What is the difference between a pad-mounted substation and a traction substation?

Pad-Mounted Substation (Box-Type Substation)

Definition:
A pad-mounted substation, also known as a prefabricated substation or pre-assembled substation, is a compact, factory-assembled power distribution unit that integrates high-voltage switchgear, a distribution transformer, and low-voltage distribution equipment according to a specific wiring scheme. It combines functions such as voltage step-down and low-voltage power distribution into a single unit housed in a fully enclosed, movable steel enclosure that is moisture-proof, rust-resistant, dust-proof, rodent-proof, fire-resistant, theft-resistant, and thermally insulated. It is especially suitable for urban power grid construction and upgrades, representing a new type of small-scale substation that emerged after traditional civil-built substations. Typically, it steps down high voltage to low voltage—for example, converting 10 kV to 380 V for industrial or residential use.

Pad-mounted substations are widely used in mines, industrial plants, oil and gas fields, and wind power stations, replacing conventional civil-built distribution rooms or substations and serving as a new type of integrated transformer and distribution assembly.

A pad-mounted substation (commonly called a “box sub” or “box-type sub”) generally consists of three compartments: a high-voltage room, a transformer room, and a low-voltage room. It is a relatively simple transformer and distribution device. The selection of its transformer follows general engineering guidelines, with typical capacities not exceeding 1,250 kVA.

Temporary box substations refer to units installed for short-term use—such as a temporary transformer at a construction site—that will be removed after the project ends and are not intended for permanent installation.

Function:
Pad-mounted substations are used in high-rise residential buildings, luxury villas, plazas, parks, residential communities, small-to-medium factories, mines, oilfields, and temporary construction power applications, serving to receive and distribute electrical energy within distribution systems.

Location:
First, they are primarily deployed in urban residential neighborhoods and along streets. During peak electricity demand periods when additional capacity or voltage support is needed, pad-mounted substations are utilized.
Second, they are widely used for temporary power supply—such as at construction sites where existing buildings require electrical system upgrades. They are also commonly used in field operations, including construction sites, ports, airports, and similar locations.

Traction Substation

Definition:
A traction substation converts three-phase 110 kV (or 220 kV) high-voltage AC power from the regional grid into two single-phase 27.5 kV AC outputs, which then supply the overhead contact lines (rated at 27.5 kV) on both the up-track and down-track directions of a railway. Each side of the contact line is referred to as a "feeding arm." The two arms operate at different voltage phases and are normally separated by a phase-break insulator. Between adjacent traction substations, the contact line voltages are usually in-phase; in addition to phase-break insulators, a sectioning post (or switching kiosk) is installed between them. Through circuit breakers or disconnect switches at the sectioning post, bilateral (or unilateral) power supply modes can be implemented.

The traction substation receives electric power from the regional power system and converts it into a form suitable for electric railway traction based on specific current and voltage requirements. The converted power is then delivered to the overhead contact wires above railway tracks to supply electric locomotives, or to underground metro or urban tram systems to power subway trains or trolley cars.

Along an electrified railway line, multiple traction substations are installed, typically spaced about 40–50 kilometers apart. In long-distance electrified railways, additional "booster" or "intermediate" traction substations are installed approximately every 200–250 kilometers to segment the high-voltage transmission line and limit fault impact zones. These intermediate substations not only perform standard transformation functions but also distribute incoming high-voltage power via their busbars and feeders to other intermediate substations downstream.