| dc.description.abstract | Multilevel converters can reach medium-voltage operation increasing the efficiency
of high-power applications. Among the available multilevel converter topologies, the
invention of Modular Multilevel Converter (MMC) has gained considerable attention of
academia and industry in recent times. There are attractive advantages of this converter
in terms of modularity, high-quality waveforms, low device rating, fault tolerant
capacity and voltage scalability. Moreover, the main advantage compared to cascaded
multilevel converters is the lack of an input transformer which results in a reduction of
cooling requirements, size, and cost. The converter concept has already been introduced
in the market for high voltage dc transmission (HVDC) applications. A considerable
research is going on presently for using this topology for dc-ac power conversion e.g.
for ac drives.
The main focus of this research work will be based on identifying and solving some
of the challenges of medium and high voltage converters for drives and power grid.
Reliability and fail safe operation of these converters are primary requirements. Apart
from this, condition monitoring and health diagnosis of the converter including the load
is also essential.
This thesis studies mathematical models to predict the behaviour of the converter
under steady state and dynamic conditions. The obtained models can be useful to know
the transient response inside the converter during faulty conditions and make it possible
to design the components for this converter for abnormal operations. Exploring the fault
operation of cells and bypassing in case of a fault have been investigated by simulation
and laboratory experiments. Moreover, in MMC, there is a strong harmonic content in
the arm current that essentially controls the dynamic behavior of the converter. In this
project an explicit derivation of the equations in the frequency domain has been
developed based on the switching functions concept. The salient features of this analysis
are (i), how the second order harmonic in the circulating current influences capacitor
voltage ripples, and (ii) to find the most critical parameters related to capacitor voltage
fluctuations.
Pulse width modulation techniques are widely employed for the synthesis of ac
voltages at the terminals of a voltage sourced converters. The new level-shifted PWM
method presented in the thesis offers reduced switching frequency of the cells, no need
to calculate the duty cycles based on the reference waveforms as well as easy real-time
implementation in a DSP.
This research also contributes toward introducing circuit arrangements to start-up the
converter from a de-energized condition without adding auxiliary voltage source. Some
modification in the existing circuitry for dc-ac power conversion is also explored where
a transformer has been introduced inside the MMC. The proposed circuit helps to
reduce the voltage rating of the power devices and the capacitors to half when compared
to the conventional MMC. Additionally, the transformer also helps in limiting the
circulating current.
An improved decoupling current controller is proposed that seeks to tackle some
drawbacks of the conventional d-q current controller, while keeping the successful
reference tracking and disturbance rejection.
One of the specified objectives of this work is to develop and implement an efficient
and robust capacitor voltage observer. The observer will work as a software sensor
working in parallel with the hardware measurement part. It could be used for condition
monitoring for capacitors, predictive control, fault detection and parameter estimation.
All concepts and proposed approaches in this thesis have been analyzed and validated
through simulations and experimental results, as relevant issues are discussed.
Finally, this thesis addresses the software and hardware design of two laboratory
setups for four-level MMC-based inverter as part of a project. These setups can be used
effectively to test the above mentioned ideas as well as enable them for future practical
use. The control circuit will be built based on DSP and FPGA platforms which are
utilized to produce the gate control signals. | nb_NO |
