Apparent Current Limiting For Grid Droop Control; Notes For Active Line Modules Of The Booksize Format - Siemens SINAMICS S120 System Manual

Commissioning
7.10 Commissioning supplementary controls
7.10.5

Apparent current limiting for grid droop control

In grid control operation (p5401 = 1) with respect to the grid, the Active Line Module acts as
voltage source. This is responsible for maintaining and controlling grid voltage and
frequency, and to provide the power required for the various loads/consumers. For an Active
Line Module overload condition, the grid control must be adapted by the higher-level power
controller so that a sensible and stable load distribution is achieved in the island grid.
The Active Line Module protects itself against overcurrents by limiting the apparent current
(or the total current). When the limit is reached, the output voltage and frequency are
automatically adapted so that no inadmissible currents flow.
The apparent current limit can be separately set for normal operation and clearing short
circuits using p5478.
Typically, for normal operation, a lower current limit is selected (characterized by relatively
low grid voltage dip). For instance, this allows heavy-duty drives to be started in island grids
to be supplied: the current limit results in a decreased output voltage (in addition to the grid
droop function) and therefore to a slower start and a reduction of the currents to a level that
can be realized by the Active Line Module for the duration of the heavy load.
In the short circuit state for single generator operation (status signal r5402.4 = 1, threshold
values p5459[2, 3], single generator operation p5451 = 0) current limit p5478[1] is effective,
and facilitates briefly increased maximum currents (time limit p5458[1]), to support clearing a
grid short-circuit.
7.10.6

Notes for Active Line Modules of the Booksize format

In grid control operation, the short-circuit current p5478[1] is limited to 100 % of the
maximum current (r0209) (presetting, 90 %).
For active dynamic grid support, the Active Line Module apparent current p5509[5] is limited
to 80 % of the maximum current r0209.
The reactive current presetting for dynamic grid support is zero (p5506[..] = 0). The same is
true for the active and negative sequence currents during a grid fault
(p5509[7] = p5509[8] = p5509[11] = 0). Using these pre-settings, a grid fault is ridden
through in an almost no-current condition.
The local Siemens office can be contacted regarding any deviation from the presettings.
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System Manual, 04/2015, A5E03347436A
Grid infeed