Hydro-Québec TransÉnergie’s transmission system — with more than 34,000 km (21,127 miles) of overhead lines — was erected mainly in the 1960s and 1970s. As transmission line components age and degrade over time, strategic asset management becomes of paramount importance. Various local signs of aging can be detected visually, such as lightning damage, bird caging, wear and outer-strand breakage. Ground and air patrols are effective for visual detection, but close-range robotics can be advantageous in specific, more challenging instances.
For this reason, the inspection and maintenance robotics unit of Hydro-Québec’s research institute — Institut de recherche d’Hydro-Québec (IREQ) — has been active in developing and applying LineROVer, LineScout and drone technologies since 2000. Considering the internal damage that may occur in aluminum conductor steel-reinforced (ACSR) conductors, other solutions are required. Systematic conductor sampling and testing those samples is one solution, but the wise use of nondestructive testing (NDT) techniques should be considered to improve the inspection approach and generate significant savings.
Indeed, different technologies were selected and tested over the past five years at IREQ under a comprehensive research and development program addressing the key internal-aging mechanisms. Commercially available sensors, such as X-ray and magnetic flux detectors, were fitted onto portable, field-ready inspection systems. For some applications, however, new technologies had to be developed or improved. This was the case for LineCore, a compact, lightweight eddy current device that assesses loss of the protective zinc layer surrounding the steel core strands as a way to detect early signs of ACSR conductor corrosion.
It is important first to consider how the properties of ACSR conductors change over time in service, recognizing galvanic corrosion as the main degradation mechanism. In the presence of an electrolyte, such as saltwater or raindrops contaminated by air pollutants, the zinc on core strands will dissolve — or corrode — until it vanishes completely. Without this protective zinc layer, the steel/aluminum will start to corrode and ACSR conductor conductivity will decrease slightly.
This corrosion degradation is a continuous process occurring at an unknown rate. Environmental conditions are the most important predictive factors to consider in establishing a monitoring plan: precipitation, acidic rain, shorelines, seawater, highways, industrial areas and so forth. Conductor structure also is important because electrolytes can penetrate the aluminum and reach the galvanized steel core faster if the conductor has two layers of aluminum rather than three. Greased conductors are designed to maximize corrosion protection in locations where environmental conditions are harsher.
Once the zinc layer is reduced to a few microns in thickness and spots of bare steel appear, multiple degradation scenarios are possible. Typically, the aluminum strands will start to corrode. Steel strands may show some signs of local corrosion, and under severe conditions, the steel core may corrode entirely. It is crucial to determine this condition because it drastically lowers the strength of the ACSR conductor.
There are apparently two distinct rates in the loss of residual strength during the life of an ACSR conductor:
• A very slow phase with marginal loss in strength before complete loss of zinc
• A phase of accelerated steel/aluminum corrosion that may reduce the effective cross section and, thus, residual strength.
Evaluation of the pivotal point between these phases (T1) is key in predicting the end of life for a given conductor on a given span. From an asset management perspective, LineCore technology helps to predict when the pivotal point of total zinc loss will be reached.
LineCore technology is based on eddy current testing, which typically requires electromagnetic coils to be wrapped around the conductor. By inducting eddy currents in the ACSR structure, the measured response can be related directly to local thickness of the zinc layer, which screens the ferromagnetic steel strands. As zinc oxidizes and the coating thins, eddy currents penetrate more deeply into the steel base material and the LineCore response shifts proportionally.
The main design challenge was to build a sensing coil that could be opened into two halves to facilitate installation and removal but wound in such a way as to appear seamless. This makes it possible to cross obstacles, such as spacers or splice joints. With such a coil design and motorized opening mechanism, LineCore can inspect an entire span, including bundles of conductors, provided it is mounted on a system that can cross obstacles, something the IREQ-developed robotic platforms can do.
To save weight and optimize performance, a custom eddy current circuit board was designed. Key components were chosen to minimize energy consumption so that up to eight hours of continuous operation on a single battery charge is possible. A wheel-mounted encoder indicates the current location on the inspected span.
LineCore can be controlled remotely from the ground with a Wi-Fi transmitter and data downloaded from it to the ground station in real time. Opening and closing the head, starting and ending a scan, and all other operations are performed remotely by the ground operator using specialized software. Following a few years of field operation, the LineCore sensor head was improved to make it easier to manufacture and maintain, reduce sensitivity to temperature changes during inspections, simplify the interpretation of sensor readings and scan a larger range of conductor sizes, from 15 mm (0.6 inches) up to 40 mm (1.5 inches) in diameter.
The software is easy to operate and requires only a few simple steps to select the conductor type, calibrate the instrument and acquire data. Several views and tools are available to extract the information needed to assess the condition of the inspected span, with a resolution of a few millimeters. Once data is collected on a given span, the file can be sent to a more experienced user for further analysis and advanced reporting, if required.
The software categorizes the condition into one of four stages of the galvanic protection degradation:
• As new, no observed degradation
• Normal degradation, possibly with minor loss of zinc
• Partial loss of zinc but the conductor is still fit for continued service
• Severe or total loss of zinc, with incipient corrosion of the steel or aluminum strands.
Once the galvanic protection is entirely lost, an experienced analyst can determine the extent of degradation of the steel core and aluminum cross-sectional area, which can lead to two additional categories:
• Corrosion of the steel strands
• Corrosion of the aluminum strands.
A practical way to display the data is to color-map conductor condition along the span. Histograms are displayed for each span, indicating the percentage of data points falling into each of the six categories. The overall condition of a span then can be assessed and compared to other spans or inspection data sets.
