Overhead Structures .1 Asset Description

Overhead structures comprise poles and towers used to support overhead lines and equipment in the transmission and distribution networks. They form the largest group of network assets, numbering over 790,000 individual assets. Poles are categorised and managed by design type (pole or tower), material type (wood, concrete, metal) and network type (transmission or distribution). Stay wires, which brace the poles, and cross arms and insulators, which support the conductors, as well as foundations and earthing are included as part of the structure.

The expected service life of structures (and extended life for wood poles) and population statistics are presented in Table 7.35.

Table 7.35: Overhead structure population statistics and service lives Transmission Distribution

Structure type

Life (years)

Population Life (years)

Population

Wood pole 45 - 55 30,485 35 - 50 628,891

Concrete pole 55 1,043 55 11,628

Metal pole 55 4,846 55 1,250

Transmission Distribution Structure type

Life (years)

Population Life (years)

7.8.2 Failure modes and impact

Structure (pole) failures can have significant consequences for network safety and performance. Poles that fall or lean excessively can result in live conductors coming in contact with other conductors, vegetation, people, buildings or vehicles. Such failures generally result in network outages. In some cases failures impact property and/or the public where the protection system has failed. In these instances where protection has not operated there is a risk of live conductors falling to the ground with the potential for electric shock and fire start.

Pole tops and insulators can fail by physically breaking in which case the impact is similar to structure or pole failure. Insulators can also fail resulting in leaking current which can result in pole and ground fires. The consequences can be significant particularly in areas of high bush fire risk.

7.8.3 Age and condition

Age profiles for selected pole types are shown in Figure 7.17 and Figure 7.18. Wood poles are a key focus as a large number of distribution poles are operating beyond their expected 40 year service life and further significant volumes are nearing their expected service life.

0

NUMBER OF DISTRIBUTION WOOD POLES

0

NUMBER OF TRANSMISSION WOOD POLES

Distribution Transmission

Figure 7.17: Age profile of distribution and transmission wood poles

Whilst a small number of metal streetlight poles are operating beyond their 45 year design life, another 5,000 poles are 35-40 years old and will reach or exceed their design life in the next 5 – 10 years.

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000

0-5 6-10

11-15 16-20

21-25 26-30

31-35 36-40

41-45

46-50 51+ AGE [YEARS]

NUMBER OF STREETLIGHT POLES

Figure 7.18: Age profile of metal streetlight poles

The bulk of the concrete pole population and lattice towers are well below their expected service lives.

7.8.4 Performance Level

The performance of structures impact directly on supply reliability, network security and network safety outcomes. Reliability and security requirements are established through the Access Arrangement and the Electricity Industry (Network Quality and Reliability of Supply) Code 2005.

Safety requirements are established through several legislative instruments and also through a specific order (001-2009) in relation to wood poles issued by EnergySafety.

The primary performance indicator used to manage wood poles is the Pole Integrity Index (PII) which equals the number of unassisted pole failures per annum per 10,000 poles. This parameter is measured separately for the transmission (TPII) and distribution (DPII) pole assets and the targets are set at levels provided by national industry benchmarks. Performance against these targets is shown in Table 7.36.

Table 7.36: Wood pole performance against targets

Indicator Target Performance

2010/11

Performance gap

TPII

(Transmission)

1.0 5.69 4.69

DPII (Distribution) 1.43 (1.0) 1.22 No gap

Within the last 12 months, both the TPII and DPII have reversed their downward trend and are now tracking upwards. The latest TPII is well above the target, and the DPII is above the industry benchmark and nearing the target.

Overhead structures other than wood poles do not have specific performance targets set or indicators measured, but are managed according to results collected during regular inspection and maintenance programs.

7.8.5 Asset management strategies

Poles have a limited life span and can fail for a variety of reasons. The strategies for overhead structures are designed to enable poles to deliver the required functions and levels of performance in a sustainable way, at optimum whole of life cost without compromising health, safety, or environmental performance. This will be achieved by an increase in pole replacement and reinforcement rates to achieve a sustainable pole replacement rate over a ten to fifteen year period.

Asset maintenance

Routine, preventative and corrective maintenance activities are carried on overhead structures to maintain their service integrity. This is achieved through the regular inspection of these assets over their life time.

7.8.6 Overview of plan

A summary of the strategies used to manage Overhead Structures is shown in Table 7.37.

Table 7.37: Summary of strategies

Issue Strategy Planned Outcomes

Deteriorating condition of wood poles: The recent replacement rate of wood poles shows it is insufficient to address the portion of pole population identified as unserviceable.

Increase pole replacement and reinforcement involving:

Replace sufficient distribution poles to continue to clear the condemned pole population, to reduce the DPII to better than national industry benchmark of 1.0 in accordance with the EnergySafety Order.

Replace and/or reinforce additional distribution and transmission poles to achieve sustainable pole

management over the next 15 years (by end of AA5).

Increase wood pole replacement and

reinforcement rate over the 2012/13 to 2016/17 period in accordance with the

overhead structure volumes below.

Pole top fires: A number of pole top fires have occurred as a result of electrical tracking across insulators or cross arm failure.

Replace deteriorated cross arms (and insulators) in high fire risk areas with steel cross arms.

Apply silicon to the insulators to prevent electrical tracking.

Apply silicon to 10,000 pole top structures per annum in the period 2009/10 to 2011/12 and increase to 15,000 per year from 2012/13.

Non-compliant distribution metal poles: Metal poles are non-compliant with AS7000 step and touch potential requirements and pose a potential public safety hazard.

Re-build the Kambalda West overhead network which will remove 550 metal distribution poles.

To complete over the 2012/13 and 2014/15 period.

Stay wires underrated: 8,402 stay wires are known to be

underrated, providing insufficient support.

Replace stay wires based on priority.

75% replaced by the end of June 2017.

Stay wire insulators missing:

3,246 stay wires are lacking insulators which poses a safety hazard should a stay wire fail.

Retrofit insulators. 78% retrofitted by end of June 2017.

Issue Strategy Planned Outcomes 870 transformer poles have

been identified as having insufficient reinforcement.

Replace transformer poles. All replaced by the end of June 2017.

140 PI structures have been identified as having a span length exceeding 350m, the wood pole PI structure design limit.

Replace all PI structures with a span of over 350m.

Complete within 2012/13 to 2016/17 period.

Table 7.38 summarises the planned treatment (replacement/

reinforcement) volumes for Overhead Structures. These volumes include the replacements planned to rectify identified key issues as well as any replacements resulting from the inspection program.

Table 7.38: Overhead structure treatment volumes

7.9 Circuit Breakers