Modular Multilevel AC-AC Converter Without a Centralized DC Link
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Solution Overview
Problem
Conventional AC-AC modular multilevel converters require multiple converter structures and centralized DC links, leading to increased semiconductor effort, losses, and a larger footprint, especially in direct AC voltage level conversion applications.
Innovation Solution
A modular multilevel converter using a single converter structure with half-bridge submodules that manipulates circulating DC currents for internal capacitor energy balance, enabling direct AC-AC voltage level conversion with reduced semiconductor rating and losses, and a smaller footprint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If conventional back-to-back MMC structures are used for AC-AC conversion, then power conversion capability is achieved, but semiconductor rating and device footprint increase
Solution Approach 1:
The converter is divided into multiple identical modular units, each handling a portion of the total power. Each module contains its own half-bridge submodules and can operate independently, allowing the system to achieve high power conversion capability while keeping individual module complexity low. The modular structure enables scalable deployment where multiple smaller units work in parallel rather than requiring one large complex converter.
Solution Approach 2:
The invention processes only the necessary portion of power through active semiconductor switching, while allowing circulating currents to handle internal power transfer between modules. This partial processing approach reduces the semiconductor rating required compared to conventional designs that must handle full power conversion through active devices.
2Power
If multiple converter structures with centralized DC links are used, then AC-AC power conversion is achieved, but energy losses increase
Solution Approach 1:
The invention extracts and eliminates the centralized DC link from the conventional back-to-back MMC structure. By removing this intermediate DC connection, the system avoids associated losses and simplifies the power flow path. Power is transferred directly between AC systems through the modular converter units with circulating currents managing internal energy balance without requiring a centralized DC energy storage and transfer mechanism.
Solution Approach 2:
Each modular unit autonomously manages its own power conversion and internal energy balance through circulating currents. The modules self-regulate their operation without requiring centralized control of DC link energy, reducing control complexity and associated losses. The circulating current mechanism allows modules to automatically balance internal capacitor energies and maintain stable operation independently.
3Adaptability or versatility
If conventional AC-AC converter topologies are used, then voltage level conversion is achieved, but device footprint and manufacturing cost increase
Solution Approach 1:
The modular converter structure serves multiple functions simultaneously: it performs voltage level conversion, enables bidirectional power flow, provides internal energy balancing, and allows flexible topology reconfiguration. The same half-bridge submodule architecture handles both power conversion and internal circulating current management, eliminating the need for separate dedicated components for each function and reducing overall footprint.
Solution Approach 2:
The converter employs a nested modular structure where identical submodule units are repeated and combined to build larger converter systems. Each module contains nested layers of switching devices, capacitors, and inductors that are systematically organized. This nesting approach allows scalable voltage level conversion by simply adding more modular units rather than designing entirely different converter topologies for different voltage ratios.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution achieves efficient direct AC-AC power conversion with reduced semiconductor requirements, lower losses, and a smaller footprint compared to conventional back-to-back MMC systems, validated through simulation and experimental results.
Implementation Method 1
The filter comprises one or more components adapted to impede flow of DC current from the first and second connection point of the circuit loop to the output terminal, while allowing flow of AC current from the first and second connection points of the circuit loop to the output terminal
Data Source
AI summary
A modular multilevel AC-AC converter is provided which allows for the direct conversion of AC power between different voltage levels using half-bridge submodules. The converter has a partial power processing topology that exploits internal circulating DC currents to facilitate charge balance of submodule capacitors. This power transfer mechanism eliminates the centralized DC link required in conventional back-to-back modular multilevel converter (B2B-MMC) systems, thus enabling reductions in the semiconductor effort and submodule capacitor energy storage requirements typically required in a B2B-MMC solution. A modulation scheme is also provided for different AC voltage conversion ratios. The converter operation and controls are validated by simulation, and experimentation.