If severe degradation is detected on specific sections of a line, then taking a sample at the weakest point (that is, the most degraded location measured with LineCore) will provide a complete set of properties: zinc coating thickness, degree of corrosion, electrical conductivity and mechanical strength. The condition of the entire line then can be assessed more precisely by comparing the destructive analysis results with the LineCore NDT inspection data.
As more and more spans are scanned with LineCore, in collaboration with utilities across the world, key field results are collected. Experience is gained, and the data compiled has proved to be very useful for asset management. In some cases, lines to be dismantled revealed how corrosive the environment was above highways; in other cases, very old greased conductors were found to provide a high level of protection against coastal salt air. In some areas, span-by-span histograms of old lines clearly built a case for complete conductor restringing; in other areas, degradation was limited to just a few spans.
The initial version of LineCore, the stand-alone LineCore (SAL), consists of a passive device with remote-controlled doors that can be positioned around the conductor. Once installed, the stand-alone LineCore is pulled along the conductor by a motorized trolley, such as Hydro-Québec’s LineROVer. Combining visual inspection using the robot’s camera and LineCore inspection data provides a complete assessment of the current state of the conductor. The visual inspection data can be archived for later review.
The LineCore sensor head also can be mounted on Hydro-Québec’s LineScout robot, which can clear obstacles along the line such as suspension clamps, spacers and vibration dampers. This makes it possible for LineCore to assess several successive spans or conductors in bundled configurations without removing any spacers.
Successful integration of LineCore into various robotic platforms and recent progress at IREQ’s inspection and maintenance robotics laboratory made possible a game-changing means of LineCore deployment: unmanned aerial vehicles (UAVs), or drones. IREQ has demonstrated that a drone can safely land on an energized conductor. Given the increased payload of commercial drones, it is feasible to mount a motorized LineCore beneath a customized drone, thus diversifying the means of deployment.
With minimal modifications to the sensor head, mainly to optimize its weight and connect its eddy current circuit board to the drone power supply, the motorized LineCore was mounted under the drone using basic supports acting both as guides and suspension. A separate antenna was added to communicate directly with the ground station computer for data acquisition and real-time analysis using the existing LineCore control software.
Using such an airborne system to bring a sensor into direct contact with the transmission line significantly accelerates and simplifies the conductor inspection process. Furthermore, as the motorized LineCore can move along the conductor using very little power, the inspection can cover many spans without recharging the battery, relying on the drone to fly over obstacles such as towers, clamps and sleeves. This means of deployment also makes it possible to move quickly from one phase conductor to another for detailed assessment of all conductors on strategic spans.
Because ACSR represents a fair fraction of the cost of transmission lines, evaluating the apparent age of an ACSR conductor is considered by several power transmission utilities a key driver for a proper life-cycle management program. By collecting otherwise unavailable data, NDT sensors support better decisions that optimize the costly replacement of overhead line conductors.
In service since 2013, LineCore has been deployed on six transmission systems located in three countries. It has been used to gather information to improve capital investment planning, ensure the reliability of strategic lines and quantify the condition of conductors otherwise targeted for replacement. It also has provided objective information for utilities to make optimal maintenance decisions. After deployment of this sensor using robotic solutions, feedback from linemen and end users has helped to produce an intuitive, easy-to-use device.
Wise use of LineCore data can minimize the need for time-consuming and costly sampling of conductors. Also, conductor sampling results in two additional conductor joints that typically weaken the transmission line: it only provides information about the sample location while other sections may be at a more advanced stage of degradation, and it provides no data about the subsequent rate of degradation. The most recent milestone in the deployment of LineCore and other sensing technologies is undoubtedly the development of drones that target very specific and challenging missions.
The authors would like to thank all of the LineCore project team members for their kind cooperation, and specifically Silvia Cecco, LineCore software engineer, and Matthieu Montfrond, drone project leader, for their continuous and dedicated collaboration. ♦
Eric Lavoie holds a BSME degree and an MS degree in robotics from Laval University. In 1992, he joined IREQ as a researcher and engineer. He has been working on designing and integrating robotic instruments and sensors for application in the electric utility industry. Lavoie has headed many projects in the areas of nuclear power, hydropower generation and T&D systems, some resulting in products now commercially available.
Gilles Rousseau holds a BS degree in engineering physics and an MS degree in metallurgy from Laval University. He worked for various nondestructive technology (NDT) manufacturers and for the Canadian NDT certification agency from 1984 to 2000. He joined Hydro-Québec in 2000 as the lead expert in reactor component inspection for the Gentilly-2 nuclear power plant, where he developed several key NDT solutions for CANDU reactor life extension. He joined IREQ as a researcher in 2013 and focuses on applying electromagnetic techniques for power transmission asset management.
Nicolas Pouliot is a mechanical engineer with an MS degree in robotics and mechanism design. After an initial five years in the aerospace industry, he joined IREQ in 2002. As a research and development project leader in the inspection and maintenance robotics unit, he is responsible for developing, testing and deploying various solutions for challenging power line inspections, including LineScout and several nondestructive testing (NDT) technologies.
Serge Montambault holds a BSME degree and a PhD degree in robotics from Laval University. In 1997, he joined IREQ as a researcher. In 2013, he was appointed manager of expertise for the inspection and maintenance robotics unit.
Editor’s note: A video of this drone demonstration can be seen at https://www.youtube.com/watch?v=z3Hh_AlYJ3o&feature=youtu.be
Check out the February 2018 issue for more articles, news and commentary.