Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 76

3.6.1.1 DFIG wind turbine control
Control of a DFIG wind turbine is achieved through converters C1 and C2
(Figure 3.18). A control scheme implemented by a number of manufacturers uses
converter C1 to provide torque-speed control, together with terminal voltage or
power factor (PF) control for the overall system. Converter C2 is used to maintain
the DC-link voltage and provide a path for rotor power flow to and from the AC
system at unity power factor. Although reactive power injection can also be
obtained from the stator-side converter C2, for DFIG voltage control schemes the
rotor-side converter C1 is likely to be preferred to converter C2. This is largely due
to the fact that the reactive power injection through the rotor circuit is effectively
amplified by a factor of 1/ s .
For purposes of analysis, simulation and control, the favoured way of repre-
senting a DFIG is in terms of direct and quadrature ( dq ) axes, which form a
reference frame that rotates at synchronous speed ( w s ¼ 2 p f s ). Adjustment of the
dq -axis components of the rotor voltage provides the capability of independent
control over two generator variables. This can be achieved in a variety of control
schemes. A control methodology known as current-mode control is commonly
employed where the d -axis component of the rotor current is used to control
terminal voltage (reactive power), and the q -axis component is used to control the
torque of the generator (active power).

3.6.1.2 Torque control
The aim of the torque controller is to optimise the efficiency of wind energy cap-
ture in a wide range of wind velocities, keeping the power generated by the
machine equal to the optimal defined value. A typical wind turbine characteristic
with the optimal torque-speed curve plotted to intersect the C p max points for each
wind speed is illustrated in Figure 3.21a. The curve T opt defines the optimal torque
of the device (i.e. maximum energy capture), and the control objective is to keep
the turbine on this curve as the wind speed varies. The curve T opt is defined by
T opt ¼ K opt w r ð 3 : 26 Þ

where K opt is a constant obtained from the design of the wind turbine.

Maximum torque
curve ( T opt )

Rated
torque

Rated
torque

E

D

Cut-in
speed

Shutdown
speed

Speed
limit

C v = 12 m/s

v = 10 m/s

v = 8 m/s

v = 6 m/s

B

v = 2 m/s v = 4 m/s

A

Generator speed

ω

s

Generator speed

(a) (b)

Figure 3.21 Wind turbine characteristic for maximum power extraction