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Adapting Cable Accessories for Onshore & Offshore Substations

June 10, 2017 • ARTICLE ARCHIVE, Cable Accessories, Substation
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Integrated Surge Arrester

In a second step, an option has been created for a ‘combination product’ by replacing the post insulator by an integrated surge arrester. This solution offers significant potential for space and cost saving during substation design. The combination product requires the space as one conventional termination and is fixed onto the same support.

The cable screen is no longer connected to the base plate of such a combined product since a surge arrester needs a direct earthing point. Bonding of the cable screen can be applied at the bottom of the termination or via a link box.

Fig. 4: Design of dry cable termination combined with surge arrester, cable accessories Adapting Cable Accessories for Onshore & Offshore Substations Screen Shot 2016 07 15 at 14

Fig. 4: Design of dry cable termination combined with surge arrester.
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Fig. 5: FEM calculation at various distances of dry termination combined with surge arrester. cable accessories Adapting Cable Accessories for Onshore & Offshore Substations Screen Shot 2016 07 15 at 14

Fig. 5: FEM calculation at various distances of dry termination combined with surge arrester.
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To evaluate the ideal arrangement for this combined product, an FEM calculation has been done using two different programs to limit calculation errors. The main findings were:

• distribution of the electric potential along the metal oxide discs of the surge arrester is more homogenous and the voltage along the upper discs is reduced due to the influence of the cable termination. This effect becomes stronger for shorter distances;
• electric field strength along the silicone sheds of the surge arrester is normally higher at the upper part next to the high voltage potential. Depending on smaller distances from the cable termination, this effect can be equalized or even inversed (i.e. higher electric field on the sheds at the bottom);
• with smaller distances between surge arrester and cable termination, electric field strength at the termination is forced from the upper part towards the lower part and is significantly higher in the area close to the arrester.

These findings suggest that there is an optimum distance for the arrangement of the combined product. Apart from influence of electric field distribution in such a combined product, consideration also has to be given to different customer requirements. Both solid earthed and coil-earthed systems are used in energy transmission and this leads to two different surge arrester solutions.

Apart from dealing with the two earthing systems, another issue is dealing with different philosophies among network operators. The “safety first and cost is not the main issue” opinion, for example, focuses on tube type surge arresters that offer explosion resistant design in case of failure. In addition, these offer a high cantilever force that supports the cable termination with a high mechanical strength. At the same time, these benefits mean a higher investment cost.

On the other hand, focusing mainly on cost effectiveness leads to using a cage type surge arrester. In the rare event of an incident, the worst case would see the function of the cable termination damaged, meaning that both products will need to be replaced. The cantilever forces of this option are lower compared to a tube type surge arrester but still high enough to ensure a combined product with sufficient mechanical strength.

Additional Tests on Dry Type Outdoor Terminations & Material

Standard testing of dry type outdoor terminations is normally performed according to IEC 60840 or IEC 62067. Due to increasing demand for this type of accessory, especially for substation projects, additional tests have been performed in order to satisfy special customer requirements. One of these is the salt fog withstand test according to UX LK 208 Rev. 01 and based on IEC 60507 (second edition 1991-04). This was successfully passed for the high voltage cable termination type shown in Fig. 6, confirming the required specified salinity of 112 kg/m3 at test voltage of 98.1 kV (170 kV/√3).

Fig. 6: HV cable termination type during salt fog withstand. cable accessories Adapting Cable Accessories for Onshore & Offshore Substations Screen Shot 2016 07 15 at 14

Fig. 6: HV cable termination type during salt fog withstand.
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Fig. 7: HV cable termination type during wet power frequency withstand voltage test. cable accessories Adapting Cable Accessories for Onshore & Offshore Substations Screen Shot 2016 07 15 at 14

Fig. 7: HV cable termination type during wet power frequency withstand voltage test.
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The same termination type has also been subjected to and successfully passed a wet power frequency withstand voltage test according to UX LK 208 Rev. 01, which consists of application of 2.5 U0 (218 kV) for 15 minutes while the test object is under artificial rain, as stated in IEC 60060-1.

Moreover, during the development stage, a test to evaluate resistance to tracking and erosion was performed according to IEC 60587 (third edition, 2007-05) and yielded giving satisfactory results.

Fig. 8: Material samples after test for evaluating tracking and erosion resistance. cable accessories Adapting Cable Accessories for Onshore & Offshore Substations Screen Shot 2016 07 15 at 14

Fig. 8: Material samples after test for evaluating tracking and erosion resistance.
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The mechanical behaviour of the self-supported termination was also verified several times by means of specific tests on the support insulator as well as on the complete termination.

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