Siemens SINAMICS S120 System Manual page 61

● The harmonic content of the output voltage depends very heavily on the modulation depth
r0074, i.e. the ratio of the voltage at the AC terminals of the inverter with respect to the
DC link voltage. Optimum grid harmonics are obtained at approx. r0074 = 94 %. The
harmonic content increases when the output voltage changes with the DC link voltage
remaining constant. Using the supplementary setpoint r5433 for the DC link voltage
controller (e.g. {VECTOR}p3511 = {ALM}r5433), the harmonic content can therefore be
optimized depending on the operating point.
The modulation depth is not limited in the default setting. As a consequence, especially in
the overmodulation range (r0074 > 100 %) significant current components can occur,
even with low grid harmonics. Limiting the modulation depth with p5432[1] can prevent
this happening. In the case of overmodulation, limiting the output voltage results in an
increased reactive current with the basic fundamental frequency corresponding to the U
droop characteristic.
● Especially for non-linear loads in the grid, low grid frequency harmonics (especially the
5th and 7th) of the output voltage, caused by harmonic currents in the internal
impedances of the power generating system, are reduced. It is activated using p5440.
● Using p5401[0], it is possible to change between normal current-controlled operation (the
Active Infeed Module is then mostly assigned to the DC link voltage control) and droop
operation as voltage source. This change can be made in operation at any time.
When switching from closed-loop current control to grid control it should be ensured that
the set droop parameters match the instantaneous line frequency and amplitude.
Otherwise, high active and reactive powers are obtained corresponding to the droop.
When changing from grid control to current control it should be observed that during a
transition ramp, the no-load points internally shift the droop, so that the transition can be
carried out in an almost no-power condition. For this short transition time, it is possible
that other power generating units in the island grid provide consumer loads. Such a
change is always possible if an island grid is connected with an external power grid, or if
other, sufficiently powerful power generating units can ensure that the frequency and
voltage are maintained.
● When required, the droop characteristics can be enabled or inhibited using p5401[1]. For
active grid control (p5401[0] = 1), but with the characteristics inhibited, the no-load points
are continually internally adapted to the grid so that over the average, the power
generating unit is operating under no-load conditions. However, in turn, other power
generating units must control the grid frequency and amplitude. For instance, this function
can be used to very simply implement a hot standby function for the power generating
system. When a fault occurs, the system can supply the grid without any interruption.
Further, the characteristic inhibit can be used to already change into droop operation,
even without correctly adapted droop parameters (no-load points, gradients); the suitable
droop settings can then be made afterwards.
● When activating the grid control, the V
component. For example, this can be a battery buffering system or PV modules. For drive
applications, V
connected, for example, to the inhibit input of the V
object.
Grid infeed
System Manual, 04/2015, A5E03347436A
control using a motor inverter is typical. Status signal r5402.0 can be
dc
Detailed descriptions of the function modules
6.2 Controls for grid droop operation
control must be realized using another DC link
dc
controller p3513 in the vector drive
dc
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