Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 118

of a sequence of events that the software control program did not anticipate. The procedure for shut-
down was to cut off the load and apply the mechanical brake. A high wind gust called for shutdown;
however, it was a short gust, and the software said to release the brake, but the load was not recon-
nected because the time delay had not been reached. The turbine went into high rpm and the brake
was applied again; however, due to the high power the brake soon burned up and the rotor was in
the runaway condition, and within a short period, one blade broke loose and cut the guy wires and
the unit fell.
All the blades need to have the same pitch setting or there will be a cyclic forcing function, which
will then affect the drive train, etc. The extreme case was a 40 kW wind turbine where the three
blades had a dihedral spar with change in position to feather for shutdown, and a rapid change to
feather for overspeed. An attachment mechanism that connected rods to the middle of each blade
had some play in it, so the pitch of each blade changed on every rotation and it was different from
one side to the other. In moderate winds, the stable rotor position was yawed 45° to the wind, and
besides the wear problem that presented, the unit did not produce much power.
Another concern is the yaw rate, especially for flexible blades. For example, the rotor has angular
momentum, and when the brake is applied, the wind turbine will tend to rotate about the yaw axis.
The rate of yaw, which is motor driven on large turbines, is limited, and on some smaller turbines
the rate of yaw is limited by a yaw damper. The rate is limited because a change in angular momen-
tum gives a torque.

T Δ L /Δ t (6.24)

where the torque is in the direction perpendicular to the plane of rotation of the rotor. Therefore, a
large change in angular momentum of the rotor, due to a large change in wind direction or a change
in yaw due to shutdown for overspeed, results in a force perpendicular to the plane of rotation. For
flexible blades, this force could be large enough such that the blades could strike the tower. In the
worst case, the blades break off at the root. Another example of fast yaw rate is for small wind tur-
bines with flexible blades, downwind, with coning. Suppose the wind turbine is not operating due
to no or little wind at night. The next day the winds are from the opposite direction and the unit
starts with the rotor in the upwind orientation, which is possible, and the rotor will even track the
wind; however, it is an unstable condition and eventually the wind direction changes or wind speeds
increase enough for the rotor to suddenly change from the upwind to the stable downwind condition.
This very fast yaw rate results in large flat forces on the blades, which means the blades are bent a
large amount. The solution is to have a yaw damper, move the rotor farther from the tower, or have
stiffer blades.
The guided tour of the Danish Wind Industry Association is excellent, and they have a section on
testing wind turbine blades [40]. One problem with fatigue testing of large blades by vibration is the
long time required to reach enough cycles where fatigue becomes noticeable.
Nondestructive testing by using acoustic emission is one way to monitor the progression of
fatigue, and may even predict where the failure will occur [41]. Two fiberglass blades were tested by
dynamic loading on a full-scale blade testing facility. The acoustic emission signatures focused on
counting, amplitude distribution, and location, which provide assessment of damage status, failure
modes, and failure locations. The damage development in composite laminates under fatigue pro-
gresses from matrix cracking, crack coupling with interfacial debonding, delamination, fiber break-
ing, and fracture. A general observation is that low acoustic emission amplitudes are associated with
matrix damage, while high acoustic emission amplitudes are related to fiber failure. Mechanical
properties such as natural frequency, elastic modulus, and tip deflection were measured during the
fatigue tests, and a change in those properties indicates degradation.
Blades are loaded to static failure in flapwise bending, and some blades are tested to failure
for edgewise bending. The National Wind Technology Center, NREL, has a facility for static and
dynamic load testing of blades, which includes nondestructive techniques such as photoelastic stress