Anhui Erzheng Electric Co., Ltd. introduces six distinct application features of box-type substations.

1. Application Background

Box-type substations differ from conventional brick-and-mortar distribution rooms; their main characteristics are:

(1) The substation is designed, manufactured, and installed at the manufacturer’s facility, with internal electrical wiring completed.

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

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

2. Overall Structure of the Box-Type Substation

In a modular substation, the compartment housing the high-voltage switchgear is typically referred to as the high-voltage room; the compartment containing the transformer is usually called the transformer room; and the compartment accommodating the low-voltage distribution equipment is known as the low-voltage room. These three compartments can be arranged in two configurations within the packaged substation: the “well”‑shaped layout and the “T”‑shaped layout—commonly referred to as the European‑style and American‑style arrangements, respectively.

3. Medium-voltage switchgear inside the pad-mounted substation

In box-type substations, for terminal‑type connections, a combined switchgear comprising a load switch and a fuse is used; for ring‑network connections, a ring‑network power supply unit is employed. A ring‑network power supply unit typically includes load switches and is configured as a switchgear assembly consisting of two load‑switch cabinets serving as incoming and outgoing lines, along with a transformer protection cabinet that integrates a load switch and a fuse. Given that China has been promoting ring‑network power supply in urban grid construction and renovation to minimize service interruptions, the ring‑network power supply unit is expected to experience significant growth.

SF6 ring‑main unit features significantly enhanced personal safety, along with easy installation, minimal maintenance, space savings, and substantial economic benefits. Distribution cabinets: high‑voltage distribution cabinet, low‑voltage distribution cabinet, combined high‑ and low‑voltage distribution cabinet, JP cabinet, integrated distribution box, and packaged transformer.

A ring‑main unit consists of bays, typically comprising at least three bays: two cable inlet/outlet bays and one transformer circuit bay.

Urban distribution networks typically employ ring‑main units. Where overhead lines are used, these can be routed to the vicinity of the ring‑main unit and then connected via cable for both incoming and outgoing feeders. In general, busbars and switchgear are enclosed within an SF6‑filled housing. The sealed enclosure protects the equipment from external environmental influences, enhancing its reliability; moreover, SF6 insulation enables a compact footprint. Additionally, the low gas pressure simplifies sealing and other related design challenges.

4. Transformer Matching and Thermal Management

The transformers used in packaged substations are step-down transformers, typically reducing 10 kV to 380 V/220 V. Transformer capacities generally range from 50 to 1,600 kVA, with common ratings between 80 and 630 kVA. Preferred choices include oil-immersed transformers of the S11 class or higher—fully sealed, maintenance-free, low-noise, and cost‑effective (noise level ≤ 50 dB)—or newer dry-type transformers such as the SCB10 series. When dry-type transformers are selected, the transformer room must be equipped with a ventilation and heat‑dissipation system. Currently, provincial regulations mandate the use of S11‑series distribution transformers; some applications also employ amorphous‑alloy transformers, which offer significantly lower no‑load losses—only about one‑quarter to one‑third of conventional designs—but at a price 1.3 to 1.6 times higher. As manufacturing technologies advance and costs decline, amorphous‑alloy transformers are expected to assume a dominant market position.

Transformers can be installed in two ways: one is to expose the transformer to the open air while keeping it enclosed within a housing, which facilitates easy replacement (European‑style pad-mounted transformer arrangement); the other is to mount the transformer inside a sealed compartment (American‑style pad-mounted transformer arrangement). In the latter case, to prevent solar radiation from raising the compartment temperature, the walls are insulated with thermal‑insulating materials and constructed as double‑layer panels, while the roof is designed as an air‑cushion or insulated skylight structure. An internal ventilation duct is provided, equipped with an automatic forced‑exhaust ventilation system—either an axial fan or a centrifugal fan. Typically, the operation of this system is automatically controlled by a temperature monitoring device in the transformer room, with the setpoint set at 80%–90% of the allowable operating temperature; under normal indoor conditions, heat dissipation relies on natural ventilation.

5. Functional Features of the Low-Voltage Side

On the low-voltage (0.4 kV) side, the outgoing line protection may be equipped with multiple air circuit breakers or knife‑fusible switches; the automatic cyclic switching reactive power compensation device typically has a compensation capacity equal to 30% of the transformer’s rating; and an electric energy metering cabinet is provided.

Low-voltage switchgear equipped with an intelligent controller can provide early-warning functionality, offer signaling and operational control, and also incorporate metering capabilities.

6. Enclosure Manufacturing Process

Poor adhesion and short service life of the surface protective coating limit the use of conventional steel sheets in outdoor transformer substations. 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 boards, glass-fiber-reinforced plastic panels, and colored panels—is becoming a prevailing trend. When using glass-fiber-reinforced cement boards or glass-fiber-reinforced plastic panels, a smooth‑surfaced, waterproof drip edge can be incorporated into the roof to ensure adequate daylighting and prevent rainwater ingress.