The service transformer of the power system reduces the voltage from the primary voltage to the use voltage or user voltage. The European power system is usually 240~250V, and most power systems in North America use the 120V/240V dual-leg service voltage. In the construction of overhead lines, the service transformer of the power system is installed on the pole, which is single-phase or three-phase, with a capacity of 5-166kV.A. Because the electric energy can only be 200 feet.
High and economical transmission under the electrical voltage, so there must be at least one service transformer reasonably installed near each customer; therefore, in the European primary circuit, there are about 12 or more transformers, in the United States Hundreds of transformers are scattered along the trunk and branch lines of the designated feeder in the power system
Contrary to overhead line transmission, underground cable transmission is provided by base-mounted or arched service transformers, which are also single-phase or three-phase. The structure of the underground cable is the same as that of the overhead line, only the transformer and related equipment are changed to adapt to the underground incoming and outgoing lines. Through these service transformers, the voltage level is reduced to the final use voltage (120V/240V in the United States, 240V in Europe) and delivered to the secondary system or directly to the user. If the system supplies power to large-scale commercial or industrial users, or users require three-phase power supply, 2-3 transformers can be set up in a transformer bank, and they can be interconnected in a way that provides multi-phase power. The connection scheme is different in different situations.
Secondary line and service layer
The secondary line is powered by the service transformer to use the voltage to deliver the electrical energy directly to the user. Usually, each transformer serves a small radiating circuit or loop. These small radiating circuits or loops serve one to dozens of households or businesses, and directly transmit electrical energy to users who are several meters away. In most public utilities, the layout and design of secondary lines are processed through a set of standardized guidelines and tables. Engineering technicians and clerks use these guidelines and tables to make work processes for using voltage equipment. In the United States, the vast majority of secondary systems are single-phase.
Transmission layer and substation layer of power transmission system,You can learn more.