TB 797 Sheath bonding systems of AC transmission cables - Design, testing & maintenance
Sheath bonding is required for all cable systems to ensure an effective bond to earth of the cable system metallic sheath, armouring, and semi-conductive outer sheath covering. A sheath bonding system is used to protect the sheath insulation of the various cable components against normal induced operating voltages and transient over-voltages from lightning, switching, and fault surges.
Correct application is critical to reduce failures and safety incidents as well as improve overall performance.
Incorrectly performing sheath bonding systems may lead to cable system failures or pose a human safety risk. In addition, the bonding and earthing is used to eliminate or reduce sheath currents, which in turn will reduce losses and improve the current rating of the cable. CIGRE Working Group B1.50 was established to review design, testing and maintenance of the sheath bonding systems in light of recent developments and practice improvements in system designs, standards and regulations. The Working Group has now produced Technical Brochure (TB) 797. The Australian member for the WG was George Bucea and the Convenor of Australian Panel B1 is Russell Wheatland, who produced this article.
The basic information and requirements needed to design sheath bonding systems are included in several documents such as Electra 128-1990, TB 189-2001, TB 268-2005, TB 283-2005, TB 347-2008, TB 403-2010, TB 556-2013, and TB 680-2017. TB 797 provides a comprehensive explanation of the current practices in this field.
The different chapters discuss:
- An overview of the sheath bonding system functions and requirements through review of existing documents and other engineering information related to sheath bonding systems
- Service experience feedback received in terms of sheath bonding system configurations, schematics, standing voltages, and voltage withstand requirements
- The most commonly used sheath bonding system methods used for the design of AC cable systems (i.e., single point, multiple point (solid), and cross-bonding) and the challenges regarding the protection (insulation) of cable system sheaths
- Basic knowledge rules on voltage withstand requirements, current rating, and energy absorption for the selection and implementation of bonding leads, link boxes and sheath voltage limiters, depending on the cable system parameters, sheath bonding methods and earthing connection
- Guidelines for performing insulation co-ordination technical studies related to the design and voltage withstand requirements of sheath bonding systems, and the cable system models that can be used for computer software overvoltage calculations and simulations
- The design and protection of sheath bonding systems, describing the standard type of known sheath bonding system designs; calculation formulas, calculation formula references and examples for sheath induced voltages and circulating currents on sheath bonding systems, sheath voltage limiter selection and application guidelines
- Sheath bonding system overvoltage calculation models and software references for power frequency and transient network conditions
- The testing requirements of sheath bonding systems including type testing and commissioning testing tables and references for known testing standards or CIGRE TB’s.
- The need to undertake an insulation co-ordination study for each specific project to establish the sheath bonding system voltage withstand ratings required for both overvoltage and transient conditions.
- After installation tests
- Maintenance of the sheath bonding systems by providing recommendations on industry best practices for maintenance of sheath bonding systems (including sheath voltage limiters and testing criteria)
- Condition monitoring options for the sheath bonding systems
The Technical Brochure (TB) is available via e-cigre free for members and €220 for non members