MMC Converter Cell Control for Stable Circulating Current
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Solution Overview
Problem
Existing power conversion devices with modular multilevel converters (MMC) face challenges in controllability, particularly when a small number of converter cells are used for functions other than basic conversion operations, leading to instability in circulating current control.
Innovation Solution
The power conversion device enhances controllability by setting the frequency of control signals for converter cells used in circulating current control higher than those for arm voltage control, ensuring stable operation even with a small number of converter cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a small number of converter cells are used for functions other than basic conversion operations, then device complexity is reduced, but circulating current control stability deteriorates
Solution Approach 1:
The patent changes the control parameter by setting different frequencies for different control signals. Specifically, the control signal for converter cells performing circulating current control is set at a higher frequency than the control signal for converter cells performing arm voltage control. This parameter differentiation enables stable circulating current control even when using a reduced number of converter cells, resolving the contradiction between device complexity and control stability.
2Adaptability or versatility
If converter cells are used for multiple functions including circulating current control, then adaptability is improved, but control precision deteriorates
Solution Approach 1:
The patent applies local quality by differentiating control frequencies for different functional groups of converter cells. Converter cells dedicated to circulating current control receive high-frequency control signals, while converter cells performing arm voltage control receive lower-frequency signals. This localized differentiation of control quality ensures precise circulating current control while maintaining the versatility of converter cells for multiple functions.
Solution Approach 2:
The patent changes the control frequency parameter to distinguish between different control functions. By setting the control frequency for circulating current control higher than that for arm voltage control, the system achieves precise control for each function even when converter cells are shared across multiple purposes, thereby resolving the contradiction between adaptability and control precision.
Data Source
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Figure 2(A)~2(C)
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AI summary
In one embodiment, a power conversion device that converts power between a DC circuit (4) and an AC circuit (2) includes a plurality of leg circuits (8a, 8b, 8c) connected in parallel between first and second DC terminals (Np, Nn) and electrically connected to the AC circuit. Each of the plurality of leg circuits includes at least one first converter cell (6c, 6d) and a plurality of second converter cells (6a, 6b) other than the first converter cell. A first control signal that controls switching of a semiconductor switching element included in at least one first converter cell is higher in frequency than a second control signal that controls switching of a semiconductor switching element included in each of the plurality of second converter cells.