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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