Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 83

effective in controlling the generator during transient operation. Hence an alter-
native way to control the generator is to employ vector control techniques

(Morimoto et al ., 1994; Tan and Islam, 2004).

3.6.2.6 Vector control
Vector control techniques are implemented based on the dynamic model of the
synchronous generator expressed in the dq frame. The torque of the generator is
defined as the cross product of the stator flux linkage vector, Y s and the stator
current vector, I s , giving

T ¼ k ð Y s I s Þ ð 3 : 33 Þ

where k is a constant related to the generator parameters. The stator flux linkage
vector Y s is given by

Y s ¼ L s I s þ Y fd

ð 3 : 34 Þ

where L s is the stator self inductance and Y fd is the field flux vector. Thus (3.33)
can be written as

T ¼ k ð L s I s þ Y fd Þ I s ¼ k ð Y fd I s Þ ð 3 : 35 Þ

Assuming a permanent magnet synchronous generator (with constant field flux
Y fd ), the torque of the generator can be controlled by adjusting the stator current
vector I s (3.35). When modelling the synchronous generator it is usual to select a dq
frame where the d -axis is aligned with the magnetic axis of the rotor (field). Hence,
as the d component of the stator current, i ds , is aligned with the field flux vector,
Y fd , the cross product of these two quantities is zero, and the torque is controlled
only by means of the q component the stator current, i qs ,as

T ¼ k Y fd i qs ð 3 : 36 Þ

The reference value of the stator current in the q -axis, i qs ref , is calculated from
(3.36) and compared with the actual value, i qs . The error between these two signals
is processed by a PI controller whose output is the voltage in the q -axis, v qs ,
required to control the generator-side converter. The required voltage in the d -axis,
v ds , is obtained by comparing the reference value of the stator current in the d -axis,
i ds ref , with the actual current in the d -axis, i ds , and by processing the error between
these two signals in a PI controller. The reference i ds ref may be assumed zero for the
permanent magnet synchronous generator.

3.6.2.7 Control of the grid-side converter
The objective of the grid-side converter controller is to maintain the DC-link vol-
tage at the reference value by exporting active power to the grid. In addition, the
controller is designed to enable the exchange of reactive power between the con-
verter and the grid as required by the application specifications.