Basics of Power Cable

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Wire & Cable Technical Library: Technical Papers

The Basics of Power Cable

W.F. (BUDDY) POWERS, JR., P.E. Senior Development Engineer Wire & Cable Technology SOUTHWIRE COMPANY Carrollton, Georgia

ABSTRACT

Engineers should keep abreast of the latest changes in the equipment they use. While many components are exciting and fun to learn about, the more mundane components, like wire and cable, can be very important. Engineers weather new or experienced need good technical information on the component of the system to make a good installation. It is the purpose of this paper to give basic information about wire and cable supplemented by up to date wire and cable issues.

INTRODUCTION

Insulated power cables can seem to be complicated. In fact, many cables are electrically sophisticated. To simplify we must understand the cable's components and basics of conductor cables. These fundamentals apply to multiple conductor cables.

There are two basic components in a low voltage cable, the conductor and the electrical insulation (dielectric). There are different types of conductors and insulations. Sometimes jackets or armoring are added.

As voltage increases other components are added to handle higher electrical stresses. The purpose of this paper is to discuss each component, to help an engineer specify the type components required for specific applications.

CONDUCTORS

Conductor selection depends on ampacity, voltage, physical properties, flexibility, shape, and economics. The most commonly used metals are copper and aluminum. Conductors may be solid or stranded. A conductor's size is specified by cross sectional area. US practice is to specify area using American wire gage (AWG) up to 4/0 and by thousand circular mils (kcmil), for larger. Standard International practice is to specify area in square millimeters (mm^2). For stranded conductors, the area is the sum of the area of each strand.

A circular mil is the area of a circle having a diameter of one mil (one mil is equal to 0.001 inches). To determine the cmil area of a solid conductor, square the diameter in mils.

Stranded conductors are more flexible but have a larger diameter than a solid of the same area. Diameters of stranded conductors vary depending upon the exact type of construction. These constructions include concentric round, compressed, compact and compact sector. The number of strands is usually based on a geometric progression of single strand layers (1, 6, 12, 18, etc.).

Class B and C Stranded Conductors

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Class B and Class C conductors are constructed according to ASTM 2.03, Standards B8, B609, and B231. Class B stranding is for normal use. Class C uses more strands.

It is used when more flexibility is required. Class B or C can be compressed up to 3% of its uncompressed diameter. Each layer of strands of a Class B or Class C conductor is applied in a spiral. Each layer of a conductor is an opposite hand spiral to the layer beneath it.

19 Wire Combination Unilay Stranded Conductors

19 wire combination unilay strand has a diameter equal to the compressed strand of the same size. It is made according to ASTM Volume 2.03, Standards B786 and B787. The strands of a unilay strand are in the same direction.

Copper

Drawing is the process of pulling a wire through a series of smaller and smaller dies. Drawing large copper rod into a finished wire results in the work hardening. Work hardening causes a soft temper rod to become a higher temper wire. Soft temper copper conductors are desirable because of flexibility.

Annealing the wire after drawing or stranding makes the conductor softer. Annealing is done in an oven or by in-line annealers on the drawing machines.

Copper can be provided in three tempers based on ASTM. These tempers are soft (or annealed), medium-hard-drawn, and hard-drawn. Soft copper is used for insulated conductors. Medium hard-drawn and hard-drawn copper is used in overhead due to higher breaking strengths.

Coating copper strands can protect against corrosion. A commonly used coating is lead alloy coating (LAC) specified by ASTM Standard B189. Another coating is tin specified by ASTM Standard B33. Tinning is usually done for conductors being drawn down to a very fine gage as those found in the electronics industry. LAC is usually done to protect against environmental corrosion.

Aluminum

Drawing aluminum rod into a wire also results in the work hardening. Annealing may be used to reduce the hardening. There are several alloys of aluminum commonly used for electrical conductors. 1350 (formerly EC grade) and 8000 series aluminum alloys meet ASTM Standards B233 and B800 respectively. 8000 series alloys are used because they are harder and make better connections. 1350 is used by utilities for overhead and underground cables. 8000 series alloy is used for 600 volt UL cables. The 1990 NEC mandates the use of 8000 series aluminum alloys. Aluminum can be provided in five tempers based on ASTM.

