Environmental Engineering Reference
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are easier to ramp up and down and are more often used to meet shoulder load and peak
demand. The need to satisfy peak-demand results in the building and maintenance of more
electricity generation capacity than would be necessary if demand was more level and
stable. As previously noted, industry experts compare this to sizing a church parking lot
for Easter Sunday, or building a mall parking lot to cope with Black Friday customers (the
day after Thanksgiving, which is the busiest shopping day of the year in the United States).
Recent low natural gas prices have disrupted this order somewhat, as natural gas plants in
some locations were for a short while cheaper to operate than coal facilities.
Once electricity is generated, it flows in either direct current or alternating current.
Direct current (DC), electricity flowing consistently in the same direction between positive
and negative terminals, flows from batteries, solar cells, and fuel cells. The electricity that
is generated in a conventional power plant, on the other hand, is alternating current (AC),
which means that the direction of the current reverses, or alternates, at regular intervals.
The standard AC current in the United States grid is 120 reversals or sixty cycles per
second (or Hertz, Hz) and 110-20 volts, while Europe's is 50 Hz and 220-40 volts. The
main advantage of AC power is that voltage can easily be changed using a transformer.
3.2.2 Transmission in Legacy Systems
Electricity generation usually produces power at relatively low voltages ranging from 2 to
30 kiloVolts (kV) depending on the size of the unit, but once electricity is generated, its
voltage is stepped up before transmission. A critical step between electricity generation
and long-distance transmission involves a step-up transformer to increase voltage. The
efficiency of transmitting electricity is greatest when the voltage is high and the current
is low; higher voltage and lower current minimizes line losses (which are directly
proportional to the square of the current). Often, power generation occurs far from the
places where the electricity is needed, making long-distance high-voltage transmission
lines a crucial part of the electric system. Long-distance transmission voltages range from
115 kV to 1200kV, so the step-up transformer plays a crucial role in increasing the voltage
for transmission. Almost all long-distance transmission lines are mounted aboveground,
overhead. This is primarily due to the lower cost of installing ($1-2 million per mile)
and maintaining aboveground lines, which are roughly ten times less expensive than
underground lines. In some urban or otherwise sensitive areas, however, transmission lines
are sited belowground.
Transmission lines have typically used high-voltage three-phase alternating current
(AC), although high-voltage direct current (HVDC) is increasingly being used to enhance
efficiency for long-distance transmission and to reduce the potential for disruption of
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