thickness is commonly chosen. In addition, the 133 percent insulation level is recommended by standards where fault-clearing times on wye- connected systems are in excess of one minute but less than one hour. The additional insulation thickness will also reduce the electrical stress within the insulation and, hence, prolong cable life, which many utilities find advantageous. One disadvantage of an increase in insulation thickness is that the additional insulation volume increases the opportunity for contamination. However, this is not a realistic concern for mod- ern cable manufacturing facilities. Also, the addi- tional insulation, shield, and jacket materials needed because of the increased diameter will increase the final installed cable cost. This is due to the increased cost of the cable, the increased pulling and training effort, and the increase in duct size required. Finally, 173 percent insula- tion is used for cables on a system, usually delta or resistance-grounded, which may have a clear- ing time of more than one hour.
It should be noted that the performance of 175-mil direct buried distribution cables on 12.5/ 7.2 kV systems proved unsatisfactory in early underground systems. This was due to treeing of the insulation which could in part, be attributed to the higher voltage stresses present in the 175-mil insulation. This was particularly true in smaller (e.g., #2 AWG) conductor sizes. For this reason, RUS mandates the use of 133 percent insulation thickness (220 mil) for 15 kV class cables.
RUS is currently refining its Specifications for
Underground Primary Cables in Bulletin No.
1728F-U1, which updates and supersedes former Bulletin 50-70 (U1), dated December 22, 1987. In this new bulletin, RUS adopted the insulation thickness shown in Table 2.4 and these will be specified in the pending bulletin.
Voltage Insulation Thickness
Class (kV) Thickness (mils) Level (%)
15 220 133
25 260 100
35 345 100
TABLE 2.4: RUS Insulation Thickness.
INSULATION MATERIAL CHARACTERISTICS
An individual selecting a particular cable insula- tion should be familiar with the basic physical and electrical characteristics of various materials. Each of these characteristics affects the suitability of an insulation material for a particular applica- tion. Selecting a cable construction involves compromise as most materials have different strong and weak points.
Physical characteristics of the insulating layer affect the resistance of a cable to mechanical damage under normal operating conditions. Situ- ations imposing mechanical stresses on cable in- clude the following:
• Soil pressure in direct-burial installations,
• Sidewall pressure on cables pulled into con- duit,
• Flexure during switching operations for elbow-connected apparatus,
• Expansion/contraction in ducts, and
• External clamping action on risers.
Some of the pertinent physical properties are listed below.
Hot Creep
This is a measure of the plasticity of a material at elevated temperatures. It shows the ability of an insulating material to resist deformation at elevat- ed operating temperatures. For thermosetting in- sulations, the hot creep is generally measured at 130°C (266°F), which is the maximum emergency operating temperature. The hot creep is deter- mined by measuring the tensile stress (pounds per square inch, or psi) needed to stretch the in- sulation sample to 200 percent of its original length. See Figure 2.6 for a relative comparison of the hot creep of HMWPE (thermoplastic), XLPE (thermosetting), and EPR (thermosetting).
High-Temperature Aging Characteristics
Electrical insulation in power cables must retain good physical properties after being subjected to high temperatures. High-temperature aging evalu- ations usually compare tensile strength and elon- gation remaining after seven days (168 hours) of exposure to temperatures ranging from 120°C to 180°C (248°F to 356°F).
ELECTRICAL CHARACTERISTICS OF INSULATION MATERIALS
The electrical characteristics of cable insulation are just as important as the physical characteris- tics. After all, if a cable is mechanically durable but will not withstand the applied voltage, the cable no longer serves its intended purpose. Electrical characteristics include insulation resis- tance, insulation power factor, and dielectric constant. Basic electrical characteristics of cable insulation are discussed extensively in Section 4, Equipment Loading.
FIGURE 2.6: Comparative Hot Creep vs. Temperatures for Cable Insulation Materials.
Adapted from ANSI/ICEA T-28-562. 100% Hot Modulus Temperature (°C) 20 75 90 130 250 EP XLPE H MWP E
Insulation
Fabrication
All contemporary cables use extruded dielectric insulation. The manufacturing processes gener- ally are similar for different insulation materials and different voltage classes. The most complex manufacturer’s process involves primary voltage cables that have not only extruded insulation but also extruded conductor shields and extruded insulation shields. Secondary cables have similar construction methods, but employ only an insu- lating layer or, in the case of “ruggedized” styles, possibly two layers.
