Introduction to Wind Turbine Technology - Small Wind Turbines: Analysis, Design, and Application - page 21

Fig. 1.12 Ratio of cogging
to rated torque for a number
of small permanent magnet
generators. The vertical axis
is logarithmic

10 -1

10 -2

Ginlong PMGs from web site
Ginlong PMG measured
Skystream 2.4 kW
AWP 500 W
Seoltech web site

10 -3

0 5 10 15 20 25 30

Rated Power(kW)

and 7 analyse the starting and low wind behaviour of small turbines and consider
the impact of cogging and other resistive torques. It will be shown that the above
estimate of the cogging torque is optimistic: it must be less than 1% of the rated
torque to have minimal effect on the starting performance.
On most turbines, the variable frequency and voltage, three phase power from
the generator is immediately rectified to a sufficiently high DC voltage which is
fed either to a battery bank or to an inverter. An inverter produces mains-frequency
voltage A.C. for grid connection or for using standard domestic and other
appliances.
What is not shown in Figs 1.10 and 1.11 is the controller that regulates the
generator and the D.C. voltage to feed the batteries or inverter. Most controllers
implement some form of MPPT to attempt to achieve optimal power production as
the wind speed varies. With a microprocessor housed in the controller, MMPT
often combines a look-up table that gives the maximum power possible at each
generator frequency with a strategy to move towards the optimum trajectory if the
current power is lower as explained in Chap. 11 . Note that frequency and power
are easy to measure, the former from the generator frequency, and the latter from
the current at the DC bus after the rectifier.
In addition some controllers provide the turbine's primary form of overspeed
protection by forcing the blades to operate away from their optimum point (in
terms of C P versus k) in high winds to limit the output power. In other words,
MPPT is modified as the wind speed approaches the rated value to give the shape
of the power curves shown in Figs. 1.4 and 1.5 . This strategy can work well, but
needs some other method for backup in case the load is lost. Typical power
electronics and control strategies are covered in Chap. 11 , where reference is made
to specialist literature for more information.
As small turbines are being increasingly used for grid connection, more and
more of them use stand-alone or monopole towers shown in Fig. 1.2 . These are