Modular Multilevel Converter Control for Capacitor Voltage Hold
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Solution Overview
Problem
In modular multilevel converters, when both cell groups perform different operations, the voltage at the capacitor in one cell group cannot be maintained, leading to potential converter control failures, especially when active and reactive power output is small.
Innovation Solution
A power conversion device with a control system that manages the voltage of each cell group by controlling the first converter cells based on circulating current and the second converter cells using specific modes to maintain capacitor voltage, including stopping switching operations when necessary to prevent discharge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the other cell group controls only the circulating current, then the circulating current control is simplified, but the voltage at the capacitor cannot be maintained when both active power and reactive power output are small
Solution Approach 1:
The control device changes the control parameters of the other cell group based on capacitor voltage status. When capacitor voltage is within the threshold, the cell group controls only circulating current; when voltage exceeds the threshold, the control device switches to a mode that prioritizes voltage maintenance, thereby adapting control parameters to different operating conditions and resolving the contradiction between control simplicity and reliability
Solution Approach 2:
The control device implements feedback control by continuously monitoring the capacitor voltage in the other cell group and adjusting the switching operations accordingly. When voltage deviates from the threshold, the feedback mechanism triggers appropriate control actions to restore voltage, ensuring reliable operation while maintaining simplified control during normal conditions
2Reliability
If the converter cell performs switching operation to maintain capacitor voltage, then the voltage control is improved, but the switching frequency increases causing additional losses
Solution Approach 1:
The control device applies partial action by selectively controlling only certain converter cells in the other cell group when capacitor voltage exceeds the threshold. Instead of uniformly controlling all cells, it targets specific cells that need voltage maintenance, thereby achieving reliable voltage control while minimizing unnecessary switching operations and associated energy losses
Solution Approach 2:
The control device implements periodic monitoring of capacitor voltage and applies switching operations only when necessary (when voltage exceeds threshold). This periodic control approach maintains voltage reliability while avoiding continuous switching that would cause excessive energy losses during normal operating conditions
Data Source
AI summary
A power conversion device includes a power conversion circuit unit including a plurality of leg circuits, and a control device. Each of the leg circuits includes a plurality of first converter cells each having a capacitor and connected in series to each other and a plurality of second converter cells each having the capacitor and connected in series to each other. The plurality of first converter cells are controlled not based on a circulating current circulating between the plurality of leg circuits, and the plurality of second converter cells are controlled based on the circulating current. The control device stops a switching operation of at least one second converter cell in the plurality of second converter cells when a voltage at the capacitor in the second converter cell becomes less than a first threshold.