Electricity transmission towers (or ‘pylons’) and overhead lines are perhaps the most familiar face of electricity transmission and distribution to the public. However, while demand for electricity is constantly increasing, the general public is less and less accepting of the transmission towers that carry it.
Concerns over transmission towers fall into three basic areas:
1. Aesthetics: transmission towers are often considered a blot on the landscape
2. Space: in densely populated areas, it is increasingly difficult to find room for large transmission towers and the conducting lines that connect them
3. Magnetic fields: there is increasing concern among the public over the possible effects of living close to high magnetic fields.
With these concerns in mind, TenneT, the Dutch national transmission system operator (TSO) asked DNV GL to assist it in developing an innovative new transmission tower family. The project started in 2006, with the aim of developing a new family of transmission towers that is more visually appealing to the general public, has a smaller electromagnetic (EM) field zone and minimises the right of way for overhead lines.
In 2010, the first three of these Wintrack towers were erected and commissioned along the A12 motorway, between Krimpen and Bleiswijk. Subsequently, a complete high-voltage line from Wateringen to Bleiswijk, comprising 33 Wintrack tower sites, was rolled out in 2012. And during the course of 2015 and 2016, a high-voltage (380 kV) line will be constructed with almost 160 Wintrack tower sites linking Bleiswijk, Vijfhuizen and Beverwijk.
This first Wintrack design was presented at the 2013 INMR WORLD CONGRESS in Vancouver, Canada. This article, by Peter Kolmeier of DNV GL Kema, reported on a subsequent upgrade to the original Wintrack design.
Updating the Wintrack Design
The original Wintrack tower design included a double 380 kV circuit and a combination line of two 380 kV circuits and two 150 kV circuits. However, projected grid utilisation in the Netherlands for the coming years suggested that 4-circuit 380 kV towers would also be required. In particular, two 4-circuit 380 kV, 4000 A lines were planned to run from Diemen to Eemshaven in the northern Netherlands (NW380) and from Borsele to Tilburg in the southwest Netherlands (ZW380).
DNV GL was asked by TenneT to assist in a feasibility study on the development of the 4-circuit 380 kV Wintrack towers. In updating the Wintrack design to accommodate the 4-circuit 380 kV, 4000 A lines, two additional considerations had to be taken into account:
• maintenance must be possible on one circuit while the other three are in service
• the new design must be able to withstand a much higher mechanical load due to
– the double 380 kV circuit on each pole
– the severe icing that is likely along part of the route of the NW380 line.
The requirements of the new application mean the suspension insulators will be put under much greater mechanical stress than in the previous design. Hence dimensioning the new suspension insulators and verifying whether insulator manufacturers are able to meet the mechanical and electrical requirements would be a major focus for the project.
The design parameters for the 4 x 380 kV line were the same as for conventional lattice towers: four circuits carrying 4000 A at 380 kV. In addition, the new design would also be required to fulfil the same criteria as the original Wintrack design i.e. reduce the magnetic fields and right of way, minimise the impact on the environment and increase public acceptance of transmission towers.
The Basic Wintrack Bi-Pole Tower Design
The basic design of the new towers is essentially the same as the original Wintrack design. Each tower comprises two slim poles spaced about 16 metres apart, with the poles tapering in diameter from bottom to top. This gives the pole the necessary mechanical strength and, combined with the light grey colour, helps reduce its visual footprint on the skyline, thus making it more aesthetically acceptable. The conductors are supported by braced V-shaped insulators. These feature C-shaped frames which hold the conductors in position. In addition an extra earth wire (return conductor) is installed below the lowest phase bundle to reduce the ground potential rise in case of a short circuit.
Two important parameters in the design are the magnetic field zone and the (vertical) phase distance in relation to galloping. These two parameters are related as larger vertical distances result in a larger magnetic field zone. The vertical distances between phases were selected based on the Dutch National Normative Aspect (NNA) of the NEN-EN 50341/3 standard as well as previous experience in the Netherlands regarding ice-induced galloping.
The basic dimensions of the new Wintrack designare summarised in Table 1.
Magnetic Field Zone
A key driver for the Wintrack tower design is the desire to reduce magnetic fields around transmission lines. The magnetic field strength at a given point P close to a transmission tower is determined by the amplitude of the phase current and the distance to that point. This is illustrated for a conventional lattice tower in the donau configuration in Figure 1.
