Civil Engineering Reference
In-Depth Information
Electrical drives and electrically heated users are discussed in this chapter;
it is advisable to consult specialized technical literature.
7.2
Electric Distribution Losses
Electric distribution losses in the site are mainly due to the Joule effect which
depends on the square of the current and on the line's resistance. The basic
relationship can be expressed as follows:
I
2
(W)
P
losses
¼
n
R
where
n
¼
number of phase conductors.
R
¼
resistance of the phase conductor (
).
Ω
I
¼
RMS or effective current (A).
If the current flows through a series of
n
electric conductors, the total resistance
is expressed as the sum of the single conductor's resistances:
X
n
1
j
l
j
S
j
ðÞ
R
total
¼
ρ
j
where
ρ
j
¼
resistivity of the conductive material which is defined as resistance per unit
length of unit area at a given temperature (reference or operating temperature).
S
j
¼
cross section of the
j
-conductor.
l
j
¼
length of the
j
-conductor.
In the case of a line having
n
electric conductors connected in parallel sharing the
total current, the lower the resistance the higher the current flowing in each
conductor is. The total resistance is expressed as the reciprocal of the sum of the
reciprocals of the resistances:
l
R
total
¼
P
1
j
ρ
j
1
l
j
1
S
j
In practice, if
n
conductors are in parallel and they have the same section, the
resistance of the line is (1/
n
) multiplied by the resistance
R
of each conductor:
l
n
R
total
¼
R
