DC to AC Power Tapping via Series-Connected Converter Modules
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Solution Overview
Problem
Existing power transmission systems face challenges in distributing appropriate AC and DC voltages, maintaining cell capacitor voltages within specific ranges, enabling boost mode operation, and balancing voltages across DC blocking capacitors while mitigating harmonics in HVDC transmission networks.
Innovation Solution
The solution involves a series-connected power transfer module arrangement with each module comprising a string of converter cells and a capacitor, where a control unit generates a reference voltage signal by combining AC and DC voltage contributions to control the cells, ensuring balanced operation and reduced converter current, and employs a method to mitigate harmonics using negative feedback and active filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If power is tapped from DC power line to AC power line using series-connected power transfer modules, then efficient power transmission is achieved, but voltage distribution and capacitor voltage balancing become complex
Solution Approach 1:
The system divides the power transmission function into multiple series-connected power transfer modules, where each module handles a portion of the voltage transformation. This segmentation allows independent control of each module's converter cells, simplifying the overall voltage distribution control while maintaining high transmission efficiency.
Solution Approach 2:
The control unit implements feedback control by continuously monitoring the voltage across DC blocking capacitors and adjusting the converter cell switching accordingly. This feedback mechanism automatically balances capacitor voltages and maintains proper voltage distribution across series-connected modules, reducing control complexity.
2Reliability
If DC blocking capacitors are used in series with transformer secondary windings, then DC voltage blocking is achieved, but voltage balancing across capacitors becomes difficult
Solution Approach 1:
The control unit monitors the voltage across each DC blocking capacitor and uses feedback control to adjust the converter cell switching states. This ensures automatic voltage balancing across all capacitors while maintaining their DC blocking function, eliminating the need for manual intervention.
Solution Approach 2:
The control system dynamically changes the switching parameters of converter cells to regulate capacitor voltages. By adjusting switching duty cycles and timing, the system maintains capacitor voltages within specific ranges, making voltage balancing straightforward despite the series connection configuration.
3Stability of the object's composition
If converter cells are controlled to maintain voltage within specific ranges, then stable operation is achieved, but control complexity increases
Solution Approach 1:
The control unit employs feedback control mechanisms that automatically adjust converter cell switching based on real-time voltage measurements. This maintains cell capacitor voltages within specified ranges while simplifying operation, as the feedback loop handles the complexity of coordinated control.
Solution Approach 2:
The control system uses self-service control where each power transfer module's control unit independently regulates its converter cells based on local voltage conditions. This distributed self-service approach maintains voltage stability without requiring complex centralized coordination, reducing overall control complexity.
4Adaptability or versatility
If boost mode operation is enabled where sum of cell voltages exceeds DC potential, then power transmission flexibility is improved, but control difficulty increases
Solution Approach 1:
The control system dynamically adjusts the operating mode of converter cells based on power transmission requirements. It can seamlessly transition between standard operation and boost mode where the sum of cell voltages exceeds the DC potential, providing operational flexibility while managing control complexity through adaptive control algorithms.
Solution Approach 2:
The control unit changes operating parameters dynamically to enable boost mode operation. By adjusting converter cell switching patterns and duty cycles, the system achieves variable voltage output that can exceed the DC potential when needed, while the parameter-based control approach keeps the control system manageable.
Data Source
Figure 1~2
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Figure 5~6
AI summary
An arrangement (23) for tapping power from a DC power line (12) to an AC power line comprises power transfer modules (24A, 24B, 24C) between two DC potentials, each comprising a first branch with a string of converter cells (CA1, CA2, CB1, CB2, CC1, CC2) in parallel with a second branch comprising a capacitor (Csa, Csb, Csc) and being connected to an AC phase (22A, 22B, 22C). There is at least one control unit (26A, 26B, 26C) that controls the arrangement considering one or more of a) distributing appropriate AC and DC voltages in converter output voltages of all series connected modules, b) maintaining/setting cell capacitor voltages in specific range and allowing boost mode operation, c) performing possible balancing of the introduced capacitor and.d) employing an alternate approach of using passive filters to mitigate low order harmonics.