Introduction: The following project address rating issues related to National Grid’s cable and overhead line assets.

A project has been initiated to improve the delivery of cable ratings calculations through the use of more flexible and accurate Finite Element Analysis (FEA) modelling methods. The project will demonstrate the advantages of finite element analysis of cable rating problems compared to the currently used finite difference method. Analyses performed by BICC (prior to their take over and closure) demonstrated the advantages of this method. FEA is the only readily available method for accurately modelling problems such as cable ratings and can be extended to include phenomena such as thermal dry out. The Auckland black-out demonstrated the need for utilities to understand and be able to model the ratings of their cable systems to avoid thermal runaway and consequent failures.

Objectives: Verification of the rating methods used for cable ratings under various laying conditions and considering cable joint rating methods. A review will be carried out using FEA methods to confirm or otherwise existing methods.

Rating methods employed in the design of both forced cooled and naturally ventilated cable tunnels. The objectives of this project are to:

• Review the ratings methods used to design cable tunnels

• Assess existing operational data from cable tunnels, including Distributed Temperature Sensor (DTS) data

• Develop a specification for a rating method for cable tunnels installations with independent cable circuits.

Probabilistic overhead line ratings: This project aims to review whether the level of risk that is currently accepted by National Grid’s ratings model is correct. This has not been reviewed for approximately 20 years and with improved instrumentation and measuring methodologies it is thought that the level of risk currently used may be shown to be conservative.