Modular Multi-Level Converter DC Fault Blocking
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Solution Overview
Problem
Conventional modular multilevel converters (MMC) face challenges in effectively blocking DC failures, leading to increased switching losses and potential damage from fault currents, particularly when using full-bridge circuits, which are more costly and inefficient compared to half-bridge circuits.
Innovation Solution
The MMC incorporates a combination of full-bridge and half-bridge submodules within each converter arm, utilizing a circuit opening unit to manage and block DC fault currents by turning off power semiconductors and using a disconnector to isolate the circuit when the fault current reaches zero, thereby reducing switching losses and protecting internal components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If full-bridge circuits are used to block DC fault current, then DC failure blocking capability is improved, but switching loss increases by 30% or more
Solution Approach 1:
The converter arms are segmented into two types: first converter arms with full-bridge submodules for DC fault blocking and second converter arms with half-bridge submodules for normal operation. This segmentation allows the system to achieve DC failure blocking capability only where necessary while maintaining efficiency in other parts.
Solution Approach 2:
Different circuit configurations are applied to different converter arms based on their functional requirements. The first converter arms use full-bridge circuits with bidirectional switches for DC fault blocking capability, while the second converter arms use simpler half-bridge circuits for efficient normal operation, optimizing local characteristics for specific functions.
2Reliability
If full-bridge submodules are used for all converter arms, then DC fault current blocking is achieved, but device complexity and cost increase
Solution Approach 1:
The system is divided into first converter arms requiring DC fault blocking (using full-bridge submodules) and second converter arms for normal operation (using half-bridge submodules). This segmentation reduces the total number of full-bridge submodules needed, thereby reducing device complexity and cost.
Solution Approach 2:
Half-bridge submodules are used in second converter arms as a simpler, more economical alternative to full-bridge submodules. These half-bridge submodules perform adequately for normal operation without providing DC fault blocking, reducing the overall component count and system cost.
3Device complexity
If half-bridge circuits are used, then device simplicity is maintained, but DC fault current cannot be blocked
Solution Approach 1:
The converter system is segmented into first converter arms with full-bridge submodules for DC fault protection and second converter arms with half-bridge submodules for simple normal operation. This segmentation ensures that DC failure protection is provided only where required while maintaining simplicity elsewhere.
Solution Approach 2:
The full-bridge submodules in the first converter arms serve dual functions: normal power conversion and DC fault current blocking. This multi-functionality allows a single component type to address both operational requirements, eliminating the need for separate protection devices.
Data Source
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AI summary
The present invention provides an MMC and a DC failure blocking method therefor, the MMC and the method being capable of DC failure blocking and reducing loss using a combination of half-bridge submodules and full-bridge submodules for converter arms of the MMC. According to the present invention, there is provided a modular multilevel converter (MMC) including multiple converter arms, each converter arm having: N (N ≥ 2, an integer) submodules connected to each other in series; and a circuit opening unit connected to the N submodules in series to open a circuit of the converter arm, wherein the N submodules are n (n < N) submodules including full-bridge circuits and N-n submodules including half-bridge circuits.