Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 77

The complete generator torque-speed characteristic used for control purposes
is shown in Figure 3.21b. The optimal torque-speed curve is characterised by
(3.26), which corresponds to section B-C. Within this operating range, during low-
medium wind speeds, the maximum possible energy is obtained from the turbine.
Due to power converter ratings, it is not practicable to maintain optimum power
extraction from cut-in up to the rated wind speed. Therefore, for very low wind
speeds the model operates at almost constant rotational speed (A-B). Rotational
speed is also limited, e.g. by aerodynamic noise constraints, and at point C the
controller allows the torque to increase, at essentially constant speed, to rated tor-
que (C-D). If the wind speed increases further the control objective follows D-E,
where pitch regulation limits aerodynamic input power. For very high wind speeds,
the pitch control will regulate input power until the wind shutdown speed limit is
reached.
The implementation of the DFIG torque control is illustrated in Figure 3.22a.
Given a rotor speed measurement, w r , the torque-speed characteristic
(Figure 3.21b) is used to obtain the optimal torque, T opt , which after some manip-
ulation generates a reference current i qr ref . Comparing the reference current, i qr ref ,to
the actual value of the rotor current in the q -axis, i qr , an error signal is obtained.
Although i qr imposes the effect of torque control, the converter C1 (Figure 3.18)
is a controlled voltage source. Hence, the required rotor voltage v qr is obtained
by processing the error signal with a standard PI controller and adding to the output
a compensation term to minimise cross-coupling between torque and voltage
control loops.

T

i qr ref

T opt v qr

Torque
to current
transformation

PI
controller

Σ Σ

+

+

- +

ω

i qr

Compensation
element

ω

r

(a) Torque control loop

V s ref i dr ref v dr

Voltage or
power factor
control

PI
controller

Σ Σ

Σ

+

+ +

-

-

+

i dr

V s

Compensation
element

(b) Voltage control loop

Figure 3.22 Current-mode control scheme for a DFIG wind turbine