Figure 2 shows the magnetic field at mid-spn (i.e. where the conductors are at their lowest height) for this type of tower as a function of distance from the central line. The Dutch government recommends a limit for magnetic fields in “sensitive areas” of 0.4 μT. For typical tower dimensions, it is clear that the magnetic field exceeds this guideline in a zone that measures 200-300 metres across.
To maintain balanced loads, most conventional lattice towers in the Netherlands have a triangular phase configuration. Consequently, for double circuit towers, at least six phases contribute to the magnetic field at P. The contribution from each phase differs according to its distance from P. Hence, the magnetic field at P can be reduced simply by switching the phases and optimising the phase configuration as shown in Figure 3. Optimising the phases in this way can shrink the “above 0.4 μT” zone for a conventional tower with a triangular phase configuration by around 30-40% (see figure 4).
The magnetic field can be further reduced by bringing the phases closer together. The minimum allowable distance between phases is determined by numerous factors including electrical clearance for various wind condition, clearance for maintenance and clearing for galloping. The allowable distance between circuits can be minimised by an arrangement that ensure there is “no tower” between the two circuits. This observation led to the basic bi-pole design of the Wintrack towers shown in Figure 5. Again the magnetic field can be reduced by optimising the triangular phase configuration as shown in Figure 6.
Figure 7 shows how the magnetic field for the Wintrack bi-pole design compares with that of an optimised donau-type tower. It is clear that boththe maximum field and the zone in which the field exceeds the 0.4 μT guideline are greatly reduced.
With no cross-arms on the towers, the conductor suspension arrangement is as shown in Figure 8. The insulator set consists of a line post insulator suitable for compression forces, a tension insulator and a newly designed C-frame to fix the conductors. Both insulators are pivoted to allow longitudinal movement.
The dimensions of the insulators are designed to support the normal conductor loads plus any additional forces due to line angles of up to 5°. Thanks to these properties, fewer tension towers are required. This simplifies line routing, reduces the total land usage for towers and provides more flexibility for lines to follow existing infrastructure such as roads and waterways, thereby reducing the impact on the landscape. The electrical properties are compliant with the international standards.
The New 4-Circuit 380 kV Wintrack Towers
For the planned NW380 and ZW380 lines, some changes were required to the original Wintrack design. However, the new 4 x 380 kV towers employ the same basic Wintrack bi-pole design, as shown in figure 9.
When extending the Wintrack family to include 4-circuit towers, one of the key requirements was the ability to carry out maintenance on one circuit while the other three are still in service. This means the distance between the conductor and the tower had to be extended.
To achieve the necessary horizontal separation to meet the maintenance requirement, the phases of the 380 kV line are tensioned onto cross-arms for the angle and termination towers of the 4-circuit line. This is contrary to the approach taken in the original Wintrack design, where they were tensioned directly onto the body of the tower. As a result, the horizontal jumper used in the 2 x 380 kV and 2 x 380/150 kV line (see Figure 10) is no longer necessary. A standard hanging jumper is sufficient (see Figure 11).
Higher Mechanical Loading on the Braced V Insulator Set
Two factors in the line specifications led to increased loading on the insulator sets. The first was an increase in the maximum design span from 400 to 450 metres. The second was the location of the NW380 line in the northeast of the Netherlands, where icing is a larger risk. The Dutch NNA of the NEN-EN 50341 standard dictates a higher ice load regime in this part of the country and so the ice load per conductor was increased from 1.0 kg/m in the original Wintrack tower design to 2.9 kg/m in the new design.
The higher loading of the suspension of the insulator sets as well as in depth studies on the stability of Wintrack towers under unbalanced loading led to a change in the appearance of the braced V in the Wintrack towers. The swing angle and length of the post insulator were adjusted and the C-frame replaced with a more traditional armour rod suspension (see Figure 12). The stability and unbalanced load of the braced V was reported at the CIGRE 2014 Session2.
Due to these changes, the insulator type tests were repeated. Tests were performed according to the relevant standards. The following 5 electrical tests were carried out:
1. Radio interference (RI) test (acc. IEC 60437, clause 13)
2. Corona voltage test (acc. IEC 61284, clause 14)
3. Dry lightning impulse voltage test (acc. IEC 60383-2, clause 9)
4. Wet switching impulse voltage test (acc. IEC 60383-2, clause 11)
5. Wet power-frequency voltage test (acc. IEC 60383-2, clause 10)
Following the successful electrical and mechanical testing of the insulator sets, the realisation of the 4 x 380 kV Wintrack towers is a step closer. The first 4 x 380 kV Wintrack towers are scheduled to be installed between Eemshaven and Vierverlaten. Commissioning of this connection is planned for 2018.