Civil Engineering Reference
In-Depth Information
7
Electric Distribution Systems
from Facilities to End Users
7.1
Introduction
Electric energy, which is generally delivered to sites by means of utility electric
networks at high or medium voltages, is distributed to end users in the site through
medium- and low-voltage networks. Depending on the power demand of loads and
on the process layout, transformer substations (see Chap.
5
), whose main task is to
reduce the supply voltage downstream of the utility-delivering node, are located at
the site boundary or distributed around the site itself.
The choice among different distribution systems, such as radial or loop-feeder
systems, shown in the diagram in Fig.
7.1
, is based on technical and economic
evaluations that generally do not consider energy-saving targets, because of the
insignificant amount of the energy losses involved.
As a general indication, only a small part of the total electric power
travelling along the internal lines, not more than 2-3 %, is lost as Joule
and additional losses, both related to the square of the flowing current.
If compared to the heat distribution losses that can reach 10-20 % of the
total thermal energy flowing along pipelines (see Chap.
8
), this figure
shows that these problems can easily be regarded as of secondary impor-
tance in an energy management program.
Most electric losses occur in end users which can be grouped basically as
(1) electrical machinery and drives; (2) electrically heated users such as furnaces,
ovens, boilers, induction heating equipment, resistors, and microwave equipment;
(3) lighting; and (4) others such as electrochemical equipment and control and
communication systems.
