Siemens SINAMICS S120 System Manual page 60

Detailed descriptions of the function modules
6.2 Controls for grid droop operation
Figure 6-5
Typically, the primary control in grids is a fast P control, which cannot control system
deviations down to zero. Steady-state accuracy is achieved using slow higher-level,
secondary controls. These shift the no-load points of the droop characteristics so that the
rated grid frequency is reached as average value. To achieve this, it may be necessary to
manually switch-in additional power generating units.
Application
The "Grid control" function module must be activated in order to use the "U/f droop control"
functionality (p5400 to p5479).
● Suitable droop settings for the no load frequency p5405, no load voltage p5415 as well as
the gradients of characteristics (p5407, p5417) must be entered for the particular
application. In a grid with several power generating units, the energy management for the
island grid can assume this task, for example. BiCo signal sinks for synchronous setpoint
inputs are available for this purpose.
If an island grid is operated with only one power generating unit, then the gradient of the
droop characteristics can also be set to 0.
Typical gradients for operation with several power generating units lie in the 3 % range.
Excessively high gradients result in high frequency and voltage fluctuations. This can
lead to stability problems in the grid, especially if additional self-commutated inverters are
operated in this grid in the current controlled mode. The reason is that they cannot follow
these grid fluctuations fast enough.
● Using signal sinks p5403 and p5404, the actual values for grid current and voltage are
defined, which are used to calculate the droop input variables – i.e. active power and
reactive current. The output variable of the voltage droop is the voltage at the inverter
terminals. The load currents result in voltage drops across the impedances between the
inverter and the connection point of the power generating system (for instance, at the line
filter of an upstream transformer). The additional voltage controller p5425 is used to
compensate these voltage drops, and to set the voltage setpoint according to the droop
at the required interconnection point. Integration time p5427 of the voltage controller is
typically selected to be higher than the smoothing time p5419 of the droop function.
60
Common operating point of several power generating units in the grid
System Manual, 04/2015, A5E03347436A
Grid infeed