Environmental Engineering Reference
In-Depth Information
Figure 4.46
Power fl ows in induction machine
Through the asynchronous link, the capability exists of injecting rather than extracting
power into the rotor circuit, thus reversing the sign of
P
r
. The consequence of this is that as
P
m
and
P
ag
are negative the slip now must be positive and in the range 1
0. The induc-
tion generator can therefore be made to operate in a
subsynchronous
speed. In this mode of
operation
P
m
>
s
>
P
ag
and hence the electrical power fed into the rotor from the mains is returned
to the mains through the stator. Due to the possible bidirectional fl ow in the rotor circuit this
machine is often referred to as being
doubly fed
.
It can be shown that the rating of the asynchronous link in the rotor circuit is proportional
to the required speed shift from synchronous; for example a speed shift of 30% will require
an asynchronous link rated approximately at 30% of the nameplate rating of the induction
generator. The advantage of modifying the slip energy should now be obvious. By having a
bidirectional link capable of extracting or injecting 30% of the rated power in the rotor circuit,
a 60% speed variation can be achieved. For the purposes of reaping the advantages of vari-
able speed operation this is almost as good as full speed control. Also, because only 30% or
less of the power is going through the converters, energy losses in the converters and har-
monic generation are reduced. It is not surprising that a number of major manufacturers of
wind turbines have chosen this technique for their multimegawatt wind turbines.
<
DFIG Wind Turbines
[4]
DFIG stands for a
double fed induction generator
. Figure 4.47 shows a simplifi ed schematic
of a wind turbine connected to a doubly fed induction generator. The rotor-connected voltage
source IGBT converter is linked to a mains-connected IGBT converter through a DC link
containing a capacitor. Both converters are capable of
four - quadrant
operation; i.e. they can
be controlled to transfer active and reactive power in either direction. The rotor-connected
converter has a more onerous task to perform than the mains-connected one.
The rotor-connected converter looks after the wind turbine speed control so that energy
extraction is maximized. The generator is controlled on the basis of
vector control
, a complex
technique that requires substantial on-line computational requirements. This technique
'decouples' the active and reactive power control functions permitting the induction generator
to be controlled with exceptional fl exibility. It also offers superior transient response permit-
