New small turbine technologies - Wind Power Generation and Wind Turbine Design - page 406

must wait until stronger wind starts to rotates the rotor into operation. Power assist
system can catch the chance to bring the WT into operation when wind speed is
near cut-in wind speed. It works just the same as variable pitch system. The power
assist system is driven by a motor, which means it needs additional power source.
This may lose certain energy but such energy loss would be just marginal since the
start-up mechanism will not be activated during constant rotation operation, and
that if improved rate of operation is considered, the gain will be larger.
8.2 Power/speed control
In the whole operation wind speed range, power/speed control must be reliable
and fail-safe. Therefore, the control system provides more than two independent
control or brake mechanisms.
8.2.1 Aerodynamic methods
There are two aerodynamic control methods. One is traditional stall regulation. In
this method, when a gust attacks the rotor, a separation will occur on a blade which
decreases the torque resulting in power/speed regulation, preventing over speed of the
rotor as well. It is considered stall regulation method is more suitable in the condition
of relatively stable wind of small turbulence. However, active stall system has been
developed that is more reliable and available for higher turbulent wind conditions.
According to fail-safe concept, a stall regulation is usually supported by other
systems such as mechanical brake. Collapsible rotor blade tip is an aerodynamic
device for over-speed limitation.
Variable pitch control or pitch regulation is also aerodynamic method. The gen-
erated torque of the rotor as well as the lift on the blades is controlled by the attack
angles by changing the pitch angle. Hydraulic power or motor(s) are necessary to
change the pitch angle(s) by way of some movable mechanical elements such as
links. Variable pitch control is reliable and works also at starting-up at cut-in state
and shutdown at cut-out state.
8.2.2 Mechanical methods
All kind of mechanical brakes are possible. A mechanical method need consider-
able power source to shut down the system without any aerodynamic method. Dur-
ing the shutdown, all the kinetic energy lost from the rotor is converted to thermal
energy that will be absorbed into the brake materials.
8.2.3 Electro-magnetic methods
Electro-magnetic method is also possible. The lost kinetic energy from the rotor is
converted to thermal energy and temperature monitoring/control is very important.
One 1 kW SWT with such an electro-magnetic regulation has an experience that
the WT operates during high wind under a typhoon attack. The turbine was generat-
ing power at 300-650 W under the wind speed range of 27-37 m/s until just before
5 s when the maximum wind speed of 47 m/s blew through the site as illustrated
in Fig. 10. The turbine immediately responded to the extreme wind by falling into