Power Providers & Utilities

The Flow of Current

Electrons leaving a power supply are always trying to return to the same power supply. This concept contradicts the popular misconception that the current is trying to always go into the earth. For example, applying an alternating current to the primary of a transformer induces a voltage in the secondary. Causing electrons to leave one end of the transformer's secondary, travel over the circuit's conductors through the load, and return over the remaining circuit's conductors to the other end of the transformer's secondary.

Utility Neutral Current Path

Power Providers ground the utilities' primary and secondary neutral conductors to the earth at multiple locations. Grounding creates parallel paths, reducing the return neutral current path's ac resistance (impedance). Creating a multipoint grounded neutral to help minimize primary utility neutral voltage drop. In addition, it assists in clearing line-to-neutral/ ground utility faults and reducing elevated line-to-neutral voltage caused by line-to-neutral/ground faults.

Utility Ground-Fault Current Path

Metal parts of electric utility equipment (e.g., transformer and capacitor cases, guy wires, roadway lighting) get bonded to the utility neutral, which is grounded to the earth—providing a low-impedance (ac resistance) path to the power supply to clear a line-to-case fault.

Generating Plants

The electrical system begins at a generating plant. Then, it converts energy from fossil fuels (coal or oil), water, wind power, or nuclear power to steam, which turns the turbine into an electric generator. Typically, a steam turbine is directly connected to an electrical generator, producing 13.80 kV, three-phase power.

Step-Up Substation at Generating Plant

The most economical way to transfer electrical power over long distances is with high voltage. For a given power, the current decreases directly to the increase in voltage level. Thus, conductor voltage drop and power losses (wasted energy) are lower when transferring power over great distances. Step-up substations located at the generating facilities transform the 13.80 kV generator output to 69 kV, 500 kV, or even higher.

Transmission Line

High-voltage transmission lines carry the 69 kV or higher voltage from the generating plant step-up substation to various step-down substations. In addition, high-voltage transmission lines often are connected to transmission lines from other generating plants. This connecting high-voltage lines transmission is an "interconnected system" or an "electrical power grid."

Step-Down Substation

Step-down substations reduce the voltage from the high-voltage transmission lines to 34.50 kV, 14.40 kV, or sometimes as low as 4,160V. This reduced voltage is known as "primary distribution voltage."

Primary Distribution Feeders

Distribution feeders transfer the primary distribution voltage to distribution transformers, often mounted on poles or the ground (pad-mounted) next to buildings. Distribution Transformers reduce the primary distribution voltage from 34.50 kV or 13.80 kV to secondary distribution voltage, such as 120/208V, 120/240V, or 480V.

Secondary Distribution Line

Power is transferred from the utility's distribution transformer to the customer via an overhead service drop or underground service lateral. Overhead service-drop conductors are typically installed and maintained by the electric utility power company. In contrast, underground service-lateral conductors are installed and maintained by the customer.