An integral element of the T-pylon, as with any electricity pylon, is the insulators that hold the three phase wires in place below each cross arm. Whilst in Denmark in April, we assembled and erected two types of diamond shaped insulators onto the suspension T-pylon.
Just as the T-pylon has to stand up to the rigours of weather extremes, the insulators must be mechanically robust and they must also be shown to keep an earth wire and the three live phases safely separated under a wide variety of wind, ice-loading and other conditions, such as wire galloping.
So, in addition to erecting the prototype tension pylon in April, to stand alongside the previously constructed suspension pylon, we also erected two diamonds to both assess the visual impact of the units and mechanically test them. I would say a pair but they are actually slightly different.
The diamond or Insulator Unit (IU) is nominally 13m by 8m and is sized to cater for the phenomenon of galloping (“skipping rope effect on the wires” discussed in post….). At an assembled weight of circa 2,500kg (including corona rings, arcing horns, nuts and bolts) we expect to construct it with the crane that assembles the monopole. We have worked with Bystrup (http://www.bystrup.dk/)to design tools that will enable the individual elements to be extracted without the need to lower the complete assembly and connecting wires to the ground. If damage occurred to the cast iron yokes, due to their size and weight, it is likely we’d lower the complete assembly to change them.
Mechanically, the IU is essentially formed from 4 diagonal insulators which are typically in tension with a central horizontal bracing insulator. The horizontal insulator is typically under compressive and bending loads and is much larger in size and weight than the diagonals. At the corners of the IU, smoothed and rounded cast iron members provide the connection point into which the insulators are bolted securely. Each of the diagonals is designed with a safety factor of 1.6, to withstand a tension load of 55 tonnes (or 550kN). We are reviewing these values as we know that we have lower loads in the bottom triangle and so can reduce the rating and hence size of the insulators.
Electrically, we need to manage the difference in voltage levels in the top (phase to earth – 231, 000V) and bottom (phase to phase – 400, 000V) triangles. We need to do this to ensure that we reduce the likelihood of faults occurring due to lightning strikes or switching events and hence improve the reliability & availability of the overall line. The length of the insulators seeks to balance this requirement alongside that of managing the effects of pollution accreting to the insulators effectively nullifying the insulation effects in part.
The first diamond erected, IU1 (Insulator Unit 1 developed with Lapp,Germany, www.lappinsulators.com & Mosdorfer, Austria & UK, www.mosdorferccl.com) has different length insulators on the four diagonals of the diamond earring. The uniform diamond shape is achieved by installing additional metallic straps to achieve the requisite diagonal length. The top half of the diamond features two insulators which are 3.9m long while the bottom two are 4.9m long. The horizontal member is formed from 2 large diameter members coupled together with a steel section for mechanical strength. (For the mechanical tests, IU1 was supplied without the electrical insulating sheds.)
In, IU2, (by Pfisterer Sefag, Switzerland, www.sefag.ch) the four insulators or of equal length each measuring 5.9m. These insulators as well as being longer than those in IU1 have electrical insulating sheds with a creepage distance dictated by international standards. The horizontal member is formed from two sets of triple insulator posts flanged together for mechanical strength i.e. 6 insulators.
At this time, IU3, (by Allied Insulators,UK, www.alliedinsulators.com) has a design similar to IU2. The only key difference is in the proposed horizontal member which can be formed from either a flanged large diameter glass fibre rod as in IU1 or a hollow core post which could provide significant weight savings. We will be seeking to work closely with Allied Insulators to help develop our UK test line in the future and so we will seek to design an optimised solution based on electrical & mechanical properties, which can then be rolled out to the two other suppliers.
Erecting the diamond
IU1 & IU2 were built at ground level before being raised into position. We first assembled the horizontal member which weighed in at ~1 tonne (with cast iron side junction yokes) on trestles. A sling from the crane was then used to support the top cast yoke whilst we introduced the different parts that formed the top triangle. The horizontal member was then introduced and bolted to the casts at which point the crane raised the assembly further to enable the installation of the bottom triangle. The complete diamond assembly with insulators, corona rings, arcing horns, cast yokes, bolts and nuts weighed in at just under 3 tonnes. As you can imagine, it is a lot easier to connect the various parts of each diamond at ground level than 30m or so up in the air.
When IU1 was lifted into place, to be connected to the ‘horn’ at either end of the crossarm with the crane that had erected the tension pylon, we had to take account of the configuration of the horn. The horn offers an angle of ~45 degrees yet the diamond earring is flat. This meant we had to tip the earring to prevent fouling and enable the connection with the M72 bolt to be made easier.
When we stood back to admire the completed suspension pylon, we found that IU1 looked similar to how we’d pictured it in photomontages. IU2 was erected the day after and seeing photographs of both solutions on the same pylon gave us confidence that the light grey colour of the insulators alongside the uncoated (e.g. weathering steel) pylon blended well into the landscape.
These are exciting developments because it is only by mechanically testing the insulator units, that we can understand how the design works in real life and take the project forward to the next stage. We have also undertaken preliminary electrical development tests on IU1 & IU2 at a test lab, STRI in Sweden (www.stri.se) – more on that in the next post.