EoW March 2012

Technical article Quality materials can improve reliability of distribution cables

By Peter Pang, Shawn Miao, Simon Leung and Simon Sutton, of Dow Electrical & Telecommunications, a business unit of the Dow Chemical Company

Introduction Not all materials are created equal. It’s a simple but true concept that needs to be considered and embraced throughout the value chain. Performance is essential to the overall functionality of the grid and can only be delivered by materials that undergo thorough testing. Specifiers who require distribution cables that will deliver decades of reliable electrical performance must stay abreast of material developments. Choosing materials for any reason other than performance, such as price, availability or regional origin, could result in early and frequent failures. This means additional time and expense required for repair and/or replacement as well as a decrease in consumer confidence. Medium voltage cables using TR-XLPE materials have been shown to maintain excellent electrical properties, such as ACBD strength, after many years of service, thus guaranteeing long life and improving reliability for distribution grids. which can inherently retard water trees, semi-conductive shield materials with high cleanliness have also been shown to improve electrical properties of cables aged under laboratory conditions. To achieve long-term success, it is recommended that cable makers and end-users (utilities) specify cables that are constructed with high-quality materials that meet specific test criteria. Installation should be done by experienced professionals who also run post-installation tests to ensure system-wide performance. Pricing can and should be evaluated by a life-cycle cost analysis. Besides insulation materials

Failures and prevention Early power cable failures are usually caused by defects that are not detected during factory testing and by poor workmanship associated with preparation and installation of cable joints and terminations. During the majority of the life of the cable system, failures tend to be caused by external factors such as accidental dig-ins. In the later stages, cables fail usually by aging of insulation materials. Typical late-stage failure mechanisms include initial formation of water trees that subsequently generate electrical trees, causing partial discharge, and cable failures.

The presence of water trees of various lengths in field-aged cables have been found in recent studies by Shanghai Municipal Electric Power Company of SGCC. The formation of water trees can be prevented by the inclusion of water barriers in the cable, like metal shields and filled conductor strands, or by the use of insulation and semi-conductive shield materials that have been shown to significantly slow water tree growth. This is seen in cables aged in actual field conditions or under laboratory accelerated aging conditions. Accelerated Water Treeing Test (AWTT) protocols such as those found in industrial

▼ ▼ Figure 1 : Comparing retention of dielectric strength of TR-XLPE and XLPE cables

(KV/min)

AC Breakdown Strength

Service Years

184

EuroWire – March 2012