Power Conversion Device Bypass Circuit for Short-Circuit Protection
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Solution Overview
Problem
In modular multilevel converters, when a short-circuiting failure occurs in a DC circuit, the capacitors in full-bridge converter cells can be damaged due to excessive short-circuiting current, especially when the number of full-bridge converter cells in each arm circuit is small, leading to capacitor breakdown.
Innovation Solution
Incorporating a bypass circuit for each cell block in the arm circuit, which redirects the short-circuiting current away from the capacitors in full-bridge converter cells, ensuring that the current flows through the bypass circuit during a short-circuit event, thereby protecting the capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the number of full-bridge converter cells in each arm circuit is decreased, then the device complexity is reduced, but the reliability deteriorates because capacitors may break due to short-circuiting current
Solution Approach 1:
A bypass circuit is introduced as an intermediary component connected in parallel with each converter cell. This bypass circuit provides an alternative current path that mediates between the short-circuiting current and the capacitor, allowing the current to flow through the bypass circuit instead of charging the capacitor during short-circuit conditions, thereby protecting the capacitor while enabling reduced number of full-bridge cells
2Object-generated harmful factors
If all switching elements are controlled to open during DC short-circuit failure, then the harmful current propagation is stopped, but the short-circuiting current continues to flow into capacitors causing breakdown
Solution Approach 1:
The arm circuit is segmented into multiple converter cells, each with its own bypass circuit. This segmentation allows the short-circuiting current to be diverted at the cell level through individual bypass circuits rather than affecting the entire arm circuit, enabling selective current redirection that protects individual capacitors while maintaining overall system functionality
Data Source
Figure 1
Figure 2
Figure 3(a)~3(c)
AI summary
Each arm circuit (6) of a power conversion device includes a plurality of cascaded cell blocks (20) and a plurality of bypass circuits (30) connected in parallel to the respective cell blocks (20). Each cell block (20) includes: a first connection node (40p) on a high potential side and a second connection node (40n) on a low potential side for connection to another cell block; and a plurality of converter cells cascaded between the first and second connection nodes (40p, 40n), each converter cell containing an energy storage (24). The plurality of converter cells include at least one first converter cell (21F, 21HYB) of a full-bridge (or hybrid) configuration and at least one second converter cell (21H) of a half-bridge configuration.