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The Magazine

Offshore wind

An offshore substation slims down

Three words tell the story: simplified, standardized, and shrunk. Behind these qualities of an Offshore Transformer Module (OTM), a new AC substation, is one big reason: the need to cut the costs of wind power offshore.

Wonderful as wind power is, its offshore version is challenged to bring the price down. From 2015 to 2020, European governments want to see a one-third reduction in the tab for generating a kilowatt hour at sea. That’s a mighty cut, and Siemens is at the forefront of making it happen. Cue to a key initiative: the alternating current (AC) Offshore Transformer Module, or OTM.

Its genesis came from Siemens’ first AC offshore platform, the 300-megawatts Thanet project off the east coast of England that went online in 2009. While this showed increasingly detailed specifications, better functionality and higher power ratings, it also brought an increase in costs. This prompted Siemens to convene a task force, with representatives of all relevant divisions, to draw on their extensive offshore wind experience as well as to consider the best ideas from a broad cross-section of customers, partners, suppliers, and other key industry stakeholders.

Over eleven months in 2014 to 2015, the group created a design for future AC platforms. According to Andreas Barth, Siemens’ Head of Grid Access Solutions, their OTM is the new gold standard. Four key features set this third-generation model apart from conventional, more bespoke substations currently in the market:

•    Simple design, with no moving parts;
•    more environmentally friendly, by making the use of mineral oil obsolete;
•    one-third less weight and volume than a conventional AC platform; and thus,
•    a cost reduction of up to 40 percent.

Meanwhile, the OTM has morphed from idea to order. Scottish energy giant SSE has contracted to install the first of these AC transformers in its 588-MW Beatrice wind
power plant
. To provide the required transmission capacity, the OTM’s will be linked together. The new wind power development will go live in 2019, off the northernmost coast of Scotland.

The innovations and advantages of the new Offshore Transformer Module in a nutshell.


Other industries have already noted and leveraged the advantages of a standardized, modular approach in complex engineering projects. Rather than building major equipment directly on the deck of an offshore platform, which brings increased interfaces and critical paths, the gear is instead built into standard-sized containers. Such containers can then be mounted on a simple platform deck at the fabrication yard. The process is analogous to the “plug-and-play” principle in the consumer electronics industry. Modular assembly cuts out an enormous amount of cost, Barth points out, and it means Siemens can leverage a true standardized approach across multiple projects at all levels of the supply chain.

Repairs done on land are about ten times less expensive than doing the same on the platform itself.
Andreas Barth, Siemens’ Head of Grid Access Solutions

Maintenance is also simplified. Rather than bringing specialists to malfunctioning equipment, a modular approach allows the customer to decide whether to rectify faults offshore in minor cases, or to bring the equipment onshore, replacing it with an immediately available spare unit. Repairs done on land, Barth notes, “are about ten times less expensive than doing the same on the platform itself.” Downtime is nearly eliminated, because units can be exchanged altogether with replacement units held as spares – another case of “plug-and-play”.



A more compact AC substation, the Offshore Transformer Module helps cut operational and maintenance costs.



Smaller, leaner switchgear
Both high- and medium-voltage transformers have been optimized, increasing capacity while minimizing bulk. Fans and pumps have been completely eliminated. Neither do the transformers require any mineral oil. Instead, they employ a non-flammable, highly biodegradable fluid called ester liquid. It has two advantages over oil, as it allows an elimination of the fire-suppression system and is a much lower risk for marine life.
Siemens’ task force also realized that cutting total mass to below 1,000 tonnes could bring about a step change in cost reduction. Below that threshold, the OTM can be installed by conventional ships and cranes already deployed on the wind farm for wind turbine and foundation installation, as opposed to large specialized vessels that are much more costly, much less available, and subject to greater weather restrictions in their operations.
So how did the task force get below a kilotonne?

Firstly, the new solution obviates the requirement for dedicated compensation reactors –
a large piece of equipment that is no longer needed offshore – by better utilizing the given capabilities of the wind turbine generators.

The second step, not obvious at first but brilliant in retrospect, was to split the substation into two smaller ones, each of which can be mounted on separate foundations that are almost identical to those used for the wind turbines themselves, thus eliminating the need for a large, bespoke foundation. Smaller OTM’s also allow the utilization of a much broader, potentially more local supply chain for construction, something that many customers find valuable.

Combined savings
Another future installation option is to attach the OTMs to two wind turbine foundations. No additional foundation is needed to support the two modules.

All told, each new substation module weighs around 630 tons (generating 250 megawatts each), a reduction of nearly 60 percent from conventional designs. For larger capacities, multiple modules can be used. Given that two thirds of offshore capital is typically spent on building and installing platforms, this translates into a serious reduction of future project costs.

Eric Johnson writes about technology, business and the environment from Zurich.
Picture credits: Siemens AG