Environmental Engineering Reference
In-Depth Information
5.3.3.2 Super-Synchronous Kramer System
The doubly fed asynchronous generator system has been mentioned as a standard
solution of today. An earlier configuration is the static Kramer system known in the
form of the subsynchronous motor cascade in drive technology, but is also applicable
as supersynchronous generator cascade. It may serve here to explain the basics of
slip power recovery.
The stator winding is directly connected to the grid, while the rotor winding feeds
a converter having an intermediate d.c. circuit containing an inductor, see Fig. 5.9.
While the machine-side inverter is an uncontrolled diode bridge rectifier, the grid
side inverter is a phase controled thyristor device. The latter is generally connected
to the grid via a transformer for voltage level adaption. The converter is designed
for slip power conversion.
To investigate the performance of the cascade, the simplified machine model
Fig. 5.8 is used, neglecting the stator resistance. It is understood that secondary side
quantities are transformed to the primary (stator) side. The grid-side current source
inverter is controled to provide a specific negative d.c. side voltage U
by setting
/ 2 <
W < (
π β
the phase angle
α
W ; where
π
α
). To secure stability of the inverter
/ 6.
Regarding the machine-side converter, and neglecting any phase displacement
between rotor voltage and fundamental rotor current, U 2 must be in phase with
I 2 . In this case the imaginary part of the apparent rotor power vanishes, and the
following implicit equation applies:
performance, the minimal extinction angle
β
is normally kept about
π
s
·
cos
ϕ 2 + s k ·
sin
ϕ 2 = k 2
(5.6)
When the rotor voltage is of constant magnitude, i.e. k 2 = const, the same circle
applies to the current locus of I 1 , with only a specific slip division characteristic for
the given k 2 which is the new no-load slip of the cascade. The current locus circle
Fig. 5.9 Static Kramer
system, basic circuit diagram
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