Page 384 - Wind Energy Handbook

358 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

should act on the rotor or low-speed shaft, but DS 472 goes further and requires
that one should be an aerodynamic brake.
Normal practice is to provide both aerodynamic and mechanical braking. How-
ever, if independent aerodynamic braking systems are provided on each blade, and
each has the capacity to decelerate the rotor after the worst-case grid loss, then the
mechanical brake will not necessarily be designed to do this. The function of the
mechanical brake in this case is solely to bring the rotor to rest, i.e., to park it, as
aerodynamic braking is unable do this.

6.8.2 Aerodynamic brake options

Active pitch control

Blade pitching to feather (i.e., to align the blade chord with the wind direction)
provides a highly effective means of aerodynamic braking. Blade pitch rates of 108
per second are generally found adequate, and this is of the same order as the pitch
rate required for power control. The utilization of the blade pitch system for start-
up and power control means that it is regularly exercised with the result that the
existence of a dormant fault is highly unlikely.
In machines relying solely on blade pitching for emergency braking, independent
actuation of each blade is required, together with fail-safe operation should power
or hydraulic supplies passing through a hollow low-speed shaft from the nacelle be
interrupted. In the case of hydraulic actuators, oil at pressure is commonly stored in
accumulators in the hub for this purpose.

Pitching blade tips

Blade tips which pitch to feather have become the standard form of aerodynamic
braking for stall-regulated turbines. Typically the tip blade is mounted on a tip
shaft, as illustrated in Figure 6.13, and held in against centrifugal force during
normal operation by a hydraulic cylinder. On release of the hydraulic pressure
(which is triggered by the control system, or directly by an overspeed sensor), the
tip blade flies outwards under the action of centrifugal force, pitching to feather
simultaneously on the shaft screw. The length of the tip blade is commonly some 15
percent of the tip radius.
The ability of the control system to trigger blade tip activation is of crucial
importance. On a number of early machine designs, the blade tips were centrifug-
ally activated only, so there could be long periods without overspeed events when
they did not operate. As a result there was a risk of seizure when operation was
eventually required. With the now commonplace arrangement enabling the control
system to activate the tip as well, the system can be routinely tested automatically.
The penalty is that the low-speed shaft needs to be hollow to accommodate the feed
to the hydraulic cylinder.