Siemens SINAMICS S120 System Manual page 56

Detailed descriptions of the function modules
6.2 Controls for grid droop operation
Figure 6-2 Basic control concept
Overview
In island grid operation, a generating system must play its role in maintaining the voltage and
frequency (grid former). Stiffly impressing voltage and frequency would be conceivable using
inverters. However, this would not facilitate stable, common operation with other power
generating systems for a specific load distribution without the use of additional, complex
communication.
For primarily inductive grids (X>>R), from a general apparent power perspective for an
electrical "two-port" between two grid connection points, a change of the voltage angle or the
frequency especially influences the exchanged active power P; on the other hand, the
amplitude of the voltage influences the reactive power. As a result of the commutation
inductances of the inverters and the inductive behavior of generators and many loads, the
precondition X>>R can be assumed to be applicable in typical microgrids.
For the setpoints for frequency f
are applied to the inverter output voltage (see the following diagram):
● f
● U
Instead of the reactive power, for the voltage droop characteristic, reactive current i
This means that in the case of grid voltage dips (high reactive current, low reactive power), a
sensible setpoint input is achieved. Typical values for characteristic gradients k
3 %; however, these are very dependent on the specific application.
By smoothing the droop outputs, the behavior of rotating machinery is emulated to such an
extent so that for mixed power generation involving generators and inverters, a stable,
steady state grid is generated – as well as the required transition response for load steps in
the grid. Time constant T
generator. The higher T
turn means that the inverter system must initially provide more power. Using a fast
supplementary droop T
avoid these types of power peaks. This results in a damping effect, which opposes any
power oscillation in the grid. Using time T
56
(t) = f -k p(t) with no load frequency f
droop 0 f
(t) = U -k i (t) with no load voltage U
droop 0 U b
f1
, the lower the frequency change at the start of a load step, which in
f1
a temporary frequency change can be implemented in order to
f2
and amplitude U
droop
0
0
in the frequency axis corresponds to the mechanical inertia of a
for the voltage axis, the inverter system can adapt
U
, the following droop characteristics
droop
System Manual, 04/2015, A5E03347436A
is used.
b
and k are
f U
Grid infeed