Co-habiting with 220 kV transmission lines has mostly been for the birds. Techniques have been developed, however, for humans to safely work on the high voltage transmission lines - when live. The ability to provide maintenance services without shutting down the line has many benefits to the network owner such as less requirement for line redundancy and no discontinuity in the electricity supply. ABB has trained experts to handle this work high above and carefully isolated from ground zero. New Zealand’s national electricity grid owner, Transpower New Zealand Limited, has awarded ABB with long term contracts for maintaining transmission assets.
ABB Ltd in New Zealand provides its customers live line maintenance services for high voltage transmission lines. Though live line capability is an essential tool in the modern contractor’s arsenal, and is perceived as a modern concept, its roots are as old as the high voltage transmission industry itself.
By using live line techniques to maintain transmission line infrastructure, circuits and transmission lines are able to remain in service while maintenance tasks are carried out. This is
a major advantage to transmission asset owners because less redundancy is needed in the transmission network. Electricity consumers who are supplied by spur lines (single circuit supplies typically in rural areas) also benefit from live line work. They do not suffer the inconvenience of a cut in their electricity supply every time maintenance is carried out on their supply lines. Given the high cost of transmission lines and the impact that transmission lines have on the environment, there is a major advantage in being able to avoid duplication of assets purely for maintenance purposes.
History of live line maintenance in New ZealandNew Zealand’s national electricity grid owner, Transpower New Zealand Limited, has been carrying out live line transmission line maintenance since 1989. At that time a major insulation replacement
| program was undertaken on the inter-island HVDC link from Benmore power station in the South Island to Hayward’s sub-station in the North Island. Because the inter-island transmission link is such a critical component of the New Zealand electricity grid, the de-energized alternative to live line maintenance for this project was untenable - the circuits would have been required to be out of service for between two and three months to complete the work. |
Since then, live line maintenance has become commonplace on the New Zealand Electricity Grid as it is around the world. Almost all line maintenance tasks can be undertaken with the circuits in service. In the 1920’s and 1930’s, live line work was carried out in New Zealand on the on the 110kV transmission lines from the newly commissioned Mangahao and Arapuni hydro power stations. As the electricity network was expanded, with duplicated circuits common, outages for maintenance purposes became readily possible, and live line maintenance was discontinued in New Zealand until the late 1980’s.
Basic methods of live line maintenanceThere are two basic live line methodologies for high voltage work, which in industry terminology are called ‘hot-stick’ and ‘bare-hand’ methods.
Using hot-stick methods, direct human contact with live components is avoided. Line workers use tools fastened to insulated fiberglass poles to carry out the work, and always keep themselves at a safe distance from the live components.
In contrast, when using bare-hand methods the line worker is positioned in direct contact with the live components, and is livened or raised to the electrical potential of the conductor being worked on. For transmission voltages in New Zealand, this is 110,000 volts (110kV) and 220,000 volts (220kV). This may sound chilling, but for well-trained live line mechanics it is all in a day’s work - much like the birds that land on power wires all the time and survive.
The same principal applies to bare-hand live line workers. The complex part of the job is getting the line mechanic on to and off the wires safely. Though this sounds simple in theory, in practice live line work is potentially very hazardous, and must be undertaken in a very methodical manner with highly trained workers in a carefully planned and controlled work environment.
When accessing live conductors for bare-hand work, it is critical that the live line mechanic does not at any time bridge the gap between the live conductor and any earthed object (including the tower or pole that supports the conductor). There are several ways of achieving this. One way is to raise the live line mechanic from the ground or a part of the tower below the working position using live-line (insulating) rope. Using this method, the live line rope is run from the ground, up to a pulley block on the tower, and back down to the worker, where it is attached to the worker’s body harness. The worker is then raised by pulling the rope up using an electric capstan (winch). The live line rope can contact the live high voltage conductors without risk to workers operating the winch or working on the tower. To ensure that all parts of a bare-hand worker are raised to the same potential as the live conductor he/she is in contact with, the worker wears a special conductive suit complete with hood and conductive socks.
When he/she gets into position adjacent to the live conductor, a solid electrical connection is made to the live conductor using a conducting ‘wand’ to bond to the conductor. It is not unusual to raise an electrical arc between the wand and the conductors when first making contact.
Safety and processTo ensure the safety of workers involved in live line work, a rigorous set of rules and guidelines have been developed, and made mandatory in New Zealand under the Electricity Act 1992 in the form of the New Zealand Electrical Code of Practice for High Voltage Live Line Work.
management systems to control all aspects of live line work, including tool management, training of staff, weather conditions and their impact on the work, and the development and approval of work procedures. All work tasks must have a fully documented work procedure that has been trialed and proven before being approved for use on live assets.
|The code of practice has been developed with inputs from all areas of the industry, including employers, industrial unions, asset owners and professional interest groups. In New Zealand, Transpower New Zealand Limited also sets specific requirements for contractors to meet before granting them approval to undertake live line work on the national electricity grid. Both the Code of Practice and the Transpower Standard require contractors to have comprehensive|
ABB New Zealand live line workABB Limited was first awarded transmission line maintenance contracts in New Zealand by Transpower in 2000. This was a three-year term contract commencing in July 2000 for the provision of transmission line maintenance services for the West cost of the South Island. The contracts include maintenance activities on transmission lines, condition monitoring, undertaking routine maintenance and providing a fault response service and managing and undertaking vegetation control adjacent to transmission lines. Commencing in July 2003, ABB was given the opportunity by Transpower to enter into a five plus five-year maintenance contract covering the same area, with expansion to include the northern part of the South Island.
Transpower’s transmission line network covered by ABB’s current line maintenance contract includes more than 1000 km of transmission lines, comprising approximately 3,110 pole structures and over 1,240 steel towers. The lines are of 66kV, 110kV and 220kV design. Wood pole structures are the most maintenance intensive types of transmission line and the high number of wood poles in ABB’s region ensures many complex live line challenges for the ABB teams.
line work experience after certification as a line mechanic, before being accepted as a trainee for live line work. A certified trainer must give training in live line work methods, and only a certified trainer can confirm competence. Competency certificates with a twelve-month validity period are issued to workers who meet the training and work experience criteria and have passed the competency evaluation. These must be re-issued annually after refresher training and evaluation of fieldwork. It costs an employer about $18,000 to $20,000 in training before a line mechanic is able to be certified as a live line mechanic. That money is well invested!
|The type of work routinely undertaken by ABB using live line techniques includes; replacement of complete pole structures, replacement of individual poles on two pole structures, replacement of cross-arms, replacement of insulation only (carried out on tower and pole lines), and enhancement works such as installation of vibration dampers onto live conductors.|
The training and work experience requirements for budding live-line mechanics is rigorous. New Zealand Codes of practice require a worker to have documentary evidence of two years of general