A prefabricated substation (or simply prefabricated substation) is a compact, complete power distribution unit that integrates high-voltage switchgear, distribution transformers, low-voltage switchgear, power metering equipment, and reactive power compensation devices into one or more enclosures according to a specific wiring scheme. It is suitable for use in 10/0.4kV three-phase AC systems for power transmission and distribution.
Compared to European-style prefabricated substations of the same capacity, American-style prefabricated substations have a more rational structure. Because European-style prefabricated substations house the transformer and ordinary high-voltage electrical equipment in the same metal enclosure, the transformer temperature is very high, causing heat dissipation difficulties and affecting output. Furthermore, the use of ordinary high-voltage load switches, fuses, and low-voltage switchgear within the enclosure results in a larger size for European-style prefabricated substations. American-style prefabricated substations differ structurally from European-style substations. In terms of layout, their low-voltage compartment, transformer compartment, and high-voltage compartment are not arranged in a U-shape, but rather a triangular arrangement. Structurally, this prefabricated substation is divided into front and rear sections. The front section contains high- and low-voltage operating bays, including high- and low-voltage terminals, load switch operating handles, no-load tap changers, plug-in fuses, and oil level gauges. The rear section houses the oil tank and heat sink, where the transformer windings, core, high-voltage load switch, and fuses are placed.
Oil-immersed metal oxide surge arresters are also used. The transformer eliminates the oil conservator, employing a sealed oil tank design based on the principle of constant oil-air gap volume. The oil tank and heat sink are exposed to air, eliminating heat dissipation difficulties. Molded case circuit breakers are used as both the main and outgoing circuit breakers for low-voltage circuit breakers. Due to the simplified structure, the footprint and volume of this prefabricated substation are significantly reduced. Because of its small size and single-sided opening, its required floor space is only 1/4 that of a European-style prefabricated substation, and its volume is only 1/5 to 1/3 that of a European-style prefabricated substation of the same capacity. American-style prefabricated substations use fuses for high-voltage protection and molded case automatic circuit breakers (MCCBs) for low-voltage protection, similar to the protection methods used in Chinese 10kV distribution transformers. High-voltage fuses protect against internal transformer faults, while MCCBs protect against overcurrent, short-circuit, and undervoltage faults on the low-voltage side. However, American-style prefabricated substations have several drawbacks. Because the load switch is immersed in oil, the oil is carbonized and decomposed by electric arcs, producing harmful gases such as acetylene, leading to performance degradation and past accidents. The lack of a visible disconnection point makes maintenance inconvenient. Furthermore, because the low-voltage protection and metering methods differ between the US and China, American-style prefabricated substations do not inherently include low-voltage protection and metering. To meet the needs of domestic users, domestically produced prefabricated substations add a low-voltage distribution cabinet to the low-voltage side. However, prolonged exposure to direct sunlight affects the heat dissipation of the MCCBs, preventing them from properly interrupting loads and short-circuit currents, potentially causing high-voltage side faults.