Hybrid Modular Multi-Level Converter Topology
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Solution Overview
Problem
Modular multi-level converters for medium voltage drive applications are costly and have a large footprint due to the high number of converter cells and energy storage requirements, and they face challenges during low-speed operations where cell capacity and ripple issues arise, leading to current derating.
Innovation Solution
A modular multi-level converter topology with two rows of converter cells, where the first row (light cells) has reduced capacity and operates in quasi 2-level mode, and the second row (normal cells) has standard capacity, reducing total stored energy and number of power modules while maintaining high efficiency and scalability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a modular multi-level converter uses a high number of converter cells with standard capacity to ensure reliable operation, then the converter achieves good voltage scalability and high quality output voltage, but the cost and footprint increase significantly
Solution Approach 1:
The converter cells are divided into two distinct groups: full-capacity cells and reduced-capacity cells. This segmentation allows the system to use fewer total cells while maintaining reliability through the strategic placement of full-capacity cells in series strings, thereby reducing the overall footprint and cost.
Solution Approach 2:
Different converter cells are assigned different capacities based on their specific position and function within the converter architecture. Full-capacity cells are placed where maximum energy storage is needed, while reduced-capacity cells are used in positions where lower capacity suffices, optimizing both reliability and space utilization.
2Object-generated harmful factors
If the converter uses standard cell capacity to maintain low voltage ripple during operation, then the output voltage quality is maintained, but the cost and number of power modules increase
Solution Approach 1:
The cell population is segmented into full-capacity and reduced-capacity types. The full-capacity cells handle the majority of the energy storage function, while reduced-capacity cells contribute to voltage synthesis. This segmentation reduces the total number of cells needed while maintaining acceptable voltage ripple through coordinated switching of both cell types.
Solution Approach 2:
The reduced-capacity cells perform a partial energy storage function, relying on the full-capacity cells to compensate when needed. This partial action approach allows the system to use fewer total cells while maintaining performance through the excessive capability of the full-capacity cells during critical operation phases.
3Speed
If the converter operates at low speed with high currents, then the machine drive requirements are met, but the cell capacity must be increased which increases ripple and requires current derating
Solution Approach 1:
Full-capacity cells are strategically positioned in series strings to handle the high current and energy storage demands during low-speed operation, while reduced-capacity cells assist in voltage synthesis. This local quality differentiation enables the converter to meet high current requirements without increasing the capacity of all cells, avoiding current derating.
Solution Approach 2:
The converter dynamically switches between different cell configurations and operating modes depending on the speed and load conditions. During low-speed high-current operation, the full-capacity cells are actively engaged to provide the necessary energy storage and current handling, while during normal operation, the reduced-capacity cells suffice, optimizing performance across the entire operating range.
Data Source
Figure 1~3
Figure 4~5
Figure 6~7
AI summary
A modular multi-level converter (10) for converting a DC voltage into an AC voltage comprises a first row (14) and a second row (18) of converter cells (16, 20), each converter cell (16, 20) comprising a cell capacitor (Ccell, Ccell') and semiconductor switches (34, 36, 34', 36') adapted for connecting the cell capacitor to an output of the converter cell (16, 20) and for bypassing the cell capacitor. The first row (14) of converter cells (16) interconnects a positive DC link connection point (22a) and a negative DC link connection point (22c), wherein the first row (14) of converter cells (16) comprises an upper pair and a lower pair of series-connected strings (24a, 24b, 24c, 24d) of series-connected converter cells (16), wherein the upper pair of strings (24a, 24b) connects the positive DC link connection point (22a) with a neutral DC link connection point (22b) and provides an upper intermediate connection point (26a) between the strings (24a, 24b) and the lower pair of strings (24c, 24d) connects the negative DC link connection point (22c) with the neutral DC link connection point (22b) and provides a lower intermediate connection point (26b) between the strings (24c, 24d). The second row (18) of converter cells (20) comprises a pair of strings (28a, 28b) of series-connected converter cells (20) interconnecting the upper intermediate connection point (26a) and the lower intermediate connection point (26b) and provides an AC connection point between the strings (28a, 28b). The converter cells (16) of the first row (14) have a first cell capacity (Ccell') and the converter cells (20) of the second row (18) have a second cell capacity (Ccell) higher than the first cell capacity (Ccell'). The converter cells (16) of the first row (14) have a capacitor switch (36'), which interconnects the cell capacitor (Ccell') with the outputs (38) and which has a lower current rating than a main switch (34') of the converter cells (16) of the first row (14) connected in parallel to the cell capacitor (Ccell'). The converter cells (20) of the second row (18) have a capacitor switch (36) and the current rating of the capacitor switch (36') of a converter cell (16) of the first row (14) is smaller than a current rating of the capacitor switch (36) of a converter cell (20) of the second row (18).