Electricite de France develops airborne techniques to maintain and repair transmission lines. Electricite de France (EDF) has always looked for new solutions to improve, inspect and maintain high-voltage and extra-high-voltage transmission lines. More than 50 years ago, EDF engineers thought that helicopters could decrease overhead maintenance and repair problems. Experimental applications and development work progressed, and in 1989, the Services et Travaux Heliportes (STH) Operational Division was created. STH now plays a major role in the reinforcement, inspection and maintenance of the French power transmission network.

EDF's Transmission Network The French transmission system is a dense power grid of more than 100,000 km (60,000 miles) of overhead lines that supply electricity to remote regions of the country. The transmission system includes circuit voltages of 63 kV, 90 kV, 225 kV and 400 kV. The EDF engineering teams' 1950s vision of delivering power without interruption contributed to the now common use of helicopters for a variety of applications. The teams identified the helicopter's advantages in maintaining and repairing transmission lines, including the helicopter's ability to transport men and materials to remote locations, inspect transmission lines, and rapidly locate line faults.

STH's Organization The STH group consists of three managed units: Heliborne Operations, Research and Development and Aeronautical Regulations. The Heliborne Operations unit's major role is to optimize the use of the helicopter in the reinforcement, inspection and maintenance of the transmission network. The Research and Development unit provides support to the line maintenance units by conceiving, developing, testing and validating new methods, tools and robots for transmission network maintenance, including overhead lines, electric power stations and heliborne maintenance. The Aeronautical Regulations office is responsible for EDF's specifications and security regulation definition for helicopter use; for selection of private operators able to perform heliborne operations that support the STH organization; and for supervision of STH's compliance with quality regulations.

Maintenance and Repair Missions Maintenance activities include various tasks that can be helicopter-aided when inspection or repair of the transmission system is required in difficult-to-reach places or when saving time is a major advantage. Line inspections to detect failures, possibly before they happen, can be performed visually or with the aid of an infrared camera. The entire transmission network is inspected once a year, and lines routed through environmentally sensitive areas are inspected twice a year (Fig. 1). This activity yields about 5000 flying hours a year. STH also uses a system that combines GPS positioning equipment and onboard data processing units to detect damaged conductors, broken insulators and the proximity of lines to buildings and trees. To prevent potential connector failures, STH developed an infrared camera inspection method to identify hot spots or faulty connections. All circuits are inspected with this equipment every three years. The operation accounts for about 500 flying hours per year and detects approximately 1000 hot spots. This procedure allows EDF to replace equipment in advance of failure.

Fault repair missions include location of the fault position and transport of repair teams and materials to the site to replace damaged conductors and components. Flight diverters are attached to conductors to limitthe number of circuit outages because of bird strikes on conductors constructed through bird habitats and in the flight path of migrating birds (Fig. 2). This operation is undertaken as a live-line, helicopter-aided mission.

Seven regionally based teams perform the annual line inspection program. Each team of STH personnel and local linemen is equipped with a STAR AS 350 helicopter and crew (pilot plus mechanic) for the inspection and repair missions.

Construction Operations EDF's network construction program has declined in recent years, but the division continues to use and refine the techniques developed over many years in the construction of new circuits and in the reinforcement of existing facilities. The helicopter missions range from transporting and assisting with tower erection in locations where crane access is difficult or impossible (Fig. 3 a & b), to the installation of conductors (both energized and de-energized work). EDF has developed a special device to facilitate conductor stringing from a helicopter. The device allows personnel to string the conductors with or without mechanical tension (Fig. 4).

STH also uses a wide range of underslung buckets to accomplish a variety of tasks. Underslung buckets and trolleys are designed to carry one or several linemen for single conductor and bundled conductor transmission line operations. STH has advanced this technique to satisfy flight regulations that allow helicopters to carry linemen as external loads (Fig. 5 a & b).

To perform this type of live-line heliborne operation, EDF must satisfy aeronautical regulations governing non-jettisonable external loads. These operations involve the use of a twin-engine helicopter manned by two pilots, which is capable of perfect hovering accuracy. In addition, two different slings hold the underslung bucket used for this work. This specific technique enables EDF to carry out 80% of its repair operations in the live-line mode, which significantly reduces maintenance costs and avoids the equivalent of 1000 days of circuit interruptions every year.

Helicopter Maintenance STH is responsible for the mechanical inspection and maintenance of each helicopter in the fleet. EDF staff, supervised by a chief mechanic and a quality manager, performs this work (Fig. 6). Maintenance is performed on each helicopter after every 500-flight hours of operation.

Personnel Training and Equipment The use of these professional techniques requires a highly motivated, well trained team and reliable equipment. The helicopter staff consists of four pilots from whom teams for the two-pilot crew's operational missions are specifically trained. Their annual refresher training is undertaken in real conditions and focuses heavily on safety. Regular training is given on the procedures required following an engine failure. The other half of the team consists of 50 carefully selected linemen from EDF's regional live-line maintenance units.

STH chose the recently certified TWINSTAR AS 355N-Eurocopter, a modern and reliable twin-engine helicopter, to provide the aerial performance needed for its missions. The helicopter is specifically equipped for utility work and is maintained like the rest of STH's fleet in the division's technical facilities. Currently, STH operates a helicopter fleet that consists of one TWINSTAR AS 355N, one LAMA and seven A-STAR AS 350 helicopters as well as various other helicopters contracted from approved operators on demand.

Conclusion STH constantly strives to improve this innovative use of helicopters by designing the most appropriate solution to any overhead power transmission network problem via a new tool or method.

With safety as a top priority, each item of equipment and every operational procedure are regularly tested before implementation. All heliborne operations are subjected to technical assessment and validation before introduction. Every aspect of this specialist technique has been developed to ensure optimum safety for the linemen working under the helicopter to the daily performance of pilots, in terms of skillful and precise positioning at all work locations.

Operations on a transmission system carried out from the ground often involve switching off the power. STH acknowledges that this is detrimental to power delivery and cost effectiveness. With cost efficiency as their goal, EDF spends over FFr 80 million (US$13 million) annually on helicopter-aided activities.