Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
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172 Conclusions and Outlook<br />
8.2 Outlook<br />
None of the new topologies have be<strong>en</strong> implem<strong>en</strong>ted in hardware, as the NP and FC control<br />
problems are the same as with the ANPC topologies and hardware implem<strong>en</strong>tation issues have not<br />
be<strong>en</strong> in the focus of this thesis. However, if any of the new topologies should be chos<strong>en</strong> for<br />
implem<strong>en</strong>tation, PE <strong>de</strong>sign issues will need to be investigated. Namely the commutation properties<br />
differ a lot from any of the main topologies pres<strong>en</strong>ted in this thesis. Differ<strong>en</strong>t protection schemes<br />
would need to be <strong>de</strong>velopped.<br />
All the proposed modulation schemes have be<strong>en</strong> tested with passive loads on low power<br />
prototypes. This is regar<strong>de</strong>d suffici<strong>en</strong>t as proof of concept and qualitatively indicates that the<br />
schemes will also perform in real applications un<strong>de</strong>r severe operating conditions (unbalance,<br />
distortions). However, for quantitative statem<strong>en</strong>ts on the performance un<strong>de</strong>r special operating<br />
conditions, the experim<strong>en</strong>ts would need to be continued un<strong>de</strong>r more realistic load conditions.<br />
The last modulation scheme proposed optimizes PWM sequ<strong>en</strong>ces online. This is clearly an<br />
area with a lot of pot<strong>en</strong>tial. An MPC based optimal sequ<strong>en</strong>ce approach can theoretically provi<strong>de</strong> the<br />
same or ev<strong>en</strong> higher NP curr<strong>en</strong>t control capacity than the virtual vectors. However, this is only<br />
possible if the prediction horizon is long <strong>en</strong>ough and the suitable sequ<strong>en</strong>ces are ev<strong>en</strong> consi<strong>de</strong>red.<br />
This could not be fully explored and <strong>de</strong>monstrated within the frame of this thesis. MPC is a logical<br />
candidate for the modulation and control of ML converters as there is a large number of redundant<br />
states and multiple quantities need to be controlled; ther is a lot of room for optimization. A<br />
powerful <strong>de</strong>monstration of MPC in PE has already be<strong>en</strong> giv<strong>en</strong> in [52] where DTC has be<strong>en</strong><br />
improved for the NPC. However, the number of states in a ML converter scales ess<strong>en</strong>tially with a<br />
cubic function of the number of levels. Also the number of quantities to be controlled is rising<br />
with the number of levels. Therefore, the computational hurdles are high. Another hurdle may be<br />
the fact that ML converters allow g<strong>en</strong>erally for higher appar<strong>en</strong>t output switching frequ<strong>en</strong>cy than<br />
lower level converters, as more cells can be operated in interleaved mo<strong>de</strong>. The calculation time at<br />
disposition gets shorter. However, with future control hardware platforms and sophisticated<br />
implem<strong>en</strong>tation of the control algorithm (e.g. on an FPGA), much more will be possible than<br />
today.<br />
Other possible topics of continuation are:<br />
1. Integration of findings regarding modified and virtual vectors directly in a 5-L MPDCT<br />
(mo<strong>de</strong>l predictive direct torque control) scheme.<br />
2. Integrate the proposed schemes with line curr<strong>en</strong>t harmonic injection schemes for NP<br />
control [91]<br />
3. Some operating points g<strong>en</strong>erate a minimum low frequ<strong>en</strong>cy ripple in the NP based on<br />
physical constraints. A prediction of this minimum ripple and an accordingly modified NP<br />
voltage refer<strong>en</strong>ce could improve overall performance<br />
4. Back to back configurations offer additional opportunities for NP control. Not only can<br />
both converters contribute to the control task, they could ev<strong>en</strong> be linked such that new<br />
types of virtual vectors using both input and output converter could be <strong>de</strong>fined.<br />
5. The special application of harmonic filtering has special requirem<strong>en</strong>ts as the harmonic<br />
curr<strong>en</strong>ts may be dominant over the fundam<strong>en</strong>tal. The harmonic relationships pres<strong>en</strong>ted in<br />
chapter 4 are not valid anymore. This questions at least the 6 th harmonic injection scheme.