Utilities are installing more and more electric power circuits underground. They do this for many different reasons: to meet the requirements of regulatory agencies, to avoid changing the landscape or simply because there is no room for an overhead line. Fortunately, with today's materials and manufacturing techniques, a properly selected and installed underground cable system is expected to operate reliably for decades.
Naturally, cable is a major component of an underground system, and the insulating compound used in a cable construction is critical to its performance. Choosing an insulation compound may appear easy; however, upon closer examination, one generally finds it is a protracted, difficult process. Issues include:
Insulation Type. At first glance, there are two basic choices: tree retardant crosslinked polyethylene (TRXLPE) or ethylene propylene rubber (EPR). Closer examination, however, shows several choices exist within each of these categories. (Note: Cables can be constructed with standard XLPE, but that insulation compound is used only occasionally today. In the past, XLPE performance on many utility systems was demonstrably inferior to other compound types.)
The selection of TRXLPE or EPR is a highly debated and controversial subject. The same is true for the subchoices within each category. The following discussion provides basic information about the two fundamental compound types (categories) while avoiding the commercial controversy involved with TRXLPE, EPR or the choices within each category.
Insulation Thickness. Each voltage class features two thickness choices, 100% and 133%. The 133% (thicker) level is suggested for some delta systems and for situations that require longer life. However, more insulation costs more money. Is it worth the extra cost? That decision varies from utility to utility. However, most utilities choose the 100% thickness because they believe it provides the best overall value (reasonable life at a reasonable cost). The issues related to thickness are the same for both EPR and TRXLPE.
Flexibility/Ruggedness. Each insulation compound type has advantages and disadvantages. Cables with EPR insulation generally are more flexible than cables insulated with TRXLPE. The degree of flexibility also depends on cable size, conductor type (solid or stranded), conductor material (copper or aluminum), metallic shield type and jacket type.
EPR insulation is softer than TRXLPE insulations, and some utilities have reported that EPR-insulated cables require slightly more care during installation than TRXLPE-insulated cable to avoid damage. In all cases, these subtle differences are considered secondary not primary performance characteristics.
For many years, only one TRXLPE insulation compound was commercially available. The HFDA 4202 compound, produced by Union Carbide, became available in the early 1980s and has a good field performance record. Today, there are more commercially available TRXLPE insulation compounds to chose from, including:
Union Carbide HFDA 4202. Introduced in the 1980s, millions of feet are in service with a good performance record. However, it soon will be phased out and replaced with HFDB 4202.
Union Carbide HFDB 4202. Introduced in 1997 as a “new and improved” version of HFDA 4202, this compound is said to be similar to HFDA 4202 but with better extrusion characteristics. Many if not most users of HFDA 4202 have switched to HFDB 4202.
Nova Borealis LE 4212 is new to the market with very little cable in service. Accelerated water treeing tests indicate a performance similar to HFDA 4202.
Pirelli PTR is new to the marketplace with very little cable in service and may not be commercially available at this time. Accelerated water treeing tests indicate a performance similar to HFDA 4202.
AT Plastics AT 320 TR is new to the marketplace with small amounts in service. Accelerated water treeing tests indicate a performance somewhat different from HFDA 4202.
Union Carbide HFDA 8202 is new to the marketplace. No cable is currently in service but should be commercially available in the near future. In addition, Union Carbide HFDA 8202 uses new polymer technology. Cables made with this insulation compound are more flexible than cables made with traditional TRXLPE insulation compounds. Accelerated tests are currently underway.
In general, the cleanliness of TRXLPE insulation compounds has improved significantly over the last 10 to 15 years. Cleaner raw materials, improved manufacturing techniques and improved handling techniques have all contributed to these improvements. It is difficult to pick one of these insulations over the other. Cable users must review field data and accelerated laboratory test data to make a decision based on their interpretation of the results.
Three basic types of medium-voltage EPR insulation materials are available on the market. Some have undergone changes over the years, but the changes are not considered dramatic new developments. Each EPR compound manufacturer uses ingredients, processes and other individualized technology that are promoted as superior features. Millons of feet of EPR insulated cable are in service with a good performance record. Over the years, the cleanliness and dispersion of the ingredients in the EPR compounds have improved significantly. The known commercially available EPR insulation materials are outlined as follows:
3728. This compound is used by a variety of cable manufacturers — mostly by those who do not make their own EPR insulation compound. Formulation was developed by DuPont and was sold for years by the Schulman Co. using a DuPont base crystalline polymer.
