Anhui Erzheng Electric Co., Ltd. presents 6 distinct application features of box-type substations.

1. Application Background

Box-type substations differ from conventional civil‑built distribution rooms; their main characteristics are:

(1) The substation has completed design, manufacturing, and installation at the manufacturer’s facility, and its internal electrical wiring has been finished.

(2) The substation has passed the prescribed type‑test assessment.

(3) The substation has been verified through factory acceptance testing.

2. Overall Structure of the Box-Type Substation

In a modular substation, the compartment housing the high-voltage switchgear is generally referred to as the high-voltage room; the compartment housing the transformer is generally referred to as the transformer room; and the compartment housing the low-voltage switchgear is called the low-voltage room. These three compartments can be arranged in two configurations within a box-type substation: the “well” (or “eye”) configuration and the “character ‘pin’” configuration—also known, respectively, as the European-style substation layout and the American-style substation layout.

3. Medium-Voltage Switchgear Inside the Pad-Mounted Substation

In box-type substations, if terminal wiring is used, a switchgear assembly consisting of a load switch and fuse is employed; if ring network wiring is used, a ring network power supply unit is adopted. The ring network power supply unit is typically equipped with load switches, which are configured as a switchgear assembly comprising two load switch cabinets serving as incoming and outgoing lines, along with a transformer protection cabinet (load switch plus fuse). Given that China has been promoting ring network power supply in urban grid construction and renovation to minimize power outages, the ring network power supply unit is expected to see significant development.

The SF6 ring main unit boasts significantly enhanced personal safety, along with easy installation, minimal maintenance requirements, space savings, and substantial economic benefits. Distribution cabinets include high-voltage distribution cabinets, low-voltage distribution cabinets, high–low voltage distribution cabinets, JP cabinets, comprehensive distribution boxes, and packaged substations.

The ring network power supply unit is composed of bays, typically consisting of at least three bays: two cable incoming/outgoing bays and one transformer circuit bay.

City networks typically use ring cables; where overhead lines are employed, the overhead lines can be routed to the vicinity of the ring network power supply unit, and then connected to and disconnected from the system via cables. In general, busbars and switches are enclosed within SF6‑filled housings. Because the housings are sealed, they are unaffected by external environmental conditions, which enhances equipment reliability. Moreover, thanks to SF6 insulation, the equipment can be made compact in size. Additionally, since the gas pressure is low, issues related to sealing are relatively easy to resolve.

4. Transformer Matching and Heat Dissipation Design

The transformer used in a box-type substation is a step-down transformer, typically reducing 10 kV to 380 V/220 V. Transformer capacities generally range from 50 to 1,600 kVA, with commonly used capacities between 80 and 630 kVA. Transformers of the S11 series or higher—fully sealed, maintenance-free, low-noise, and cost‑effective oil-immersed transformers (with noise levels ≤ 50 dB)—or new-generation dry-type transformers such as the SCB10 are often selected. When dry-type transformers are used, the transformer room must be equipped with a cooling system. Currently, provincial regulations require the use of S11 series distribution transformers; some projects have also adopted amorphous alloy transformers, which offer the advantage of extremely low no-load losses—only about 1/4 to 1/3 of those for conventional transformers—but come at a price 1.3 to 1.6 times higher. However, as manufacturing technology continues to advance, once the cost of amorphous alloy transformers comes down, they are expected to dominate the market.

There are two common configurations for transformers: one where the transformer is exposed to the air but remains housed within a cabinet, offering convenient replacement (European-style pad-mounted substation layout); the other where the transformer is installed in a sealed compartment (American-style pad-mounted substation layout). To prevent solar radiation from raising the temperature inside the transformer room, the walls on all sides are fitted with insulation materials and constructed using a double‑ply panel structure. The roof is designed as a gable roof with an air cushion or insulated layer, complete with internal ventilation ducts and equipped with automatic forced exhaust ventilation—either axial fans or centrifugal fans. Typically, the operation of these devices is automatically controlled by temperature monitoring systems within the transformer room, with setpoints adjusted to 80%–90% of the allowable temperature; under normal indoor temperatures, heat is dissipated through natural ventilation.

5. Functional Features on the Low-Voltage Side

On the low-voltage (0.4 kV) side, outgoing line protection can be equipped with multiple air circuit breakers or knife-fuse switches; an automatic cyclical switching reactive power compensation device is used, with a compensation capacity typically equal to 30% of the transformer’s rated capacity; and an energy metering box is provided.

Low-voltage switches equipped with intelligent controllers can provide early warning functionality, as well as signal units and operational control, while also offering metering capabilities.

6. Enclosure Manufacturing Process

Poor adhesion and short service life of surface protective coatings limit the use of ordinary steel plates in outdoor switchgear cabinets. While some enclosures are made from stainless steel, aluminum alloy, or polystyrene composite panels, the adoption of non-metallic enclosures—such as glass fiber–reinforced cement panels, glass fiber–reinforced plastic panels, and colored panels—has become a prevailing trend. When using glass fiber–reinforced cement panels or glass fiber–reinforced plastic panels, the roof can be designed with a smooth, waterproof drip edge to ensure adequate daylighting while preventing rainwater from entering.