06.05.2021 | Efficient solution for voltage conversions
Increasing grid voltage improves performance and reduces power losses. As a result, Axpo is continually expanding its 50-kV distribution grid. However, voltage increases can result in lengthy planning approval processes and require large investments when the mast positions have to be changed. This can be avoided with a clever Axpo innovation: Thanks to insulated supporting tension chains (iTAK), voltage can be increased without changes to masts and suspension points.
Axpo has been steadily converting its 50-kV distribution grid to 110 kV in order to transmit more capacity over existing power lines and reduce losses. Three quarters of Axpo's grid has been converted. In many cases, mast construction does not allow a voltage increase without modifications. If the mast arms are too narrow or too close together, the spacing is insufficient for a voltage of 110 kV. The mast must be replaced, or equipped with new mast arms. This is not only expensive, but can also lead to lengthy planning approval processes if residents file objections.
Axpo developed the insulated supporting tension chains for this purpose. The chain comprises three insulators with a Y form arrangement and an insulated loop. This, at first glance complex design is necessary so that the suspension points and static forces on the mast remain uniform (see box).
The line voltage can be increased to 110 kV using iTAK without having to modify mast construction or the suspension points. Existing infrastructure can continue to be used as best possible and a voltage increase can be realised with a shorter planning approval process because the line falls under the category of an "old installation" according to non-ionising radiation regulations (NISV).
In 2020, insulated supporting tension chains were successfully tested in a pilot installation. Feedback from the technicians involved in the project was positive: Supporting tension chains are easy and fast to install. In most cases, a mast can be converted in half a day. In the course of 2021, more Axpo lines (approx. 110 masts), which are longer than those in the pilot project, will be retrofitted with insulated supporting tension chains.
Axpo also offers iTAK to other grid operators. Survey findings from the year 2020 indicate a keen interest in this innovative solution: For example, to solve problems spacing problems whilst maintaining the same suspension point during voltage conversions. iTAK can also be used for high thermal loads when the conductor cable does not fulfil the required minimum ground clearance. The suspension points can be raised by means of insulated supporting tension chains.
With insulated supporting tension chains, Axpo has developed an economical, efficient, environmentally friendly solution to optimise existing grid installations. Mast geometry can be maintained in compliance with statutory clearance requirements. This avoids costly modification of mast infrastructure and extensive planning approval processes. As indicated by feasibility studies, Axpo's patented solution can be used for various voltage levels and adapted if necessary.
Axpo's 50-kV distribution grid comprises numerous masts, some with a mast arm width of 1.4 m and a vertical mast arm distance of 2.5 m. If the existing insulators (50 kV) were replaced with a standard insulator for 110 kV, the ground clearance would decrease because of the longer insulator. The distance to the lower cross arms would also decrease and strong winds could deflect the line too close to the masts. In any case, an expensive planning approval process would result because the suspension points have to be changed.
In place of one individual insulator, two insulators are place placed horizontally to the line. A conductor loop connects the ends of the insulators (see image above). The line is now suspended at a higher level and the conductor loop distance to the lower cross arm sufficient. This is also the case for the distance to the mast. However, the horizontal forces of the line would go directly to the mast arm and the masts were not designed for such forces.
In order to alleviate horizontal forces, a third insulator is added and serves as the suspension point for the two horizontal insulators. In this way, the horizontal forces remain within the design scope of the masts. However, the conductor loop is now lower – too close to the lower cross arm and to close to the mast under strong crosswind conditions. Apxo developed an insulated conductor loop for this reason. It prevents flashovers to the mast and the lower mast arms. The insulation also protects large birds from electric shock when they land on the masts.
The supporting tension chain insulators are manufactured in porcelain or silicon. Conductor loop insulation is made from long-lasting, cross-linked polyethylene. The life span is about 40 to 50 years.
Insulated supporting tension chains were tested by the High Voltage Testing and Engineering Commission (FKH) in Dänikon. The chain fulfilled tests for lightning impulse voltage prevention of 550 kV (positive/negative) and standard AC testing voltage of 230 kV in both dry and wet weather conditions.