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The generation, transmission, and distribution of power involve electrical facilities, apparatus, and components to carry the electrical energy from its generating site to where it is utilized. An important part of this power system is the medium-voltage cable system that is used exclusively to carry power from the main substation to secondary substations at load centers. Low voltage cable is used to distribute power from the load centers in conduits and ducts, even though other methods such as cable trays, direct burial for outdoor applications, and aerial cable are used. Electrical, mechanical, and environmental considerations are the main factors in selecting and applying cable systems for distribution and utilization of electrical power. The splices and terminations of medium-voltage cables or connections of different type of cables (such as aluminum and copper) require careful consideration and evaluation during installation, as well as throughout their service life. Correct installation and preventive maintenance of cable systems will assure continued electrical power service.
2. Cable Construction and Classification
It is difficult to select and apply cables to power systems without some knowledge of the cable insulation system and of cable components. Therefore, it is important to review some basic considerations and fundamentals of cables for their application to power systems. The following materials are presently used for cables.
The copper material used in the manufacture of cable is pure electrolytic copper, which has 100% conductivity. This means that a wire 1 ft long and one circular mil (1/1000 of in.) in cross-sectional area has a resistance of 10.371 Ohm at 20°C. Tinning of copper is also required for many rubber and rubber-like insulation compounds to prevent corrosion of copper due to the sulfur that is used in the vulcanizing process.
Aluminum conductors are made from 99% pure aluminum, which has a conductivity of 61%. Normally, three-fourths aluminum is used for construction of aluminum conductors. Three-fourths hard drawn aluminum has strength of 17,000-22,000 psi. Some of the disadvantages of aluminum are low conductivity; high resistance of aluminum oxide, which forms very rap idly when aluminum is exposed to air; cold flow characteristics; and galvanic action when connected to dissimilar materials.
2.1 Types of Conductors
The following types of conductors are in use in power distribution systems.
Solid conductors: Normally, solid conductors are available up to size 6 American wire gauge (AWG). However, they can be made available up to size 4/0 AWG.
Stranded conductor: Most systems use concentric stranding for the applications discussed here.
Overhead cable conductors
The strandings available for this application are type AA and A confined to bare conductors.
Power cable conductors
The concentric stranding is most commonly used for power cable conductors. The construction of concentric-type cable consists of a central core surrounded by one or more layers of helically applied wires. The first layer has six wires and each subsequent layer has six more wires than the preceding layer. In this type of cable construction, the core consists of single wire and all of the strands have the same diameter. The first layer over the core contains 6 wires, the second contains 12 wires, the third 18, and so on. The following types of strandings are used in this application.
Class B: This class of stranding is used exclusively for industrial power cables for application in 600 V, 5 kV, and 15 kV power systems. The cable stranding usually consists of 7 (#2 AWG), 19 (#4/0 AWG), 37 (500 kcmil), or 61 (750 kcmil) strands.
Classes C and D: These classes are used where a more flexible cable is required. Class C uses 19, 37, 61, or 91 strands and class D uses 37, 61, 91, or 127 strands for the #2 AWG, #4/0 AWG, 500 kcmil, and 750 kcmil cable construction, respectively.
Classes G and H: These classes are used to provide more flexible cable than class D. Classes G and H are also known by rope or bunch stranding. Class G uses 133 strands and class H uses 259 strands for cable construction. Examples of cables in these classes are welding and portable wire for special apparatus or large cables.
Cables used for utility systems are of somewhat specialized construction.
Some of these are as follows:
Compact strand (compact round): This type of construction allows for smaller diameter and less weight than solid conductor.
Annular: This construction uses a hollow space or rope core in the center of the cable.
Segmental: Consists of four segments stranded together and operated in parallel.
Concentric cable: Consists of an inner and outer conductor with equivalent cross sections, which are separated by insulation.
Sector: A multiconductor cable in which the conductor is shaped like a sector in a circle.
2.2 Conductor Arrangement
Conductors may be arranged in various ways to form a cable. The common arrangements used in power system applications are the following:
Single-conductor cable: A single conductor of either shielded or nonshielded construction.
Three-conductor cable: Three single conductors bound together with a nonmetallic tape. Three-conductor cables may also be bound together by interlocking galvanized steel, aluminum, or bronze tape. This type of cable is known as interlocked armor cable. Three-conductor cables may come with ground wires, which are used for system ground or equipment ground. Ground wires are usually located in the interstices of the three-conductor cable. Three-conductor cable can be either shielded or nonshielded.
