Environmental Engineering Reference
In-Depth Information
Step-up
Transformer
Step-down
Transformer
Turbine
Generator
Load
Power plant
Transmission line
Distribution line
Figure 4.6
A sketch of the elements of an electric power system for generating, transmitting, and
distributing power to end-users.
Electric power is transmitted via electric cable consisting of two wires that conduct electrical
current at different electric potentials.
9
In a
direct current
(DC) system the electric current in each
wire flows in one direction only. On the other hand, in an
alternating current
(AC) system, where the
potential difference and current are sinusoidal functions of time, the currents in each wire reverse
direction every half-cycle. In either case, the instantaneous electrical power
P
el
transmitted in the
cable is the product of the potential difference
between the wires and the current
I
flowing
in them,
P
el
=
()
I
(4.6)
The time-averaged power
P
el
is
P
el
=
()
2
(
I
)
2
cos
φ
(4.7)
where the overline indicates a time-averaged value and
φ
is the phase angle between the current
I
and the potential difference
for alternating current.
In both DC and AC transmission systems there is a loss of electric power in the transmission
line in the amount
I
2
R
, where
R
is the electrical resistance of the line. To minimize this loss, the
line resistance can be reduced by using large size copper wire and the current minimized, for a given
power, by increasing the potential difference. Long-distance transmission lines operate at hundreds
of thousands of volts to reduce the transmission loss. But high voltages are impractical and unsafe
for distribution to residences and commercial users so the voltage in AC distribution systems is
reduced to much lower levels by transformers. Because DC power cannot be transformed eas-
ily to a different voltage, its use is restricted to special applications, such as electric rail trains.
The use of AC power predominates in modern electrical power systems. Nevertheless, high-
voltage DC (
∼
500 kilovolt) transmission lines may be used for very long distance transmission
(
1500 kilometers) to reduce energy loss.
The energy efficiency of electric power transmission and distribution is almost entirely deter-
mined by economic choices. To increase the efficiency, more money must be invested in copper
wire and transformer cores, which is only justified if the value of the saved electric energy exceeds
the amortization costs of the increased investment. Transmission and distribution losses in electric
power systems are usually held to 5-10%.
∼
9
Electric utility transmission and distribution lines usually include more than two wires, one of which is a
common ground return.
