Negative Sequence System Current Controller - Siemens SINAMICS S120 System Manual

When the magnetizing inductance of the transformer is not known (type plate, data sheet),
then the magnetizing inductance can also be measured (p5480 = 11). It should be taken into
account that the identification result r5491, especially for transformers with low magnetizing
current (large magnetizing inductance L
quantities. The reliability when determining L
filter currents using a VSM10 (r3671, r3672). To do this, filter monitoring must be activated
using p3678 > 0.
If there is any doubt regarding the validity of the set magnetizing inductance p5492, then we
recommend that the gain of the DC component controller is set to somewhat lower values
(e.g. p3650 = 30 %), to avoid any instability.
Note
The DC component control increases the computational load and reduces the ruggedness of
the control, as additional natural resonance effects occur. As a consequence, the controller
should only be activated when required.
6.1.4

Negative sequence system current controller

Background
In the case of unsymmetrical grid faults or voltage dips, some grid codes specify that
symmetrical grid currents should be injected, i.e. the same current amplitude in all three grid
phases. In conjunction with a fluctuating grid voltage, this automatically results in a power
oscillation, where the DC link capacitor is used as energy buffer.
The current is firstly broken down into symmetrical components, where each 3-phase system
can also be defined with complex amplitudes for a zero component, a positive sequence
system and a negative sequence system. In the time domain, the negative sequence system
corresponds to a vector rotating in the opposite direction, i.e. a coordinate transformation
with a negated grid phase angle -φ.
For a negative sequence system setpoint of "0", the controller modulates the converter
output voltage with a signal component at twice the grid frequency, so that a symmetrical 3-
phase current is obtained. By suitably entering negative sequence system current setpoints,
when required, individual grid phases can also be supported. In fact, in an extreme case with
identical positive sequence system and negative sequence system setpoints, a no current
grid phase can be achieved.
Application
In the frequency spectrum of the phase currents, an existing negative sequence system is
noticeable as a result of a frequency characteristic at twice the grid frequency – and possibly
multiples of this. This current dissymmetry is corrected by activating the negative sequence
system control (p3640 = 1) with a setpoint of zero (p3641). The gain of the negative
sequence system controller corresponds to the gain factor of the standard positive sequence
system current control, and therefore cannot be separately adjusted. On the other hand, the
integral time can be adapted when required (p3639).
Grid infeed
System Manual, 04/2015, A5E03347436A
Detailed descriptions of the function modules
6.1 Additional closed-loop controls for current-controlled operation
) is very sensitive with respect to disturbing
H
can be improved by separately sensing the
H
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