Modular Multilevel Converter Control for Low-Power Capacitor Balance
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Solution Overview
Problem
In modular multilevel converters, when both active and reactive power output are small, the voltage at the capacitor in one cell group cannot be maintained, leading to potential converter control failures, as the existing methods only control circulating current in the other cell group.
Innovation Solution
A power conversion device with a control system that manages the output voltages of converter cells in each arm based on specific instruction values, including a combination of DC and AC components, to maintain capacitor voltage balance across different operating cell groups.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If one cell group controls only circulating current while the other performs AC-DC conversion, then the device complexity is reduced and operation is simplified, but the capacitor voltage in the circulating current control cell group cannot be maintained when active and reactive power output are small
Solution Approach 1:
The cell group that originally controlled only circulating current is given dual functionality: it continues to control circulating current while also performing AC-DC conversion. This allows the same cell group to maintain capacitor voltage through power conversion operations, resolving the voltage maintenance issue without adding separate dedicated cell groups for each function.
Solution Approach 2:
The control method dynamically adjusts the output voltage of converter cells based on capacitor voltage deviations. When capacitor voltage drops below the threshold, the control device increases the output voltage of converter cells in the circulating current control cell group to recharge the capacitor, thereby maintaining voltage within acceptable ranges while preserving the simplified operational structure.
2Reliability
If both cell groups perform AC-DC conversion, then capacitor voltage is maintained reliably, but the device complexity and control difficulty increase
Solution Approach 1:
Instead of making both cell groups perform identical AC-DC conversion operations, the invention applies different control strategies to different cell groups based on their specific roles. The circulating current control cell group receives additional voltage instructions tailored to its dual function, while the dedicated AC-DC conversion cell group maintains its original control method, thereby maintaining reliability without uniformly increasing complexity across the entire system.
3Reliability
If the circulating current control cell group increases power conversion operation, then capacitor voltage is maintained, but the circulating current control performance may be affected
Solution Approach 1:
The control device acts as an intermediary that coordinates between circulating current control and capacitor voltage maintenance. It calculates voltage instructions based on capacitor voltage deviations and combines them with circulating current control requirements, ensuring that power conversion operations support voltage maintenance while circulating current control precision is preserved through proper coordination of control signals.
Data Source
AI summary
A power conversion device includes a plurality of leg circuits and a control device. The control device controls an output voltage at a first converter cell, which is controlled not based on the circulating current, based on a first voltage instruction value. The control device controls an output voltage at a second converter cell using a first value based on a deviation between a circulating current and a circulating current instruction value and a second value based on a deviation between a capacitor voltage and a capacitor voltage instruction value in the second converter cell. When the capacitor voltage at the second converter cell is less than a first threshold, the control device linearly combines an auxiliary voltage instruction value including at least one of a DC component and a fundamental AC component of the AC circuit with the first value and the second value.