Modular Multilevel Converter AC Fault Handling
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Solution Overview
Problem
Modular multilevel converters face challenges in handling AC side faults, particularly phase-to-ground faults, which require high voltage ratings and lead to oversizing of converters, and can result in temporary high AC side voltages and overcurrents, necessitating additional components or redundancy.
Innovation Solution
Implementing a protective control mode that converts bipolar voltage contribution cells to unipolar operation during faults, blocking faulty phase legs and controlling healthy phase legs to mitigate fault effects, thereby reducing converter voltage rating and addressing AC side fault consequences without additional components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If full-bridge cells are used to handle DC fault currents, then DC fault voltage blocking capability is improved, but converter size increases due to oversizing for AC side faults
Solution Approach 1:
The patent implements dynamic cell blocking strategies where cells are selectively blocked based on fault type and location. During AC side faults, only affected phase legs are blocked rather than the entire converter, allowing the system to adapt its protection scope dynamically. This prevents unnecessary oversizing while maintaining DC fault blocking capability when needed.
Solution Approach 2:
The patent changes operational parameters by switching cells between bipolar and unipolar modes depending on fault conditions. During AC faults, cells in healthy phases continue operating in bipolar mode to maintain voltage blocking capability, while cells in faulty phases are blocked. This parameter change allows the converter to handle AC faults without requiring all cells to be oversized for simultaneous AC and DC fault protection.
2Reliability
If full-bridge cells are used to block DC fault voltage, then DC fault handling is improved, but temporary high AC side voltages occur during faults
Solution Approach 1:
The patent applies preliminary anti-action by pre-configuring the converter with full-bridge cells capable of bipolar operation before faults occur. These cells are designed to block both polarities of voltage, creating a preemptive defense against both AC and DC faults. When AC faults occur, the blocking capability prevents the generation of temporary high voltages by maintaining proper voltage polarity control.
Solution Approach 2:
The patent converts the potential harm of high AC side voltages during faults into a benefit by using the same full-bridge cell structure that causes the voltage blocking requirement to also provide fault protection. The bipolar capability that necessitates higher voltage ratings also enables the cells to block fault voltages effectively, transforming the design constraint into a protective feature.
3Weight of stationary object
If asymmetrical converter design is used to avoid oversizing, then converter size is reduced, but device complexity increases due to different cell types in phase arms
Solution Approach 1:
The patent embraces asymmetry by allowing different phase legs to have different operational states during faults. Healthy phase legs continue full bipolar operation while faulty phase legs are blocked. This asymmetric fault response allows the use of full-bridge cells without requiring symmetrical oversizing of all phase legs, reducing overall converter size while maintaining protection capability.
Solution Approach 2:
The patent achieves universality by designing all cells as full-bridge cells with bipolar capability, making them multi-functional. These cells can handle both AC and DC faults, operate in both bipolar and unipolar modes, and provide voltage blocking in all phase legs. This universal cell design eliminates the need for different cell types in different phase arms, reducing structural complexity while maintaining size efficiency.
4Weight of stationary object
If bypass switches are added to full-bridge cells to handle AC faults, then converter size is reduced, but device complexity increases due to additional components
Solution Approach 1:
The patent implements self-service by enabling full-bridge cells to handle their own fault protection functions without requiring external bypass switches. The bipolar capability of full-bridge cells allows them to block both polarities of voltage and protect themselves during AC and DC faults. This self-protection mechanism eliminates the need for additional bypass components, reducing device complexity while maintaining size efficiency.
Data Source
Figure 1~3
Figure 2
Figure 4~6
AI summary
A modular multilevel converter comprises a number of phase legs comprising cells (C1p1, … C5p1, C1n1, … C5n1) having a bipolar voltage contribution capability. The modular multilevel converter (14) also comprises a control unit (22) configured to enter a first protective control mode upon the detection of an operational disturbance of the converter, the first protective control mode comprising controlling all the bipolar voltage contribution cells to operate as unipolar voltage contribution cells, and if the operational disturbance is identified as a fault on a phase of a connected AC link, to block the cells of the phase leg connected to the phase for the remainder of the first protective control mode and control the cells of the phase legs connected to the healthy phases of the AC link to handle effects of the fault.