66kV Inter-Array Connection Creates ‘Chicken and Egg’ Dilemma

At its National Renewable Energy Centre in Blyth, ORE Catapult recently held a workshop on switching from 33kV to 66kV in offshore wind farm inter-array operating voltage. Following the workshop, ORE Catapult provided Offshore WIND with information on the industry’s work towards switching from 33kV to 66kV.

Offshore wind farms have traditionally used 33kV systems for inter-array power distribution and up to 170kV cables for exporting power to shore. More recently, as wind farms have become larger and generator capacity has increased, the case for doubling the inter-array operating voltage to 66kV for these large-scale installations is widely understood and, in theory, compelling.

At a time when the renewables industry is under great pressure to prove its economic viability, the higher-voltage route offers significant cost-saving opportunities. In spite of this, the change-over to 66kV has not yet happened.

According to recent studies, carried out by organizations such as the Carbon Trust’s Offshore Wind Accelerator (OWA), wind turbines interconnected at 66kV contribute to cost reduction and increase the power density through the cables, creating more cost effective cable systems. This is also more efficient and cost effective for transmitting power to shore.

In spite of this, the change-over to 66kV has not yet happened.

Alex Neumann, Head of HV at the Offshore Renewable Energy (ORE) Catapult, explained why and what is being done to change this.

As offshore developers consider these options, they will naturally seek assurance that all necessary technology is available, certified and with delivery times in line with their project expectations. This has introduced a classical ‘chicken and egg’ dilemma and created the industry’s fundamental barrier to progress. As the OWA states: “Developers will not design a 66kV wind farm without the certainty that certified equipment is available on the market, while the supply chain is hesitant to invest in higher voltage equipment without assurance that a market is available.”

In addressing this, the OWA project is designed to reduce energy costs by 10% in time for Round 3. Their programme of market-oriented R&D involves nine developers. A key aspect of this programme, the Higher Voltage Engineering Design Study, also identified subsea cables as the underlying cause of the ‘chicken and egg’ barrier.

ORE Catapult is also helping industry to move forward. As one of seven Catapults that share £1.7bn investment, it works with industry, academia, research and government with a technical focus on high voltage insulation, switchgear, power conversion techniques and T&D system optimisation.

While the technical know-how being generated by the ORE Catapult and OWA initiatives is essential to the industry’s progress, it must be supported by relevant guidelines, specifications and standards. Some gaps and misalignment exist in these, yet developers and manufacturers alike rely on them to proceed with confidence into equipment manufacturing and deployment. A relatively fast-track route to closing these gaps is offered by CIGRÉ, the Council on Large Electric Systems, which is currently looking to develop approved guidelines which are expected to take between six-nine months.

The technical development initiatives, demonstrators and standards generation work underway should be considered alongside the views and current position of the providers within the supply chain. These vary widely according to whether the provider is involved with equipment manufacture, cable manufacture, or cable installation and termination.

Switchgear manufacturers see an increasing role for 66kV as individual turbine capacity grows to over 8MW. Cable manufacturers are scaling up existing 33kV ‘wet’ designs for 66kV application, rather than using a ‘dry’ design that is typically associated with transmission system cabling applications.

While switching to 66kV creates challenges in terms of subsea cable design and manufacture, it also makes new demands related to the cables’ handling and connection into offshore WTG platforms. HV termination is a skilled job, and poorly implemented connections are likely to fail, while the ensuing costs are higher than for lower-voltage systems. However the expenses of training in terms of both time and money are high, and there is only a limited pool of jointing expertise in the UK.

Additionally, there is no industry standard for jointing; a matrix of industry best practices provides the only available reference points, and different suppliers have different standards.

Subject to these technical and standards-related barriers being overcome, developers certainly want to move ahead. Scottish Power, for example, is seeing offshore wind projects grow and WTGs increase in capacity, and accordingly regards 66kV capability as their primary focus.

[mappress mapid=”14642″]

Source: ORE Catapult; Image: ECN (Illustration)