Nowadays the Modular Multilevel Cascaded Converter (MMCC) is a family of
emerging high-voltage multilevel converters that are configured with a
cascaded connection of identical submodules with low-voltage ratings by
distinct topological structures. The MMCC system is featured with a high
quantity of coupled system variables (converter currents and floating
submodule voltages) and abundant discrete control inputs (submodule
switching states). To guarantee a stable and optimal system operation,
it is a fundamental challenge to fully model and control these
variables. This thesis addresses two frameworks for the control-oriented
MMCC modeling as well as the hierarchical analysis. The first framework
in Part I presents a comprehensive classification of MMCC topologies,
analyzes them by replacing converter branches with continuous
controllable voltage sources and develops a unified modeling procedure
for current and branch energy, aiming for a general understand of MMCC
in the context of continuous system theory. The second framework aims to
develop an explicit relation between submodule switching states and MMCC
system variables, which preserves the characteristics of discrete
switched system. Two practical direct control methods, e.g., fast
reduced control set and event-based method, are proposed, which achieves
comparable harmonic performance and obviously improved submodule voltage
balancing under the premise of the same switching frequency as the
conventional submodule-voltage-sorting method.
