MEDIUM AND HIGH VOLTAGE WIRES AND CABLES

MEDIUM AND HIGH VOLTAGE WIRES AND CABLES

What are the types of conductors/cables used for power distribution?

There is no consensus among standard governing bodies like (i.e., IEC, ANSI, IEEE, UL, NEC and others) concerning the classification of voltage level.Thus, for clarity of this manual we will utilize IEEE voltage level classifications wherein 601V to 69,000V is medium voltage and 69,001V to 230,000V is high voltage.Furthermore, conductors are also classified according to their degree of insulation covering (i.e. bare, covered, and insulated). Basically, construction of the wires and cables is the same or similar for medium and high voltage applications.

Bare Conductors

Bare conductorsare those without covering and primarily used for overhead power transmission and distribution application. Insulating medium is air wherein the conductors are spaced from each other and any grounded object based on the system voltage. Insulators (e.g. porcelain, glass, and polymers) are used to support theconductors and insulate these from the supporting structure such as tower or pole.

Copper and aluminum conductors are commonly used for this application. However, there are instances where economics dictate the use of conductors with low conductivity such as galvanized steel, copper-clad steel (copperweld) or aluminum-clad steel (Alumoweld) in the distribution system. In such cases, the coductor losses are lower than the cost of recovering the investment in the distribution line if copper or aluminum conductor is used. In this field of application, the most dominant conductor used by the industry is the aluminum conductor steelreinforced (ACSR).

Covered Conductors

Covered conductors are bare conductors with thin insulation covering used for overhead power distribution system. These are used for power distribution circuits that transverse along routes with heavy tree growth. The covering does not fully insulate the conductor but it is thick enough to reduce the chances of flashover whenever a tree branch falls between the conductors. Covered conductor is also commonly known as tree wire. Also, it helps minimize faults caused by animals and enable distribution utilities to utilize conductor configurations with tight spacing. COvered conductors are commonly used as a cost-effective method for increasing overhead line reliability.

The conductor materials are typically copper or aluminum or other conductors designed to give a balance between strength and conductivity such as ACSR. Tree wire is commonly covered by insulating materials such as polyethylene,XLPE, or EPR. Insulation thickness typically ranges from 30 to 150 mils. Tree wires must always be treated as bare conductors. However, cover spacings are allowed for this type of conductor.

Insulated Cables

Majority of insulated cables are utilized for underground transmission and distribution systems. Being insulated for voltages higher than 2kV, that these cables are typically shielded.

Construction

The fundamental difference between non-shieldedand shielded cables is the inclusion of outer conducting components in the cable system. The nasic components of a shielded cable are:

• conductor - the conductors used in shielded cables are basically the same as those used in non-shielded cables, with copper and aluminm as the conductor. 
• conductor shield or screen - the coductor shield is usually a semi-conducting material applied over the conductor circumference to shield out the surface irregularities of the conductor.
• insulation - this is the part of the cable that is relied upon to insulate the conductor from other conductor or conductive object or from ground.
• insulation shield or screen - this absorbs the symmetrical radial stresses and discharges on the surfaces of the insulation. It protects the cables from induced potentials. Shield help attenuate, make uniform and reduce the surge potential stresses on the insulation. It increases safety to humans and removes the risk of fire due to electrical discharges on the cable surface.