WEL Networks | 2018 Asset Management Plan 142 ASSET REPLACEMENT AND RENEWAL 8 ASSET REPLACEMENT AND RENEWAL This chapter describes our renewal and maintenance approach for the AMP period. It details our planned renewals and maintenance work and how we have forecast the associated expenditure. 8.1. OVERVIEW OF ASSET REPLACEMENT AND RENEWAL Delivering our performance objectives, as described in Chapter 5, requires the right balance between expenditure on maintenance and investment in renewals. In striking this balance, we have considered the whole of life cost of our assets, and required interventions during their lifecycle. As established by our asset management framework, described in Chapter 3, we have taken a risk based approach to renewals with the implementation of Condition Based Risk Management (CBRM). All major asset groups are now contained within the CBRM model as shown in the list below: Key Asset Classes with CBRM Model 1. Sectionalisers and Reclosers 2. Network Switches 3. Battery and Power Supply Systems 4. Circuit Breakers 5. Distribution Transformers 6. 11kV Overhead Line Conductors 7. LV Pillars 8. Poles 9. Protection Relays 10. Ring Main Units 11. Crossarms and Insulators 12. Zone Transformers For assets such as HV fuses (DDOs) that don’t have a CBRM model, WEL uses information obtained from inspection and reliability tools such as Failure Mode Effects and Criticality Analysis (FMECA) to assess risks and prioritise the renewal programme. WEL uses SAIDI as one of the key measures of network reliability. Consequently it is a reliable indicator of the effectiveness of CBRM in managing network performance risk. SAIDI has been used to monitor risk trends following adoption of the CBRM methodology. There has been a steady reduction in the SAIDI impact attributed to equipment related failures as described in Chapter 5. Of most significance is the reduction in 16mm2 copper conductor failures. While the number of these failures is reducing, those that do occur are predominately due to storm events or bird line clashes. Although WEL is achieving positive results in SAIDI reductions in the equipment failure category, planned SAIDI has had to increase to enable the asset replacement program to be carried out. We have also seen an increase in planned SAIDI due to the introduction of the Electricity Engineers’ Association proposal for HV Live Line safe work practice. This has increased the amount of work on HV lines to be undertaken de-energised. The resulting maintenance works and renewal plans are described below.