Analysis: Siemens' radical substation plan

GERMANY: Siemens announced new significant potential for cost saving in grid technology for offshore wind at the EWEA Offshore conference and exhibition in Copenhagen last week.

The company is marketing an alternating current (AC) technology solution for nearshore projects — these use only AC technology —and for the AC part of farshore projects.

Siemens also heralded a new high-voltage direct current solution for farshore projects, available from 2016, also with substantial cost cutting potential.

All this is not before time. "Society can't afford offshore wind at the current levels," said Michael Hannibal, Offshore CEO at Siemens Wind Power. "It is no use leading in an industry if the industry is not surviving," he added in the same session.

Elsewhere, delegates heard that UK Round Three projects may not materialise at the current level of high-voltage direct-current grid costs. In Germany, the government may pull the plug on offshore wind — most of it being farshore as nature protection areas largely rule out wind energy use nearshore — because it is not competitive with other types of renewables.

Cutting grid costs

The grid portion of offshore wind accounts for 32% of the current levelised cost of offshore wind generation (LCOE) of €0.145/kWh, according to Siemens data. The company aims to cut the grid share to 28% of a maximum LCOE of €0.10/kWh by 2020. This means grid costs have to drop from €0.0464/kWh to €0.028/kWh, or by €0.0184/kWh, which is equivalent to a nearly 40% reduction.

A new Siemens innovation in AC connections, used up to about 100 kilometres from shore, will achieve a grid system cost reduction of 30-40%, compared with the traditional solution, said Tim Dawidowscky, CEO of transmission solutions at Siemens Energy. This could also mean a 10% reduction in the whole offshore system.

This could account for roughly one third of the 31% cut in LCOE required between 2015 (€0.14.5/kWh) and 2020 (€0.10/kWh).

Siemens' radical step is to do away with the AC substation in an offshore wind farm, dividing the necessary equipment into 250MW transmission capacity modules dubbed offshore transformer modules (OTM).

These are mounted on special "balconies" fitted to turbine foundation transition pieces. A 500MW wind farm would have two turbines fitted with the equipment on their foundation structures after the necessary structural load adjustments have been made. Alternatively the OTM is fitted on a separate turbine foundation (without a turbine).

Each OTM is just one third of the weight of an AC platform, eliminating the need for special heavy-lift vessels and shortening the installation time by about 20% due to better availability of standard service vessels, the company said at the product launch.

Changing converter technology

At far-shore projects, sole use of AC technology is not an option, due among other reasons, to the substantial losses over longer distances. Yet the huge high-voltage AC to DC (direct current) voltage source converter platforms are an enormous cost factor. The structures stand to a height of 54-metres above sea level, or twice as high as Berlin's Brandenburg gate, on top of 30-metre deep foundations under water. The technology works — but the cost is prohibitive.

The design, developed from an idea originated at the University of Valencia in Spain, replaces the air-insulated converter and air-insulated DC switch gear — which requires much space and costly air conditioning — with a diode rectifier unit (DRU) solution.

"Diodes are simple, robust, with low losses, can be stacked one after the other, and can be encapsulated," enthused Peter Menke, head of new DC access development at Siemens. A 0.9GW voltage source converter (VSC) requires 50,000 cubic metres of space.

The diode rectifier unit with a 1.2GW rating requires three platforms each with a pair of DRUs converting from 66kV to 106.7kV strung together to add up to 320kV as provided by the VSCs used today. The three platforms have a total volume of just 6,500 cubic metres, or a nearly 90% volume saving on the VSC platform.

One large VSC platform weighs 26,000 tonnes. Each new DC platform with 2 DRUs weighs just 3,000 tonnes, Menke said.

Limitations

Whether wind farm planning can be adjusted to the optimal 1.2GW transmission capacity remains to be seen. For instance, in Germany from 2021, only 800MW of new capacity is envisaged each year.

A drawback is that diodes work in only one direction, to export power. But wind turbines need auxiliary power to maintain systems, for example when there is a grid outage. An additional AC cable is therefore needed running parallel to the DC cables from the onshore network to supply this power when needed.

"We have cooperated with cable manufacturer Prysmien on this with a cost-effective solution in which the additional cables don't significantly increase the cable costs," Menke said.

On top of that, rectifiers are simple instruments unable to perform some of the tasks currently perfomed by the VSCs. With the VSC replaced by the diode solution, software changes will be required of the wind turbine controls to maintain voltage and frequency. "This can be done, but it is a new development to ask the turbine to do it," Menke said.

A joint industry approach is needed here, he said, but overall, the new development "will cut the Gordian knot of farshore grid transmission costs".

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