Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 245

loads but rely on the generator absorbing power and heat when shutting down for
''anti-islanding'' if the grid connection is lost. Alternatively a smaller dump load
can be used. 6 This has the power rating of the turbine but can only absorb the
power for a short time while the turbine brakes or is brought to rest by other
means.

Example 11.1 The brake on the Aerogenesis 5 kW turbine mentioned in con-
nection with Fig. 11.1 will stop the blades from rated speed in 2 s. What are the
power and energy absorption requirements for the dump load for anti-islanding?

Answer The power requirement is obviously 5 kW. A conservative assumption is
that 5 kW will be absorbed over the time required to stop so the heat capacity of
the dump load is 2 9 5000 = 10 kJ.

11.6 Manual Shutdown and Condition Monitoring

Many owners of small turbines rely on the energy produced by the turbine. Hence
any method to allow the turbine to generate safely, even with some failures, is
desirable. It is therefore important that the controller can be over-ridden in the
event of a controller fault. If the controller fails in a dangerous manner (say it
leaves the turbine unprotected in strong winds) there should be a way of rescuing
the turbine system manually.
For most small turbines, the rectifier must operate in order for the turbine system
to function. However, if one phase of the rectifier is damaged, it is possible to
isolate the relevant incoming phase, and depending on the type of damage, continue
to operate on two phases only. This will cause more vibration and possibly the
turbine will need to be partially furled to limit its speed, as its load is now reduced.
If circuit breakers are used, they can be part of a manual override system or for
disconnecting parts of the turbine system. The ability to manually initiate soft-
braking is also desirable. Some turbines, such as the Skystream, use shorting of the
generator output to slow it down. Other generators may either burn out or
demagnetise if shorted. However, a shorting switch on the generator output is
useful in the absence of a brake, as it allows the wind turbine to remain parked.
That is, if the turbine has been slowed by the controller, then it can be stopped by a
short circuit and often will not start up in substantial winds. Careful design may be
required to prevent accidental parking from near-rated power.
Condition monitoring is a rapidly growing area especially for large off-shore
wind turbines. The aim is to simply and cheaply monitor characteristic signals
from turbine components in order to assess the machine condition, need for
shutdown, and maintenance. Example signals include vibration and generator
temperature. Vibration levels can increase significantly if a blade is lost,

6 The data sheet for one range of these resistors can be found at http://www.welwyn-tt.com/
pdf/datasheet/WDBR.PDF (accessed 20 Sept 2010).