PCS6000 wind converter for 5 MW offshore wind turbines

　　Wind power has experienced stupendous growth levels over recent years. Not only are countless new installations going online, but the turbines are growing larger and more powerful. The largest turbines now generate 5MW of power and stand almost as high as the spires of Cologne’s worldfamous cathedral.

　　Wind farms are increasingly being built offshore or in sparsely inhabited regions, making not only their construction more and more challenging, but also their connection to the grid. ABB’s PCS 6000 converter addresses the latter issues. The converter fits neatly inside the mast of the turbine. Besides converting the generated power to the desired frequency and voltage, it helps support weak grids by supplying or absorbing reactive power.

　　The Alpha Ventus offshore wind farm is a pioneer project undertaken jointly by E.ON Climate and Renewables, EWE and Vattenfall Europe. Situated some 45 kilometers north of the German North Sea island of Borkum, in water about 30 meters deep, Alpha Ventus is the first German wind farm to be erected at sea under genuine offshore conditions. The design, construction, operation and grid integration of the Alpha Ventus research project as a test field will help build up fundamental experience with a view to future commercial use of offshore wind farms.

　　The plan is to erect six Multibrid M5000 and six Repower 5M wind turbines. An offshore transformer station will be located at the south-eastern corner of the wind farm. An on-shore control room will be set up to supervise the operation of the turbines. The feeding of power into the German grid will be handled by the transmission system operator E.ON Netz GmbH.

　　The total capacity of offshore wind energy is estimated to be between 20 and 40 GW. This will be harnessed in the European Union by 2020.

　　The first six wind turbines will be erected on an area of four square kilo meters. They will be arranged in a rectangle with four north-south parallel rows of three turbines each. In the resulting grid-like formation, the wind turbines will stand approximately 800 meters apart.

　　Including its rotor, each wind turbine is about 150 meters tall, almost as high as Cologne Cathedral. The six wind turbines supplied by Multibrid will be anchored to the seabed by a tripod structure. The depth of the water at this site is around 30 meters. It would take 56 men to surround the triangular area of 255 m2 on which the tripod stands. The amount of steel in each unit (around 1,000 tons) weighs as much as 200 adult elephants or 22 railcars. The rotor receives the wind over an area about one and a half times the size of a football pitch. When the rotor is turning at maximum speed, the tips of the blades cut through the air at around 300 kilometers per hour.

　　Technology from ABB

　　Wind turbines whose synchronous generator is excited by a permanent magnet need to be connected to the utility grid via a full-scale converter.This arrangement allows wind turbines to be utilized at their optimal point of operation, and transmit the energy to the grid with high efficiency. With larger turbine unit sizes, medium-voltage converter systems are most suitable for handling the corre sponding high power. The flexibility of industrial full-scale converter-based systems allows easy adaptation to different operation modes and grid requirements. Three important issues are of concern in using a power electronic system. These are reliability, efficiency, and cost

　　System description

　　Today, turbines with a power greater than 2 MW are mainly variable-speed turbines. For offshore applications, where low maintenance requirements are essential, a turbine whose generator is excited by a permanent magnet is widely considered the preferred solution. This solution calls for a fullscale converter. The conversion efficiency of this system is very competitive, especially in partial load operation

　　In order to benefit from the broad experience gained from this kind of application, the converter is based on a standard industrial design. This will increase the reliability of the new turbine generation owing to the reduced failure rate achieved by the simple system structure.

　　The PCS6000 wind converter is based on modular power electronics building blocks (PEBB) using high-power semiconductors. This approach enables the development of remarkably compact converters

　　For 5 MW wind turbines, ABB made a four-quadrant PCS6000-based converter. It uses standard IGCT (integrated gate-commutated thyristor) – PEBB technology. The complete converter is based on a PEBB platform, a control platform and a platform for mechanics. This brings high advantages in terms of costs, quality and reliability. The PCS6000 combines two NPC (neutralpoint connected) phases. It achieves an advantageous power density – an important factor for converters located inside the wind turbine. shows the PCS6000 5 MW converter placed inside a tower of the offshore installation .

　　The PCS6000 Wind converter needs only one platform inside the tower with all necessary auxiliary components such as the water cooling system, grid filter and generator dv/dt filter. The very compact design allows access from all sides for service and maintenance. The converter system is controlled by the ABB AC 800PEC Programmable Logic Controller.1) All measurement and control connections from the control system to the medium-voltage compartment are isolated via fiber optic links. Only one pair of fiber optic links is necessary for the communication between the PCS6000 wind-converter control and the auxiliary cabinet for the cooling and filter functions. This ensures an operation of the system that is immune to disturbances caused by EMC. The IP54 (ingression protection) design of the cabinets assures durability in the face of condensation inside the tower. Care and attention is applied to the design of all components with regard to withstanding possible vibrations. To secure the safety of maintenance and service personnel, the converter has an earthing switch and fail-safe door interlocks.

　　The PEBB-concept has significantly improved serviceability 3 . Components can be exchanged in a convenient and swift way, without disconnecting bus bars or cooling pipes. The double stack can be opened very easily by discharging a spring, then spreading the heat sinks with a tool, permitting the IGCT to be removed from the stack . The PCS6000 wind converter does not even require fuses: The intelligent protection system prevents mechanical damage caused by a semiconductor failure. Broken semiconductors cannot cause fault arcs.

PCS6000.pdf

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