A New Transformerless Modular Multilevel Converters (MMC) Based on Half-Bridge Converters at Distribution Level

Abstract

This thesis presents a new family of converters for high power interconnection of dc buses with different voltage levels. Proposed converters achieve high voltage dc-dc conversion without an intermediate ac conversion stage. This function is implemented without series connection of active switches, or the use of isolation transformers. The salient features of proposed converters are (i) design and construction simplicity, (ii) low switching losses through soft turn-on and soft turn-off, (iii) single stage dc-dc conversion without high-current chopping, (iv) modular structure, (v) equal voltage sharing among the converter modules.

Three converter circuits are investigated. The first performs unidirectional power transfer from a dc bus with higher voltage to a dc bus with lower voltage. The second performs unidirectional power transfer from a dc bus with lower voltage to a dc bus with higher voltage. Both converters are suitable for interconnecting single pole dc buses with same polarity, or double pole dc buses. A third converter is also presented which performs the function of either the first or the second converter with polarity reversal. The third converter is suitable for interconnecting single pole dc buses with different polarities, or double pole dc buses. By hybridintegration of the proposed three converters, the thesis also investigates other topologies for bidirectional power transfer between two dc buses.

Proposed converters operate only in discontinuous conduction mode and exhibit soft switching operation for the active and passive switches. A common feature between the proposed converters is the self current turn-off for the active switches at zero voltage. This allows the use of thyristors as active switches alleviating their reverse recovery losses. For each converter topology, the structure is presented, its operation principle is explained and a complete set of design equations are derived. Comparisons are performed on high-power and high-voltage design examples. The merits and limitations of each converter are concluded. Practical considerations regarding components selection, loss analysis, filter design and the non-idealities of the circuits are studied. Experimental implementation of scaled-down laboratory prototypes is presented to provide a proof of concept and validate the operation principle of the proposed converter topologies.