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

1. Application Background

Box-type substations differ from conventional civil-engineering switchgear 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 combined-type packaged 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 different layouts within the packaged substation: the “目”-shaped layout and the “品”-shaped layout—also commonly known as the European-style and American-style arrangements, respectively.

3. Medium-voltage switchgear inside the packaged substation

In box-type substations, if the terminal wiring is used, an electrical switchgear assembly consisting of a load switch and a fuse is employed; if ring network wiring is adopted, a ring-network power supply unit is utilized. Typically, a ring-network power supply unit is equipped with load switches, comprising two load switch cabinets serving as incoming and outgoing lines, plus a transformer protection cabinet (combining a load switch and a fuse) to form a single electrical switchgear assembly. Given that China has been promoting ring-network power supply systems in urban grid construction and renovation efforts to minimize power interruptions, the ring-network power supply units are expected to see significant growth in the future.

The SF6 ring network power supply unit is characterized by significantly enhanced personal safety, ease of installation, minimal maintenance requirements, compact footprint, and remarkable economic benefits. Distribution cabinets include high-voltage distribution cabinets, low-voltage distribution cabinets, combined high- and low-voltage distribution cabinets, JP cabinets, integrated distribution boxes, and packaged transformers.

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

City networks typically use ring cables. Where overhead lines are employed, these 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 an SF6-filled enclosure. Because the enclosure is sealed, the equipment is insulated from external environmental influences, thereby enhancing its reliability. Moreover, since SF6 is used for insulation, the equipment can be made compact in size. Additionally, due to the low gas pressure required, issues such as sealing become easier to address.

4. Transformer matching and thermal management

The transformer used in a packaged substation is a step-down transformer, typically reducing the voltage from 10 kV to 380 V/220 V. The transformer capacity generally ranges from 50 to 1,600 kVA, with commonly used capacities for packaged substations ranging from 80 to 630 kVA. Transformers of the S11 series or higher—fully sealed, maintenance-free, low-noise, and cost-effective oil-immersed transformers (noise level ≤ 50 dB)—or new-type dry-type transformers such as SCB10 are usually selected. When using dry-type transformers, the transformer room must be equipped with a cooling system. Currently, provincial regulations require the use of S11-series distribution transformers; some areas have also adopted amorphous alloy transformers, which offer the advantage of extremely low no-load losses—only one-quarter to one-third of those of conventional transformers. However, their price is 1.3 to 1.6 times higher. As manufacturing technology continues to improve and costs come down, amorphous alloy transformers are expected to become the market leader once their prices become more competitive.

Transformers can be configured in two ways: One involves exposing the transformer to the air while keeping it enclosed within a housing, making replacement convenient (European-style box-type substation arrangement); the other involves installing the transformer in a sealed compartment (American-style box-type substation arrangement). In the latter case, to prevent the transformer room temperature from rising due to solar radiation, the walls around the room are equipped with thermal insulation materials and feature a double-layer panel structure. The roof is designed as a ventilated attic structure incorporating an air cushion or thermal insulation material, and is fitted with internal ventilation ducts and an automatic forced-exhaust ventilation system (axial-flow fans or centrifugal fans). Typically, the operation of this system—turning on and off—is automatically controlled by a temperature monitoring device located in the transformer room; the setpoint for the room temperature is usually set at 80% to 90% of the allowable maximum temperature. Under normal room temperatures, heat dissipation relies on natural ventilation.

5. Functional Features of the Low-Voltage Side

On the low-voltage (0.4 kV) side, outgoing-line protection can be equipped with multiple air switches or knife-fuse switches. The reactive power compensation device features automatic cyclic switching and its compensation capacity typically amounts to 30% of the transformer’s rated capacity. It is also equipped with an energy metering box.

The low-voltage switch equipped with an intelligent controller can provide early warning functionality, as well as signal units and operational control, and also features 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-type switchgear cabinets. While some enclosures are made of stainless steel plates, aluminum alloy plates, or polystyrene composite panels, the adoption of non-metallic enclosures—such as glass fiber-reinforced cement boards, glass fiber-reinforced plastic boards, and colored panels—has become a popular trend. When using glass fiber-reinforced cement boards and glass fiber-reinforced plastic boards, to ensure adequate daylighting and prevent rainwater from entering, the roof can be designed with a smooth, waterproof drip edge.