How does a box-type substation work?

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A box-type substation is a compact integrated power distribution unit that combines high-voltage switchgear, distribution transformers, low-voltage switchgear, electrical energy metering equipment, and reactive power compensation devices within one or several enclosures, according to a specific wiring scheme. It is suitable for three-phase AC systems with a rated voltage of 10/0.4 KV, and is used for power transmission and distribution. 　　Compared with European-style box transformers of the same capacity, American-style box transformers have a more rational structure. Since European-style box transformers combine the transformer with ordinary high-voltage electrical equipment within the same metal enclosure, the temperature of the transformer remains very high, which makes heat dissipation difficult and affects its output capacity. On the other hand, ordinary high-voltage load switches and fuses, as well as low-voltage switchgear, are used within this enclosure, which is why box transformers have a relatively large size. American-style box transformers and European-style box transformers differ in structure. In terms of layout, its low-voltage room, transformer room, and high-voltage room are arranged in a triangular pattern rather than a cross shape. Structurally, this box-type transformer is divided into a front and a rear section. The front section serves as the high- and low-voltage operation compartment, which contains high- and low-voltage connection terminals, a load switch handle, a tap changer, plug-in fuses, an oil level gauge, etc. The rear section consists of an oil filling tank and cooling fins, within which the transformer windings, core, high-voltage load switch, and fuses are placed in the transformer oil tank. 　　Oil-immersed metal oxide arresters are also used as surge arresters. The transformer eliminates the oil conservator; a sealed oil tank is designed based on the principle of maintaining a constant volume of oil plus air gap. The oil tank and radiator are exposed to the air, so there are no difficulties with heat dissipation. Molded case circuit breakers are used as the main circuit breakers and outlet circuit breakers in low-voltage circuit breakers. Thanks to the simplified structure, the floor area and volume of this type of box substation are **reduced**. Due to its small size and the fact that it has doors only on one side, the floor area required is only 1/4 of that of European-style box substations, while its volume is only 1/5 to 1/3 of that of European-style box substations with the same capacity. The high-voltage side of the American-style box substation is protected by fuses, while the low-voltage side is protected by molded-case automatic air circuit breakers, which is the same as the protection method used for 10kV distribution transformers in China. High-voltage fuses protect against internal faults in the transformer, while automatic air circuit breakers protect the low-voltage side circuits against overcurrent, short-circuit, and under-voltage faults. Disadvantages of American-style box transformers. Since the load switch is immersed in oil, the oil is carbonized and decomposed by the arc, producing harmful gases such as acetylene, which reduces its performance and has led to accidents in the past. No obvious break point can be seen, making maintenance inconvenient. Since the low-voltage protection and metering methods in the United States differ from those in China, American-style box transformers do not come equipped with low-voltage protection and metering. To meet the needs of domestic users, domestic box-type transformers are equipped with low-voltage distribution cabinets on the low-voltage side. Since the box-type substation is exposed to direct sunlight for extended periods, it affects the heat dissipation of the molded case circuit breakers, preventing them from properly interrupting loads and short-circuit currents, which can lead to faults on the high-voltage side.