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Abstract
Multilevel converters are the preferred topologies in high voltage applications due to multiple voltage levels generated in the quantized line-to-line voltages and the fact that the power devices have to withstand only a fraction of the total voltage dc bus voltage. Amongst them, the modular multilevel converter (MMC) defines the state of the art in voltage source converters.
The MMC topology offers various advantages compared to the other multilevel converters such as the modularity of converter that enables to conceptually reach any voltage level, while capacitor voltage balancing can be achieved relatively easily. Furthermore, waveforms with low harmonic distortion are generated, fault tolerant operation can be implemented, and the dc-link capacitor can be eliminated. However, there are still challenges associated with control and modulation of the MMC. Circulating current and sub-modules capacitor voltage balancing have significant impact on the rating of the power devices, losses and capacitor voltage ripples. The above, together with the requirement for relatively low switching frequencies pose a very unique set of challenges in an effort to ensure stable and efficient operation of the MMC.
In this thesis, two control methods are proposed to minimise the rms values of the arm currents through elimination of the ac components in the circulating current. Evaluation of the controller is based on the transient response of the controller, rms value of the arm currents and the capacitor voltage ripples.
A new implementation of capacitor voltage balancing based on a dual sorting stage and feedback of the switching states of the SMs has the ability to reduce the converter switching frequency and leads to lower switching losses. A modified voltage sorting with only one transition (operation at fundamental frequency) and three transitions is analysed and the assessment is based on the average SMs switching frequency and capacitor voltage ripples.
A comparison study between carrier-based PWM (CB-PWM) and staircase modulation techniques is carried out to demonstrate that staircase modulation operates at a switching frequency higher than the fundamental one, while CB-PWM performs better at equal SM switching frequency. Furthermore, carriers interleaving, which extends the number of levels from a given MMC configuration, is also investigated in this thesis.
Finally, all the case studies are verified through extensive simulations and experiments on a low power phase-leg MMC laboratory prototype to demonstrate the proposed circulating current control and capacitor voltage balancing can reduce the power and switching losses and increase the converter reliability and efficiency.