Modular Multilevel DC/DC Converter With Current-Shaping for Charge Balance
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Solution Overview
Problem
Current converter technologies for interconnecting DC networks at differing voltage levels, such as HVDC and LVDC or MVDC, are costly and complex, limiting their application to higher power scenarios, and lack efficiency in lower power applications due to high cost and complexity, especially in tap applications.
Innovation Solution
A modular multi-level DC/DC converter topology that incorporates a current source module (CSM) to maintain capacitor charge balance, allowing for rapid power flow direction changes and reduced switching losses, enabling a high step-down ratio power exchange with reduced capacitor requirements and transformer size, and optional galvanic isolation for safety and design flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a dual-active bridge with multi-modular converter (MMC) front-end is used to interface LVDC and HVDC networks, then power transmission capability is improved, but cost and device complexity increase significantly
Solution Approach 1:
The converter is divided into modular units where each string contains multiple voltage sub-modules (VSMs) that can be independently controlled. This segmentation allows the system to handle high power transmission while maintaining manageable complexity through standardized modular building blocks that can be scaled according to power requirements.
Solution Approach 2:
The converter topology is designed to perform multiple functions: it can interface different voltage levels (LVDC-HVDC), enable bidirectional power flow, provide galvanic isolation through optional transformer integration, and maintain capacitor charge balance. This multi-functionality reduces the need for separate dedicated components for each function, thereby reducing overall system complexity.
2Power
If MMC with full-voltage rated components is used, then high power application performance is improved, but cost increases for lower power applications
Solution Approach 1:
The converter employs dynamic voltage sharing among parallel strings and dynamic capacitor charge balance control that adapts to varying power levels. The system can operate efficiently at different power levels by adjusting the number of active VSMs per string and optimizing switching patterns, allowing cost-effective deployment across a wide range from low to high power applications without requiring full-voltage rated components at all times.
3Speed
If rapid power flow direction changes are enabled, then system responsiveness is improved, but switching losses increase
Solution Approach 1:
The converter uses periodic switching patterns with optimized duty cycles to control power flow direction. By employing pulse-width modulation (PWM) techniques and coordinating switching events across multiple VSMs, the system achieves rapid bidirectional power flow control while minimizing the number of simultaneous switching transitions, thereby reducing switching losses through time-distributed switching actions.
4Reliability
If capacitor charge balance is maintained through CSM, then system reliability is improved, but device complexity increases
Solution Approach 1:
The current source module (CSM) automatically maintains capacitor charge balance through self-regulating current control. The CSM monitors capacitor voltages and adjusts its output current accordingly to equalize charge levels across all VSM capacitors without requiring external intervention or complex control algorithms, thereby improving reliability while adding minimal complexity through a dedicated self-managing module.
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
This solution enables cost-effective and efficient power conversion between DC networks, reducing the need for AC transformers, achieving higher power densities, and allowing for bidirectional power transfer with reduced volume and weight, while maintaining safety through galvanic isolation.
Implementation Method 1
The CSM comprises two or more inductors; and two or more switches
Implementation Method 2
two or more switches in some embodiments of the converter circuit
Data Source
AI summary
Embodiments are directed to a modular multi-level DC/DC power electronic converter for transferring power from or between a higher-voltage DC network and a lower-voltage DC network. The power electronic converter features a series connection of low-voltage voltage source modules (VSM) and a current source module (CSM). The series connection of the sub-module elements forms a string. The higher-voltage DC network is interfaced to the converter by connecting across the outer terminals of the string. The lower-voltage DC network is interfaced to the converter through the CSM.