Thyristor-Bypassed Submodule Power-Groups for Achieving High-Efficiency, DC Fault Tolerant Multilevel VSCs

Abstract

Achieving DC fault tolerance in modular multilevel converters requires the use of a significant number of Sub-Modules (SMs) which are capable of generating a negative voltage. This results in an increase in the number of semiconductor devices in the current path, increasing converter conduction losses. This paper introduces a thyristor augmented multilevel structure called a Power-Group (PG), which replaces the stacks of SMs in modular converters. Each PG is formed out of a series stack of SMs with a parallel force-commutated thyristor branch, which is used during normal operation as a low loss bypass path in order to achieve significant reduction in overall losses. The PG also offers negative voltage capability and so can be used to construct high efficiency DC fault tolerant converters. Methods of achieving the turn-on and turn-off of the thyristors by using voltages generated by the parallel stack of SMs within each PG are presented, while keeping both the required size of the commutation inductor, and the thyristor turn-off losses low. Efficiency estimates indicate that this concept could result in converter topologies with power-losses as low as 0.3% rated power, whilst retaining high quality current waveforms and achieving tolerance to both AC and DC faults.

Hybrid Alternate-Common Arm Converter With High Power Capability: Potential and Limitations

“…6. In other words, the maximum power ratio is achieved at the point where the rated current of the converter, as given by (21), is minimized.…”

“…• the modulation index should be lower than the one for which the discontinuity current I ds becomes higher than the peak arm current given by (21), which is represented by the symbol M ds max ;…”

A T-Type Modular Multilevel Converter

“…Four hybrid topologies combining SMs with linecommutated converters (LCCs) were reviewed in terms of active and reactive power capabilities [20]. With the design concept inherited from LCCs, various VSC configurations based on FB SMs and thyristor strings have been investigated [21]- [24]. However, different from the conventional LCC implementation, the series-connection of thyristors requires higher-performance voltage-balancing and signalsynchronizing techniques for forced-commutation approaches, and sophisticated turn-on/off processes occur at the fundamental frequency, which may result in voltage-balancing or commutation failure.…”

Double-Thyristor-Based Protection for Valve-Side Single-Phase-to-Ground Faults in HB-MMC-Based Bipolar HVDC Systems

“…As single-bypass-thyristors have been widely employed in real industrial applications to protect the system upon DC faults [13]- [14], adding one more thyristor to form double-thyristors may not be a technical problem and may not lead to a high increase of cost [15]- [16]. In addition, the double-thyristors will not cause power losses during normal operation.…”