Quasi Two-Level Operation of Modular Multilevel Converter for Use in a High-Power DC Transformer With DC Fault Isolation Capability

Abstract

DC fault protection is one challenge impeding the development of multi-terminal DC grids. The absence of manufacturing and operational standards has led to many pointto-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and DC fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode DC component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a DC to DC transformer where two converters operating in the proposed mode are coupled by an AC transformer for voltage matching and galvanic isolation. The proposed DC transformer is shown to be suitable for high-voltage high-power applications due to the low switching frequency, high efficiency, modularity, and reliability. The DC transformer facilitates DC voltage regulation and near instant isolation of DC faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.

DC fault isolation study of bidirectional dual active bridge DC/DC converter for DC transmission grid application

“…In [13], an IGBT based LCL resonant converter was studied that addressed some of the limitations of converter [12] but without isolation between the two bridges. An MMC DC-DC converter based on the concept of DC transformer is suggested in [14] that achieve DC blocking, but at the expense of large number of components and increased losses. This paper focuses on one important feature of bidirectional dual active bridge (DAB) DC-DC converter [15]: its inherent fault isolation capability without a need for a very fast controller to limit fault current.…”

An Open-phase Fault Detection Method for Six-phase Induction Motor Drives

“…The fault management in a drive can be classified in three stages: fault detection FD [13][14][15][16][17], fault isolation FI [18][19] and implementation of a fault tolerant control (FTC) [20][21][22][23][24][25]. The implementation of these three stages in the system is necessary to ensure a suitable operation and to protect the system in post-fault situation.…”

Modular input‐parallel output‐series DC/DC converter control with fault detection and redundancy

“…Currently, there is uncertainty regarding the architectures and control approaches that enable this high-capacity DC/DC power conversion. One possible solution is to use modular multilevel DC/DC converter (MMC) technology which has been widely studied for HVDC and MVDC [4,5]. Modular multilevel approaches to high-voltage conversion have achieved significant gains in HVDC applications.…”

“…Modular multilevel approaches to high-voltage conversion have achieved significant gains in HVDC applications. The effectiveness of such techniques is, however, limited in DC/DC applications [4,5]. Instead, a more compact and lighter design that uses a few modules arranged in a parallelseries topology could provide a replacement for a conventional single converter that uses high-voltage valves comprising several series-connected switching devices to enable operation at medium-voltage.…”