The UK’s Dogger Bank Wind Farm will feature GE’s Haliade-X wind turbines. At 13 MW they are currently the most powerful in the world. However, there is a next-generation already in planning that will push the power rating up to 15, 16, or even 20 MW. That means we are crossing a threshold of unit power beyond which the change from low voltage (LV) systems to medium voltage (MV) becomes an attractive solution. The MV system will solve some critical problems for turbine manufacturers in dealing with immense currents at the 10+ MW range.

MV converters are proven to deliver the combination of performance, reliability and levelised cost of energy (LCOE) demanded by high-power offshore wind turbines. ABB can speak from practical experience as the first company to put MV converters into serial production, with well over 200 units already in commercial operation.

Photo: ABB

There are some specific challenges that need to be addressed for a more widespread uptake of MV converters. First, there is the focus of the wind industry on LCOE. In the long run, being able to increase annual energy production by several percentage units will generate additional revenues. The price gap between LV and MV solutions will then close as the trend to higher powers and the benefits in turbine design, system cost and additional revenue generation possibilities for owners will justify the move to MV. That calls for a particular focus on value engineering to drive down costs to help customers stay competitive.

Second, there is the need for exceptional reliability. For example, in a traditional wind farm of say 20 turbines, each rated at 3.5 MW, the failure of a single unit results in a 5% loss of output. But if that wind farm is replaced by five mega-turbines then one turbine going offline will result in an unacceptable 20% loss of generation. At the same time, mega-turbines will tend to be installed in more remote areas, where it might not even be possible to gain access for maintenance for six months of the year.

Ensuring reliability depends ultimately on extensive testing in close to real-world conditions and the collection of big data. Through digitalization, it is now possible to measure aspects that were never measured before, such as the temperature changes and switching rates of even the tiniest components. The detailed insight gained from testing and field experience is being built into onboard analytics for MV converters as the basis for remote condition monitoring. The benefit is that any potential reliability issues can be detected at a very early stage, enabling wind turbine operators to take preventive action before it causes a failure.

ABB's MV wind power converters

ABB already has over 200 MV converters in current operation, with a further 95 units destined for Dogger Bank Wind Farm.

Designed for larger turbines, ABB's medium voltage full-power converters are characterised by low parts count, high availability, and low losses. The modular design allows easy customisation to meet customer requirements. The converters are designed for nacelle or tower base installation and enable low-cost and efficient cable installation.

ABB’s global fleet of MV converters will grow considerably over the next two years with the deployment of 95 units to GE Renewable Energy for installation at Dogger Bank Wind Farm. They will enable the 220-meter turbines to produce a total 1.2 GW in the first phase.

The majority of lower power wind turbine installations, especially onshore, will continue to use LV converters very effectively. Yet for the very important and highly demanding systems required by the new generation of large offshore wind turbines, MV is sure to emerge as the technology of choice.