2.3 Cable Types
Power cables are classified with respect to insulation as follows:
Laminated type: This type of cable uses paper, varnished cambric, polypropylene, or other types of tape insulation material. Insulation formed in layers, typically from tapes of paper or other materials or combination of them. An example of this type of cable is the paper insulated lead-covered (PILC) cable.
Extruded type: This type of cable uses rubber and rubber-like
compounds, such as polyethylene (PE), cross-linked polyethylene (XLPE), ethylene propylene rubber (EPR), etc., applied using an extrusion process for the insulation system.
The 2008 National Electrical Code (NEC), Table 310-13A, "Conductor Application and Insulations Rated 600 Volts" lists insulation types including the trade names (type letter), polymer name, maximum operating tempera ture, sizes of conductors, thickness of insulation in mils, jackets, and application. Similarly, NEC Table 310-13C, lists similar information for conductor application and insulation rated 2001 V and higher. In general, cables are classified according to their insulation system as follows:
2. Varnished cambric
4. Rubber and rubber-like compounds (polymeric insulation)
5. Mineral-insulated (MI) cables
Each of these materials has unique characteristics that render it suitable for a given application.
Paper can be wound onto a conductor in successive layers to achieve a required dielectric strength, and this insulation is generally used for cables operating at 10,000 V and higher. Paper can be impregnated in different ways, and, accordingly, cables so insulated can be subdivided into solid and oil-filled types.
Solid paper: Insulated cables are built up of layers of paper tape wound onto the conductor and impregnated with viscous oil over which is applied a tight-fitting, extended lead sheath. The three-conductor cables are of either belted or shielded construction.
Belted construction: Consists of three separately insulated conductor cables wrapped together with another layer of impregnated paper or belt, before the sheath is applied.
Shielded construction: Each conductor is individually insulated and concentrically covered with a thin overlapping metallic nonmagnetic shielding tape; the three conductors are then cabled together, wrapped with metallic or nonmetallic binder tape, and sheathed.
The purpose of the metallic shielding tape around each conductor is to control electrostatic stress, reduce corona formation, decrease thermal resistance, minimize circulating currents, and limit power under normal operating conditions. Shielding tape of only 3 mils thickness is used in cable construction.
Oil-filled cables: Oil-filled paper-insulated cables are available in single- or three-conductor cables. Single-conductor oil-filled cable consists of a concentric stranded conductor built around an open helical spring core, which serves as a channel for the flow of low-viscosity oil. This cable is insulated and sheathed in the same manner as the solid paper- insulated cable. Three-conductor oil-filled cables are all of shielded design and have three oil channels composed of helical springs that extend through the cable in the space normally occupied by filler material.
A thin plain cotton or linen fabric of fine, close weave that is applied as tape; it has a high dielectric strength, low dielectric losses, excellent resistance, good flexibility, good mechanical strength, and fair resistance to moisture. It can be used outdoors above ground and is always covered with an impervious jacket, such as flamenol. Underground installations using this type of cable require a lead sheath. It has a maximum operating temperature of 85°C at 5 kV and below 77°C at 15 kV. The maximum short-circuit is 200°C for this cable.
Asbestos-insulated wire and cable can have their principal usefulness in locations where the ambient temperature is high (over 50°C or 60°C). Their use is imperative in ambient over 85°C since this is the maximum safe operating temperature of most commonly used insulating materials, except silicone insulation.
Rubber and rubber-like compounds (polymeric insulation)
This type of cable can be classified as follows:
1. NEC compounds
The rubber and rubber-like insulated cables enjoy their popularity owing to moisture resistance, ease of handling, ease of splicing, and extreme flexibility.
Elastomers are materials that can be compressed, stretched, or deformed like rubber and yet retain their original shape. The thermoplastics materials soften when they are reheated, whereas thermosetting-type insulation has very little tendency to soften upon reheating after vulcanization. The earlier oil-based natural rubber compounds have been replaced by synthetic materials, which have better electrical and mechanical characteristics. The following synthetic rubber-like compounds are in use today:
Ethylene propylene rubber (EPR), an elastomer compound: EPR is commonly used in power cables, but is also gaining use in telecommunications and other types of cables. EPR possesses good chemical, mechanical, and electrical properties. However, it is not inherently flame retardant. It has a maximum operating temperature of 90°C, and maximum rated voltage (phase-phase) of 138 kV.