Water Blocked

Water blocked conductors prevent moisture migration along the conductor strands. A water blocking compound occupies the spaces between the strands of the conductor.

This blocking compound reduces the possibility of premature failure from water.

Conductivity

Conductivity is typically specified in percent. This percent is based on an International Annealed Copper Standard (IACS). This standard was established in 1913 by the International Electrotechnical Commission. It specifies the conductivity of copper as 100%. The conductivity of aluminum is about 61%. It is common to use 61% conductivity for aluminum in power cable.



ELECTRICAL INSULATION OR DIELECTRIC

There are many insulations used in electric power cables. Insulations are classified as thermoplastic or thermoset. Thermoplastic materials lose their form upon heating. Thermoset materials maintain their form in spite of heat. These insulations range from common thermoplastic polyvinyl chloride to advanced thermoset tree-retardant crosslinked polyethylene and synthetic rubber compounds.

Polyethylene

Polyethylene (PE) is a long chain hydrocarbon. It is a thermoplastic produced by the polymerization of ethylene gas under pressure. PE is popular being inexpensive, easily processed and resistive to chemicals and moisture, and with good electrical properties and flexible in low temperature. PE is produced in low, linear low, medium, and high densities. As the density increases, so does the hardness, yield strength, stiffness, heat and chemical resistance.

When PE cables are required to be resistant to sunlight, carbon black or other suitable ultra-violet (UV) inhibitors are added. UV radiation can degrade the physical and electrical properties of PE without UV inhibitors.

When PE cables are required to be flame retardant materials are added to make it flame retardant.

PE's electrical properties are excellent. PE is susceptible to degradation by corona discharges at high voltages. PE is susceptible to treeing when subjected to high electrical stress. Corona discharges and treeing may lead to premature cable failure. In the past PE was used in high voltage cable, but it is not used in cables over 5 kV today.

Crosslinked Polyethylene

Crosslinked polyethylene (XLPE) is a thermoset. It is produced by compounding PE (polyethylene) with a crosslinking agent, like organic peroxide. The molecules of polyethylene are "crosslinked", forming an interconnected network. The terms "cured" and "vulcanized" are also used for "crosslinked".

Peroxide can be used to crosslink only low density polyethylene. Recent silane crosslinking technology works on all densities. Crosslinked polyethylene can operate at higher temperatures and higher voltages than thermoplastic polyethylene.

Crosslinking improves polyethylene's physical and electrical properties. Additives reduce the electrical properties of the polyethylene. Medium voltage (5 to 46 kV) uses unfilled crosslinked polyethylene, with impulse strengths of 2700 V/mil.

Fillers, like carbon black, increase the tensile strength and hardness for low voltage applications. Carbon black also provides ultra-violet protection.

XLPE cables may be operated at a conductor temperature of 90°C and at 130°C during emergency conditions. XLPE has good low temperature properties, increased resistance to corona compared with thermoplastic polyethylene, and good impact, abrasion, and environmental stress crack resistance.

Recent Developments

Recent technology has resulted in Tree Retardant XLPE insulation compounds. TR-XLPE is used for medium voltage applications.

For low voltage applications, the compounding of halogen or non-halogen flame retardants into the insulation achieves the required level of flame retardancy. Non-halogen flame retardants do not generate toxic acid gases on combustion. These retardants are areas of recent developments for low voltage cables and jackets.



Ethylene Propylene Rubber

Ethylene Propylene Rubber (EPR) is a thermoset made from ethylene and propylene. EPR is used for cables from 5 kV to 35 kV and low voltage cables.

Peroxide is used to crosslink most EPR compounds. However, work has been done on the use of silane crosslinking systems. Sulfur crosslinked used to be used, but is being replaced by peroxide. EPR is filled 50% or more. Common fillers are treated clay and silicate.

EPR may be used up to 90°C continuously or 130°C during emergency conditions. EPR has good elastic properties, ozone resistance, environmental, and low temperature resistance. EPR is good for medium voltage applications.

Polyvinyl Chloride

Polyvinyl chloride (PVC or vinyl) is thermoplastic. PVC is the standard insulation for cables rated at 1000 volts or less. PVC is a mixture of PVC resin, plasticizer, fillers, stabilizers, and modifiers. The quantity and type of each determines the properties. A broad range of electrical, physical and chemical properties is possible.

