* montambault.serge@ireq.ca

Robotics applied to power line inspection and maintenance: Hydro-Québec’s experience and future applications

S. MONTAMBAULT* N. POULIOT R. DANSEREAU Hydro-Québec Hydro-Québec Hydro-Québec Canada Canada Canada

SUMMARY Among other innovations, grid owners are looking at robotic systems to assure and maintain the reliability and availability of transmission networks, as part of the solution to rising electricity demand, stricter regulations and the inevitable ageing of systems. This paper outlines Hydro-Québec’s power line robotics program, shares robotics field experiences, describes the technology’s impact and benefits, and presents aspects of future applications. Hydro-Québec’s development of robotics for live transmission line maintenance began in 1998 with the introduction of a teleoperated live-line trolley, LineROVer Technology. The next step and main technical challenge was to develop a reliable teleoperated robot capable of accessing live conductors while manoeuvring around obstacles encountered on power lines. LineScout technology was first employed on a live 735-kV line in 2006 as part of its testing and validation phase. It is now equipped with a robotic arm designed to carry sensors and maintenance tools required to perform work on the network. During the live-line interventions performed on the Hydro-Québec and British Columbia Transmission Corporation networks, the quality and value of the data collected confirmed the potential of the mobile robot. Information such as high-quality video images, audible noise, infrared images and electrical resistance measurements have proven to be extremely valuable in contributing to maintaining system reliability and in making optimal maintenance decisions. By working on energized lines, LineScout also made its mark regarding system availability and the safety of maintenance personnel. In the future, new sensors to evaluate ACSR steel core corrosion and to detect broken inner-layer strands could become the backbone of the technology’s business case. The ability to identify line components will eventually enable power line robots to perform inspections and cross obstacles autonomously.

KEYWORDS

Robot, live-line inspection, transmission line, maintenance.

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1 INTRODUCTION

Transmission line networks are undeniably strategic assets for every country. A rigorous maintenance strategy applied to transmission structures and facilities is therefore essential. Robotics has made its contribution to many fields of inspection and maintenance and transmission line maintenance should be no exception. Although not many advanced work has been implemented in systematic maintenance activities, the last few years have been fertile as many research teams are emerging with new projects [1] aiming to apply robotics to the power industry’s core businesses [2][3].

2 EARLY WORK

After Québec was severely hit by an ice storm in 1998, Hydro-Québec intensified its effort in the development of solutions to respond to such events. As part of the ensuing R&D program, robotics technology made its first appearance on Hydro-Québec’s transmission network in 2000. Initially developed as a mechanical de-icing solution for overhead ground wire, LineROVer Technology was soon used as a live-line inspection solution [4]. Used for visual and infrared inspections, measurement of the electrical resistance of splices, and live-line replacement of overhead ground wire, robotics applications in transmission line maintenance became a reality.

A single-span approach reaches its limit once an obstacle must be crossed. In 2002, the robotics team from Hydro-Québec’s research institute targeted another major challenge: accessing conductor bundles and crossing obstacles. The first prototype of RST-2X Technology was successfully tested in 2003 on a four-conductor bundle. The project was later put on hold to concentrate on a solution that would eventually cover the whole transmission network, LineScout Technology.

3 LINESCOUT TECHNOLOGY

LineScout started in 2003 with the objective of developing a teleoperated platform that could travel along live transmission lines and cross most obstacles on the Hydro-Québec network, opening the way for multi-span applications.

3.1 Crossing strategy and specifications

LineScout concept [5] and design [6] have been presented previously. As a summary, Figure 1 shows the steps that take place while crossing an obstacle, in this case a suspension clamp. As seen in the sequence, a set of arms and grippers offer a temporarily grasp on both sides of the obstacle, allowing wheels to disengage and flip down underneath the line prior to crossing to the other side. The two-minute operation is teleoperated from the ground. Details about the control algorithms and safety rules implemented to avoid mistakes and to ease the operator’s task were discussed in [7].

Figure 1: Demonstration of LineScout’s capacity to cross obstacles.

