Benefits of TR-XLPE Cable

TR-XLPE Cable

INTRODUCTION

                  Utility companies worldwide are striving to reduce the life cycle costs of their medium voltage distribution systems in response to economic and environmental drives. The use of tree retardant XLPE insulation has allowed utilities to achieve long service life under severe operating conditions. This has led to improved life cycle economics and has minimized social and environmental issues resulting from cable replacement activities.

                   It was recognized that XLPE, as well as other polymers, undergo a degradation process, called water treeing, when exposed to moisture and an electrical stress. Two different approaches were used, at about the same time, to solve this problem. In North America, a novel additive formulation approach was used to impart water treeing resistance. The resulting product, called additive TR-XLPE or “TR-XLPE,” was introduced in the early 1980’s and has shown excellent field service performance. In Europe, blends of polyethylene with ethylene alkyl acrylate copolymers were used to impart resistance to water treeing degradation. This product, called “Copolymer XLPE”, was also introduced in the early 1980’s and has had excellent field service performance.

 DISCUSSION

                   Tree-retardant crosslinked polyethylene (TR-XLPE) was designed to overcome the water treeing deficiency of high molecular weight thermoplastic polyethylene and crosslinked polyethylene (XLPE). In addition to significantly retarding the growth of water trees, TR-XLPE was designed to maintain XLPE’s high dielectric strength and low electrical loss. Laboratory testing has consistently demonstrated the excellent resistance of TR-XLPE against degradation in wet electrical aging. Accelerated cable testing methods have further proven the performance enhancement of TR-XLPE in wet environments.

                  Evaluations of field aged cable continue to support the performance advantages of TRXLPE over other insulation compounds. Unjacketed high molecular weight thermoplastic polyethylene and cross-linked  polyethylene (XLPE) cables began failing prematurely with water treeing being associated with the cable failures.

 Water Tree Growth Patterns in XLPE

 Water Tree Growth Patterns in TR-XLPE

Figure 1

 Figure 2

                    Figure 1 highlights the water tree growth shapes of XLPE and TRXLPE in the laboratory test after aging 90 days at room temperature with the microphotographs being taken at 40X magnification. Figure 2 highlights the length of the water trees grown in XLPE and TR-XLPE in this laboratory test with days of aging. This lab test has now been accepted by the industry and adopted as ASTM D6097-97. As demonstrated by the water tree shapes in Figure 1, the TRXLPE grows smaller and constrained trees compared to conventional XLPE.Following the laboratory demonstration of improved water tree resistance, TR-XLPE demonstrated improved performance.

                    If Emphasis was placed on cleanliness and retention of electrical breakdown test after aging in water. Researchers found that blends of the polyethylene resin used in XLPE with copolymers, based on ethylene alkyl acrylate copolymers, resulted in improved resistance to electrical breakdown after aging in water under electrical stress. The resulting product, called “Copolymer XLPE” has also had excellent field service performance for MV cables.

CONCLUSIONS

                   It was recognized that XLPE, as well as other polymers, undergo a degradation process, called water treeing, when exposed to moisture and an electrical stress enhancement. Additive based “TR-XLPE” was introduced in the early 1980’s and has shown excellent field service performance. Multiple accelerated wet electrical tests have consistently demonstrated the improved retention of dielectric strength achievable with TR-XLPE over other insulation materials. These tests have led to TR-XLPE being the predominant insulation used for medium voltage underground distribution cables. “Copolymer XLPE” was introduced inthe early 1980’s and has also had excellent field service performance for medium voltage cables. The TR-XLPE technology used has been shown to have comparable or better performance than copolymer XLPE standard tests such that the growth of TR-XLPE insulation is expected.The expectation for improved cable life and reliability has led to significant interest in TR-XLPE to achieve these expectations. Usage of TR-XLPE is growing as performance-based tests have been implemented. As the long life performance expectations for medium voltage underground cable systems increase, there is an increasing motivation to use TR-XLPE as the insulation of choice to achieve these objectives.

REFERENCES

• Lawson, J.H. and Vahlstrom Jr., W. “Investigation of Insulation Deterioration in 15 kV Polyethylene Cables removed from Service, Part II.”
  IEEE Trans. PAS Vol. 92, March/April, 1973, pp. 824-831.
• Bahder, G., Katz, C., Lawson, J.H., and Vahlstrom Jr., W. “Electrical and Electromechanical Treeing Effects in Polyethylene and Crosslinked Polyethylene Cables.”                                      IEEE Trans. PAS Vol. 93, May/June 1974, pp. 977-986.
• Global Trends and Motivation Toward the Adoption of TR-XLPE Cable Authors: P.J. Caronia, A. Mendelsohn, L.H. Gross, J.B. Kjellqvist
  The Dow Chemical Company, 1 Riverview Drive, Somerset, N.J. 08873