Changes to generation over the next decade mean that there will be substantial investment in expanding and upgrading the U.K.’s transmission network. National Grid has embraced the need to harness innovation to build new and upgrade existing transmission assets in order to operate a more flexible network, facilitating connection of its customers – including an increasing proportion of lower carbon generation to meet Government environmental targets. This INMR article, based on a contribution by Mike Fairhurst of the National Grid provides insight into development of unique 400 kV double circuit transmission lines across the U.K.
Lattice steel structures have been deployed in the U.K. for the last 90 years, with the first design selected based on a design competition and evolving over time. In 2011, the Department of Energy & Climate Change as well as National Grid called on architects, designers, engineers and students to come up with innovative transmission tower designs that balance energy needs and visual impact, again carried out on a competitive basis. The competition attracted 250 entries with scale models of the finalists going on public display at the Victoria and Albert Museum as part of the London Design Festival. The winning design – the ‘T’ Pylon – submitted by Danish company Bystrup was the overall winner. The basic loading criteria for the design is based on an existing lattice design consisting of a double circuit 400 kV line with a maximum demand transfer of 3800 MVA on a 3-phase, triple bundled, 500 mm spaced, 700 mm2 AAAC conductors (Araucaria)
National Grid subsequently engaged Bystrup to take the design from concept to full engineering compliance, having established that they had the necessary experience and capability in transmission tower design. The design team also engaged a number of composite insulator manufacturers to develop the insulator array. As the ‘T’ Pylon moved from concept to detailed design, National Grid worked closely with the design team to develop a tower family, based on the original concept, so that it could become a viable alternative to traditional lattice designs for transmission routes of the future.
In moving from concept to full design, a great deal of effort was put into the mechanical performance and manufacturing processes. This involved producing full-scale prototypes and developing procedures to fully test mechanical performance under a number of extreme conditions. The diamond shaped design has to withstand high compression and torsional forces under broken wire conditions as well as extreme wind and ice.
In parallel with this developmental work, the electrical performance needed to be fully understood since this was a completely new insulator arrangement that incorporates composite line insulators along with substation composite post insulators – both solid and hollow core. No international standards yet exist for this arrangement and National Grid therefore engaged two domestic universities to carry out electrical performance studies. In addition, European-accredited test institutions were engaged in association with National Grid and the designer in compiling a robust test protocol to prove the electrical performance of the insulator array.
National Grid also has responsibility to ensure that the erection, maintenance and repair of this novel design of transmission line can be carried out safely and efficiently. To that end, a full-scale training facility was constructed that incorporates the entire ‘T’ Pylon suite of towers. The National Training Centre at Eakring in Nottingham allows development of erection and stringing techniques as well as maintenance and repair procedures in advance of any new build. A plan and photo of this facility is shown below in Appendix 1.
At the project’s onset, the team looked at existing tower structures and their capability to determine what tower types would be required to design and construct a new line. In the U.K. a family of towers is typically designed with a selection of standard height suspension towers with an array of extensions, a range of tension towers with various maximum angles of deviation (typically 10°, 30°, 60° and 90°). In addition, there are two types of terminal (end) towers, double and single circuit. In many cases, a double circuit junction tower was also designed to enable existing lines to be tied in to create a 3-ended circuit. Since the winning entry in the competition was a suspension tower, the team had to produce similar tension tower designs. The aim was to keep the same conductor geometry by staying with the diamond shaped configuration while producing a design capable of restraining the mechanical loads and forces imposed when deviating from a straight line.
Outline designs were produced for 30°, 60° and 90° deviations; however it soon became apparent that for monopole designs with angles above 30°, towers would have to be extremely strong, large and wide to meet conductor loading criteria. The resulting structures, compared to the suspension tower that has a slender, light and uplifting visual appearance, looked heavy, wide, cumbersome and less aesthetic. In order to retain the original visual concept, further development on early tension tower designs greater than 30° was frozen and the team looked to design a suspension tower with a maximum deviation of 10°. This tower, in combination with the D30 tension tower, allows the line to deviate in sweeping bends rather than the traditional method of turning corners. The deployment of this combination has the effect that the ‘T’ Pylon follows through the landscape rather than crossing it in a number of sharp turns. By adopting this principle, the number of tower types was reduced to just 5, with 360 m span standard and height being 35 m.
‘T’ Pylon Tower Types
Suspension (Straight Line) Tower
Flying Angle F10: Suspension tower with a maximum deviation of 10 degrees
Tension D30: Angle tower with a maximum deviation of 30 degrees
Double Diamond Terminal Tower: Two independent diamond-shaped structures
Single Circuit Gantry – Terminal Tower: Low level ‘goal post’ gantry tower
Outline drawings and images of these are shown in Appendix 2
Suspension (Straight Line) Tower
Insulation Array consists of two 600 kN standard design suspension long rod composite insulators to form the top section of the array and two 300 kN extra creepage suspension long-rod composite insulators to form the bottom section; composite hollow core or solid core composite compression insulator to provide phase to phase insulation between the points of the diamond.
Flying Angle F10 – Suspension Tower with Maximum Deviation of 10°
Insulation Array consists of two 1100 kN solid core composite standard post insulators to form the top section of the array, two 1100 kN extra creepage solid core composite standard post insulators to form the bottom section; composite hollow core or solid core composite compression insulator to provide phase-to-phase insulation between the points in the diamond.
Tension D30 – Angle Tower with Maximum Deviation of 30°
Insulator Array consists of a standard 300 kN, 400kV composite long rods in a triple formation – one for each sub-conductor with standard sag adjuster fittings. Fitted with solid core composite insulators on the inner and outer phases to ensure jumper connections maintain 400 kV phase-to-earth (ground) clearance.
Double Diamond Terminal Tower – Two Independent Diamond-Shaped Structures
Insulator Array – same as D30
Single Circuit Gantry
Insulator Array – As D30
See Appendix 3 – Outline insulator drawings
During development of the ‘T’ Pylon, a number of workshops took place with all OHL contractors and National Grid maintenance teams (including Eakring OHL training staff), with respect to build issues and repair techniques. These workshops highlighted the main areas of concern, that in the main were concentrated on the suspension and flying angle towers and associated insulator arrays. Main issues arising from the construction and line repair workshops associated with the Eakring ‘Need Case’ included:
1. Tower construction & assembly;
2. Tower access & egress;
3. Conductor stringing;
4. Performance of suspension tower insulator array imbalance when using traditional, one phase at a time, tension string techniques;
5. Erection of insulator array;
6. Earth wire (ground wire) stringing and induced currents;
7. Conductor running blocks;
8. Clipping in new conductor;
9. Raising and lowering of insulator array;
10. Changing defective suspension insulators.
Working on tension towers and terminal towers will, in principle, use current techniques and practices since tower attachment points are fixed and will carry conventional composite insulators string arrangements. Since there is no requirement to climb ‘T’ Pylon structures, access methods will deploy Mobile Elevated Working Platforms (MEWP’s) and techniques will therefore be developed at the training facility.
Technical Specification Development
Initial electrical specification was developed on the basis of existing National Grid’s technical specifications, which are derived and adhere to IEC. However National Grid sought verification and input from an independent, internationally accredited test institution to carry out a review and make further recommendations as well as to develop a suite of mechanical and electrical test protocols.
Three suppliers were engaged to produce insulators and fittings to meet mechanical and electrical performance criteria. All 3 suspension insulator designs were then subjected to the test regime. Once all the results are compiled, the information and findings will then form the basis for the insulator technical specification and the type test protocol. This will enable National Grid go out to the global market on a commercial basis.