Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 200

range, such as enabling the number of poles of the stator winding to be changed
(adjusts the no-load synchronous machine speed); inserting controllable external
rotor resistance (allows the torque-speed characteristic to be shaped). However,
such machines can still be classified as fixed-speed, and hence they will provide an
inertial response.
In contrast, direct-drive, variable-speed wind turbine generators, based on
synchronous machines, offer several advantages over fixed-speed machines. Using
an AC-DC-AC converter the synchronous machine and the wind turbine rotor are
mechanically decoupled from the power system. Thus the rotor rotational speed can
be varied in accordance with wind speed, and independently of system frequency.
As discussed previously in Section 3.6 this increases energy capture, particularly at
lower wind speeds, as aerodynamic efficiency can be maintained over most of the
operating range. The further ability to cope with gusts by adjusting rotor speed
implies a reduction in tower stresses, and hence a reduction in capital cost, as well
as voltage flicker. The direct-drive arrangement also allows the gearbox between
the wind turbine rotor and generator to be eliminated, along with the associated
energy losses and maintenance costs. One disadvantage of this arrangement, how-
ever, is that an inertial response will not be provided, since the turbine rotor speed
is now independent of the system frequency. However, in principle, this link can be
restored by suitably modifying the torque setpoint of the power converter control
system, following (5.2) as follows:

D t ¼ D P gen

w ¼ 2 H gen S max

df
dt

Nm

w f 0

where H gen is the inertial constant of the wind farm. Figure 5.33 illustrates a
modified version of the torque control loop for a variable-speed turbine from
Chapter 3 (Figure 3.22a), with the torque setpoint, t , modified by an input depen-
dent on the ROCOF. Hence, a fall in the system frequency will cause an increase in
the torque setpoint, leading to a transient increase in the electrical output of the
machine - an inertial response. Since this mechanism is provided electronically
rather than mechanically, the user value selected for H gen could actually exceed the
physically defined limit. During the initial stages of an event, as much energy as
required could be extracted from the rotor inertia, subject to limits on the capacity

df
dt

Inertial
compensation

(Rate of change of
system frequency)

t (ref)

i rotor (ref)

v rotor

+

t

w t

Controller

K

+

+

_

w

i rotor

Figure 5.33 Modified DFIG torque-control loop