Modular multilevel converters (MMCs) are the technology of choice for the new generations of high power
drives. They have many advantages which have already been reported in the literature. For instance it is
simple to provide fault redundancy using the modularity of the converter; it is also relatively simple to upscale
the converter to reach high power and voltages. Moreover, high effective switching frequencies (leading to
low distortion in the input/output currents) are usually achieved with this topology. Some commercial
solutions offered by Siemens and Curtis are available in the market, for marine propulsion, ore conveyors,
hydro-generation, etc. The application of MMC-based high power drives to SAG mills, which are typically
used in copper mining, has also been proposed.
There are certainly several issues and control challenges which have to be addressed to further improve
the performance of high power drives, increasing the efficiency and the power/size ratio of this technology.
Some of the main control challenges still faced by MMCs are related to the regulation of the voltages in a
large number of floating capacitors and the compensations of the relatively large capacitor voltage oscillations
produced at some operating points. In this proposal new control systems and hardware topologies are
discussed, which can produce a significant impact on the design and performance of MMC drives. The
proposed research can be summarised as:
1) A novel AC-to-AC back-to-back (BTB) topology based on M2C is proposed in this work. There are not
many AC-to-AC solutions for MMC drives discussed in the literature, and in this proposal a new one is
discussed. For this topology in both, the grid-side MMC as well as the machine side MMC, a mixture of fullbridge cells and half-bridge cells are considered. Using this arrangement it is possible to reduce or completely
eliminate the common-mode voltages typically required in M2C-based speed drives operating at low speed.
This eliminates some machine-health issues related to the application of common-mode signals.
2) A holistic approach is proposed for the design of the control systems and hardware topologies. For
the proposed BTB topology it is possible to implement new control systems based on the regulation of the
dc-port voltage between the converters, (i.e. this voltage is varied according to the drive operating point)
augmented with the application of ac voltage and currents superimposed in the dc-port connecting both
converters. The proposed topology could be used for variable speed wind energy conversion systems and
drives for cage/synchronous machines. The performance of this AC to AC drive for low voltage ride through
(LVRT) and the impact of weak grids in the performance of the BTB topology are also going to be studied.
3) New control systems are going to be investigated in this work which can applied to the topology
presented in this work, as well as, other topologies proposed in the literature. For instance closed-loop vectorcontrol strategies to regulate the voltage oscillations of the floating capacitors inside a pre-defined voltageband will be investigated. For variable cage machines it is also proposed to reduce the voltage fluctuations
by regulating the machine flux using optimisation methodologies.
4) New control systems for the modular multilevel matrix converters (M3C) are going to be investigated
in this project. The application of closed-loop vector-control systems to regulate the voltage fluctuations in
the matrix converters, as well as the investigation of new algorithms to limit the capacitor voltage ripple
inside a pre-defined voltage-band are going to be pursued in this work. The control system will be tested in
the whole speed operating range, including some operating point where the M3C is prone to instability.
5) The application of the M3C in variable speed wind energy systems based on DFIGs will be investigated.
The topology has some potential advantages, e.g. fault redundancy, scalability to high power/voltages, better
performance for LVRT control, etc.
Some of the potential benefits of the research discussed in this proposal are: Reduction in the magnitude of
the circulating currents in the MMC (improving the converter efficiency); reduction in the common-mode
voltages applied to the machine; improved dynamic response of the control systems, etc. The methodology
proposed in this research is based on at least four consecutive stages. Firstly a theoretical study is pursued
considering literature review and modelling/analytical studies of the proposed control systems and
topologies. Secondly simulation studies will be realised analysing the performance of the proposed systems.
In the third stage experimental validation will be carried out. Finally dissemination of the new knowledge
obtained from the research project will be realised through publications in high impact journal papers and
international IEEE/EPE conferences.