Series-Connected MMC Topology With Fewer Modules for High-Voltage Conversion
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing MMC converters face challenges with high module count, large size, and low power density due to limited withstand voltage of commercial semiconductor devices, leading to increased costs and reduced reliability.
Innovation Solution
A series-connected MMC topology with a reduced number of modules, comprising a three-phase bridge circuit, half-bridge valve strings, a three-phase filter inductor, and a three-phase grid frequency transformer, where high-voltage switch strings are connected in series and parallel, and half-bridge valve strings are used to achieve stable AC output and balanced capacitor voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If the number of submodules is increased to support higher AC/DC bus voltage, then the voltage level is improved, but the device size and cost increase significantly while power density decreases
Solution Approach 1:
The patent divides the high-voltage bridge arm into multiple series-connected switch strings, where each string contains multiple semiconductor switches in series. This segmentation allows the system to achieve high voltage capability without requiring a single high-voltage switch or a large number of parallel submodules, thereby reducing device size while maintaining voltage support capability.
Solution Approach 2:
The patent transitions from the conventional topology where voltage is achieved through parallel submodule multiplication to a topology where voltage is achieved through series connection of switch strings within each bridge arm. This dimensional change in the voltage-building approach reduces the number of submodules needed and consequently reduces device volume.
2Stress or pressure
If the number of submodules is increased to support higher AC/DC bus voltage, then the voltage level is improved, but the cost increases and reliability decreases
Solution Approach 1:
By segmenting the voltage support function into series-connected switches within each bridge arm, the patent reduces the total number of submodules required. Fewer submodules mean fewer failure points and simplified control, thereby improving system reliability while maintaining high voltage capability.
Solution Approach 2:
The patent changes the operational parameters of the semiconductor switches, specifically operating them at lower individual voltage ratings but in series configurations. This parameter change allows the use of more reliable, commercially available switches while achieving the required total voltage through series multiplication rather than parallel submodule expansion.
3Stress or pressure
If the number of submodules is increased to support higher AC/DC bus voltage, then the voltage level is improved, but the power density decreases
Solution Approach 1:
The patent segments the voltage-building function into series-connected switch strings, which reduces the number of submodules required. This reduction in submodule count directly decreases device volume while maintaining power output capability, thereby improving power density.
Solution Approach 2:
The patent shifts from achieving voltage through horizontal expansion (more parallel submodules) to vertical expansion (series-connected switch strings within each bridge arm). This dimensional change reduces the footprint and volume of the converter while maintaining voltage and power output, thus improving power density.
Data Source
AI summary
The present disclosure relates to the field of electric power system, and more particularly to a topology of a series-connected MMC with a small number of modules, where the topology is composed of a three-phase bridge circuit, half-bridge valve strings, a three-phase filter inductor, and a three-phase grid frequency transformer. The topology of a series-connected MMC with a small number of modules in the present disclosure needs only two half-bridge valve strings, thus greatly reducing the number of the submodules as compared with the conventional MMC structure. When achieving the same high DC voltage output, the present disclosure can improve the power density of the MMC, realize stable three-phase AC output voltage, and further achieve balance of capacitor voltages in the two half-bridge valve strings. Compared to the conventional MMC topology, the topology in the present disclosure can reduce the number of submodules by nearly ⅔, and has a greater AC-DC voltage transfer ratio, thus reducing the cost of the MMC converter, reducing the device size, and improving the power density.