Modular Multilevel Converter Inductance Mismatch Control
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Solution Overview
Problem
Conventional power conversion systems with modular multilevel converters face challenges in maintaining stable control when the positive and negative arms have different inductance values, leading to interference between current control systems and unreliable performance.
Innovation Solution
A power conversion system that generates accurate voltage commands for both the positive and negative arms, using a control device with a voltage command generating unit and PWM circuit to manage inductance differences, ensuring independent control of AC and circulating currents through the distribution of voltage components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional control methods are used with different inductance values in positive and negative arms, then the system structure is simple, but control stability deteriorates due to interference between current control systems
Solution Approach 1:
The voltage command is segmented into distinct components: a common voltage command applied to both arms and a differential voltage command applied only to the arm with different inductance. This segmentation allows independent control of each arm's current, preventing interference while maintaining overall system stability.
Solution Approach 2:
The control approach applies local quality by providing differentiated voltage commands to specific arms based on their individual inductance characteristics. The arm with different inductance receives an additional differential voltage component tailored to its specific needs, while the other arm receives only the common voltage command.
2Adaptability or versatility
If the same number of chopper cells are connected in cascade in both arms, then the converter structure is symmetric and simple, but the system cannot handle different inductance values without control interference
Solution Approach 1:
The control system achieves universality by handling both symmetric cases (equal inductances) and asymmetric cases (different inductances) through a unified control framework. The common voltage command handles the basic power conversion function for both arms, while the differential voltage command provides additional functionality for arms with different inductance values.
3Reliability
If voltage commands are generated without considering inductance differences, then the control system is simple, but AC current and circulating current controls interfere with each other
Solution Approach 1:
The differential voltage command acts as an intermediary that mediates between the common voltage command and the arm with different inductance. This intermediary component ensures that the voltage commands are properly coordinated, preventing interference between AC current control and circulating current control while achieving reliable operation.
Data Source
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AI summary
In a power conversion system in which one or more converter cells (10) are connected in series to form an arm (5, 6) for each phase, a control device (20) includes a voltage command generating unit (21) for generating a positive arm voltage command Vp+ and a negative arm voltage command Vp- for each phase. The voltage command generating unit (21) includes an AC current control unit (23), a circulating current control unit (24), and a command distributing unit (25). On the basis of input voltage commands, the command distributing unit (25) subtracts voltage drop portions due to inductance values in the arms (5, 6) from respective voltages assigned as outputs of the positive arm (5) and the negative arm (6), to distribute voltage components, thereby determining the positive arm voltage command Vp+ and the negative arm voltage command Vp-.