Dc Component Controller - Siemens SINAMICS S120 System Manual

Detailed descriptions of the function modules
6.1 Additional closed-loop controls for current-controlled operation
6.1.3

DC component controller

Background
Especially when used in power generating systems, grid inverters must normally maintain
extremely low limit values for the DC component in the phase currents.
Already low DC components can have a negative impact, for example on the operation of
grid transformers ["Verfahren und Vorrichtung zur Ausregelung von Verzerrungen im
Magnetisierungsstrom eines Transformators, der mit einem selbstgeführten Stromrichter mit
abschaltbaren Leistungshalbleiterschaltern verknüpft ist", EP0896420, H. Weiss, Siemens
AG]:
Already standard active and reactive current controllers significantly reduce the DC
components in AC currents through the proportional element, therefore guaranteeing reliable
and straightforward operation of converter transformers.
Further, in order to satisfy the requirements of the grid codes – also without connection
transformer – a dedicated additional controller is available to compensate any remaining DC
components.
It is worth noting that a DC component in the phase currents should not be confused with
what is known as a zero component when breaking down three-phase currents into
symmetrical components. The latter does not involve a DC component when generating a
mean value over one line period (in an individual conductor), but involves a current
component, which flows with the same phase in all 3 conductors – and that returns, e.g. via a
ground conductor.
Application
The "Line transformer" function module must be activated in order to use the DC component
controller (p3648 to p3654).
The controller is activated by selecting the proportional gain p3650 > 0. The value
p3650 = 100 % corresponds to the recommended default setting for a correctly set
magnetizing inductance of the transformer (p5492).
52
A DC current component in the phase currents shifts the magnetic operating point of a
transformer. As a consequence, an unsymmetrical current characteristic is obtained in
operation, where the dissymmetry depends very heavily on the amplitude of the shift. If
a transformer is operated with the grid voltage, it is normally very highly utilized mag-
netically; this means that the magnetizing current already manifests visible symmetrical
distortion. This distortion is symmetrical and only causes harmonics with uneven order
numbers. If a DC current component is injected into the transformer, then even very low
DC current amplitudes can result in a very high distortion of the magnetizing current. For
modern transformers, the limit values for DC components may only be a per mill of the
rated AC current. This distortion is no longer "hidden" in the generally very high primary
current, but instead generates very high magnetizing current spikes, which then visibly
impact the primary current. The effects range from noise as a result of the electromag-
netic current forces, through harmonics in the output voltage with even order numbers
up to insulation faults as a result of high voltage peaks occurring across resistances
and/or inductances.
System Manual, 04/2015, A5E03347436A
Grid infeed