By Chris Poynter, Division President, ABB System Drives
With 13 MW, the Haliade-X wind turbines that GE supplies for the Dogger Bank wind farm in the UK are currently the most powerful in the world. However, the next generation turbines will push the power up to 15, 16 or even 20 MW. This means that we are crossing the threshold beyond which low voltage (LV) systems might find it difficult to cope with the higher currents and losses in generators, converters and cables. This is leading to a trend towards medium voltage (MV) converters which can deliver the combination of performance, reliability, and discounted cost of energy (LCOE) demanded by large offshore wind turbines.
BT is certainly a simple and efficient technology for use at lower power levels. It is also technically feasible at higher powers. The challenge is that the high currents involved require multiple converter modules to be connected in parallel. The space occupied by the conversion system then increases roughly in proportion to its power. This has the effect of greatly increasing the size and weight of the turbine nacelle, as well as complicating the mechanical stability and logistics when mounting the turbine.
In industrial power applications, it is well known that LV is most cost effective at low power levels, while MV is superior at high power levels. The same goes for wind turbines. As their power increases, MV converters become more competitive. The higher voltage means lower currents in the electrical transmission, which in turn allows the use of smaller cables with a smaller converter footprint and less weight. In addition, MV converters improve overall turbine efficiency through the use of IGCT (integrated gate-commutated thyristor) semiconductor technology.
Although the technical advantages of MV converters are clear, their widespread adoption still poses challenges. The first is simply that there is only one driving force in renewables these days: how to make it cheaper than conventional energy. This manifests itself in increased pressure on the prices of turbine manufacturers and their suppliers. Everyone is talking about total cost of ownership (TCO), and that is of course an important consideration. Yet it is the initial capital cost that is most often the deciding factor.
Proving the financial attractiveness of MV technology must take into account the investment costs for the entire electric kinematic chain. This includes the converter, switchgear, cooling systems, control and other ancillary equipment. Other associated costs, such as wiring and the impact of converter size and weight should also be included in the calculation. Ultimately, this requires a special focus on value engineering to reduce costs and help customers stay competitive.
The second challenge 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 production. But if that wind farm is replaced by five mega-turbines, taking one wind turbine offline will result in an unacceptable loss of production of 20%. At the same time, mega-turbines tend to be installed in more remote areas, further offshore, where it might not even be possible to access them for maintenance for six months of the year.
MV converters have an inherent advantage in terms of reliability, as they use fewer components than LV models. Manufacturers strive to take this to the next level with an emphasis on design for reliability. A small example is the use of self-healing components, such as capacitors which can restore their insulation properties after a failure. Encoders and fuses are also examples of components that tend to fail due to aging. MT converters can be designed to operate without encoders, using software control instead. Additionally, fuses can be removed using advanced circuit breaker control algorithms.
Ensuring reliability ultimately depends on extensive testing in near real-world conditions and the collection of big data. Thanks to digitization, it is now possible to measure aspects that had never been measured before, such as temperature changes and the switching rates of the smallest components. Detailed information gained from testing and field experience is incorporated into on-board analyzes for MV converters as the basis for remote condition monitoring. The advantage is that any potential reliability issue can be detected at a very early stage, allowing wind turbine operators to take preventative action before it causes failure.
Although MV converters are the technology of the future for larger offshore wind turbines, they are already a well-established business proposition. In fact, ABB currently has more than 200 units in operation in the North Sea, the Baltic and off the coast of China. This fleet is set to grow considerably over the next two years with the deployment of 95 units at GE Renewable Energy for installation at the Dogger Bank wind farm. They will allow the 220 m turbines to produce a total of 1.2 GW in the first phase.
LV converters will continue to dominate the majority of installations, especially onshore. On the other hand, MV converters will always remain a niche product of relatively low volume. However, in the very important niche occupied by the largest offshore wind turbines, MV is sure to establish itself as the technology of choice.
Chris Poynter is Division President, ABB System Drives. The System Drives division is a global supplier of high power, high performance drives, drive systems and packages for industrial process and large infrastructure applications. In addition, the division offers global support to help customers, partners and equipment manufacturers improve asset reliability, improve performance and energy efficiency in mission-critical applications.
Chris originally joined ABB in 1980 working with industrial motion and energy technology clients in the marine, utilities and process industries. He has held various management positions in ABB’s drive, process automation and service businesses, working in Australia, Canada and Switzerland. Chris holds a degree in Electrical Technology and an Executive MBA in Marketing.
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