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Title: Modelling and control of a grid-connected full bridge modular multilevel converter for integration of wind energy conversion systems
Author: Allu, Ahmed
ISNI:       0000 0004 7972 0608
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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The research work of this PhD thesis presents a performance analysis of the grid connected full bridge modular multilevel converter FB-MMC operating in the buck and boost modes, considered different levels of the dc link voltage with respect to constant grid voltage. The main objective of this research work is to analyse the influence of the ratio between the converter input and output voltages on behaviour of voltage and current harmonic components in the FB-MMC, with giving emphasis of this analysis on the 2nd order circulating current, capacitor voltage ripples and THD of the phase output voltage. The contributions of this research works involve development of the phaseshifted carriers PSC- PWM strategy through defining ac and dc modulation indexes, which allow to investigate the operation of the FB-MMC in the boost mode over a wide range the dc link voltage and to explore those of the optimal operating points. The steady-state analytical model is also developed to fully understand how the dc link voltage level influences low order harmonic components of the FB-MMC voltages and currents. The 2nd harmonic circulating current and resonant inductance equations are calculated in the boost mode using this analytical model. The results of this analysis expose an optimal operating point when the FB-MMC operates in the boost mode at 60% of the nominal dc link voltage, in which the 2nd order circulating current is minimized without using the circulating current suppression control, and consequently, of a decrease in the capacitor voltage ripples. As design consideration, this research work may suggest that this optimal operating point can be exploited to reduce the FB-submodule capacitor size, as the capacitor voltage ripple is considerably lowered. A further analytical model is proposed in this research work. In particular, it is to identify high order harmonic contents of PWM voltage waveforms of the FBMMC. Thus analytical expressions of this model are derived to describe an interaction of high order frequency harmonics and different dc link voltage levels. Based on this analytical model, performance harmonic is evaluated using THD of the output voltage. In the boost mode, the obtained results expose a nonlinear behaviour of THD regarding different operating points of the dc link voltage. These results also reveal that the optimal dc link levels that used to achieve better THD value are highly depended on a number of FB-submodules of each arm, capacitor voltage regulation approaches and displacement angle. Under two different capacitor voltage regulation approaches, the results obtained from both analytical models are validated to those of simulated switching models of the FB-MMC, that built in MATLAB/Simulink environment. In addition, the main findings of this performance analysis are studied and compared using both the real-time simulation and experimentation. In which a down-scaled prototype for the single-phase FB-MMC inverter is built, and its obtained results are verified to those of a simulated switching model, that designed using a digital simulator of OPAL-RT OP4500.
Supervisor: Odavic, Milijana Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available