PVC has good electrical properties. It is tough and resistant to flame, moisture, and abrasion. Resistance to ozone, acids, alkalies, alcohols, and most solvents is adequate. PVC can be made resistant to oils and gasoline. PVC can have temperature ratings from 60°C to 105°C.

PVC has the disadvantage of having a high dielectric constant and dissipation factor. Also plasticizer loss can cause hardening and cracking. PVC compounds stiffen as temperatures decline, and are not recommended for flexing below -10°C, although special formulations will allow flexing to -40°C.

Chlorosulfonated Polyethylene

Chlorosulfonated Polyethylene (CSPE) is a thermoset, commonly called by DuPont's trade name "Hypalon." CSPE is polyethylene plus chloride and sulfonyl groups. This changes stiff PE into a rubbery polymer. It is crosslinked with organic peroxides and sulfur.

CSPE is a mixture of polymer, fillers, modifiers and crosslinking agents. The quantity and type affecting the final properties. CSPE contains a Halogen (Chlorine) and is flame retardant. CSPE can be rated 90°C and has excellent tensile strength and abrasion resistance. It has good weather, oil, chemical, and fluid resistance.

SHIELDING

Dielectric Fields

In all electrical cables there is a dielectric field when the conductor is energized. The density of the field is dependent upon voltage on the conductor and the thickness of the insulation. The denser the field the more the insulation is "worked".

In a low voltage cable the insulation is not "worked" enough for further design considerations. As voltage increases the insulation is "worked" more. The field becomes crowded. It becomes important that a cable have conducting (and semi-conducting) components to smooth and evenly space the equipotential lines. The main difference between nonshielded and shielded cables is having conducting components in the insulation system.

Shielding uses conducting and semi-conducting stress control layers to make the dielectric field within the insulation symmetrical. For 2 kV, shielding is required only over the conductor. At 5 kV, shielding is required over the insulation too. If the insulation



shield is at ground, all of the voltage is confined to the insulation.

Conductor Shield

The conductor shield is a layer of semi-conducting material. Semi-conducting materials do not conduct electricity well enough to be a conductor but will not hold back voltage.

It "smooths" out the surface irregularities of the conductor. The conductor shield makes the voltage on the inside of the insulation the same. Industry specifications define the performance of conductor shield.

The conductor shield should come loose from the conductor without "pickoffs", while bonding to the insulation. Good insulation shields are extruded in tandem with the insulation.

Insulation Shield

The insulation shield consists of two components. These components are the extruded (auxiliary) shield and the metallic (primary) shield.

The extruded shield consist of a semi-conducting layer similar to the conductor shield. It makes the voltage on the outside of the insulation the same.

The primary shield can consist of metal tape, drain wires or concentric neutral (CN) wires. Grounding the primary shield makes the voltage on the outside of the insulation ground. The copper of the shield is usually bare, but may be coated with lead or tin. Some primary shields consists of drain wires and tape. Aluminum and lead can also be used as the shield.

Concentric neutral wires serve a two fold purpose. They function as the metallic component of the insulation shield and as a conductor for the neutral return current. Their cross sectional area must be sized in order to function as the neutral conductor.

ICEA and AEIC provide the required characteristics of the insulation shield.

JACKETING, SHEATHING, AND ARMORING

The terms jacketing, sheathing and armoring can be confusing and sometimes redundant.

Jacketing is usually a plastic cover. PVC is a common jacket material. Jackets provide mechanical, thermal, chemical and environmental protection. They normally perform no electrical function. They ease installation and routing concerns by enclosing multiple conductors.

Sheathing includes tubes, tapes, or wires for armor and a built-in grounded conduit. The term "sheathing" is typically a tubular metallic covering. Lead or aluminum are common sheathings.

Armoring is used to protect the cable from penetration by sharp objects, crushing forces, and damage from gnawing animals or boring insects. High tensions may damage armor. Armoring can be an interlocked tape of a continuous tube.

Sometimes it is hard to distinguish between jackets, sheaths, armoring and shield. For example: An overall seamless metal covering is usually called a sheath. It can act as armor and electric shield. It can also be a jacket

A plastic covering, usually called a jacket, can act as armor providing mechanical protection.