Early design specifications of the robot were decided in collaboration with Hydro-Québec field personnel. The fundamental decision relates to which line components must be crossable, and which may not. First, it must be possible to simply roll over small obstacles such as splices and vibration dampers. Then, based on a thorough line component survey, the 0.76-m diameter spherical aerial marker was identified as the logical choice for the largest obstacle that could be crossed, as its size exceeds most other common obstacles (e.g., simple or double suspension clamps, corona rings and smaller aerial markers). Lastly, some less common, oversized or overly complex obstacles, like dead-end towers, were not attempted in order to aim for a reasonably compact system. Other LineScout specifications are as follows:

Operate on live circuits of up to 735 kV at 1,000 A

Roll along the ground wire, a single conductor, or one of the lower conductor in a bundled configuration (double, triple, quad or more)

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Climb a 25-degree slope

Negotiate 12-degree horizontal turns at suspension towers

Work safely in winds of up to 40 km/h

Adapt its crossing strategy to obstacles up to 0.8 m in length and also to series of adjacent obstacles, such as suspension clamps spaced about 1.0 m apart

Cross obstacles in less than 2 minutes

Operate at temperatures of -15°C to +30°C

Roll at an average speed of 3.0 km/h

Tolerate light rain

Accommodate a modular robotic arm designed to reach all conductors in a quad bundle (seen on Figure 2 – left)

Weigh 98 kg in basic configuration and 110 kg when equipped with the modular robotic arm

Have an overall length of 1.36 m and hang 0.8 m beneath the conductor

Have a 20-minute typical installation time once at the tower

Up to now, two complete systems are assembled: one dedicated to live-line work in a stable and validated state, the other serving as a platform for R&D purposes (seen on Figure 2 – right).

Figure 2: LineScout on a lab mock-up and on a full-scale experimental conductor bundle.

3.2 Implemented Applications

Cameras are required for visual feedback to the operator but are also a potent means of inspection. For that purpose, LineScout is equipped with up to four powerful zoom CCD cameras mounted on programmable orientation mechanisms, as presented in detail in [8]. Other forms of feedback to the operator useful for inspections include: sound captured by microphones, GPS coordinates, pitch and roll values from inclinometers, and battery charge level.

As mentioned above, a modular robotic arm can reach all conductors in a bundle. Also, it serves as a base for the quick-connect mounting of various tools, including:

A sensor for measuring the electrical resistance of splices

A tool for installing custom-made annealed aluminum clips to temporarily repair broken strands

An electric torque wrench for tightening or loosening bolted line components

An infrared camera was recently added as a complementary means of inspection. Although this type of inspection can be made from the ground or a helicopter, having direct access to the line components of interest increases tremendously the spatial resolution of measurements and sometimes offers otherwise impossible points of view.

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3.3 Peripheral systems for field deployment

The ground control unit (GCU) comprises a military-grade portable PC and a pair of industrial 3-axis joysticks for operator input. The video signal transmitted by LineScout can be enhanced by an image stabilizer and displayed on a 15-inch sun-readable screen. It is also archived on mini-DV magnetic tape. All these components, seen in Figure 3 - left, are packaged in a robust case for field transportation. LineScout and its crew are usually deployed in a dedicated 4x4 truck but other means of access to the installation site have also been used, including ATV and snowmobile. For longer distances, a sturdy shipping crate was built for ground or air transportation. LineScout Technology and all the above equipment proved to be well suited to field operations.

Figure 3: Peripheral systems used to transport and operate LineScout.

4 EXPERIENCE IN THE FIELD

LineScout underwent a rigorous set of validation tests that confirmed its reliability under different conditions [9]. Ultimately, in early 2006, it was considered appropriate to install and operate the technology on live transmission circuits of up to 735 kV. These early, successful field trials also provided a good opportunity to document deployment logistics, including drawing up a list of necessary equipment and backup systems, and the final pre-installation checklist. It also gave Hydro-Québec line maintenance personnel the opportunity to draft LineScout installation procedures, along with standard live-line work methods. These trials proved useful in raising the crew’s level of confidence in the technology.

Upon completing validation tests, LineScout equipment was deemed adequate to undertake actual field work. In all the examples reported below, LineScout was carried about, deployed and operated by the R&D team. However, the work was planned, robot installed and inspection performed with the invaluable collaboration of line maintenance personnel. This split in roles was a logical and efficient way to value each group’s expertise.

4.1 Visual inspection of components on live lines

A typical task for LineScout is gathering high-quality close-up visual information about line components without the need to de-energize the circuits. Such an inspection is being performed in Figure 4 - left, with LineScout crossing the suspension tower of a 735-kV circuit. The objective was specifically to inspect all four rubber bushings (Figure 4 - right) located inside the spacer-damper clamps, looking for cracks, loosened assemblies or missing locking hardware.