Neoprene, an elastomer compound: Neoprene is one of the most common materials in use for cable jackets. It is used where service conditions are usually abrasive. Since neoprene is not inherently flame retarding, it is usually compounded with the necessary flame retarding chemicals when used as cable jackets.
Hypalon, an elastomer compound: Hypalon is also a commonly used material for cable jackets. It has properties similar to neoprene, and in addition exhibits better thermal stability and resistance to ozone and oxidation.
Polyvinyl chloride (PVC), a thermoplastic compound: It is flexible, has good electrical properties, and requires no external jacket. Cables using this insulation are rated up to 600 V; maximum operating temperature is 60°C for power applications; maximum short-circuit rating temperature is 150°C. NEC designation is T, TW. It is avail able in several colors and is mainly used as low-voltage cable systems.
Polyethylene (PE), a thermoplastic compound: It melts at very low temperatures (i.e., 110°C). It is also severely affected by corona. It has a high coefficient of thermal expansion. However, it has excellent electrical and moisture-resistance properties. It has a low cost. Its maximum operating temperature is 75°C and maximum short-circuit temperature is 150°C. It is used in low- and medium-voltage applications.
Buna, a thermosetting compound: It combines the most desirable properties of low-voltage insulation. It has the advantages of heat and moisture resistance, excellent aging qualities, and good electrical characteristics. However, it lacks resistance to ozone. NEC designation is RHW. Its maximum operating temperature is 75°C and short circuit temperature is 200°C.
Butyl, a thermosetting compound: It has a high resistance to moisture, heat, and ozone. NEC designation is RHH. It has a maximum operating temperature of 90°C and short-circuit temperature of 200°C.
Silicone rubber, a thermosetting compound: It is extremely resistant to flame, ozone, and corona. It has a maximum operating temperature of 125°C and a maximum short-circuit temperature of 250°C. It has poor mechanical strength.
XLPE, a thermosetting compound: It has excellent electrical properties and high resistance to moisture and chemical contaminants. It is severely affected by corona and has an operating temperature of 90°C. Its short-circuit temperature is 250°C. It can be applied on up to 35 kV distribution systems.
Mineral insulated (MI) cables
The design of MI cable differs very widely from conventional types of cable.
Basically, it consists of a single- or multiple-conductor insulated cable with magnesium oxide and sheathed with copper tubing. NEC operating temperature designation for this cable is 85°C. However, it can be used up to 250°C operating temperature.
Teflon is used where high temperatures, moisture, and chemicals are present. Teflon is also resistant to oils. It is rated up to 200°C.
2.5 Shielding and Semiconducting Tape
Power cables at voltages above 2000 V usually have shielding and semi conducting tape. Cable shielding system consists of strand shield and insulation shield system.
Insulation shield system
The insulation shield system is comprised of two conductive components: a semi-conductive layer called "semi-con" and metallic (conductive) layer. The insulation shield system is installed on the outer surface of the insulation and hence is called "the outer shield." The purpose of the semi-con is to remove air voids between the metallic shield and the insulation.
Shielding is accomplished by wrapping a thin (0.005 in.) copper tape spirally around the insulation to form a continuous shield along the entire length of the cable. This tape may or may not be perforated to reduce losses and is held to ground potential by suitable grounding.
Shielding is necessary on medium and HV cables to
1. Prevent damage from corona.
2. Confine dielectric field to the inside of cables or conductor insulation.
3. Give symmetrical stress.
4. Reduce induced voltages.
5. Provide increased safety to human life.
The shield must be grounded at one end and preferably at more than one point. The usual practice is to ground the shield at each termination and splice. Shielding is discussed in more detail under Section 6.6.
Strand shielding (semiconducting tape)
Except on 600 V rubber and varnished cambric cables, semiconducting tape is used to separate the conductor from the rubber insulation to prevent possible damage of the insulation from corona and ionization. The solid line in FIG. 1 shows how voltage stress may develop in the air spaces between conductor strands and insulation, thereby causing the ionization of air and breakdown of cable insulation. The application of semiconducting tape smooths the voltage stress, as shown by the dashed lines, and keeps such voltage stress constant and to a minimum. This application of the semiconducting tape is known as "strand shielding." Modern cables are generally constructed with an extruded strand shield.