Many aspects of the manufacturing process are very important. Some of these are the following:
• Purity of the insulation material,
• Lack of voids in the insulation and shields,
• Smoothness of the conductor shield and conductor,
• Adhesion between the conductor shield and the insulation,
• Cleanliness of the conductor shield-insulation interface,
• Smoothness of the insulation outer surface,
• Adhesion between the insulation and the insulation shield,
• Cleanliness of the insulation–insulation shield interface,
• Maintenance of uniform dimensions and con- centricity along the cable, and
• Inclusion of agglomerates, gels, and ambers. Failure to adhere to any of these requirements at any point in the manufacturing process will lead to defective cable that is unsuitable for util- ity applications.
MATERIAL HANDLING
One of the most important requirements of cable manufacturing is cleanliness of the raw materials. The cable manufacturer receives insu- lating and shielding materials, particularly poly- ethylene compounds, as pellets. These pellets must be handled very carefully at both the cable plant and at the insulation manufacturing plant to ensure there is no contamination. Quality control tests that meet, or exceed, industry stan- dards must be made on each batch of pellets to ensure cleanliness. In addition to normal quality control sampling, some plants use optical scan- ning to continuously sample pellets before they enter extruding equipment. This sampling is beneficial because contaminated pellets are re- jected before being extruded into the cable.
Resin suppliers now employ online pellet in- spection devices. Some manufacturers inspect 100 percent of their product. From this, a new generation of XLPE and TR-XLPE materials that bear designations of extra clean, ultra clean, or
super clean has emerged. However, a precise definition of each designation based on per-unit volume contamination is not available, nor is a comparison between compound manufacturers. The opaque nature of EPR does not permit a similar determination of cleanliness.
Cable manufacturers, in turn, have implemented materials-handling systems to prevent contamina- tion during the course of manufacture. For ex- ample, Class 1000 clean rooms have been in- stalled in most cable manufacturing plants and separate handling facilities for insulation and semiconductor materials have been implemented.
Supersmooth semiconducting shields were first introduced in 1988, resulting from better dispersion of the acetylene carbon black in the polymer base. Better dispersed semiconducting shields provide for a much smoother interface between the insulation and the shields, leading to much longer service life. Utility acceptance of the cleaner and smoother compounds has been rapid, as most utilities specified these materials in 2004.
Polyethylene manufacturers have focused on material purity, improvement in the compound- ing and process design, and quality assurance and quality control improvements. In addition, delivery systems have dramatically improved over the past 15 years. Using dedicated reactors, upgrading reactor clean out and defouling proce- dures, and monitoring each run for ambers and gels have improved manufacturing technology. Increasing the raw material cleanliness, filtrating all process air and water, and operating under a sealed loop strategy have helped to ensure a better product. In addition, handling systems now use gravity feed and dense phase, as well as dedicated, sealed rail cars in good condition.
Polyethylene is manufactured by compound suppliers and shipped in pellet form to the cable manufacturers for extrusion onto the full-sized cable. Contamination is possible at any step along the way. Most manufacturers carry out op- tical pellet inspection, but usually only about two percent of the total amount of compound is inspected. Needless to say, many contaminants are missed, as recent statistics suggest that even the cleanest compound can contain contaminants above 12 mils, and these may be removed with
100 percent pellet inspection. Ideally, the pellet inspection should take place as close to the manufacturer’s extruder head as possible and not contribute to further contamination.
Currently, pellet inspection devices are avail- able for use at the cable manufacturer’s plant. The inspection devices remove loose contami- nants and surface contaminants as well as pel- lets containing embedded contaminants. All models come with a self-enclosed air filtration system that provides a Class 1000 environment under a plastic curtain surrounding the unit. Re- moval of contaminants starts at three mils and optimizes at 12 mils.
Inspection of EPR is more difficult, as the ma- terial is opaque. Tape inspection devices can also be used for surface inspection of extruded EPR sample tapes.
Also available are inspection devices for gels in polymers and for small defects in interfaces. Although interfacial inspection does not occur until after the cable is manufactured, this latter device does provide an opportunity to identify, locate, and remove interfacial problems before shipment.