Figure 4: Visual inspection on a live 735-kV circuit.

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All Hydro-Québec 735-kV circuits are in four-conductor bundles, in a horizontal configuration. The supporting towers can be climbed by linemen, who attach pulleys to the structure above the conductor where LineScout is to be installed. The robot is then hoisted up to slightly above one of the bottom conductors of a bundle following standard live-line work methods. Once maintained in position by linemen using hot sticks from the structure, the robot is progressively lowered so that its wheels sit properly on the conductor. This installation method can be used on most suspension towers, regardless of the circuit’s voltage level.

4.2 Vibration damper retrieval

One major 315-kV double circuit supplies Montreal Island and passes over a highway, city streets and neighbourhood backyards. It has an optical fibre ground wire (OPGW) equipped with vibration dampers at each tower. Over time, four of such dampers had come loose, slipped and ended up at mid-span. To obtain de-energized access to such strategic circuits would have been a huge challenge. Furthermore, if de-energizing one circuit were possible, using a crane to retrieve the dampers over the highway would have blocked traffic for a significant amount of time. To circumvent these issues, it was instead decided to send LineScout to retrieve the dampers.

Different checks were made prior to this job. The first were to validate a new installation method since no structure was in place to support a pulley above the overhead ground wire. As seen in Figure 5 - left, the pulley was therefore attached directly onto the OPGW and LineScout was raised attached by its grippers. Once pulled up close to the wire, the wheels were folded back, controlled from ground. Other checks ensured that safe clearance was maintained at all times: when hoisting the robot between the two energized circuits, and when LineScout was at mid-span and sag at its maximum due to the robot’s weight.

LineScout was equipped with a special fork designed to drag the damper by its clamp’s sides. Figure 5 - centre shows LineScout once it had crossed the mid-span damper, and is about to make contact and push the damper back up the span. Plans also called for loosening the spacer-damper clamp with the modular torque wrench on the robotic arm’s end-effector had it been too tight to drag along the wire (Figure 5 - right). All four dampers were successfully, and quickly, retrieved using this robotic method.

DamperDamperDamperDamper

Figure 5: Vibration dampers retrieved over a highway.

4.3 Multi-span inspection of aerial markers

When a particular type of suspended aerial marker (Figure 6 - left) raised concerns due to its faulty attachment, the question was asked whether LineScout could be used to assess the conditions of such markers installed on the overhead ground wire of ten consecutive spans over a total distance of about 3.5 km. This type of marker was also installed on the bottom conductors of one river-crossing span (Figure 6 - middle).

LineScout cannot cross this uncommon type of marker, which has a suspended length (below the conductor) significantly greater than the 0.40 m allowed. In this case, thanks to the camera’s powerful 26X optical zoom and an image stabilizer, it was suggested that LineScout could be installed on the top conductor of one of the circuits. This lower position would then provide a good view of marker attachments, as shown in Figure 6 - right, during a lab simulation with the LineScout camera located 10.8 m away from the attachment. The inspection scenario was to install LineScout at one end using

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the standard hoisting method and to operate it to cross the nine supporting structures, ending on the river-crossing span.

Figure 6: Multi-span inspection of aerial markers.

4.4 Long-Span Inspection

A collaborative agreement was reached in 2007 between Hydro-Québec and British Colombia Transmission Corporation (BCTC) for technology improvements and the use of LineScout Technology. Objectives mainly focused on the inspection of long crossing spans.

The first inspection was made on a 69-kV triple circuit where mid-span conductor damage was suspected. The 996-meter long span ran over a highway, a railway and a large water inlet. Since both adjacent structures were of the dead-end type, and because of the relatively low voltage, it was possible to quickly install LineScout on the outer circuit’s lower conductor using a boom truck equipped with an insulated jib (Figure 7 - top left). Once located, the multi-strand damage was thoroughly inspected, thanks to the peripheral view provided by the camera mounted on the modular arm (Figure 7 - top right). Following this inspection, BCTC had visual data to recommend conductor replacement and the installation of insulated phase spacers.

Figure 7: Inspection of a long water-crossing span.

