Thyristor/Diode-Bypassed Submodule Power Groups for Improved Efficiency in Modular Multilevel Converters

Abstract

The half-bridge Modular Multilevel Converter (MMC) is a Voltage Source Converter (VSC) with high efficiency, controllability and modularity. The topology is weak to DC side faults unless bipolar sub-modules are used, but this results in decreased efficiency. Power-Groups (PGs), a thyristor augmented multilevel structure, have been proposed as a way to reduce the power-loss increase arising from achieving DCfault-tolerance. This paper investigates whether the PG concept can also achieve significant efficiency improvements in VSCs that are not required to be DC fault tolerant. A Single Sub-Module Voltage (SSMV) method of controlling the turn-on/turn-off of the thyristor assembly within each PG structure is presented and the differences with the previously detailed Dual Sub-Module Voltage (DSMV) technique are described. Two thyristor-based PG structures for use in non-DC-fault-tolerant MMCs are proposed, one using SSMV and the other using DSMV. A comparison is made considering the required semiconductor device count, the impact on thyristor snubber design, and the overall powerlosses achieved. A further, simplified, variant using a diode bypassed PG structure is presented which results in powerloss reductions during rectifier mode only. Results show that power-loss reductions of ∼20-25% can be achieved by using the proposed PG structures to augment a half-bridge MMC.

Modeling and control of the modular multilevel converter (MMC) based solid state transformer (SST) with magnetic integration

“…6), while the amplitude of the MF component (83A) equals to the amplitude of icA. These all match with the arm current expression in (15). The voltage and current of the LF and MF ports are also demonstrated.…”

“…10(c) shows the arm current iLA. It can be found that iLA contains the LF and MF components, matching with the expression in (15). Fig.…”

“…For example, when the LVDC short-circuit occurs, the secondary voltages vsy (y=a, b or c) would be clamped to zero. In this case, the MF voltages vMFx should be reduced to match with vsy, so as to limit the fault current; and when the DC bus of MMC is short-circuit, the SM should be protected by extra protection circuit such as thyristors [15]- [17]. The detail analysis and design will be done as the future work.…”

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

“…where A pk is given by (17), while K um and K mo are the coefficients shown in (18) and (19), respectively. Finally, by combining the definition (20) with (22), the power ratio of the converter can be expressed as…”

“…Both circuits reduce the total rating of the semiconductors and the energy storage elements compared to the MMC. The authors of [18], [20]- [22] propose variations of an FB-MMC where thyristor (or diode) switches are used for bypassing a converter arm or small groups of seriesconnected FB submodules (SMs), which leads to a significant loss reduction. Moreover, a thyristor-based half-bridge SM was proposed in [23], [24] for constructing a thyristor-based MMC with reduced conduction losses.…”