Aluminum conductor steel reinforced (ACSR) conductor used in overhead electric power transmission lines is subject to corrosion from environmental exposure and degradation of protective coatings. Power transmission and distribution companies typically have conservative replacement criteria for their overhead conductors, replacing large sections of conductor well before its optimal service lifetime has been reached. Quantitative Fitness-for-Service assessment using the procedures and criteria of API 579-1 is not possible for transmission line conductor due to non-systematic influences of climate and power loading. The decision to replace conductors is usually based on:
- Age of the conductor
- Number of failures a particular section of line has experienced
- Visual inspection
This limited decision model may lead to unnecessarily replacing serviceable conductor. Quest Reliability offers the alternative of a condition assessment (centered on a damage evaluation) and remaining life prediction (specific to a damage mechanism) that provides a more objective basis for line replacement planning. The general model for remaining life determination is illustrated in the diagram below which recognizes several damage mechanisms, the state of which can be directly measured where measurement is possible, and some of which may be reasonably inferred from historical and current operating condition data.
Data used in this model includes the following:
Qualitative:
- Line environment
- Maintenance history
- Visual damage
- Corrosion products
- Corrosion distribution
Quantitative:
- Grease degradation
- Embrittlement tests
- Coating weight test
- Loss of cross section
- Mechanical strength
Acquisition of the above quantitative data types is problematic, because these data normally are obtained in destructive testing which typically involves unfavorable logistics (the line has to be taken out of service) and revolves around a necessarily small sample size. To overcome these constraints, Quest Reliability has developed an in-situ inspection tool (the “LineCrawler™”) that can detect pitting, fretting, surface damage and surface area loss to 0.5%, and will detect broken strands in the second aluminum layer. In addition, the system has the ability to detect localized defects in aluminum strands. This device, which is still in development, has been demonstrated to work in the laboratory, and is being upgraded and refitted for near term use in the field.
Until the LineCrawler™ is operational and can be used to collect comprehensive condition data on a transmission line’s conductor, the analysis is dependent on the qualitative data listed above, and limited destructive testing of line sections indicated by visual inspection. Using these data, Quest Reliability computes corrosion rates and grease degradation rates and then uses these to construct a life profile for each line segment, from which we can base a remaining life prediction. In addition to this critical piece of information, these analyses will also yield the following insightful findings:
- Breaking load predictions
- Environment ranking
- Estimates of time to conductor swelling
- Statistical analysis
For further information about Power Transmission Lines, please contact us.