Modular Multilevel Converter Carrier Assignment
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Solution Overview
Problem
Conventional Modular Multilevel Converters (MMC) face issues with operation when sub-modules fail, as they require continuous carrier shifting, leading to increased switching frequency and complexity in controller implementation, and not all sub-modules participate in switching due to unbalanced carrier ranges.
Innovation Solution
The MMC system includes N sub-modules and M redundant sub-modules, where all N+M sub-modules participate in switching with carrier signals oscillating between -A and +(A × (N + M) / N, allowing for balanced participation and phase-shifting, eliminating unnecessary switching and simplifying controller implementation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If M redundant sub-modules are added to N sub-modules and carriers are continuously shifted to maintain N operational sub-modules, then reliability is improved through redundancy, but switching frequency increases and controller complexity increases
Solution Approach 1:
The patent pre-assigns all N+M carrier signals to their corresponding sub-modules at the initial stage, establishing a fixed mapping relationship. This preliminary assignment eliminates the need for dynamic carrier shifting when sub-modules fail, as the redundant sub-modules are already configured with their carriers and can immediately take over operational roles without requiring controller reconfiguration or carrier reallocation.
Solution Approach 2:
The patent makes all N+M sub-modules universally capable of participating in switching operations by assigning carriers to all of them. Instead of designating specific sub-modules as purely operational or redundant, the system enables any sub-module to function in switching based on its carrier signal state, thereby simplifying the controller's task of managing carrier assignments while maintaining reliability through redundancy.
2Reliability
If carriers are continuously shifted to maintain N operational sub-modules among N+M sub-modules, then reliability is improved, but unnecessary switching increases leading to higher switching frequency
Solution Approach 1:
The carrier assignment is performed preliminarily and statically at the system initialization stage, with each of the N+M sub-modules having a predetermined carrier signal assigned to it. This eliminates the need for continuous dynamic carrier shifting that would cause unnecessary switching transitions. The fixed assignment allows sub-modules to remain in stable states while still providing redundancy capability.
3Device complexity
If N carriers are assigned to N sub-modules without redundant sub-modules, then device complexity is reduced, but reliability decreases when sub-modules fail
Solution Approach 1:
The patent segments the carrier assignment function into two independent parts: (1) static carrier signal generation and (2) dynamic sub-module selection based on operational status. The carrier signals themselves remain simple and unchanged, while the selection of which sub-modules to operate is dynamically determined. This segmentation allows redundancy to be added without complicating the carrier signal structure or generation logic.
Data Source
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AI summary
The present invention provides a Modular Multilevel Converter (MMC) in which M redundant sub-modules are additionally arranged in addition to the N sub-modules that are needed for operation, and the N+M sub-modules are controlled so as to participate in switching in turn. The MMC according to an embodiment of the present invention includes multiple sub-modules connected in series with each other and a controller for controlling on/off switching of the sub-modules. Here, the multiple sub-modules include N sub-modules that participate in the operation of the MMC and M redundant sub-modules for replacing a failing sub-module when at least one of the N sub-modules fails, and the controller switches on the sub-module if the carrier signal assigned thereto is higher than a preset reference signal, and switches off the sub-module if the carrier signal assigned thereto is lower than the reference signal.