Air space; Conductor insulation Conductor strands Voltage stress with semiconducting tape Voltage stress without semiconducting tape
2.6 Finishes and Jackets
A wide variety of finishes are used; they are referred to as jackets, sheaths, armors, and braids. These coverings are required primarily because of the physical or chemical characteristics of the particular insulation involved and the required mechanical protection. Finishes can be divided into two categories: (1) metallic finishes and (2) nonmetallic finishes.
Metallic finishes Metallic armor should be applied where a high degree of mechanical protection is required along with protection from rodents, termites, and the like.
All metallic sheaths are subject to electrolytic damage. Metallic finishes are subdivided into the following:
Lead sheaths: One of the earliest types of metallic sheaths still in use.
Flat-band armor: Consists of jute bedding, two helical tape wraps, and a protective jute covering over the tapes. The tape may be either galvanized or plain steel.
Interlocked armor: Consists of galvanized steel, aluminum, or bronze strip (0.750 in. wide and 0.020-0.030 in. thick) over the cable in such a way as to provide excellent protection.
Aluminum-sheathed cable: A recently introduced cable that offers advantages such as lightweight, resistance to fatigue, good corrosion resistance, and positive moisture barrier.
Wire armor: Available in two types, round and basket-weave or braided wire.
Round wire armor offers extremely strong cable and has high tensile strength for vertical applications. Braided or basket-weave wire armor consists of a braid of metal wire woven directly over the cable as an outer covering where additional mechanical strength is required.
Most of the nonmetallic finishes include PVC, PE, neoprene, hyplon, and EPR.
1. PVC: This covering (i.e., finish) offers excellent moisture-resistance characteristics, but does not provide mechanical protection.
2. PE: It has excellent resistance to water, ozone, and oxidation. It is resistant to gasoline, solvents, and flames.
3. Neoprene: It is commonly recommended where service conditions are usually abrasive and extreme. By itself, it is not flame retardant.
4. Hyplon: It possesses similar properties as neoprene, but also has better thermal stability and resistance to ozone and oxidation.
5. EPR: It exhibits excellent weathering properties and is resistant to ozone. It has good chemical and mechanical properties, but is not inherently flame retardant.
6. Braids: Generally, present-day trends are away from the use of non metallic braid coverings. Braids may be of the following types:
a. Heat- and moisture-resistant cotton braid
b. Flame-resistant cotton braid
c. Asbestos braid
2.7 Cable Construction
Low-voltage cables (less than 2000 V): The low-voltage cable consists of the center conductor surrounded by an insulation layer which provides the electrical insulation. The insulation may be covered by a protective jacket. This type of cable construction is also known as nonshielded construction.
Medium-voltage cables (2001-35,000V): The majority of medium-voltage cables are constructed as shield construction. However, it is not uncommon to find medium-voltage cables up to 5 kV being of nonshielded construction.
The shielded construction cable can be classified into two major types. For direct burial use, such as underground rural distribution (URD), the cable is known as concentric neutral cable. It consists of the conductor, strand shield (semiconductor), insulation, neutral, and jacket as shown in FIG. 2a. The other shielded construction-type cable consists of the conductor, strand shield (semiconductor), insulation, insulation shield (semiconductor), metallic shield, and jacket as shown in FIG. 2b. The purpose of the strand shield is to minimize corona by equalizing the voltage gradients across the air gap between the conductor strands as explained in Section 6.2.5. The semiconductor over the insulation serves a similar purpose as the strand shield by enclosing the insulation completely and equalizing the electric field within the cable. The metallic shield serves as an electric shield which equalizes and confines the electric field inside the cable. By equalizing the electric field in the cable, the voltage stress is uniformly distributed around the cable, thereby eliminating any high points of voltage stress in the cable. This enhances the cable life and reliability.
