Transformer Substation

an electric substation that steps up or steps down the voltage of an AC power system, and also distributes electric power.

Step-up substations, which are usually built at electric power plants, transform the voltage produced by the generators into a higher voltage (of one or more values) that is necessary for the transmission of electric power over power lines. Step-down substations transform the primary voltage of electric power systems to a lower secondary voltage. Step-down substations may be of the regional, principal, or local (plant) type, depending on their purpose and the values of the primary and secondary voltages. Regional substations take electric power directly from the high-voltage power lines and transmit it to the main step-down substations, from which—after reduction to 6, 10, or 35 kilovolts (kV)—it is fed to local and shop substations, where the last stage of transformation is accomplished (with step-downs to 690, 400, or 230 V) and the electric power is distributed to consumers.

A transformer substation usually has one or two power transformers, distribution equipment, control and protective devices, and auxiliary equipment. Autotransformers are used in a number of high-power step-down substations (for 220, 330, 500, and 750 kV), since they provide reductions of 30–35 percent in power losses, 15–25 percent in the consumption of copper, and 50–60 percent in the consumption of steel. The distribution equipment may have one or two busbar systems, or it may have none. The most common substations have one busbar system, usually with section switches and circuit breakers; in some substations there are also bypass busbar systems that permit the performance of preventive maintenance and repair work without cutting the power supply to consumers.

Transformer substations are usually constructed in plants and delivered to installation sites in completely assembled form or as individual units. Such substations are called prefabricated types. In the USSR, prefabricated transformer substations are manufactured for powers of 20 to 31,500 kilovolt-amperes, with primary voltages of 6, 10, 35, 110, and 200 kV and secondary voltages from 0.22 to 10 kV. A promising application is the use of transformer substations in which sulfur hexafluoride (SF₆), which has high electric strength and arc-arresting capacity, is used as insulation in the high-voltage switchgear. This permits a substantial reduction of the size of the high-voltage apparatus and of the overall dimensions of the substation.

The site of a transformer substation depends on the station’s purpose and the nature of the loads. Substations having secondary voltages of 6,10, 35, and 110 kV are usually centrally located with respect to the consumers they serve, thereby reducing power losses in transmission, as well as the consumption of construction materials for the distribution system. Among the factors considered in the siting of plant substations are the layout of the production areas, the arrangement of equipment, the environmental conditions, and fire safety requirements. The substation equipment may be outdoors or in an enclosure, such as a separate building.