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4.6 Controls
WT control systems enable and facilitate smooth operation of the WT and the
overall WPP. At the turbine level, the control system is responsible for reacting to
changes in local wind speeds to generate the optimum level of power output with
the minimum amount of loads. At the operational level, control systems collect
data for automating decisions and for a number of downstream analyses. They also
support communication and information fl ow throughout the turbine and power
plant components. At the power plant level, control systems integrate all of the
above with grid conditions. Control systems continue to rapidly evolve, resulting
in steady performance enhancements from existing turbine hardware. These same
advancements are providing new opportunities for future large turbine designs.
4.6.1 Turbine control
A WT control system is used to control and monitor the turbine sub-systems to
ensure the life of components and parts, their reliability and meet the functional
performance requirements. The fundamental design goals are safety, reliability,
performance and cost. The turbine must produce the energy advertized for the
wind conditions actually experienced - and do so for the life of the machine at the
cost provisioned in the customer pro forma.
Variable-speed pitch controlled WTs are the most advanced control architecture
and state of the art for today's MW WTs (see WT Type C, Fig. 9). To operate a
WPP under optimum conditions, the individual turbine rotor-generator speeds are
controlled in accordance with the local wind speeds using generator torque (elec-
tric current control) or blade pitch angle adjustment. Figure 35 illustrates the main
considerations for a typical control scheme used in today's MW WTs.
A turbine is in the standstill state with the high-speed rotor brake applied when
the turbine is down for maintenance.
The typical turbine condition with little or no wind is the idling mode with the
blades feathered and the rotor near standstill or gently pin-wheeling.
The spinning state can best be characterized as increasing winds starting from
dead calm and approaching cut-in wind speed, while the blades are pitched to
an intermediate blade angle (see sketch /B/ of Fig. 17) so that the rotor can
proceed to accelerate during the run-up condition.
Figures 35 and 36 can be cross-referenced to better understand the continued
sequence of control steps for the example case of increasing wind speeds (e.g. the
approach and development of a storm front):
Once the cut-in rotor speed has been achieved (condition (1) of Fig. 36), the
blade pitch angle is advanced to the full operational position (see sketch /C/
of Fig. 17).
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