How the power distribution grid works

Electrical power travels from the power plant to the industrial plant through the power distribution grid. All commercial electrical generators of any size generate three-phase ac.

03/01/2001


Electrical power travels from the power plant to the industrial plant through the power distribution grid. All commercial electrical generators of any size generate three-phase ac. The power transmission and distribution grids use ac because it has significantly less line loss than dc when transmitted over long distances. Also, transformers must have ac to operate. The power distribution grid depends on transformers.

The power plant produces three phases of power simultaneously. These phases are offset electrically by 120 deg. Four wires come from every power plant: three phases plus a neutral at ground potential, electrically, which is common to all three. An advantage of three-phase power is that at any given moment, one of the three is nearing a peak. Because of this situation, equipment such as high-power industrial three-phase motors and welding machines has smooth power output.

"Ground" in the power distribution grid is literally "the ground." The power company uses the earth as one of the wires in the power system. The earth is a good electrical conductor because it makes a good return path for electrons.

Three-phase power leaves the generator and enters a transmission substation at the power plant. This substation uses large transformers to convert the generator's voltage, which is at thousands of volts, to extremely high voltages for long-distance transmission on the transmission grid. Typical voltages for long-distance transmission are in the 155,000-765,000-V range to reduce line loss. A typical maximum transmission distance is about 300 miles.

Voltage is stepped-down from the transmission grid to the distribution grid. The conversion from "transmission" to "distribution" occurs in a power substation, which typically performs several functions.

  • Transformers step transmission voltages (in the tens or hundreds of thousands of volts range) down to distribution voltages (typically less than 10,000 V).

  • A bus splits the distribution power off in multiple directions.

  • Circuit breakers and switches disconnect the substation from the transmission grid or the distribution lines from the substation when necessary.

    • Power goes from the substation transformer to the distribution bus. This bus distributes power to separate sets of distribution lines, some possibly at different voltages. Transformers attached to the bus step the power down to standard line voltage (usually 7200 V) for one set of lines, while power leaves in other directions at different voltages if necessary. The higher-than-standard voltages are stepped down at other substations or in small transformers somewhere down the line.

      Regulator banks prevent undervoltage and overvoltage conditions, and are located along the line either underground or in the air. Typically, there are switches that allow the regulator bank to be disconnected for maintenance when necessary.

      At the plant

      Plants typically have a substation that receives the incoming power, typically at 7200 V, and steps it down to the highest voltage used. Most plants use three-phase electrical power at 460-V ac. However, there are some plants that use 575-V ac and others 208-V ac.

      Some plants require voltages much higher than 460-V ac, usually to run large motors. At the plant's substation, another feed is provided for a separate electrical service. The voltage for this extra connection is typically 6600-V ac. However, there are some plants that require 4160-V ac.

      The stepped-down, three-phase 460-V ac electrical power enters the plant through switchgear, which provides for controlling, switching, metering, electrical protection, and power regulation. The power is then fed throughout the plant and distributed wherever electricity is needed.



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