The figure below shows the concept of
typical energy transmission and distribution systems. The generating
station produces the electric energy. The generator voltage is around
15 to 25 kV. This relatively low voltage is not appropriate for the
transmission of energy over long distances.
At the generating station a transformer
is used to increase the voltage and reduce the current. In Fig. 4.3
the voltage is increased to 500 kV and an extra-high-voltage (EHV)
line transmits the generator-produced energy to a distant substation.
Such substations are located on the
outskirts of large cities or in the center of several large loads. As
an example, in Arizona, a 500-kV transmission line connects the Palo
Verde Nuclear Station to the Kyrene and Westwing substations, which
supply a large part of the city of Phoenix.
The voltage is reduced at the 500
kV/220 kV EHV substation to the high-voltage level and high voltage
lines transmit the energy to
high-voltage substations located within cities.
At the high-voltage substation the
voltage is reduced to 69 kV. Sub-transmission lines connect the
high-voltage substation to many local distribution stations located
within cities. Sub-transmission lines are frequently located along
major streets.
The voltage is reduced to 12 kV at the
distribution substation. Several distribution lines emanate from each
distribution substation as overhead or underground lines.
Distribution lines distribute the
energy along streets and alleys. Each line supplies several step-down
transformers distributed along the line.
The distribution transformer reduces
the voltage to 230/115 V, which supplies houses, shopping centers,
and other local loads. The large industrial plants and factories are
supplied directly by a subtransmission line or a dedicated
distribution line as shown in the figure.
The overhead transmission lines are
used in open areas such as interconnections between cities or along
wide roads within the city. In congested areas within cities,
underground cables are used for electric energy transmission.
The underground transmission system is
environmentally preferable but has a significantly higher cost. In
the figure the 12-kV line is connected to a 12-kV cable which
supplies commercial or industrial customers.
The figure also shows 12-kV cable
networks supplying downtown areas in a large city. Most newly
developed residential areas are supplied by 12-kV cables through
pad-mounted step-down transformers as shown in the figure.
No comments:
Post a Comment