MMC Capacitor Balancing Control Under Low AC Power Conditions
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Solution Overview
Problem
Existing MMCs face issues with unbalanced capacitor voltages due to inefficient individual control when AC power input/output is small, leading to unnecessary power consumption and potential overvoltage/undervoltage protection, which can cause the converter to stop operating.
Innovation Solution
A power conversion device with a control device that performs phase shift pulse width control, calculating an evaluation value for capacitor voltage variations and adjusting the output time of positive voltage based on current direction to stabilize and efficiently manage capacitor voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If circulating current for individual control is always fed when AC power input/output is small, then capacitor voltage balance is maintained, but unnecessary power is consumed
Solution Approach 1:
The control method dynamically adjusts the circulation current injection strategy based on real-time evaluation of capacitor voltage variations. When variations exceed a threshold, circulating current is injected for balance control; when variations are within acceptable range, circulating current is suppressed to reduce power consumption. This dynamic adaptation resolves the contradiction between maintaining voltage balance and minimizing energy loss.
Solution Approach 2:
The system changes the control parameter (circulating current injection) based on the evaluation value of capacitor voltage variations. By comparing the evaluation value against a threshold, the control strategy switches between active balance control and power-saving mode, optimizing the trade-off between voltage balance reliability and power consumption.
2Reliability
If circulating current for individual control is fed when AC power is small, then stable individual control is achieved, but control complexity increases
Solution Approach 1:
The control method segments the control strategy into distinct modes based on operating conditions. By evaluating capacitor voltage variations and comparing against a threshold, the system selects between different control approaches (circulating current injection vs. suppression), simplifying the overall control logic while maintaining stability.
Solution Approach 2:
The system uses feedback from the evaluation value of capacitor voltage variations to adjust the control strategy. This feedback mechanism enables automatic adaptation to changing operating conditions, achieving stable individual control without requiring complex predetermined control schemes.
3Reliability
If capacitor voltage control is performed by individual control and all voltage control, then voltage balance is maintained, but control response time increases
Solution Approach 1:
The control system is segmented into hierarchical levels (individual cell control and overall voltage control), with the evaluation value mechanism providing a rapid assessment layer. This segmentation allows the system to quickly evaluate voltage variation status and trigger appropriate control actions, improving response time while maintaining voltage balance through coordinated control at different levels.
Data Source
AI summary
In an MMC-type power conversion device, a control device calculates an evaluation value representing the degree of variations in voltage of capacitors of individual converter cells. When this evaluation value exceeds a threshold value, the control device controls the converter cells as follows: (i) when current in a positive direction flows through a first converter cell with a voltage of the capacitor greater than a mean value, reducing an output time of positive voltage, (ii) when current in a negative direction flows through the first converter cell, increasing an output time of positive voltage, (iii) when current in the positive direction flows through a second converter cell with a voltage of the capacitor smaller than a mean value, increasing an output time of positive voltage, or (iv) when current in the negative direction flows through the second converter cell, reducing an output time of positive voltage.