A pad-mounted substation, also called pedestal-type or garden-type, is a distribution transformer in a closed metal cabinet, mounted on a concrete pad and fed by underground cables. Its medium-voltage and low-voltage compartments are separated and locked, so it can be installed in public view without a fence. It is used in housing developments, shopping centers, plants and solar installations with an underground network.
What a pad-mounted substation is
A pad-mounted substation is a distribution transformer designed to sit on a concrete pad, fed by underground cables and enclosed in a metal cabinet with all energized parts inside locked compartments. It is also called a pedestal-type or garden-type substation.
Its great advantage is that it needs no room or fence: it can go on a sidewalk, in a parking lot or next to an industrial building, in public view, because nobody can touch an energized part without opening the cabinet with a tool and a key. It is one of the most used types of industrial substation when the service entrance arrives through ducts. It is the usual solution in housing developments with an underground network, shopping centers, industrial plants with an underground service entrance and solar plants.
Main parts
- Tank with the active part: core and windings immersed in mineral oil or in a high fire-point liquid.
- Medium-voltage compartment: terminals for the cable connectors, fuses and, depending on the version, the loop switch and the surge arresters.
- Low-voltage compartment: output terminals to the main switchboard and, on many models, the off-load tap changer and the indicators.
- Concrete pad with ducts for the cables and the ground connection.
- Grounding of the tank and the neutral.
Dead-front models have all medium-voltage parts insulated: the cables are connected with separable elbow connectors that can be operated with a hot stick. Live-front models leave energized parts exposed inside the compartment and are used less today.
In the United States and in many specifications elsewhere, three-phase transformers of this type are governed by IEEE C57.12.34 (pad-mounted, compartmental-type, self-cooled).
Radial or loop feed
| Version | How it is fed | When it is used |
|---|---|---|
| Radial | A single set of medium-voltage terminals: the cable arrives and ends at the transformer | A load at the end of a circuit, when an alternate supply is not needed |
| Loop feed | Two sets of terminals and an internal switch: the cable comes in and carries on to the next transformer | Underground networks in an open loop, where a fault in one section is isolated and the rest is fed from the other end |
The loop feed version is the basic piece of the open-loop medium-voltage configuration used by utilities in urban areas and many industrial parks.
Protection
Medium-voltage protection of a pad-mounted unit is usually by fuses inside the cabinet. A common combination is a bayonet-type expulsion fuse, replaceable from the front, in series with a backup current-limiting fuse that acts only on high-current internal faults; the details depend on the manufacturer. See medium-voltage fuse.
On the low-voltage side, the main breaker of the switchboard protects the feeders and must coordinate with the medium-voltage fuses. Surge arresters go on the medium-voltage terminals or at the transition point from overhead to underground network. The fuse ratings should come from a coordination study that ensures the transformer is protected; the utility publishes the table of fuses it accepts.
When the plant needs relay protection, medium-voltage metering or several outgoing circuits, the pad-mounted unit falls short and a compact medium-voltage substation with switchgear panels is used instead.
Local code and utility requirements
Construction and safety requirements for these substations, such as separate primary and secondary compartments, the order in which the doors can be opened, padlocking and hot-surface warnings, are set by the local code and by each utility’s construction standard. The utility also defines the capacities, fuses and pad details it accepts. Specify the unit and its installation to those requirements from the start.
When to use it in a plant
- Does the plant only need to transform and protect with fuses?
- Yes: Is the service entrance underground and is there outdoor space?
- Yes: Pad-mounted unit, radial or loop feed depending on the network
- No: Compact substation with panels, or pole-mounted if the network is overhead
- No: Compact substation with panels, with circuit breaker and protection relay
- Yes: Is the service entrance underground and is there outdoor space?
For capacities of a few hundred kVA and a single outgoing circuit to a switchboard, the pad-mounted unit is a compact and safe solution. When the plant expects to grow, needs several medium-voltage outgoing circuits or selective protection with relays, consider a substation with switchgear panels from the design stage. The energization sequence of a compact substation is described in energization of a compact substation.
Testing and maintenance
Before energizing, check the insulation, the transformation ratio, the winding resistance, the off-load tap changer position, the continuity of the grounding and the condition of the medium-voltage connectors and cables. Underground cables and their terminations are the most common weak point: a badly installed elbow or an ungrounded screen ends in a fault. See cable diagnostic tests.
In operation, maintenance includes inspection of the cabinet (corrosion, locks, seals), the oil level and condition when accessible, thermography of the low-voltage connections under load and periodic electrical tests.
How we help
At Tiza Group our team does testing and commissioning of pedestal-type, compact and conventional substations, at medium, high and extra-high voltage and in industrial plants, in Colombia or elsewhere on request. We test the transformer, the medium-voltage cables, the grounding and the protection. We also design the substation and the protection coordination when the project starts from scratch, remotely and worldwide.
See our testing and commissioning services and write to us with the capacity and location of your substation.
Related guides
- Industrial substation: parts, types and protection
- Compact medium-voltage substation
- Pole-mounted substation: parts, limits and protection
- Open-loop medium-voltage configuration
- Medium-voltage fuse
- MV/LV distribution transformer
- Power cables
- Energization of a compact substation: sequence
References
- IEEE C57.12.34, Standard for Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers.
Frequently asked questions
What is a pedestal-type substation?
It is a distribution transformer housed in a locked metal cabinet, mounted on a concrete pad and fed by an underground network. The medium-voltage compartment and the low-voltage compartment are separate, and the medium-voltage one can only be opened after the low-voltage one. In English it is called a pad-mounted transformer.
What is the difference between a pad-mounted and a compact substation?
The pad-mounted unit is a transformer with its connection and protection accessories integrated in the same cabinet. The compact substation brings together, in one enclosure or kiosk, independent medium-voltage switchgear panels, the transformer and the low-voltage switchboard; it allows more switching, metering and relay protection functions, and takes more space.
What does radial or loop feed mean for a pad-mounted unit?
In the radial version the transformer has a single medium-voltage cable entry. In the loop feed version it has two sets of terminals and a switch that lets the cable come in and carry on to another transformer, so the circuit can be fed from either end.