Energy transmission and distribution guide

Transmission and distribution concept

The purpose of the electric transmission system is the interconnection of the electric energy producing power plants or generating stations with the loads. A three-phase AC system is used for most transmission lines.

The operating frequency is 60 Hz in the U.S. and 50 Hz in Europe, Australia, and part of Asia. The three-phase system has three phase conductors. The system voltage is defined as the rms voltage between the conductors, also called line-to-line voltage.

The figure shows the Phoenix area 230-kV system, which interconnects the local power plants and the substations supplying different areas of the city. The circles are the substations and the squares are the generating stations.

The system contains loops that assure that each load substation is supplied by at least two lines. This assures that the outage of a single line does not cause loss of power to any customer.

For example, the Aqua Fria generating station (marked: Power plant) has three outgoing lines. Three high-voltage cables supply the Country Club Substation (marked: Substation with cables). The Pinnacle Peak Substation (marked: Substation with transmission lines) is a terminal for six transmission lines. This example shows that the substations are the node points of the electric system. The system is interconnected with the neighboring systems.

As an example, one line goes to Glen Canyon and the other to Cholla from the Pinnacle Peak substation.

In the middle of the system, which is in a congested urban area, high-voltage cables are used. In open areas, overhead transmission lines are used. The cost per mile of overhead transmission lines is 6 to 10% less than underground cables.

The major components of the electric system, the transmission lines, and cables are described briefly below.

Generation Stations

The generating station converts the stored energy of gas, oil, coal, nuclear fuel, or water position to electric energy. The most frequently used power plants are:

Thermal Power Plant

The fuel is pulverized coal or natural gas. Older plants may use oil. The fuel is mixed with air and burned in a boiler that generates steam. The high-pressure and high-temper- ature steam drives the turbine, which turns the generator that converts the mechanical energy to electric energy.

Nuclear Power Plant

Enriched uranium produces atomic fission that heats water and produces steam. The steam drives the turbine and generator.

Hydro Power Plants

A dam increases the water level on a river, which produces fast water flow to drive a hydro-turbine. The hydro-turbine drives a generator that produces electric energy.

Gas Turbine

Natural gas is mixed with air and burned. This generates a high-speed gas flow that drives the turbine, which turns the generator.

Combined Cycle Power Plant

This plant contains a gas turbine that generates electricity. The exhaust from the gas turbine is high-temperature gas. The gas supplies a heat exchanger to preheat the combustion air to the boiler of a thermal power plant. This process increases the efficiency of the combined cycle power plant.

The steam drives a second turbine, which drives the second generator. This two-stage operation increases the efficiency of the plant.