Environmental Engineering Reference
In-Depth Information
As the turbine approaches rated power or condition (4), the blades are com-
manded to begin to pitch slightly back towards feather to lower the angle of
attack and reduce peak loads. This prescribed schedule is also known as a
“peak shaver.”
Implementation of a peak shaver comes with a reduction in generator output, so
that the turbine designer must balance the benefi ts for reducing peak loads,
material savings for the affected components, and the lower energy yield.
From conditions (2)
(3), a torque command regulates the RPM for the optimal
rotor tip speed ratio (TSR). The torque command is equal to a prescribed
function of RPM.
From conditions (3)
−
(4), a torque command regulates RPM based on the
converter setting and the generator current.
From condition (4)
−
(4”), where (4”) is the turbine shutdown or cut-out wind
speed, a current control maintains rated power output and the blades are pitched
more and more towards feather to regulate the rotor RPM. This pitching is done
to unload the blades with higher and higher wind speed to reduce WT loads by
shedding the excess power that would have otherwise been captured (with
massive loads to the turbine) beyond the generator rating.
A typical cut-out wind speed for a modern MW WT is 25 m/s. When the
−
machine reaches (4”) and shuts down, the blades are pitched to the full-feathered
position and the nacelle continues to be yawed into the wind. This keeps the
turbine in a low drag confi guration to ride out the storm.
Referencing a WT designed to IEC TC1 criteria (as an example), wind speed
increases beyond cut-out are provisioned throughout the turbine structure for
survival wind speeds of 50 m/s (10-min average) and 70 m/s (3-s average).
This is equivalent to hurricane intensities in the border region of category
II
−
III and category IV
−
V in accordance with the Saffi r-Simpson [ 36 ] scale,
respectively.
Boes and Helbig [37] give an alternative description of this process, and is pro-
vided here for additional context - “There are two modes of operation: speed
control at partial load operation (control of torque) and speed control at full load
operation (pitch control). Torque control: To achieve the optimum power yield, the
speed at partial load is adjusted to obtain an optimum ratio between the rotor speed
at the circumference and the wind speed. The blades are set to the maximum pitch.
The counter-torque at the generator controls the speed. Pitch control: After reach-
ing the maximum counter torque at the generator (nominal power) at nominal
wind speed, the speed cannot be maintained at the operating point by further
increasing the generator torque. Thus changing the pitch from the optimum value
reduces the aerodynamic effi ciency of the blades. After reaching the maximum
generator torque the blade pitch thus controls the speed.”
Future control systems for large WT are likely to involve real-time measured
signals in combination with physics-based control environments. Turbines would
sense the actual imposed and reacted conditions and feed-forward these into
smart proactive control actions. This type of approach should permit lowering
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