its own part. The manufacturer proves the station to IEC 62271-202, and the installation around it is designed to IEC 61936-1.
What a secondary substation request needs
- Whether a network operator adopts the station, and which one
- The operator's specification, or its reference number
- The form the node takes on this site
- The transformer rating the area needs
- Earthing inputs where known: soil resistivity and the earth fault current
Specification template
Fill it in on screen or by hand, then upload it with your request.