Another quite spectacular inlet crossing was inspected in the summer of 2009. In this case, LineScout surveyed two spans of a 230-kV double circuit starting from a dead-end structure and extending to a

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90-m high suspension tower erected on a peninsula. The first span was 1,260 m long. The second span was 1,680 m in length and also extended to a dead-end structure with slopes of up to 16 degrees. Furthermore, there was a 7-degree horizontal turn at the suspension clamp that was successfully crossed by LineScout. A special method for installing the robot on the middle conductor of one circuit was devised jointly by Hydro-Québec, BCTC and BC Hydro line maintenance personnel. Since the hoisting pulley had to be installed on the conductor, this method was a live-line adaptation of the OPGW installation method described in Section 4.2. The video sequences collected cover all six conductors of the circuits, aerial markers and associated hardware installed on the top conductors, and tower components. It is hardly possible to shoot such footage from the ground or a helicopter, or by any other means short of sending a crew directly onto the conductor. The high image quality and degree of detail available led to important findings that are currently under investigation.

5 IMPACT AND BENEFIT

Robotics is often used for applications in the following three contexts: executing repetitive sequences of movements (industrial robots), providing access to hard-to-reach locations (underwater, space, pipelines, etc.) and keeping workers at a safe distance from dangerous operating conditions (nuclear environments, demining operations, energized components, etc.).

The drivers for applying robotics to the maintenance of live transmission lines are also related to the need for grid owners to adapt to a rapidly changing operational context in recent years. One should see the mobile platform as the mean of reaching line components so state-of-the-art sensors and tools can then be positioned to better assess their condition. Technology such as LineScout will have an impact on the strategic and economic factors below.

Reliability of the network: introduction of state-of-the-art inspection technologies into maintenance practices

Equipment sustainability: better condition assessment and monitoring, resulting in optimal maintenance decisions (targeted investment)

Workers safety Enhancement of the maintenance strategy:

- Project prioritization (refurbish vs. replace) - Proactive maintenance approaches - Better risk management - Increased efficiency and productivity in systematic inspection - Access to new data (future applications: corrosion, broken inner-layer strands, etc.) - Quantification of defects by measurement data and high-definition images - Follow-up on system condition based on archived results

Equipment availability: new possibilities in live-line work New tools and methods to adapt to a changing system operation environment:

- New regulations - Increase in domestic and import/export power flows - Ageing assets - Major climatic events

Contribution to build solid utilities commission filings Contribution to the roadmaps and innovation objectives of utilities

6 FUTURE APPLICATIONS AND CONCLUSION

This paper has briefly described LineScout Technology and some of the inspections and maintenance work performed on live transmission lines. These inspections help maintain service continuity while performing tasks that would have required working on de-energized lines. The robot also allowed safe access to damaged conductors and the gathering of valuable data that would eventually be a determining factor in maintenance decisions.

LineScout Technology is the only robot of its type in operation on live transmission lines in the world. The technical specifications that guided the design of the robot focused on two very important aspects directly related to end users:

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The business case behind its features and applications implemented The conditions under which the technology is to be operated Implementing such a technology not only means coping with a harsh operational environment but also with acceptance of the technology by end users. Since maintenance personnel were involved in the design from the beginning, the technology was not only adequate for field operation but also was perceived and accepted as the tool it is meant to be, bringing new possibilities for linemen in performing their challenging work. Standard working methods simply had to be adapted to suit LineScout use.

The future of transmission line robotics is likely to grow through collaboration among power utilities. This future includes high-value applications such as detection of corrosion of the steel core of ACSR conductors and detection of broken inner-layer strands, often located beneath a hardware clamp. As robotic technologies enter the systematic inspection programs of utilities, it is reasonable to foresee that they acquire greater autonomy eventually allowing not only autonomous crossing of obstacles, but also detection and identification of potentially defective components.

BIBLIOGRAPHY

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[2] British Columbia Transmission Corporation,, System Planning and Asset Management Division, “Transmission Technology Roadmap – Pathways to BC’s Future Grid Research and Development Program Office”, Last revised: Sept. 2008.

[3] Bonneville Power Administration, “Transmission Technology Road Map”, Retrieved Nov. 2008 from the web site: http://www.bpa.gov/

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[9] S. Montambault and N. Pouliot, “Design and Validation of a Mobile Robot for Power Line Inspection and Maintenance”, (Proceedings of the 6th International Conference on Field and Service Robotics, FSR 2007, 9 – 12 July 2007).