Dual Submodule for Modular Multilevel Converter
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Solution Overview
Problem
Modular multilevel converters face challenges in reducing the number of power semiconductor switches, line losses, and improving short-circuit robustness, particularly in high-voltage applications, where half-bridge submodules are costly and inefficient, and full-bridge submodules increase complexity and losses.
Innovation Solution
A dual submodule configuration with two interconnected asymmetric H-bridge circuits, each with a series circuit of a power semiconductor switch and a diode, connected in parallel to share current paths and reduce line losses, allowing for a mechanical structure similar to existing submodule types.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If full-bridge submodules are used to invert voltage drop across capacitor, then voltage control flexibility is improved, but device complexity and cost increase due to higher number of power semiconductor switches
Solution Approach 1:
The full-bridge submodule is segmented into two asymmetric half-bridge submodules connected in parallel. Each asymmetric half-bridge has two parallel bridge branches with different numbers of switches, allowing independent control of each branch to achieve voltage inversion functionality while reducing total switch count compared to a complete full-bridge configuration.
Solution Approach 2:
Two asymmetric half-bridge submodules are merged in parallel connection to form a dual submodule configuration. This merging provides redundant current paths and combines the advantages of both half-bridge simplicity with full-bridge voltage control capability, reducing overall device complexity while maintaining adaptability.
2Adaptability or versatility
If full-bridge submodules are used to invert voltage drop across capacitor, then voltage control flexibility is improved, but line losses increase due to two current-conductive switches
Solution Approach 1:
The current path is segmented into multiple parallel branches within each asymmetric half-bridge submodule. This segmentation allows current to flow through only one switch at a time in each branch, reducing the number of simultaneously current-conductive switches from two (in full-bridge) to one, thereby reducing line losses.
Solution Approach 2:
The two parallel bridge branches in each asymmetric half-bridge are designed with different local characteristics (different numbers of switches). This local quality differentiation allows optimal current distribution where current flows through the path with fewer switches, minimizing line losses while maintaining voltage control flexibility.
3Device complexity
If half-bridge submodules are used for simple and robust operation, then device complexity is reduced, but short-circuit robustness deteriorates due to heavily dimensioned free-wheeling diodes
Solution Approach 1:
Two asymmetric half-bridge submodules are merged in parallel to create dual submodule configuration. This merging provides redundant current paths through multiple parallel diodes, enhancing short-circuit robustness by distributing the current load across multiple components while maintaining the simplicity of half-bridge operation.
Solution Approach 2:
The parallel dual submodule configuration provides beforehand cushioning against short-circuit damage. When a short-circuit occurs, the redundant parallel paths and multiple diodes absorb and distribute the excessive current, protecting individual components from destruction and maintaining system reliability without requiring heavily dimensioned single diodes.
Data Source
AI summary
A dual submodule is created for a modular multilevel converter, whereby the dual submodule has two interconnected submodules, whereby each submodule has an asymmetrical half-bridge circuit with two parallel bridge branches, which are connected between a first and a second terminal connection of the submodule, whereby each bridge branch is formed from a series circuit of a power semiconductor switch, and a diode, whereby the power semiconductor switch is allocated to an antiparallel free-wheeling diode, and has a capacitor, which is connected in parallel with the asymmetrical half-bridge circuit between the first and the second terminal connections of the module. The submodules are connected to each other via their AC terminals to form the dual submodule. Further, a modular multilevel converter is created, comprising a number of such dual submodules in each of its converter branches.