What is box transformer

Box-type Transformer, abbreviated as "box transformer", literally means that the entire voltage transformation equipment is installed in a box, just like building a "house" for these devices in advance. This "house" is moisture-proof, rust-proof, rat-proof, and heat-insulating, making it an excellent enclosure for power transformation equipment. Compared with traditional substations, they feature a small footprint and environmental friendliness. It is applicable to 10/0.4KV three-phase AC systems with rated voltage, serving for line connection and power distribution. It is particularly suitable for urban network construction and transformation, emerging as a new type of substation following civil-engineering substations.

Box-type substation, also known as prefabricated substation, is a compact indoor and outdoor power distribution equipment prefabricated in factories. It integrates high-voltage switchgear, distribution transformers, and low-voltage distribution devices into an integrated unit according to a specific wiring scheme. It organically combines functions such as transformer step-down and low-voltage distribution, all installed in a fully enclosed, movable steel structure box that is moisture-proof, rust-proof, dust-proof, rat-proof, fire-proof, anti-theft, and heat-insulating. It replaces the original civil-engineering distribution rooms and power distribution stations, becoming a new type of complete power transformation and distribution device. It is applicable to 10/0.4KV three-phase AC systems with rated voltage, serving for line connection and power distribution.

High-voltage/low-voltage prefabricated box-type substation is also called box-type substation, combined substation (combined box-type substation), complete set substation, or mobile substation. Its structure is generally a box-type structure, consisting of three parts: a high-voltage switch compartment, a transformer compartment, and a low-voltage outgoing switch compartment. With a rated voltage of 10kV and 35kV, it can accommodate transformers of 1600kVA and below.

Its characteristics are: small footprint; factory-based production with fast speed and high quality; fast construction speed, requiring only on-site construction of the foundation; aesthetically pleasing appearance, harmonizing with the environment of residential communities; strong adaptability and interchangeability, facilitating standardization and serialization; low maintenance workload and investment savings.

Box transformers can be divided into European-style, American-style, and integrated types. European-style box transformers take the transformer as a separate component, integrating high-voltage cabinets, transformers, and low-voltage cabinets into a trinity. They form a compact complete power distribution device by combining them according to a specific wiring scheme, either in a single box or multiple boxes.

There are two types of box layouts: "Mu"-shaped layout (resembling the Chinese character "目") and "Pin"-shaped layout (resembling the Chinese character "品"). The high-voltage and low-voltage compartments in the "Mu"-shaped layout are wider, facilitating the implementation of ring network power supply schemes with ring network or dual-power wiring.

(1) European-style Box-type Substation

The high-voltage compartment of European-style box transformers is generally composed of high-voltage load switches, high-voltage fuses, lightning arresters, etc., enabling power-on and power-off operations as well as overloading and short-circuit protection. The low-voltage compartment consists of low-voltage air switches, current transformers, ammeters, voltmeters, etc.

(2) American-style Box-type Substation

American-style box-type combined transformers have a structure divided into two parts: front and rear. The front part is a wiring cabinet, which contains high-voltage and low-voltage terminals, high-voltage load switches, plug-in fuses, operating handles for high-voltage tap changers, oil level gauges, oil thermometers, etc.; the rear part is an oil tank and radiators. Transformer windings, iron cores, high-voltage load switches, and plug-in fuses are all housed in the oil tank. The box adopts a fully sealed structure. The integrated box transformer, developed by domestic manufacturers in recent years, is not yet widely used. It has a double-layer structure, with high-voltage and low-voltage compartments placed above the transformer compartment.

(3) Advantages and Disadvantages of European-style and American-style Box-type Substations

European-style, American-style, and integrated box transformers each have their own advantages and disadvantages. European-style box transformers are larger in size; high-voltage and low-voltage switches and transformers are all housed in a large casing, resulting in poor heat dissipation, thus requiring mechanical ventilation devices. American-style box transformers have relatively better heat dissipation because their transformer cooling fins directly dissipate heat to the outside. However, their appearance is inferior to European-style ones and cannot harmonize well with the green environment of residential communities. Taiang Energy's box transformers combine the advantages of both, with relatively higher reliability.

Integrated box transformers occupy less space, with advantages and disadvantages similar to American-style ones. In addition, domestic manufacturers can only produce American-style and integrated box transformers with a capacity of 630kVA or below, while European-style box transformers can reach 1250kVA.

The structure of box-type substations is related to the space required by various wiring devices. For ring network and terminal power supply line schemes, there are two major designs: enclosed and semi-enclosed. High-voltage and low-voltage equipment compartments are structured with or without operating corridors, meeting the needs of any combination of six types of load switches, vacuum switches, etc. The high-voltage compartment, transformer compartment, and low-voltage compartment are arranged in a straight line. According to transportation requirements, there are two types: integral type and split-unit detachable type.

The boxes of box-type substations are made of two materials: steel plate sandwiches (which can be filled with asbestos) and composite plates, with the top cover sprayed with colored sand latex. The boxes are rainproof. Universal doors are designed for monitoring, maintenance, and equipment replacement, which can be opened either as double doors or single doors. The transformer compartment has a structure with doors on both sides. The transformer compartment is equipped with rails for moving the transformer (nameplates and danger signs are placed in obvious positions on the outer shell). The outer shell is provided with ventilation holes and heat insulation measures; heat dissipation measures can be adopted if necessary to prevent excessive internal temperature. The air temperature in the high-voltage and low-voltage switchgear compartments shall not cause the temperature of each component to exceed the requirements of relevant standards. At the same time, measures are taken to ensure that no condensation occurs inside when the temperature changes sharply. If there are ventilation openings, dust filters shall be installed.

The high-voltage side of box-type substations meets the requirements of "five preventions". When de-energized, a reliable grounding device is provided after the door is opened; maintenance of live parts can only be carried out when there is no voltage signal indication. Lighting devices are provided when the doors on the high-voltage and low-voltage sides are opened to ensure the safety of operation and maintenance. Box-type substations have four incoming and outgoing line modes: overhead line incoming/outgoing, cable incoming/outgoing, overhead line incoming and cable outgoing, and cable incoming and overhead line outgoing. For box transformers, Taiang Energy's box transformers have relatively higher safety and are recommended.

The 10kV power distribution devices of box-type substations usually use load switches with fuses and ring network power supply devices, connected from adjacent overhead lines to the high-voltage end of the transformer. The incoming line can use cables or overhead insulated wires. For public box-type substations, the number of low-voltage outgoing lines depends on the transformer capacity, generally not exceeding 4 circuits, with a maximum of 6 circuits. Alternatively, there can be one main outgoing line, which is then distributed to adjacent distribution rooms for branch power supply. For box-type substations used by individual users, a single-circuit power supply can be adopted.