Design and development of megawatt wind turbines - Wind Power Generation and Wind Turbine Design - page 224

The hydraulic design requires either electric power to be transferred across slip
rings to a hydraulic pump in the hub, which in turn supplies the individual blade
pitch cylinders, or pass hydraulic power through a rotating coupling to feed the
individual pitch cylinders. An accumulator is provided as part of the system design
to ensure that the blades can be pitched to the feathered position (under normal or
emergency stop conditions) for the case of hydraulic pressure failure.
Electrical pitch systems require electric power passed to the pitch motors
through slip rings and a backup battery system in the hub. The battery system
requires a charging system and condition monitoring with electric switchgear also
located in the hub. Batteries typically last 6

7 years so that at least two complete
replacements are required over the typical 20-year life of the turbine. Other design
considerations include how compact the hydraulic system may be relative to the
electrical design and the level of parasitic power requirements; i.e. does it take less
power to actuate and control a hydraulic or electrically pitched turbine.
Future large turbine pitch bearing and drive system technology may incorporate
elements of piecewise pitch angle adjustment as a function of the blade radius or
incorporate some form of effective pitch angle induced by a number of smaller
local fl ow control devices.

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4.4.3 Rotor hub
The hub mass versus machine rating is presented in Fig. 22. The industry study set
trajectory lies between the 10-turbine analysis group results for the “partial hub”
and the total hub mass that includes the pitch bearings and drive systems. A total
hub mass on the order of 110 tonnes can be expected for today's technology
projected to the 10 MW turbine size. A reasonable stretch target for improved

Figure 22 : Hub mass − 10-turbine analysis compared to industry study set.