Modular Multilevel Converter Drive Signal Modulation for Thermal Stress Balancing
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Solution Overview
Problem
Traditional modular multilevel converters face challenges in effectively handling DC faults, particularly in full-bridge module units which experience high losses and costs, necessitating an optimization of performance for DC interconnection applications.
Innovation Solution
A drive signal modulation method for modular multilevel converters that alternates between two modes to balance thermal stress and current stress on power semiconductor switches, allowing for flexible fault isolation of broken-down switching transistors without stopping the submodule unit, thereby reducing fault rates and increasing system availability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a full-bridge module is used to suppress DC fault current, then DC fault handling capability is improved, but losses and costs increase
Solution Approach 1:
The patent applies periodic action by alternating between two drive modes (first drive mode and second drive mode) in a cyclic manner. The converter switches between these modes periodically, where in the first mode the full-bridge submodule operates with all four switches, and in the second mode it operates with only two switches. This periodic switching reduces average losses while maintaining DC fault handling capability through the alternating operation pattern.
Solution Approach 2:
The patent changes the operational parameters of the full-bridge submodule by switching between different drive modes. In the first drive mode, all four power semiconductor switches are activated for full fault suppression capability. In the second drive mode, only two switches are activated, reducing losses. This parameter change allows dynamic adjustment between performance and efficiency based on operational conditions.
2Reliability
If a full-bridge module is used to suppress DC fault current, then DC fault handling capability is improved, but costs increase
Solution Approach 1:
By periodically switching between full-bridge operation (first drive mode) and reduced-bridge operation (second drive mode), the system maintains DC fault handling capability only when necessary, rather than continuously operating in the more expensive full-bridge configuration. This reduces the average cost while preserving the required reliability.
Solution Approach 2:
The patent changes the operational state of the full-bridge submodule between two modes: a high-cost high-reliability mode (first drive mode with all switches active) and a low-cost adequate-reliability mode (second drive mode with fewer switches active). This parameter change optimizes the cost-reliability tradeoff by adjusting the operational configuration based on system needs.
3Temperature
If alternate drive signal is applied to balance thermal stress, then thermal stress on power semiconductor switches is reduced, but control complexity increases
Solution Approach 1:
The patent uses periodic action to balance thermal stress by alternating between two drive modes in a regular cycle. This periodic switching ensures that no single power semiconductor switch remains in a high-stress state continuously, allowing thermal redistribution and stress balancing across the device set while maintaining manageable control through a repeating pattern.
Solution Approach 2:
The patent applies dynamics by making the drive signal configuration changeable and time-varying. The system dynamically switches between the first drive mode and second drive mode, adjusting which switches are active based on a timing signal. This dynamic operation allows thermal stress balancing while the control complexity is managed through systematic switching rather than complex individual switch control.
4Productivity
If the converter switches between two drive modes, then thermal stress balancing is achieved and converter capacity increases, but operation control complexity increases
Solution Approach 1:
The periodic alternation between drive modes enables thermal stress balancing and increased converter capacity utilization while keeping control complexity manageable through a systematic repeating pattern. The regular cycling ensures fair distribution of stress and maximizes the use of available switch capacity without requiring complex real-time decision-making.
Solution Approach 2:
The dynamic switching between operational modes allows the converter to adapt its configuration to balance thermal stress and optimize capacity. The control complexity is managed through structured dynamic switching rather than static operation, enabling the system to flexibly adjust which switches are active based on thermal and operational conditions.
Data Source
AI summary
Disclosed are a modulation method of a modular multilevel converter and a fault isolation method of a submodule unit. The modulation method comprises a first mode and a second mode, and the first mode and the second mode operate cyclically. In the first mode, a first power semiconductor switch and a second power semiconductor switch are turned on alternately, while a third power semiconductor switch is turned off normally and a fourth power semiconductor switch is turned on normally. In the second mode, the third power semiconductor switch and the fourth power semiconductor switch are turned on alternately, while the first power semiconductor switch is turned on normally and the second power semiconductor switch is turned off normally. The method enables junction temperatures of the power semiconductor switches used to be equalized, increases an operation safety margin of the converter, effectively increase the capacity of the converter without increasing engineering costs, and achieve better performance in both economic efficiency and technicality.