U-F Droop Control - Siemens SINAMICS S120 System Manual

6.2.2

U-f droop control

Background
The load equalization (balancing) between the power generating units in the grid, which is
required for stable operation at an operating point with common frequency and amplitude, is
preferably realized using droop characteristics (see the following diagrams). The droop
function can also involve smoothing elements (PT1) to allow the existing time constants to
be adapted to the particular grid. For instance, the frequency of a power generating unit
decreases with increasing power output.
Every power generating unit in the grid, which actively participates in maintaining the
frequency and voltage, determines, using these characteristics, its actual setpoints for the
output voltage and frequency. In the case of conventional power generating stations, this
would be comparable with what is known as the primary control. For instance, if all of the
power generating units have the same rated power and the same characteristics, then an f-U
operating point is obtained in the grid, where all power generating units provide the same
power to cover the required overall load. This therefore results in a symmetrical load
distribution without requiring any additional communication between the power generating
units.
Figure 6-4
The principle of operation can be explained using a simple example for the reactive power
characteristic with 2 power generating units and a consumer load P
the output frequency of the first and second power generating unit, with k
characteristic gradients and identical no-load frequency f
f = f
E1
For the condition for the sum of powers P
power generating unit, the following dependency on the power of the first power generating
unit is obtained:
f = f
E2
As a consequence, the characteristic of the second power generating unit can be mirrored –
and shifted by P
show the resulting operating point with common frequency f
participates with P
Grid infeed
System Manual, 04/2015, A5E03347436A
Active power droop characteristic and reactive power droop characteristic for frequency
and voltage
+ k x P ,.....f = f + k
0 1 1 E2 0
+ k x (-P -P ).
0 2 1 V
– entered in the P-f diagram of the first power generating unit in order to
V
in the overall power required.
E1
Detailed descriptions of the function modules
x P .
2 2
+ P + P
1 2 V
6.2 Controls for grid droop operation
(following diagram): For
V
and k
1
, the following applies:
0
= 0, for the frequency of the second
. The first power generating unit
A
as
2
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