Modular Multilevel DC-DC Converter Soft-Switching Control
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Solution Overview
Problem
Current DC-DC converters for medium voltage direct current (MVDC) systems lack galvanic isolation, efficient operation, and effective DC fault ride-through capability, leading to inefficiencies and safety concerns, especially in high-power applications like offshore wind farms and shipboard power systems.
Innovation Solution
A modular multilevel DC-DC converter with a current-fed or voltage-fed dual-active-bridge configuration, utilizing a transformer and cascaded half-bridge or full-bridge submodules with integrated capacitors, and a control system for soft-switching and DC fault ride-through capabilities, enabling efficient high-frequency operation and reduced fault current injection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dual-active-bridge DC-DC converters are used for galvanic isolation, then safety and voltage conversion ratio are improved, but efficiency and power density deteriorate due to high switching losses
Solution Approach 1:
The patent employs periodic soft-switching actions where the switching devices operate at zero-voltage or zero-current switching instants. The control system generates switching signals that periodically trigger the switches at optimal moments in the AC cycle, ensuring minimal switching losses while maintaining galvanic isolation through the transformer.
Solution Approach 2:
The patent changes the operating parameters of the switching devices by implementing soft-switching modes that modify the voltage and current conditions at switching instants. The control system adjusts duty cycles and phase shifts to achieve zero-voltage switching (ZVS) or zero-current switching (ZCS), thereby reducing switching losses while preserving isolation capabilities.
2Power
If conventional DC-DC converters operate at high switching frequencies, then power density is improved, but switching losses increase reducing efficiency
Solution Approach 1:
The converter operates with periodic soft-switching cycles where each switching event is timed to occur at zero-voltage or zero-current instants. The control system generates complementary switching signals that periodically activate the switches at optimal moments, enabling high-frequency operation with minimal switching losses and maintaining high power density.
Solution Approach 2:
The patent introduces an intermediary control mechanism that coordinates the switching of multiple bridges. The control system acts as an intermediary that generates phase-shifted PWM signals, ensuring that switching events occur at optimal instants mediated by the AC voltage waveform, thereby reducing losses while maintaining high power density.
3Reliability
If DAB DC-DC converters are used for fault isolation, then fault segment isolation is improved, but controllable DC fault current capability deteriorates
Solution Approach 1:
The control system implements feedback mechanisms that monitor DC voltage and current conditions on both primary and secondary sides. When a fault is detected, the control system adjusts switching signals based on feedback information to maintain controllable fault current while isolating the fault segment. The feedback ensures coordinated operation during fault conditions.
Solution Approach 2:
The converter employs dynamic control strategies where switching patterns are continuously adjusted based on real-time operating conditions. During fault conditions, the control system dynamically modifies duty cycles and phase shifts to maintain controllable fault current capability while ensuring proper fault isolation, making the system adaptable to varying fault scenarios.
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 provides high efficiency, reduced switching losses, and effective DC fault management, enhancing power density and stability in MVDC systems while ensuring safe and robust power transport.
Implementation Method 1
a transformer with primary windings connected to primary side and secondary windings connected to secondary side
Implementation Method 2
control the plurality of switching devices in a soft-switched manner
Implementation Method 3
control the plurality of switching devices in a soft-switched manner
Data Source
AI summary
In one embodiment, a current-fed modular multilevel dual active-bridge DC-DC converter suitable for medium voltage direct current (MVDC) grid or high voltage direct current (HVDC) grid integration is described. The DAB modular converter and the current-fed DAB converter are soft-switched modular multilevel dual-active-bridge (DAB) converters having DC fault ride-through capability. In an additional embodiment a voltage-fed isolated modular dual active-bridge DC-DC converter for medium voltage direct current (MVDC) or high voltage direct current (HVDC) grids or systems is described. In specific embodiments, the converters may be coupled to a battery energy storage system (BESS), wherein the BESS comprises split-battery units and the interface of the isolated DC-DC converter connects the split-battery units to the MVDC or HVDC system. The converters can be implemented in single-phase or poly-phase configurations and can be controlled to maintain a desired DC output current under both normal and DC grid fault condition.