A box-type substation (abbreviated as a box-type substation) is a compact, integrated power distribution system that combines high-voltage switchgear, distribution transformers, low-voltage switchgear, energy metering equipment, and reactive power compensation devices within one or more boxes according to a specific wiring scheme. It is suitable for use in three-phase AC systems with a rated voltage of 10/0.4 kV, serving as a circuit and distributing power.
Compared to European-style box-type substations of the same capacity, American-style box-type substations have a more rational structure. Because European-style box-type substations house the transformer and common high-voltage electrical equipment within the same metal enclosure, the transformer's room temperature is high, making heat dissipation difficult and affecting output. Furthermore, the box-type substations utilize common high-voltage load switches, fuses, and low-voltage switchgear, resulting in a larger size. The American-style box-type substation differs structurally from its European counterpart. In terms of layout, the low-voltage compartment, transformer compartment, and high-voltage compartment are arranged in a triangle shape, rather than a "M" shape. Structurally, this box-type transformer consists of two sections: the front and rear. The front section houses the high- and low-voltage operating compartments, which contain the high- and low-voltage wiring terminals, load switch operating handles, no-load tap changer, plug-in fuses, and an oil level gauge. The rear section houses the oil filling tank and heat sink, which houses the transformer windings, core, high-voltage load switch, and fuses.
The lightning arrester also uses an oil-immersed metal oxide arrester. The transformer eliminates the oil pillow and adopts a sealed oil tank designed based on the principle of constant oil-air gap volume. The oil tank and radiator are exposed to the air, eliminating heat dissipation issues. Molded case circuit breakers are used as the main and outgoing low-voltage circuit breakers. Due to its simplified structure, this box-type substation significantly reduces its footprint and volume. Due to its compact size and the fact that the door only opens on one side, it requires only one-fourth the floor space of a European-style box-type transformer and its volume is only one-fifth to one-third of that of a European-style box-type transformer of the same capacity. American-style box-type transformers use fuses for high-voltage protection on the high-voltage side and molded-case automatic air circuit breakers for low-voltage protection on the low-voltage side, similar to the protection scheme used by 10kV distribution transformers in China. The high-voltage fuses protect against internal transformer faults, while the automatic air circuit breakers protect against overcurrent, short circuit, and undervoltage faults on the low-voltage side. However, American-style box-type transformers have disadvantages. Because the load switches are immersed in oil, the oil is carbonized and decomposed by the arc, producing harmful gases such as acetylene, which degrades performance and has caused accidents. The lack of visible disconnect points makes maintenance difficult.