Nonmetallic Jackets

Nonmetallic jackets include PE, PVC, Nylon, CSPE (Hypalon) and armoring. PE, PVC, and Nylon are thermoplastic materials. CSPE is a thermoset material. Armoring can be either. These materials conform to one or more of the standards issued by AEIC, ASTM, CSA, ICEA, IEEE, NEMA, and UL as dictated by specific requirements and applications.

Metallic Sheaths

Lead is one of the oldest sheathing materials used on power cables. Lead is fed into a cylinder. A hydraulic piston forces it around the cable. Lead is a very effective moisture barrier. This contributes to the long-term reliability of cable.

A disadvantage of lead is weight. Lead is prone to deform under continuous load due to "creep". Lead sheaths are also susceptible to fatigue failure from vibration and thermal cycling.

Aluminum is also used as sheathing. It is lighter than lead and has good mechanical properties. Aluminum sheaths may be extruded similar to lead. Aluminum sheaths can also be applied by a thick metal tape around the core and welded. After forming, an aluminum sheath can be corrugated.

Water Impervious Constructions

Water impervious constructions use a special sheath. This sheath is in the form of a metal/plastic laminate tape. This tape can be applied with two different approaches, on core and under jacket. On core use a thin lead tape laminated with a semi-conducting plastic. The tape is bonded to the insulation shield. In the under jacket approach, the lead is laminated to an insulating plastic. The tape goes over the neutral wires and bonds to the jacket. A water absorbing agent is used in the area between the insulation shield and the jacket to prevent water migration.

Failures induced by treeing in the insulation are reduced with a water impervious layer. Using a lead laminate tape accomplishes this without the added bulk, weight and cost of a thick lead or welded aluminum sheath. The cable can be handled and terminated in much the same manner as standard cables.

Armoring

Typical requirements for interlocked, flat tape, and round wire armoring appear in specifications such as ICEA S-68-516.

Interlocked armor is a flat metal tape, bent into an "S" shape then helically wrapped around a core so the edges lock together. The two most commonly used materials are galvanized steel and aluminum. An outer jacket of PVC or PE is often used.

Advantages of interlocked armor includes flexibility and ease of termination.

A big disadvantage: interlocked armor cannot be used under high tension.

Applications include commercial or industrial power, control and lighting. Interlocked armor is used in ducts, troughs, and raceways or suspended from aerial messengers. Interlocked armor is not a moisture barrier.

A flat steel, copper or bronze tape is used for armoring. It is helically wrapped around the cable core. The tape is typically protected by a jacket. Applications include commercial or industrial installations in conduit, ducts, troughs, and raceways or suspended from aerial messengers.

Round wire, when used as armoring are helically wrapped around the core of the cable.



Galvanized steel is typically used. An overall jacket may be used. Applications include submarine, bore hole, dredge, shaft and vertical riser cables.

Teck Cables

To meet CSA and Ontario Hydro requirements, a double PVC jacketed interlocked armor design is used. The CSA designation is "Teck Cable". In this construction, an additional PVC jacket is under the armor. This provides extra protection against thermal degradation, mechanical damage, and fluid penetrations. The inner jacketed core can be routed and terminated beyond where the armor is terminated.

CONCLUSION

Engineers should certainly keep abreast of all the latest developments, but learning about the new and latest equipment is frequently done at the expense of learning the basics thoroughly. It is the mundane subjects that sometimes get left by the wayside. A full understanding of wire and cable by some individual in an organization can insure all installations of highly sophisticated equipment are successful and conform to the requirements of the customer.

REFERENCES

[1] Thomas P. Arnold and C. David Mercier, "Southwire Company Power Cable Manual".

W. F. (Buddy) Powers, Jr. (M'90) graduated from the Georgia Institute of Technology. He joined Southwire's Research and Development Department more than 12 years ago, and has focused on product and process development of wire and cable technology. In 1988 he was promoted from Product Development Engineer to his present position of Senior Development Engineer and assumed the responsibility of coordinating new product development internally and with customers. He currently develops products catering to the industrial power cable market.

Mr. Powers is a member of the National Society of Professional Engineers, the Georgia Society of Professional Engineers, and the Canadian Standards Association. He was named the "Young Engineer of the Year" for the State of Georgia in 1989.

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Basics of Power Cable

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