Controls For Grid Droop Operation; Background Information - Siemens SINAMICS S120 System Manual

The status is displayed in r5482 and r5499.
If a transformer is not being used, a 1:1 transformer can be parameterized (for example, to
still be able to use other functions such as the DC component controller etc.).
6.2

Controls for grid droop operation

6.2.1

Background information

The subsequent diagram shows the principle, modular design of a power generating system
based on the kinetic energy with the generator, converter system, line filter to reduce the
pulse-frequency grid harmonics to comply with the applicable grid code and a grid
transformer. By adjusting the magnitude and angle of the output voltage, inverters with 3
conductor connection without internal energy storage device (e.g. B6 bridges) have the
ability to control two independent variables. These are used on the generator side, e.g. using
subordinate current controls in dq coordinates for the flux and the torque/speed control. For
the grid side, the currents differ corresponding to the active and reactive axes; the higher-
level Vdc controller is used for the active current control. A specific cosφ with respect to the
grid can be set using the reactive current controller.
In a first approximation, in this configuration, the grid currents are impressed, independent of
the grid state and the grid parameters. As a consequence, with respect to the grid, the power
generating system behaves like a current source, whose power only depends on the power
of the primary energy source. The grid converter continually follows the changing amplitude
and frequency of the grid voltage. This principle of operation obviously requires a grid, which
can absorb the instantaneous generated power at any time; further, it must control (closed
loop) the amplitude and frequency. As a consequence, it is suitable for feeding into power
grids.
These preconditions are violated when it comes to an island grid, if the power generating
system reaches an excessively high percentage of the overall grid capacity (power rating). In
this case, the behavior of a voltage source is required, which provides the instantaneously
required load power with a defined voltage amplitude and frequency. The two degrees of
freedom of the grid inverter are now required for these controlled variables. As a
consequence, the inverter on the generator side must control the DC link voltage, therefore
ensuring a balance between the generated and consumed power. The required Vdc
controller uses, in a cascade structure, the torque controller; in this operating case, the
speed control must be externally implemented as part of the actual application itself.
Grid infeed
System Manual, 04/2015, A5E03347436A
Detailed descriptions of the function modules
6.2 Controls for grid droop operation
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