Corrosion Surveying of Live Transmission lines - ATTAR offer a Non-Destructive, Overhead Line Corrosion Detection Services for the comparative assessment of zinc loss from ACSR/GZ and aluminium cladding loss from the ASCR/AC steel core of the conductors. This system can detect corrosion often well before it is visible or detectable by infra-red methods. The equipment is placed on the “live” conductor by a small team of linesmen and then operated remotely. The equipment can be operated on lines with voltages over 500kV
Overhead Line Transmission Corrosion Testing
Internal corrosion is probably the major factor in limiting the life of steel reinforced aluminium conductors. Early detection on live lines with Non-Destructive methods will give early warning of deteriorating conditions thereby enabling the asset manager to implement cost effective maintenance and replacement programs to extend service life. Transmission line corrosion surveys offer signiﬁcant data to aid an Asset Manager in making safe and cost effective decisions in a risk based program; measuring and monitoring the extent of corrosion in the steel reinforcing enables an assessment of entire remaining service life of a conductor. Having worked with asset owners in Australia and New Zealand for over 23 years, ATTAR has optimised systems with high accuracy to determine the extent of corrosion on transmission lines [ACSR (AC & Gz)], assisting transmission companies to assess the condition of their High Voltage Power Transmission networks. Zinc loss from the central steel core is detected in ACSR/GZ conductors with the additional beneﬁt of determining Aluminium cladding loss from the strands in the steel core of ACSR/AC conductor.
- ATTAR’s equipment is placed on the “live” conductor by a small team of linesmen and then operated remotely.
- The equipment can be operated on lines with voltages over 500kV
- No need for costly and/or complicated line outages.
This Non-Destructive Testing (NDT) method is suitable for live lines up to 550kV, with access being obtained from insulated elevating work platforms, helicopters, rope access or other safe methods. ATTAR’s TLC device is placed on the conductor and is remotely controlled and driven along the cable.
Corrosion of conductor lines is accelerated by the effects of local environment conditions. This means that a 50-year-old line may still be in the ﬁrst stage of its life cycle, while another line, of the same age, could be at the end of its life. Corrosion is caused by local environmental factors such as industrial pollution or salts from a marine or local inland source. Local inﬂuences from industrial sites may include contaminants such as grease, localised heating or harsh atmospheric pollutants. But these contributing corrosion factors cannot necessarily be generalised for a similar location as the direction of prevailing winds and low-lying terrain may contribute. Once the galvanising or Aluminium cladding on the steel structural core has been preferentially corroded, galvanic corrosion theory tells us that the Aluminium conductor immediately adjacent to the steel core will then be subject to corrosion. This corrosion model tends to be the predominant one, with the innermost strands of the Aluminium conductor of the cable being the ﬁrst attacked. When this process generates sufﬁcient corrosion product [including Al(OH)3 and Al2O3], it can become visible as bulges and /or white powder patches on the surface.
Micro environmental conditions within the cable may allow the steel to continue corroding with the pitting and loss of section of the central structurally supporting core, compromising its strength well before the corrosion is visible on the surface of the conductor. This mechanism may not signiﬁcantly affect the power transmission capacity of the cable and therefore, does not generate local heating, which may be detectable with other inspection methods. ATTAR’s team of Materials Engineers have considerable experience in testing overhead power lines and understanding the corrosion mechanisms involved; they are available to work with Line Engineers to identify environments and microenvironments which instigate corrosion in particular regions, and thus spans which have the greatest risk of reaching end of life, and target/prioritise them for assessment.
Optimised Asset Management
End of life can be considered where the galvanising or aluminium cladding on the steel structural core has been consumed from the steel surface. Early detection of the percentage loss of the galvanising or aluminium cladding sacriﬁcial coatings gives warning of deteriorating conductors before the end of life stage where the steel is unprotected and risk of failure becomes high. The percentage loss of Zinc or Aluminium cladding from the central steel core is detected with Transmission Line Corrosion Surveying; ATTAR can monitor 20% corrosion losses of sacriﬁcial coating, which, looking at a relatively large percentage of the line, enables more statistically valid life prediction to an end of life condition i.e. where no protection of the structural steel core dramatically increases risk of failure as well as loss of Al conductor capacity. Once the galvanising or aluminium cladding on the steel structural core has been consumed, the aluminium conductor immediately adjacent to the steel core should start to corrode, which can result in a large loss of aluminium and conductivity, while the structural steel remains protected.
Advantages to the Asset Manager
- Transmission Reliability & Availability
- Reduce or avoid unplanned outages on transmission lines due to equipment condition
- No line outages required during inspection
- Transmission Asset Data Adequacy and Availability
- Information on asset condition is accurate and readily accessible
- Estimate the entire remaining service life of a conductor
- Priority is given to critical assets at greatest risk of failure
- Risks can be managed: assets may be proactively maintained, and capital expenditure planned
- Maintenance is reliability-centred and condition-based
- Reliability is optimised at least life cycle cost ATTAR’s Transmission Line Corrosion Surveys, which assess live lines, will give more statistically valid data as many spans/ lines can be surveyed in a day, offering a very cost effective and comprehensive alternative to the traditional process of cutting and assessing a limited number of short sections of spans, requiring line outages.
Reports are representative of the line and easy to read. The condition of the conductor is colour coded in terms of remaining zinc (or aluminium cladding) and each span tested is reported with a colour-coded condition versus distance bar graph.
Accuracy of the instrument allows individual condition assessments to be made down to centimetres, ensuring that localised lengths of corroded conductor are identified.
Comparison with other methods
Traditional cut outs involving line outage, high labour costs, and specialised equipment is needed; however, this method offers a statistically small sample size, and therefore may not be representative of the true condition of a whole span (and consequently, may miss ‘holidays’).
Other methods, conﬁned in their estimation to the loss of mass in the steel core, cannot ascertain the remaining service life of a conductor until the end of a conductor’s life is reached. General loss of steel core is rarely the failure mechanism; rather, very localised pitting and crevice corrosion can initiate multi-strand cracking and failure. Note: single, broken steel strands may be detected in relatively good conductor.
ATTAR’s method of Transmission Line Corrosion Surveying ensures that very short lengths of corroded conductor will be detected, regardless of whether the corrosion is localised or general; traditional conductor sampling may well miss these corroded sections, giving a false assessment of the span.
David Padfield - Senior Materials and Testing Engineer
Daniel King - Materials and Testing Engineer