Electricity Asset Management Plan 2019-2029
178
Vector Limited://
PROPOSED INVESTMENT SUMMARY ($MILLION NOMINAL)
DESCRIPTION
FY20
FY21
FY22
FY23
FY24
FY25
FY26
FY27
FY28
FY29 TOTAL
CBARM
0.50
0.50
Total CAPEX
0.50
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.50
5.2.1 SUBTRANSMISSION FEEDERS While subtransmission feeders currently make up a relatively small proportion of network interruptions, failure of a subtransmission feeder can result in a large impact to SAIDI, particularly for underground circuits with low physical accessibility. It is therefore important to closely monitor the condition of subtransmission feeders and maintain risk profiles to an acceptable level.
The following section describes the project and investment proposals for subtransmission cables.
SUBTRANSMISSION CABLE REPLACEMENT PROGRAMME
NEEDS STATEMENT Vector’s subtransmission cable population is ageing but presently with a consistent trend of failures. Notwithstanding that the failure rate is not presently showing an increasing trend, in terms of design life, certain types of subtransmission cables especially PILC cables, are nearing the third part of the ‘bathtub curve’ namely, the wear out failure period where if enough units from a population of assets remain in use long enough, the failure rate begins to increase as materials wear out and degradation failures occur at an ever increasing rate. We are experiencing degradation failures and the concern is that with a population of similar age profiles approaching the end of design life that a sudden deterioration of subtransmission cables could result in loss of supply to large areas. In many instances, the two or three subtransmission cables that run to a zone substation are of the same age and hence the risk of concurrent failure is real. If one subtransmission cable to a zone substation should fail we will in many instances be able to backstop the shortfall in supply capacity via the 11kV distribution networks from neighbouring zone substations. However, if both or more subtransmission cables should fail there will likely be large and lengthy outages. Our XLPE fleet of subtransmission cables have the largest number of cable faults but is also the largest population of this asset. Vector also has a large number of ageing oil filled cables of similar age but the construction of this type of cable is such that they can be retained in service for a number of years to come under Vector’s existing maintenance regime (maintenance requirements are described in Vector standard ESM301 Maintenance of Cables). The population most at risk is our PILC subtransmission cable fleet where there is real concern that this part of the fleet is entering the wear out failure period of the bathtub curve. For certain subtransmission cables it is not the health of the cable(s) that is a concern but their location: over time with the development of new road corridors certain subtransmission cables or portions of cables can only be accessed with difficulty or not at all. In some instances, excavations of up to 4m deep are required to reach cables – this results in very lengthy outages during which time the supply to large areas of network is at risk. Such cables or portions of cables must be replaced.
TARGETED OUTCOMES
CUSTOMER EXPERIENCE
SAFETY
RELIABILITY
RESILIENCE
OPERATIONAL EFFICIENCY
CYBER SECURITY AND PRIVACY
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