Modular Multilevel Converter Three-Arm Topology Cell Reduction
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Solution Overview
Problem
Modular multilevel converters face high complexity due to the number of individual converter cells and associated switch control and capacitor charge balancing, which complicates their operation and scalability.
Innovation Solution
The design includes at least two phase units connected in parallel with upper and lower converter arms, a middle converter arm, and bridging elements, where the middle converter arm can support either arm depending on AC voltage, reducing the number of cells required and allowing for smaller capacitors, optimized switches, and redundant configurations to enhance reliability and reduce losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional modular multilevel converter configuration is used, then power conversion function is achieved, but device complexity increases due to high number of individual converter cells and associated switch control
Solution Approach 1:
The patent extracts the middle converter arm from the traditional upper-lower arm configuration, creating a three-arm topology (upper, middle, lower). This reconfiguration reduces the total number of cells required while maintaining the power conversion function, as the middle arm can share cells between upper and lower configurations depending on AC voltage conditions
Solution Approach 2:
The patent implements dynamic reconfiguration where the middle converter arm can support either the upper or lower arm depending on AC voltage levels. This dynamic switching capability allows flexible cell utilization and reduces the total cell count required, thereby decreasing device complexity while maintaining reliability
2Reliability
If more converter cells are used to maintain reliability, then power conversion function is improved, but loss of energy increases due to higher conduction losses
Solution Approach 1:
The middle converter arm serves multiple functions: it can support the upper arm during certain AC voltage conditions and support the lower arm during other conditions. This multi-functionality allows the same cells to be reused in different configurations, reducing total cell count and associated conduction losses while maintaining reliable power conversion
3Ease of manufacture
If capacitor size is reduced to decrease device complexity, then ease of manufacture is improved, but reliability decreases due to insufficient energy storage
Solution Approach 1:
The dynamic reconfiguration capability allows the converter to optimize capacitor utilization based on AC voltage conditions. The middle arm can dynamically support either upper or lower arm, enabling more efficient energy distribution and reducing the total energy storage capacity required while maintaining reliable operation
4Ease of operation
If number of cells is reduced to decrease device complexity, then ease of operation is improved, but loss of energy increases due to higher current ripple
Solution Approach 1:
The patent doubles the current ripple frequency through the three-arm configuration with middle converter arm. This frequency doubling effect reduces the amplitude of current ripple, thereby reducing associated energy losses while maintaining ease of operation with fewer cells
Data Source
AI summary
Various embodiments include a modular multilevel converter comprising: two phase units in parallel each with an upper arm and a lower arm, each arm with cells in series, each cell with an energy storage element and switching the element in or out of the series of cells; a control unit for the switches; a middle converter arm in series between the upper and lower arm, with cells arranged in series; an upper node between the upper and middle arms and a lower node between the lower and middle arms; an upper bridging element arranged between said upper node and an AC terminal; and a lower bridging element arranged between said lower node and said AC terminal.