Capacitive energy balancing of multilevel submodules for cascaded converter in STATCOM applications

Abstract

Cascaded converters also known as modular converters are used for STATCOM applications due to their small footprint, capability to achieve high voltage levels, modularity and reduced losses. A commonly used modular converter consists of a series connection of full - bridge also known as H - bridge cells or submodules. There is a wide area of resea rch that focuses in various topologies of these submodules. In this project a new multilevel submodule has been proposed. This project investigates operational princip les and capacitive energy balancing of this multilevel submodule that can be used in any type of modular multilevel converter. The investigation consists of analysis of the submodule circuit and its switching states, design and simulation of the modulation strategy as well as the capacitive energy balancing algorithm in the case when multiple submodules are connected in series forming a cascaded converter. Various methods of capacitive energy balancing that employ different ways of sorting capacitor voltage s and deciding which capacitors/submodules should be inserted in the current path are investigated in this thesis. These energy balancing methods have been simulated and evaluated in terms of their impact on the capacitor voltage ripple and switching frequ ency of the new multilevel submodule. This evaluation results in the selection of the most promising energy balancing method, which is embedded in a simulation model of a three - phase STATCOM constructed with the proposed multilevel cells. Finally, the simu lation model is used for evaluating the performance of the proposed multilevel cell and capacitive energy balancing method under steady - state and fault conditions, which would typically occur in a utility application.