How To Segregate Different Cable Types
In Substation?
Segregating low-voltage power cables,
control cables, and instrumentation cables in the substation cable
trench or cable tray system is generally not necessary. High-voltage
power cables should be segregated from all other cables. Cables
installed in stacked cable trays should be arranged by descending
voltage levels, with the higher voltages at the top.
High-voltage power cables
These cables should be installed so
that the high voltage cannot be impressed on any lower voltage
system. Methods for achieving this segregation are
a) Installation of high-voltage cables
in raceways that are separated from low-voltage power and control
cables and from instrumentation cables. Installation of different
voltage classes of high-voltage power cables in separate raceways is
also suggested.
b) Utilization of armored shielded
cables (separate raceways are still suggested).
Low-voltage power and control cables
These cable classifications may be
mixed. Consideration should be given to insulation deformation when
cable\ diameters differ greatly. When cable classifications are
mixed, the power cable ampacity is calculated as if all the cables
were power cables.
Instrumentation cables
These cables should be installed to
minimize noise pickup from adjacent circuits and equipment. Methods
for achieving segregation are
a) Installations that provide physical
separation between the instrumentation cables and any electrical
noise source [B15], [B38].
b) Installation in separate enclosed
magnetic raceways.
c) Cable construction configurations,
such as twisted conductors and shielding.
d) Installation of analog signal cables
separate from all power and control cables, and from unshielded
cables carrying digital or pulse type signals. Shielded voice
communications cable (without power supply conductors) may be
included in raceways with analog signal cables.
e) Segregation of telephone and other
communication type cables from all other substation cables.
Additional information on
instrumentation cable may be found in NEMA WC 55-1989.
Optical cable
From the outside, an optical cable
looks like any electrical multiconductor cable; however, it is
lightweight and flexible compared to metal conductor cable. Typical
optical cable diameter ranges from less than 1/8 in (3.175 mm) to 3/4
in (19.4 mm), depending on fiber numbers and cable construction.
The most common optical cable jacket
materials are polyethylene (all types), PVC, and polyurethane. The
placement of optical cable in conduit is quite common. Conduit offers
protection from crushing, ground disruption, rodents, and other
environmental abuse. In addition, the cable is easier to replace or
upgrade in the future.
Several methods and types of conduit
systems are used. For example, one configuration includes pre
manufactured segregated ducts or large ducts with multiple plastic,
high-density polyethylene “inner ducts” installed inside. The
inner ducts can be smooth walled or corrugated either longitudinally
or horizontally.
One method of installation involves a
composite optical overhead ground wire (OPGW) on a transmission line
to link substations together. The OPGW is usually terminated in a
standard splice case at a substation structure. At this splice case,
it is interfaced with the substation optical cable.
The substation optical cable should be
installed in conduit from the splice case into the substation cable
duct or trench system. The optical cable should be installed in
conduit from the substation cable duct or trench system to the
control house where it is terminated on a fiber termination panel.
There are important differences to be
considered in the handling and installation of fiber optic cable, as
compared to metallic cable. In ladder type cable tray, optical cable
may be subjected to stress due to the weight of other cables which
can induce microbending into the optical cable.
Therefore, it may be a better practice
to place the optical cable in a separate duct installed in the tray.
Optical cables in substations should be installed in the same manner
as metallic conductor cables. This\ practice requires robust optical
cables that can withstand normal construction handling and still
protect the fibers inside.
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