Doubling down

Friday, 28 August, 2015 - 10:15
´Stepping toward 66 kV cables may also have an impact on the minimum bend radius of the cable. (Photo: VBMS)
´Stepping toward 66 kV cables may also have an impact on the minimum bend radius of the cable. (Photo: VBMS)

Lower costs and less impact on the environment is the reputation that precedes the 66 kV cable. Technical problems remain to be solved, and some scepticism persists in the industry. Still, the trend is nearly unstoppable. Like it or not, 66 kV is coming.

Peter Hungershausen, a Sales Manager at Norddeutsche Seekabelwerke (NSW), is bullish about the future. “Over the next few years, it will be the big thing.” The “it” he is referring to is 66 kV inter-array cable. Hungershausen sees a clear trend toward a shift to 66 kV. Although the transition from 33 kV to 66 kV cable still seems to be in its infancy, Hungershausen remains completely un­fazed. “We’re keeping an eye on the market,” he says.

Others are less cool-headed. Nexans, which considers itself the technology leader, presented a type approved 66 kV cable at this year’s Hannover Messe. Currently, the three-wire 630 mm2 copper cable is undergoing its final long-term testing.

This forward-looking development takes place against the backdrop of rising offshore wind turbine generation capacities. Windmills in the 6 MW class are now commonplace, and 8 MW machines are in the ­pilot phase. Turbines with even higher capacities are in the works. If the industry continues using stan­dard 33 kV cable, it will have to plan for increasing the transformer capacity. Put another way, 66 kV will ­reduce the need for more transformer substations. The 66 kV cables would also make it possible to maintain the ­conventional ­offshore layout, strings of up to five wind turbines ­connected in series, which in turn would lower the cost of wind farms significantly, according to a Nexans press release.

The 66 kV cost cutter?

Klaus-Peter Lehmann, the Head of project development at Thüga Erneuerbare Energien, sees the transition from 33 kV to 66 kV cable in a less than positive light. “I think this is a systemic issue,” said Lehmann. “The whole system has to be pulled along with it.” Transmission losses are lower at higher voltages, Lehmann explains, but he points out that this benefit could be cancelled out by the higher expense associated with the new cable. He could imagine using the technology in future offshore projects but not as an upgrade to existing wind farms.

Kevin Todd, a Senior Project Manager at RES ­Offshore, points out other impacts: “The increase in the array cable voltage from 33 kV to 66 has a number of impacts on the technology employed in the transmission asset. The turbine transformer steps up to a higher voltage and therefore contains more coils. The turbine switch gear has a higher rating.” On the other hand, he says, “The array cable conductor diameter can be decreased as the losses in the cable reduce for a given cross sectional area. This means the weight of the array cable – per unit length – reduces,” he says.

This benefit aims to trim the amount of material for cabling and thus the cost of inter-array cabling. The internal cable network connects wind turbines with each other and with the substation. A significant amount of cable is used for internal lines. Cable installers laid a 60 km long network for the relatively small wind farm of Nordsee Ost, which has just 48 wind turbines.

Todd also points out some differences in laying the new cable. “The overall cable diameter may not change dramatically as the size of the dielectric insulation increases. This may also have an impact on the minimum bend radius of the cable. The step-up requirement for the substation transformer decreases and therefore the equipment contains fewer coils.”

Hard data not available for another few years

The discussion of the design of inter-array cables may be somewhat overheated at the moment. This could continue for a while as it will take a few years to gather initial experience with this technology. Suitable connector solutions are still unavailable. In order to master the technical demands of offshore wind, the industry has joined forces with the British energy ministry and, with the support of the Carbon Trust, has established the Offshore Wind Accelerator (OWA) development programme. In the summer of 2014, Nexans was among the companies receiving support under this programme. Nexans was awarded € 165,000 to continue development of its 66 kV technology. This can be seen as a signal and evidence of the relevance of this technology.

However, 66 kV is unlikely to turn the offshore wind industry on its head. “In terms of offshore cable installation, not much changes as the small change in the cable diameter may have some small impact on bending radius, but this can generally be accounted for in the design – cable handling parameters and conventional cable laying and installation techniques can still be ­employed,” says RES-Manager Todd. He adds, “The maintenance requirements for a 33 kV cable are therefore very similar to those for a 66 kV cable. By the same token, related systems, like transformers and switch gear, also follow established maintenance regimes.” But there is still plenty to discuss.

Jörn Iken

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