HVDC Bypass Circuit Impedance Reduction via Asymmetric Cell Placement
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Solution Overview
Problem
Modular multilevel converters (MMC) face challenges in protecting against DC short-circuit circulating currents in high-voltage direct current (HVDC) systems, as existing solutions do not effectively manage the impedance of bypass circuits to prevent damage to freewheel diodes during short-circuit accidents.
Innovation Solution
The power conversion apparatus incorporates bypass circuits connected in parallel with cell blocks, with cell converters at the highest and lowest potential ends arranged closer to bypass circuits than other converters, reducing the impedance of the circulating-current path through bypass circuits compared to cell blocks, thereby protecting freewheel diodes and allowing more current to flow through bypass circuits during a short-circuit event.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bypass circuits are connected in parallel with cell blocks to protect against DC short-circuit circulating currents, then the reliability of the system is improved, but the impedance of the bypass circuit path increases due to longer connection paths
Solution Approach 1:
The patent applies local quality by differentiating the positions of cell converters within cell blocks. Specifically, cell converters at the highest and lowest potential ends are positioned closer to bypass circuits than other cell converters. This creates a localized optimization where the impedance path is minimized at critical points (the ends of the cascade connection) while maintaining the overall bypass circuit architecture. This selective positioning reduces the impedance of the circulating-current path through bypass circuits during short-circuit events without requiring all cell converters to be repositioned.
2Object-affected harmful factors
If cell converters are arranged closer to bypass circuits to reduce impedance, then the impedance of the circulating-current path is reduced, but the device complexity increases due to asymmetric arrangement requirements
Solution Approach 1:
The patent deliberately introduces asymmetry in the arrangement of cell converters within cell blocks. Instead of uniform distribution, cell converters at the highest and lowest potential ends are positioned closer to bypass circuits than intermediate cell converters. This asymmetric arrangement optimizes the impedance characteristics of the bypass circuit path during short-circuit events. The asymmetry is functional and targeted, affecting only the positioning of specific cell converters relative to bypass circuits while maintaining the overall modular structure of the MMC system.
Data Source
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AI summary
A power conversion apparatus includes: a plurality of cell blocks (CLB) connected in cascade; and a plurality of bypass circuits (BC) each electrically connected in parallel with a corresponding one of the plurality of cell blocks. Each cell block (CLB) includes: a first connection node (TP) on a high-potential side and a second connection node (TN) on a low-potential side for connection to another cell block; and a plurality of cell converters (CL) connected in cascade between the first connection node and the second connection node, each of the cell converters (CL) including an energy storage device. When a DC fault current flows in the direction from the low-potential side to the high-potential side, the current path via the plurality of cell blocks (CLB) is larger in impedance than the current path via the plurality of bypass circuits (BC).