Cabling standards hold key to cutting costs

The offshore wind sector is under significant pressure to reduce costs. One of the sectors where large savings could be made is the offshore electricity transmission network. We assess the contribution the German offshore network plan will make

The 800MW HVDC converter platform for BorWin2
The 800MW HVDC converter platform for BorWin2

Cutting costs is more than a wishful aspiration for offshore wind power. Its very existence "depends substantially on whether the cost reduction potential of 40% assumed by the industry can actually be achieved", Germany’s federal environment minister, Peter Altmaier, said in a recent position paper.

Germany’s offshore-wind cable links to shore have so far been planned on an individual basis. For instance, each of four German North Sea connections by Siemens has a different capacity: HelWin 1 with 576MW, HelWin 2 with 690MW, BorWin 2 with 800MW and SylWin 1 with 864MW. This means that a tailor-made design is needed for each of the marine converter stations where alternating current delivered by the offshore wind farms is converted to direct current for cable transmission over tens of kilometres to shore.

The cable voltages also vary at 250kV for HelWin 1, 300kV for BorWin 2 and 320kV for HelWin 2 and Sylwin 1, reflecting the largest size of cable that happened to be available at the time each system was ordered.

Standardisation plan

With around 25 converter platforms needed for 25GW of planned German offshore wind, and with parts of the UK Round 3 projects to be located more than 80 kilometres from the coast, it makes sense to look at standardising cables links. This would lower costs by enabling converter platforms to be connected to each other without extra equipment or software adaptations.

The German offshore network plan — Bundesfachplan Offshore (BFO) — for the North Sea was presented on 22 February. It lays out the key elements of a standardised, efficient and coordinated layout that can deliver secure operation for direct high-voltage cables to transmit offshore wind energy to the onshore network.

Produced by the federal shipping and hydrography office, Bundesamt für Seeschifffahrt und Hydrographie (BSH), which is part of the German transport ministry, it will be updated every year to keep pace with technical and project developments.

The BFO divides the offshore projects planned for Germany’s North Sea into 13 clusters with a total capacity of 21GW, but assumes that only about 11.7GW will be installed by 2022. It sets out far-reaching new rules and conditions that German offshore wind needs to comply with in order to respond to the requirements of the Energy Industry Act. This calls for a secure, reasonably priced, consumer-friendly, efficient and environmentally compatible electricity supply that increasingly relies on renewable energy.

This underlying philosophy has led to the decision to prioritise offshore wind projects that lie within 120 kilometres off the shore. The necessary cables might otherwise be unavailable, the BFO points out. "Delivery times are already long and would be lengthened even more with increasing cable lengths," it says. Also, cable manufacturers are unable to make cables to the length required, calling for the use of connecting sockets — increasing the risk of cable failure with every extra socket.

New rules

New standard procedures and technologies introduced by the BFO are open to discussion and modification if later developments should require it. Voltage-sourced converter (VSC) technology for direct-current systems must be routinely used under the new rules. This technology allows self-start of a network connection after a fault without reactive power from a connected alternating current network. Using a direct current system without this capability would require additional power generators on the offshore converter platforms. The VSC technology also allows the use of polymeric cables, which are more widely available in the market and can be produced more swiftly than paper-oil insulated earth cables, according to the BFO.

The high-voltage direct current transmission cable voltage is to be standardised at 320kV, the maximum currently possible with polymer cable technology. Also, cable capacity of 900MW is required. At this voltage and capacity, the seabed sediment in which the cable is laid will only warm — due to cable heat losses — within environmentally acceptable limits, says the BFO. Investigations will be made on raising system capacity to 1,000MW, it adds.

Pairing up

A "mother-daughter" concept should contribute to security of electricity transmission. Pairs of converter platforms are sited 30-50 metres apart and connected to each other with an alternating current cable system. These could then share facilities such as a helicopter platform and living quarters to bring down costs, and one converter could supply emergency power if the other suffered an outage. The idea is still being explored to weigh up possible disadvantages, such as the cost of drawing up two converter platform designs (mother and daughter) and the additional risk in the case of a ship collision if the two are sited close to one another.

The BFO sets a standard of 155kV with a frequency of 50Hertz for the alternating-current inner array cabling system for wind farms. Some industry players have called for higher voltages and developed corresponding cables, but these have not yet been tested.

A higher voltage will carry an added risk of faults, for which electricity customers would ultimately end up paying via the "offshore levy" introduced at the beginning of 2013. It will also require larger converter platforms to accommodate the large switching gear and other facilities. But the platforms have already reached the limit of the sizes for which designs currently exist, states the BFO. On the positive side, a voltage of 155kV per cable allows a system capacity of up to 200MW for each circuit, which would allow groups of wind turbines to be brought online consecutively within a wind project, the BFO points out.

Future set-up

In future, each converter station is expected to have two connections for the high-voltage direct current (HVDC)system to shore, six switchgear panels for alternating current systems from the offshore wind farms, two for alternating-current connections with each other, two reserve switchgears and possibly mother-daughter and other connections.

Looking ahead, the BFO has also decided that transmission cables to shore should be connected to each other at their marine ends to improve system security. At the moment, HVDC cable technology for such connections is not available, but the converter stations should be designed to allow such connections to be implemented if and when it becomes possible.

Standardisation does not only concern the transmission systems. The BFO says the converter platform should occupy a marine area of 100 metres by 200 metres, somewhat larger than the current 65 metres by 105 metres. The extra space is needed to accommodate jack-up barges for installation purposes.

Two platforms side by side need a total area of 600 metres by 200 metres to allow space for ships to manoeuvre, while three platforms would need 600 meters by 600 metres. Transmission system operator Tennet even called for an area with a radius of up to two kilometres around converter stations to be kept free in spatial planning to allow plenty of space for cable laying and ship movements. But these demands are not compatible with the need to limit space use and to avoid creating stand-alone constructions in the marine environment, declares the BFO.

It is difficult to predict to what extent this new era of offshore technology standardisation will bring down costs, but it should be a big improvement on one-off systems that can hardly be connected with each other and that contribute nothing to the vision of a meshed offshore transmission network in the North Sea.

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