Environmental Engineering Reference
In-Depth Information
care of the longer variations in the wind, while the OptiSlip system looks after the faster
variations due to gusting. The power output is maintained at the rated value with the speed
allowed to vary between the 1 and 10% slip range. The variable slip concept produces high
quality electrical output at high wind speeds without generating any detectable harmonics.
The variable slip concept does not enhance energy capture but improves the quality of gener-
ated electricity and the stresses in the wind turbine drivetrain in the region that matters, i.e.
for wind speeds above rated.
The Principle of Slip Energy Recovery
A far superior approach to dissipating energy in external rotor resistors is to capture this
energy and transfer it to the mains. There are a variety of such
slip energy recovery
schemes.
In this section the general principles of slip energy recovery will be outlined before dealing
with its application to wind turbines. At the outset it must be remembered that the frequency
and magnitude of the induced voltages and the resulting currents in the rotor circuit of an
asynchronous generator depends on the slip. Transferring energy out of or into the rotor circuit
requires the interfacing of this variable frequency variable magnitude rotor voltage to the
fi xed frequency, fi xed magnitude voltage of the mains. This can be achieved through an
asynchronous link
consisting of an AC to DC converter followed by a DC to AC converter.
The technical problems in such a scheme are not trivial. At very low slips the rotor side
converter will have to deal with very low voltage, low frequency rotor voltages. Conversely,
at high slips, the voltage and frequency will be proportionately larger. It can be shown that
for control of speed from zero to synchronous, the rating of the rotor connected DC link will
have to be the same as the nameplate rating of the induction generator. This would involve
an electronic interface of considerable size and therefore cost. The advantages offered by full
variable speed control can be almost matched if a limited range of control is provided. This
can be ingeniously arranged by a rotor connected asynchronous link of relatively low rating.
To investigate this arrangement, the possible power fl ows in and out of the rotor of an asyn-
chronous machine need to be looked at more closely.
Figure 4.46(a) shows the power fl ow in an induction machine when motoring. The elec-
tromagnetic power
P
ag
crossing the air gap is equal to the electrical power
P
s
fed into the
stator minus the losses referred to in Section 4.4. In turn, subtracting the rotor losses from
P
ag
gives the useful mechanical power output
P
m
. Neglecting the losses, the simplifi ed
diagram of Figure 4.46(b) can be set up with the aid of Equation (4.22). The asynchronous
machine behaviour when the machine is generating will now be examined, and with the
assumption that the capability exists of extracting and injecting power into the rotor circuit
through an asynchronous link. In other words, there is an ability to adjust the magnitude and
sign, i.e. the direction of
P
r
in Figure 4.46(b). In the fi gure, the arrows indicating power fl ows
are shown in the conventional way from left to right, i.e. when the machine is motoring.
In the usual operation as a generator (Figure 4.16) the slip is negative in the range
−
1
<
s
<
0, and
P
m
fl ows from right to left, i.e. has a negative sign. As a consequence
P
ag
and
P
s
are
both negative but
P
r
is positive. By extracting the required level of
P
r
from the rotor circuit,
the induction generator speed can be controlled in the
supersynchronous
range. In this mode,
the mechanical input power is fed into the mains partly through the stator and partly through
the rotor connected asynchronous link.
