الفهرس 
Introduction to Medium Voltage Multilevel ConvertersOnly 14 pages are availabe for public view
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Abstract
The motivation behind the recent keen developments of grid-connected multilevel converters; is to fulfill the extreme demand of having an ideal energy conversion and delivery under all grid’s non-ideal conditions. Most of the continuous improvements are mainly biased on optimizing the converter’s impeded control algorithms and adopting complex strategies. However, the grid-connected converters are not yet impeccable, and practically, there are a plentiful of incompetent designs looming out, which is still leading to a performance deterioration, or even a sudden interruption, whenever the grid is instantly disturbed. Therefore, the enclosed thesis, investigates thoroughly the conventional design aspects to overcome that shortfall, and to immensely achieve the desired performance, while challenging the most severe grid disturbances.
In this thesis, a new Grid Sequence Separation (GSS) scheme is introduced, for effectively separating the grid voltage and current sequence components to perform complex converter control algorithms; consolidated with a novel frequency Adaptive Phase Locked Loop (ADC-PLL); aiming to provide an optimum synchronization under severe disturbances. Although, it is incumbent to secure a smooth synchronization to the grid during the fault incident, but on the other hand, the quick recovery of the converter operation after fault clearance is at the same importance; for that, a novel Ride-Through Sequence and a fast hybrid synchronization scheme based on the Single Input Fuzzy Logic Controller (SFLC), are successfully developed and validated.
In this thesis also, two modulation techniques are investigated and validated. The first technique is the selective Harmonic Elimination (SHE) which seems attractive despite its slow response which is not preferable for some applications. However, a method for increasing SHE accuracy when digitally implemented is proposed. The second investigated modulation technique is the Space Vector Modulation (SVM) which is found much suitable for modulating such converters under disturbed grids. A generic method to construct the SVM algorithm is given and verified. The chronic problem of capacitors voltage balancing is tackled by proposing a self-adaptive technique which acts on the dwell times of the SVM switching patterns.
The converter performance under voltage dip is improved by proposing a Compensated Dual Vector Control (CDVC) method utilizing the developed sequence separation method GSS and the proposed ADC-PLL. The CDVC has actively stabilized the power at the converter terminals under voltage dip operation and consequently obtained a constant and ripple free dc-link voltage. However, the fast active and reactive power response is mandatory for some applications in which, the CDVC cannot cope with. Accordingly, the Direct Power Control method (DPC) is proposed with several improvements to allow the efficient operation under severe disturbances; providing robust and fast response. The most efficient converter operation is obtained through the introduction of a novel technique that dynamically alters the conventional switching table of DPC. That proposed technique is mathematically and experimentally confirmed ensuring the highest possible smooth and fast active and reactive power response, as well as, supporting the grid during fault incidents, by boosting the voltage at converter terminals, through the injection of reactive power. Further improvement of the overall converter efficiency is achieved through a novel technique which is relying on the reduction of DPC switching frequency; hence improving the overall converter efficiency and increasing the semiconductor switches lifetime, and as a result, increasing its reliability.
In conclusion, the experimental results along with the intensive mathematical analysis explores a new paradigm of building a strong and solid footing techniques for a powerful and robust grid-connected converter, which is confirmed having an extensive capability to manage the most grid destructive incidents, and ride-through them successfully.