DuPont no longer supplies the base polymer. Electric Cable Co. (ECC) supplies the same 3728 insulation compound formulation, using a base polymer made by Exxon or Uniroyal. The new base polymer is very similar to the original resin made by DuPont. Most if not all manufacturers that use the 3728 compound have conducted cable qualification tests on 3728 formulation made by ECC.
Pirelli EPR, BICC EPR. Both cable manufacturers make their own EPR insulation compound. The compound formulation is quite similar to the 3728 formulation. Each manufacturer uses ingredients and processing techniques that may yield slightly different compound characteristics from each other and from the 3728 formulation produced by ECC.
Okonite Okoguard. This proprietary compound has been available exclusively from the Okonite Co. since the 1970s. The compound has a good field performance record and now carries an MV 105 rating, indicating that the cable insulation is adequate for operation at a 105°C (221°F) conductor temperature continuously with an emergency temperature rating of 140°C (284°F).
Kerite. Like Okoguard, this proprietary compound has been available exclusively from the Kerite Co. since the 1970s and has a good field performance record. Unlike all other cables, Kerite cables are not designed to be corona free. Thus, Kerite insulation compound is designed to be corona resistant. Original Kerite designs used a high dielectric constant “Permashield” material for both the conductor and insulation shields. However, most of their medium-voltage distribution cables now are made with a conductive polymeric insulation shield.
TRXLPE insulation compounds deform more readily at temperatures above 100°C (212°F) than EPR compound. This means that some accessories on TRXLPE-insulated cable might cause more deformation on TRXLPE-insulated cable than on EPR-insulated cable. However, tests have shown that well-made accessories will perform acceptably on both cable types at conductor temperatures up to 140°C (284°F).
A cable's ability to perform adequately at elevated temperatures is more a function of the cable design and operating conditions than the insulation material. The key is to be sure that accessories will perform adequately under the operating conditions and environment expected by a utility.
Cable life is one of the more arguable issues for EPR and TRXLPE insulation compounds. Although performance is variable, most (if not all) of the insulation compounds within each basic compound type will perform adequately in one or more accelerated cable-aging tests. However, cable life is affected by many different factors including the operating environment and the cable design.
Another issue to consider is dielectric losses. EPR insulation compounds have a higher dielectric loss than TRXLPE insulation compounds. The difference depends on the operating voltage, the specific type of EPR insulation compound, the specific type of TRXLPE insulation compound and the operating temperature.
Continuing technical developments in the manufacture of polymers for wire and cable applications have resulted in the ability to polymerize (chemically join) ethylene (the basic chemical component in XLPE, TRXLPE and EPR insulations) with chemical components other than just conventional propylene. This technology gives rise to the chemical designation EAM, Ethylene Alkene Copolymer.
In the future, cable manufacturers may supply a filled or unfilled EAM compound where specifications now require a thermoset material such as XLPE, TRXLPE or EPR. Utilities should be aware that basic cable insulation compound terminology is likely to change over the next 10 to 15 years, along with the insulation compounds.
The Bottom Line
At the end of the day, it's your choice. The good news is you are likely to get good performance from most of the insulation compounds discussed above. Other vendors in the global marketplace make suitable insulating compounds with formulations similar to those mentioned above. Remember to select an insulation and cable design for the application at hand. It is critical that the cable is installed and operated properly. The cable maker also must use good-quality manufacturing practices, so it is advisable to monitor the cable quality by conducting periodic quality-assurance tests. Selecting which cable to use is a difficult process. You have to dig through issues, review data and make the best choice for your application. The effort, however, is worthwhile. Utilities and their customers both will benefit from a properly designed and maintained underground power system.
Rick Hartlein is currently program manager for Underground Systems at NEETRAC, a transmission and distribution testing center at Georgia Tech, Atlanta, Georgia, U.S. He previously worked for 25 years in various positions at the Georgia Power Research Center conducting research and test programs on underground transmission and distribution systems. Hartlein is the immediate past chair of the IEEE Insulated Conductors Committee and serves as a technical consultant to the AEIC Cable Engineering Committee.