66 kV SUBMARINE
Cable SYSTEMS

Renewable energy resources are abundant, inexhaustible, and have the potential to fully meet global energy needs while reducing emissions and mitigating climate change.

Offshore wind is an essential component of renewable energy which is significantly improving its cost competitiveness over the last few years. Nevertheless, wind farm developers are always looking at ways to reduce Levelized Cost of Energy (LCE).

Among the technology developments enabling LCE reduction, several independent studies have shown that use of array cables operating at 66 kV instead of 33 kV presents considerable advantages on typical offshore wind farm systems.

The main cost reduction drivers for using 66 kV, instead of 33 kV whilst maintaining the same overall output power, are:

• Two times more power can be transported over a single array cable, which reduces the length of cable required and consequently the investment in these cables and their installation.
• Lesser number of cables are entering the offshore substation, therefore the number of J-tubes, transformers and switches, as well as the space these items require can be reduced.
• Larger turbines unit power to reduce the number of turbines and associated array cables.

Current rating for the three cable designs has been calculated under the same ambient conditions:

• Dry design, is a cable with an extruded lead sheath over insulation.
• Semi-wet/Semi-dry design, is a cable with a PE sheath over a non-fully impervious metallic screen (e.g., metal tapes or thermoglued foils).
• Larger turbines unit power to reduce the number of turbines is a cable without any polymeric sheath over a non-fully impervious metallic screen (e.g., metal tapes). The insulation material of 66 kV cables operates at higher electric stress than 33 kV cables, requiring rigorous cable design and insulation material selection

Therefore, Prysmian’s 66 kV array cable is based on wet design EPR insulation, with 50 years successful operational experience up to 72.5 kV, which is a reliable and cost effective solution for offshore cable systems.

The technical features of these cables are outstanding, with no equal among any other cable insulation types at this voltage level.

Prysmian EPR insulated cables are compatible with Prysmian’s full range of accessories including Click-Fit®, ElaspeedTM and hang-off systems.

Over-voltage performance is usually tested by “impulse testing” of cable samples at room temperature.

Results clearly show XLPE to have a peak voltage breakdown level some 20-30% higher than EPR with both stresses being well above the operating levels.

However, the effect of raising the temperature of the cable sample to the upper operating ranges is marked and shows once again the resilience of EPR compared with the degradation of performance of XLPE. The next chart shows the results of peak voltage testing of XLPE, clearly demonstrating the small difference in performance between EPR and XLPE at high temperatures.

It is worth noting the outstanding results achieved for 20 m samples of 250 mm² 145 kV Prysmian EPR cables manufactured with the latest technologies.

The average breakdown voltage for these cables was 88 kVp/mm at 95 °C, better than most results for XLPE.

Diameter expansion of 150 kV EPR and XLPE insulated cables

AC over-voltage testing, however, gives a different picture for the breakdown levels as RMS voltages are usually considered.

The breakdown test is generally carried out on a standard test cable, 20 m long with a 70 mm² conductor and rated at 12/20 kV. Once again, XLPE is seen to have higher breakdown stresses than EPR, 60-70 kV rms/mm compared to 50-55 kV rms/mm, but XLPE exhibits far greater scattering of results.

The consequence of this scattering is only apparent when different dimensions of cable are tested.

With increasing length and conductor radius, the breakdown voltage of the cable system falls more rapidly than that of EPR because of the wider spread of results (in a larger cable it is more likely that there will be some part of the cable that will fail at a lower stress).

The test parameter “b” is used to describe the amount of scatter of results (Weibull theory), with high scatter giving a lower value. The band of scatter for XLPE has a median of 10 compared to 14 for EPR, and the consequence of this can be seen in the next chart. The graph plots breakdown voltage against a coefficient proportional to the square of the conductor radius times the length of the sample. The larger gradient of the XLPE samples shows a fall in failure voltage with increased cable size and length.

It is clear from this that the larger the quantity of installed cable and the bigger the cables installed, the greater the reliability of EPR compared with XLPE.

Short term AC tests on XLPE and EPR

Floating offshore wind turbines are now entering the world stage with the potential to become a relevant part of the business.

Floating offshore wind farms remove the restriction of being installed in shallow waters and also have a major advantage as they are assembled in the port and then towed to the site by an ordinary tugboat, which can also tow them back to shore for heavy maintenance or final dismantling.

Thanks to this advantage floating technology will become competitive when operating costs go down. In particular, this segment is gaining significant ground due to the fact that floating offshore wind turbines use multiple components and similar services developed for the offshore oil & gas industry.

FROM BOTTOM FIXED TO FLOATING OFFSHORE APPLICATIONS

Energy demand is growing, wind turbine generators are growing in size thanks to progress in technology and they are located further away from shore: hence the need for dynamic cables.

Dynamic cable systems must be able to withstand maximum offsets of floating structures under the load of centennial current, swell, and wind. This is the challenge that Prysmian is part of.

Prysmian has all the in-house expertise and experience to develop a robust dynamic cable system by carefully engineering all the necessary components. Prysmian deals with dynamic cable systems during production, installation, and operation.

Building wind farms, both in shallow and deep water, as well as other marine renewable energy systems, will require more and more dynamic cables that will have to withstand repetitive dynamic forces caused by marine waves, tides, and currents.

KEY ELECTRICAL AND MECHANICAL CHARACTERISTICS

• Electrical design stress in the range of 4.5-5 kV/mm.
• WET design, all the electrical characteristics incompliance with IEC 60840.
• EPR (up to 72.5 kV) is really indicated for dynamic and mostly IAC (where WTGs tend to bend) because of its great flexibility, mechanical resistance and elasticity ensuring excellent fatigue performance and combines both mechanical & thermal stress.
• Metallic screen is made of copper wires meshed into braid.
• Double armouring layer to ensure torc balance.
• PE sheath as outer protection increases the bending stiffness which makes installation easier and it is resistant to abrasion.

The energy market has been changing dramatically in recent years, as a result of deregulation and privatisation. To face the challenge of competition, Transmission Systems Operators (TSOs) are optimising their existing resources and new investments.

To support its customers, Prysmian has evolved over the years from the traditional role of cable manufacturer to that of a Global Solutions Provider.

A TOTAL SYSTEM APPROACH

Prysmian focuses on a total system approach to give its customers the lowest cost of ownership for their new and installed cable networks and to provide them with real advantages in terms of asset optimisation.

Besides increasing activity on product innovation to lower investment costs, Prysmian is developing additional pre- and post-sales services for its customers, including network services, enhanced logistics, and engineering studies, to optimise asset management and give the best possible utilisation of transmission and distribution networks.

Following this philosophy, Prysmian provides complete turnkey solutions, including cable installation, which makes it possible to employ its own fleet without having to rely on external contractors.

Prysmian fields six of the most state-of-the-art cable-laying vessels in the world - Leonardo da Vinci, Cable Enterprise, Giulio Verne, Ulisse, Barbarossa, and Monna Lisa (under construction) - as well as its extensive range of well-proven in-house cable protection equipment to provide an enlarged and strengthened submarine cable installation capability.

They offer extended project versatility with deep-water installation capabilities of up to 3,000 m, as well as shallow-water and near-shore solutions, from single cable to bundle cables, using simultaneous lay and burial operation. The company can leverage on an integrated marine assets with full range of high tech burial tools.

Thanks to our focus on technological innovation and our project execution capabilities, Prysmian plays a leading role in partnering with customers in the strategic sector of HVDC submarine cable transmission systems.