Although inspection for contaminants is im- portant, it is also important to eliminate all possi- ble sources of contamination during the manu- facturing of not only the insulation system but also the conductor and insulation shields. This means controlling the contact of possible conta- minants, especially airborne dust particles, to raw insulation materials or to the cable during extrusion. Materials should be exposed as little as possible to the ambient air in the plant. In ad- dition, cable interface surfaces should, similarly, have minimum possible exposure to an uncon- trolled environment during the extrusion process.
EXTRUSION AND CURING PROCESSES
During cable manufacture, the various shields and insulating layers are extruded over the con- ductor. The raw material is melted and the liq- uid polymer is pumped into a die that applies a continuous and uniform layer around the con- ductor. The material is then cured at the proper temperature for the proper time. This process is repeated for various layers until the desired cable configuration is achieved.
Expediency and quality in cable manufacture can be achieved if the extrusion of different lay- ers is performed simultaneously. The industry uses multiple simultaneous extrusion processes. Figure 2.7 shows the general layout of a cable extrusion line. The conductor enters the process from the pay-off reel. The conductor first passes through the extrusion heads, where the shields and insulation are applied. The cable then en- ters the curing tube, where the extruded poly- mers are cured at a temperature between 218°C (425°F) and 293°C (560°F). Pressure in the cur- ing tubes is also maintained between 150 and 300 psi. This temperature and pressure is main- tained long enough for cross-linking to take place in the insulation and/or shields. After curing, the cable enters a cooling zone, com- monly referred to as a water bath or quenching. However, some new production lines use dry gas cooling.
The methods used to cure and cool the cable during manufacture are the subject of much re- search. Older systems used high-pressure steam for curing, which led to higher water content (5,000 parts per million) within the insulation. It is suspected that this insulation water content may contribute to the development of water
Extrusion Area – Conductor Shield – Insulation – Insulation Shield Curing Tube Water Cooling Take-Up Reel Pay-Off Reel Insulated Cable Ba re Cond uctor
FIGURE 2.7: General Layout of a Cable Extrusion Line.
trees within polyethylene. Some newer equip- ment uses dry nitrogen as a heat transfer agent in the curing tube, which eliminates insulation contact with water until it has solidified. The re- sult is lower water content (200 ppm) in the in- sulation. The few cable production lines that use dry gas for both curing and cooling achieve even lower water content (50 ppm). The signifi- cance of the lower water content is still the sub- ject of continuing investigation. It is believed that the very lowest water contents are main- tained in service only if the cables are com- pletely sealed from moisture. However, the industry has widely accepted the desirability of dry nitrogen gas curing, especially for poly- ethylene-based cables.
Steam curing is the oldest cross-linking or vul- canizing method employed in any continuous vulcanizing (CV) plant. In steam curing, the freshly extruded cable passes down the center of a long vulcanizing tube filled with saturated steam at about 20 atmospheres (300 pounds per square inch gauge (psig)) pressure and temperature of about 215°C (419°F). The cured insulation is then cooled under pressure by cold water. Most EPRs are still made with steam curing in a CV catenary process.
Dry curing, on the other hand, consists of an electri- cally heated tube filled with high-purity nitrogen gas at about 10 atmospheres (150 psig) pressure. The infrared energy emitted by the hot tubes is transferred to the
cable components. The cable surface tempera- ture can be as high as 300°C (572°F). The cured cable is cooled by passing through a cooling section containing water under the same pres- sure as the curing section to prevent void forma- tion in the insulation. A dry cured insulation contains voids in the order of 100/mm3, 1 to 10 µm (micrometers) in size, whereas steam curing generates voids of 105/mm3, 1 to 50 µm in size. Sixty percent of the investor-owned utilities now specify dry curing. Of the remainder, 33 percent
FIGURE 2.8: Typical Extrusion Methods.
are EPR users who gain little advantage in the dry cure tech- nology. Most utilities that spec- ify EPR insulation request steam curing or do not specify a curing method at all.
For UD cable production, the triple extrusion and the dry cure technology with the catenary arrangement is most common.
Extrusion heads are continuing to evolve. The simplest head configuration is the two-pass (or dual-tandem) process shown in Figure 2.8(a). A disadvantage of this arrangement is the open space between the application point for the con- ductor shield and that for the insulation. The conductor shield surface can be contaminated by airborne particles. In addition, the cable must be returned to a separate extrusion line