Extruded conducting layer
Class B compressed stranding
Bare copper tape XLP (or EPR) insulation
Conductor Jacket Neutral
Stranded bare copper
The electrical characteristics of cables are concerned with the electrical
constants most commonly required for power system calculations. These electrical constants, such as positive sequence impedance (Z1), negative sequence impedance (Z2), and zero sequence (Z0), are used in the application of symmetrical components for calculations of short-circuit currents, unbalanced voltages, and their phase relationships among sheaths and conductors, which are important in the calculation of reactance, capacitance, insulation resistance, and dielectric loss. The cable geometry for single- and three-conductor cable is shown in FIG. 3. The cable ratings provide the basic information regarding its application and use.
A basic knowledge of cable ratings is essential for correct selection and application of cables. Exceeding cable ratings or their misapplication can be hazardous to property and personnel, as well as to successful operation of the plant or facility.
The two properties of cables, as mentioned, are geometry of cables and electrical constants. A general rule is that regardless of the complexity of mutual inductive relations between component parts of individual phases, the method of symmetrical components can be applied rigorously whenever there is symmetry among phases. All three-conductor cables satisfy this condition by the nature of their construction; single conductor cable may or may not.
However, the error is very small when they are treated similarly as three conductor cables. The space relationship among sheaths and conductors in a cable circuit is a major factor in determining reactance, capacitance, charging current, insulation resistance, dielectric loss, and thermal resistance. The physical characteristics of cables can be determined from the geometry of cables, which is described next.
Geometric mean radius
The geometric mean radius (GMR) is a property usually applied to the conductor alone, and depends on the material and stranding used in its construction. One component of conductor reactance is normally calculated by evaluating the integrated flux linkages both inside and outside the conductor within an overall 12 in. radius. Consider a solid conductor that has some of the flux lines lying within the conductor and contributing to the total flux linkages, even though they link only a portion of the total conductor current.
Now consider a tubular conductor having an infinitely thin wall substituted for the solid conductor; it has a flux that would necessarily lie external to the tube. Therefore, a theoretical tubular conductor, to be inductively equivalent to a solid conductor, must have a smaller radius so that the flux linkages present inside the solid conductor, but absent within the tube, will be replaced by additional linkages between the tube surface and the limiting cylinder of 12 in. radius. A solid copper conductor of radius d/2 is equivalent to a tubular conductor radius 0.779d/2. This equivalent radius is called the GMR of the actual conductor. This quantity can be used in reactance calculations with out further reference to the shape or makeup of the conductor. The factor by which actual radius must be multiplied to obtain GMR varies with stranding and hollow-core construction.
Geometric mean distance (GMD)
Spacing among conductors or between conductors and sheaths is important in determining total circuit reactance. The total flux linkage surrounding a conductor can be divided into two components, one extending inward from the cylinder of 12 in. radius and the other extending outward from this cylinder to the current return path beyond which there are no net flux linkages.
The flux linkages per unit conductor current between the 12 in. cylinder and the return path are functions of the separation of the conductor and its return path. GMD is therefore a term that can be used in the expression for the external flux linkages, not only in the simple case of two adjacent conductors, where it is equal to the distance between conductor centers, but also in the more complex case where two circuits, each composed of several conductors, are separated by an equivalent GMD.
The positive or negative sequence reactance of a three-phase circuit depends on separation among phase conductors. If the conductors are equilaterally spaced, the distance from one conductor center to another is equal to the GMD among conductors for that circuit. The GMD for three conductor cable is GMD3c = S for an equilateral circuit where S is the distance between each conductor. If the conductors are arranged other than equilaterally as shown in FIG. 4, but transposed along their length to produce a balanced circuit, the equivalent separation may be calculated by deriving the GMD from the cube root of three distance products. This is expressed as follows:
The component of circuit reactance caused by flux outside a 12 in. radius is widely identified as the reactance spacing factor (Xd) and can be calculated directly from GMD.
When equivalent separation is less than 12 in., as can occur in cable circuits, the reactance spacing factor is negative so as to subtract from the component of conductor reactance due to flux out to a 12 in. radius.
The zero-sequence reactance of a three-phase circuit may depend on spacing among conductors and sheath.
The relation in space between the cylinders formed by the sheath internal surface and conductor external surface in a single-conductor lead-sheath cable can be expressed as a geometric factor. The factor is applicable to the calculation of cable characteristics such as capacitance, charging current, dielectric loss, leakage current, and heat transfer. The mathematical expression for geometric factor G in a single-conductor cable is…is the inside radius of sheath d is the outside diameter of conductor
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