MMC Capacitor Voltage Ripple Reduction via Dual Control Loops
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Solution Overview
Problem
Modular multilevel converters (MMCs) face challenges in minimizing capacitor voltage ripple in switching submodules, particularly in high voltage, high power applications, where conventional methods either increase volume and cost or are unsuitable for high voltage applications due to large arm currents.
Innovation Solution
The implementation of a control loop with a differential mode control loop and a common mode control loop in MMCs, which generates differential control signals based on a target modulation index to reduce fundamental voltage ripple components and injects 2nd order harmonic current to reduce 2nd order harmonic components, allowing for the use of full bridge and semi full bridge topologies to achieve a modulation index greater than 1.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional control methods are used to reduce capacitor voltage ripple, then voltage ripple is reduced, but arm currents become relatively large making the method unsuitable for high voltage applications
Solution Approach 1:
The invention changes the control parameters by implementing a dual control loop system with differential mode and common mode control. The differential mode control loop adjusts the modulation index to reduce fundamental voltage ripple components, while the common mode control loop injects 2nd order harmonic current to reduce harmonic components. This parameter change approach allows voltage ripple reduction without requiring large arm currents, making the method suitable for high voltage applications.
Solution Approach 2:
The invention employs feedback control mechanisms where the differential mode control loop uses voltage ripple detection to adjust the modulation index, and the common mode control loop uses harmonic component detection to adjust the injected current. This feedback approach enables automatic optimization of control parameters to minimize voltage ripple while maintaining acceptable arm current levels for high voltage applications.
2Manufacturing precision
If larger capacitors are used to reduce voltage ripple, then voltage ripple is reduced, but the volume and cost of the MMC increase
Solution Approach 1:
Instead of increasing capacitor size, the invention changes the control parameters through the dual control loop system. The differential mode control loop optimizes the modulation index to reduce fundamental voltage ripple components, and the common mode control loop injects harmonic currents to reduce harmonic components. This approach achieves voltage ripple reduction through control optimization rather than hardware enlargement, avoiding increased MMC volume.
Solution Approach 2:
The invention replaces the mechanical approach of using larger capacitors with a control-theoretic approach. Instead of physically enlarging the energy storage components, the system uses sophisticated control algorithms with dual control loops to achieve the same voltage ripple reduction effect, substituting control complexity for hardware complexity.
3Manufacturing precision
If larger capacitors are used to reduce voltage ripple, then voltage ripple is reduced, but the cost of the MMC increases
Solution Approach 1:
The invention achieves voltage ripple reduction by changing control parameters through the dual control loop system rather than using larger, more expensive capacitors. The differential mode control loop adjusts the modulation index to minimize fundamental voltage ripple, while the common mode control loop injects harmonic currents to reduce harmonic components. This parameter optimization approach maintains cost-effectiveness by avoiding expensive hardware upgrades.
Solution Approach 2:
The invention substitutes control algorithm complexity for hardware cost. Instead of purchasing and installing larger, more expensive capacitors, the system implements sophisticated dual control loop algorithms that achieve the same voltage ripple reduction effect through intelligent control, thereby reducing the overall manufacturing cost.
Data Source
AI summary
Aspects of capacitor voltage ripple reduction in modular multilevel converters are described herein. In one embodiment, a power converter system includes a modular multilevel converter (MMC) electrically coupled and configured to convert power between two different power systems. The MMC includes one or more phase legs having a cascade arrangement of switching submodules, where the switching submodules include an arrangement of switching power transistors and capacitors. The MMC further includes a control loop including a differential mode control loop and a common mode control loop. The differential control loop is configured to generate a differential control signal based on a target modulation index to reduce fundamental components of voltage ripple on the capacitors, and the common mode control loop is configured to inject 2nd order harmonic current into a common mode control signal to reduce 2nd order harmonic components of the voltage ripple on the capacitors.