A distribution substation steps the grid voltage down to the voltage used by customers. Common types are pole-mounted, pad-mounted, indoor or in-building, and prefabricated, each with its own requirements. All share the same building blocks: incoming line and switching, protection, transformation, outgoing circuits, grounding and maintenance.
What a distribution substation is
A substation is the single set of installations, electrical equipment and complementary works used to transfer electrical energy at voltages above 1 kV through power transformation. A distribution substation is one that steps the grid voltage down to the one the customers use: for example, from 13.2 kV to 208/120 V in North American practice, or from 20 kV to 400/230 V under IEC practice.
Voltage class names vary by country and by document: what one code calls medium voltage another may call primary distribution voltage. Always use the classification of the local code and of the utility for design, inspection and connection rules.
Types of distribution substation
Local codes set specific requirements by type. The table summarizes how they differ. The selection of the central piece of equipment, the transformer, is covered in Distribution transformer selection.
| Type | What it looks like | Guide |
|---|---|---|
| Pole-mounted | Transformer mounted on one or more poles, with overcurrent and overvoltage protection on the primary | Pole-mounted substation |
| Pad-mounted | Transformer in a closed enclosure on a concrete pad, with lockable compartments; designed for underground networks | Pad-mounted substation |
| Indoor or in-building | Room or vault with medium-voltage switchgear, transformer and panels | Medium-voltage switchgear |
| Prefabricated or packaged | Prefabricated enclosure with a naturally ventilated transformer; includes mobile units | Compact substation |
| Outdoor high-voltage yard | Busbars, disconnectors and breakers outdoors, with a power transformer | High-voltage and extra-high-voltage substations |
An urban distribution substation of a utility combines several of these elements: an outdoor or gas-insulated substation steps down from high to medium voltage and feeds circuits that end in pole-mounted, pad-mounted or indoor transformers.
What codes usually require of each type
The details change with the jurisdiction. These are the points that normally appear; we adapt the design to the local code and to the requirements of the utility.
Pole-mounted. In urban areas a transformer on a single pole, without an enclosure, is usually accepted if the pole meets the required breaking load. The transformer fastening elements must carry a safety margin over its weight. It needs overcurrent and overvoltage protection at least on the primary side, and the neutral and the tank must be solidly grounded. In rural areas, small transformers are usually mounted on poles with a minimum breaking strength set by the code.
Pad-mounted. The three-phase transformer has separate primary and secondary compartments with front doors. If it has two doors, the primary can be reached only after opening the secondary one. The latch system allows an external padlock and all energized parts are in lockable compartments. If the surface temperature rises well above ambient and the substation is within public reach, it requires a barrier and hot-surface warnings, unless a protection trips the supply on overtemperature.
Indoor or in-building. It needs adequate space, easy access from outside and doors that open outward with panic hardware. The switchgear meets the product and test requirements of the applicable standards (IEC 62271-200 / IEEE C37.20.2). If the room can flood or is a basement, it needs drainage or pumping, or the equipment must be submersible. It must be separated from the emergency generator by a wall or barrier, ensure that fire does not spread to the rest of the building, and show calculations that guarantee ventilation by means that do not depend on power from the transformer.
Prefabricated or packaged. It must be type-tested as a complete transformation installation (IEC 62271-202 for prefabricated substations), with verification on site as the local code requires. The manufacturer supplies transport, installation, operation and maintenance instructions, and the information to prepare the site: earthworks, external ground terminals, position of cable entries and drainage.
Components and what each one does
Even if the type changes, the blocks of a distribution substation are the same:
| Block | What it does | Examples |
|---|---|---|
| Incoming line and switching | Receives the grid and allows the substation to be isolated | Fuse cutouts, disconnector, line switchgear |
| Medium-voltage protection | Clears faults in the transformer and its cables | Fuse, vacuum circuit breaker with relay |
| Surge protection | Limits lightning and switching overvoltages | Surge arrester |
| Transformation | Steps the voltage down | Distribution transformer |
| Low-voltage outgoing circuits | Distributes to the loads and protects the circuits | Main panel, breakers |
| Grounding | Controls step and touch voltages | Grid or electrodes, conductors, ground bus |
| Metering | Records consumption and quality | Instrument transformers and meter |
| Control and station service | Powers relays, controls and communications | Charger and battery bank |
Surge arresters are normally required at every transformer and at every transition from an overhead line to an insulated cable. For distribution substations inside buildings, the designer should evaluate and justify whether the arresters go only at the transition to the underground service and not at the transformer. Grounding of these substations is detailed in Grounding of distribution substations.
Substations with control, protection or communications panels have additional requirements for the control room. There should be fire detection, with alarms that reach the control system. There should be indication of the real position of the switching and disconnecting elements. In unattended substations, detection and extinguishing equipment should be automatic. See the control house of a substation.
Protection and operation
Protection varies with the type. In pole-mounted and pad-mounted units, the fuse and the disconnector are usually used, coordinated with the low-voltage breaker. In an indoor substation with switchgear and in outdoor yards, the breaker with an overcurrent relay predominates. For a distribution circuit with reclosers and fuses, see distribution feeder protection and the configuration in medium-voltage open ring. The protection each type of bay carries, in larger substations, is summarized in Protection per bay: what each bay type carries.
Maintenance
A maintenance plan should include all the equipment of the substation and the interval of each item, which depends on the type of substation and on environmental conditions. For pole-mounted and pad-mounted medium-voltage transformers, that plan can be part of the maintenance plan of the distribution network. Maintenance and cleaning records and evidence should be kept, because the authorities and the utility can ask for them, and the installer hands the owner an installation manual with recommendations and intervals. More detail in substation maintenance.
Testing and commissioning
Before energizing, the substation is tested end to end: transformer, switchgear, breakers, protections, grounding and control. The order and scope are in what a substation commissioning includes.
Testing and commissioning your substation with Tiza Group
We test and prepare for operation medium-voltage distribution substations, from the transformer and switchgear to the protections and the grounding. Testing and commissioning is done in Colombia, or elsewhere on request. See our testing and commissioning service and the medium-voltage switchgear tests, or write to us through contact.
Related guides
- Electrical substation
- Pole-mounted substation
- Pad-mounted substation
- Compact medium-voltage substation
- Industrial substation
- Medium-voltage switchgear
- Distribution transformer
- Distribution transformer selection
- Grounding of distribution substations
- Protection per bay: what each bay type carries
Frequently asked questions
What counts as a substation?
It is the single set of installations, electrical equipment and complementary works used to transfer electrical energy at voltages above 1 kV through power transformation. That is why a distribution transformer installed on a pole or on a concrete pad is also a substation.
What is the difference between a pole-mounted and a pad-mounted substation?
The pole-mounted one has the transformer mounted on the pole and is fed from an overhead line. The pad-mounted one has the transformer in a closed enclosure on a concrete pad, with lockable compartments for the primary and the secondary, and is fed from an underground network.
Does a prefabricated substation need certification?
That depends on the local code and the utility. In many jurisdictions prefabricated or packaged substations must be type-tested to the applicable standard (for example IEC 62271-202 for prefabricated MV/LV substations) and verified or inspected on site before energizing. The requirements are those of the local code and of the utility.