MMC Submodule Topology for Fault-Tolerant Bidirectional Switching
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Solution Overview
Problem
Modular multilevel power converters face challenges in failure behavior and complexity due to the limited bidirectional switching capabilities of semiconductor switches, which restrict the generation of output voltages and increase the number of semiconductors required for redundancy.
Innovation Solution
The proposed submodule arrangement allows for eight switching states, including bidirectional operation, by using capacitors in parallel connection with diodes, reducing the number of semiconductors needed and enabling half the energy to be controlled in case of failure, thus simplifying and cost-reducing the design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If semiconductor switches are used with limited bidirectional switching capabilities, then the device complexity is reduced, but the reliability and availability of the power converter deteriorates
Solution Approach 1:
The submodule is divided into two identical bridge circuits (first bridge circuit and second bridge circuit), each with three semiconductor switches. This segmentation allows independent operation and failure isolation of each bridge, improving reliability while maintaining manageable complexity through modular design
Solution Approach 2:
The invention changes the switching state parameters by enabling bidirectional switching capability through the capacitor-diode configuration. The capacitors can be charged in one direction and discharge in both directions, effectively providing four-quadrant switching operation with limited bidirectional semiconductor switches, thus improving reliability without proportionally increasing device complexity
2Device complexity
If the number of semiconductor switches is reduced, then the cost and device complexity are reduced, but the ability to generate different output voltages deteriorates
Solution Approach 1:
Capacitors are introduced as intermediary energy storage elements between the semiconductor switches and the output. These capacitors store energy during charging phases and release it during discharging phases, enabling the generation of multiple output voltage levels (positive, negative, and zero) with fewer semiconductor switches, thus maintaining adaptability while reducing complexity
Solution Approach 2:
Each semiconductor switch in the invention serves multiple functions: it can conduct current in one direction and block voltage in the opposite direction, and through the capacitor configuration, enable bidirectional voltage output. This multi-functionality allows the reduced number of switches to maintain full adaptability for generating different output voltages required by the power converter
3Reliability
If more semiconductor switches are used for redundancy, then the reliability is improved, but the device complexity and cost increase
Solution Approach 1:
The capacitor-diode configuration provides beforehand cushioning by enabling alternative current paths and voltage output modes when semiconductor switches fail. The capacitors store energy that can be released through diodes even when original switch paths are compromised, providing built-in redundancy protection without requiring additional semiconductor switches for backup
Data Source
AI summary
A submodule for a modular multilevel converter has nine semiconductor switches that can be switched off, four capacitors, six network nodes, and two terminals. The components are mounted such that different voltages are generated between the terminals of the submodule by controlling the semiconductor switches. This arrangement of components substantially improves the behavior of the converter and of the submodule in the event of a fault.