The latest large offshore wind turbines are rated at 14 MW and the next generation is set to take this up to 20 MW
Operating at high power is challenging for traditional low-voltage (LV) converter systems. The answer is to switch up to medium voltage (MV) technology.
The UK’s 3.6-GW Dogger Bank offshore windfarm will become the world’s largest offshore project of its type when completed in 2026. The first two project phases are using 220-m, 13-MW Haliade-X wind turbines supplied by GE. The third phase is using the 14-MW version of the Haliade-X, the most powerful offshore wind turbine in operation today. The drive for increased power will not stop there.
To illustrate the trend for ever-higher power, the average rated capacity of turbines installed in 2020 increased to 8.2 MW (source: Wind Europe Trends and Statistics), and two thirds of all offshore windfarms used turbines even larger than this. This average capacity is up from 7.2 MW in 2019 and 6.8 MW in 2018.
Demand for higher power ratings means that wind turbines have crossed an important threshold. This is because traditional LV systems can struggle with the higher currents and losses in generators, converters and cables. That means MV converters now offer an attractive alternative capable of delivering the essential combination of performance, reliability and levelised cost of energy.
LV converters are well proven for use with lower power wind turbines and they can also be used at higher powers. However, multiple converter modules need to be connected in parallel to cope with the higher currents. This has an impact on the space occupied by the converter system, which increases roughly in proportion to the power rating. To accommodate the extra equipment, the size and weight of the turbine nacelle will increase considerably.
Experience from industrial power applications is that LV converters are most cost-efficient at low power levels. But when we move to higher power, MV is preferable. Unsurprisingly, the same applies to wind turbines. This is because, for the same power, a higher operating voltage enables the electrical drivetrain to operate at a lower current. The result is that the size and weight of the converter can be reduced along with ancillary equipment such as cables. Using MV converters also provides a boost for the overall turbine efficiency since they use IGCT (integrated gate-commutated thyristor) semiconductor technology.
However, there are challenges for the widespread adoption of MV converters. The first is that renewable schemes are being driven increasingly by the need to be cheaper than conventional energy. Indeed, while total cost of ownership is acknowledged as being important, there is a brutal focus on the initial capital cost. This translates to severe price pressure on both turbine manufacturers and their suppliers.
To prove that MV technology is financially attractive, it is important to consider the investment costs for the complete electrical drivetrain including converter, switchgear, cooling systems, controls and auxiliary equipment. The case is also helped by considering the capability of MV technology to reduce associated costs, such as cabling, as well as lowering the size and weight of the turbine assembly. Value engineering can also play an important role in helping to drive down converter costs while maintaining a high level of performance and reliability.
The need for exceptional reliability provides the second challenge. In a windfarm of 20 turbines, each rated at 3.5 MW, one unit going offline is inconvenient rather than catastrophic. The loss of output is 5% . However, if the same 70-MW capacity is replaced by five mega-turbines, each rated at 14 MW, then one turbine going offline results in an unacceptable 20% loss of generation. The situation is made even more challenging since the trend is for windfarms that utilise mega-turbines to be deployed in much more remote areas, further offshore.
A benefit of MV converters is that they have a reduced component count compared with LV models, which provides an inherent advantage with respect to reliability. A focus on design for reliability can take this to the next level. As an example, in some cases it can be possible to use self-healing components, such as capacitors that can restore their insulation properties after a breakdown. Other components that tend to fail due to ageing, such as encoders and fuses, can be eliminated from the design by managing the same functions with advanced software and advanced breaker control algorithms.
The key to ensuring the reliability of MV converters is carrying out extensive testing in close to real-world conditions and the collection of big data across every aspect of their operation. By embracing digitalisation, development engineers can measure factors that were never considered before, such as the temperature changes and switching rates of even the tiniest components.
The onboard analytics built into MV converters can use this detailed insight as the basis for remote condition monitoring. This makes it possible to detect any potential reliability issues at a very early stage, enabling wind turbine operators to take preventive action before they result in failure.
There is a clear case for MV converters as the future technology for the largest offshore wind turbines. However, they are already a well-established commercial proposition. Currently, ABB has over 200 units in operation in the North Sea, Baltic and off the Chinese coast. Over the next two years this fleet will grow considerably when GE Renewable Energy deploys 95 units for the first 1.2-GW phase of Dogger Bank.
MV converters will not replace LV converters. They are always likely to be a niche and relatively low volume product, while LV technology will continue to dominate the majority of installations, especially onshore. However, the new generation of high power offshore wind turbines is a very important niche. That is where MV technology will play a vital role in helping turbine manufacturers deliver performance and reliability at the lowest possible cost.
*Chris Poynter is division president, ABB System Drives
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