Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...

66 3-L DC Link ML Converter Properties
essentially giving the same result: medium vector application is reduced; the increased NP balance
capacity allows for an elimination or reduction of the low frequency NP voltage ripple at the cost of
increased switching frequency or output voltage distortion. Videt [90] is also applying a non
optimum pulse pattern but with the aim to reduce CM voltage. In this case, he avoids the large
vectors and there is only a minor impact on the NP balancing capacity.
A fundamentally different approach for NP control has been proposed by Marchesoni [91].
Instead of changing the switching waveforms spectrum (namely by adding a CM component) to
interact with the load current fundamental, Marchesoni proposes to inject harmonics in the load
current to interact with the switching pattern. This concept is not taken up to be developed further
in the frame of this thesis, but the considerations on harmonics interaction in the following
paragraphs help pointing out, how the concept works. It can be combined with most of the other
concepts proposed in literature and in this thesis.
Active NP control hardware circuits avoid the need for sophisticated NP control schemes. [92]
analyzes the NP current in function of input current harmonics but then proposes an additional
charge balance leg to control the NP voltage.
Most of the publications above refer to the 3-L NPC. ML split DC link topologies differ from
the 3-L NPC by the number of levels but also by the fact that the higher level converters feature
redundant states. The question is, whether the existence of such additional redundant states offers
new opportunities regarding operating limitations and NP control. For this investigation, the
analysis of the NP current characteristics is very important. A large part of the following section is
dedicated to that with special attention to CM impact, harmonics interaction and relevance of
redundant states. It will be shown that all of the cited methods can be applied to any general 3-L
DC link converter. The same controllers can be used and the same limitations apply (for a given
method). However, some of the 3-L DC link converters with higher number of output levels allow
for an operation and NP control capacity beyond what has been presented in any of publications
above.
4.5.3 FC control
4.5.3.1 FC control by variation of duty cycle (carrier shifting)
A very simple flying capacitor control scheme can be implemented based on the variation of
the individual duty cycles: A shift of those duty cycles against each other leads to non-zero average
currents in the capacitors, which can be used to control their voltage. Note that a shift of individual
duty cycles leads to an output voltage distortion. This approach is not very dynamic and can be
insufficient with very distorted loads or highly dynamic operation with a lot of dynamics. In steady
state operation, it is capable of giving very smooth FC control with minimum capacitor voltage
ripple. A suitable offset in the reference signal gives exactly the same result. If the modulation is
done by SVM, the duty cycles can easily be adapted by adding and subtracting the appropriate times
from the duty cycles.
4.5.3.2 FC control by use of redundant states
Instead of a modification of duty cycles, a suitable choice of redundant states can be used for
FC control. Vector {100} 2D is used to illustrate the concept. Any two states of the same color in
TABLE 22 results in zero average FC current in all three phases. The resulting NP current is +i a/2
for low CM voltage and –i a/2 for high CM voltages (i c + i b equals –i a). Therefore, a combination of