Modular Multilevel Converter Circulating Current Control
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Solution Overview
Problem
In power conversion devices, particularly modular multilevel converters, controlling both DC and circulating current components is challenging when an impedance is inserted in the DC circuit, leading to unstable voltage variations in DC capacitors across phases.
Innovation Solution
A power conversion device with separate arms for each phase, each comprising series-connected semiconductor switches and parallel DC capacitors, along with circulating current calculation and control units to independently manage circulating currents, thereby stabilizing voltage variations in DC capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If both DC current component and circulating current component are collectively controlled, then the control response is determined by the impedance of the DC circuit, but the circulating current component cannot be controlled independently leading to unstable voltage variations in DC capacitors
Solution Approach 1:
The patent segments the control of arm currents into two independent control loops: one for the DC current component and another for the circulating current component. This segmentation allows each current component to be controlled independently with optimized response characteristics, resolving the contradiction between voltage stability and control complexity.
Solution Approach 2:
The patent introduces circulating current calculation means as an intermediary that extracts the circulating current component from the total arm current. This intermediary enables independent control of the circulating current component without affecting the DC current control, thereby stabilizing DC capacitor voltages while maintaining manageable control complexity.
2Reliability
If an impedance is inserted in the DC circuit, then the DC circuit impedance increases affecting the control response, but independent control of circulating current becomes necessary to maintain voltage stability
Solution Approach 1:
By segmenting the control into separate DC current control and circulating current control loops, the patent enables the circulating current component to be controlled independently with optimized response speed, unaffected by the DC circuit impedance. This resolves the contradiction between voltage stability and control response speed.
Solution Approach 2:
The patent changes the control parameters by separately controlling the circulating current component with its own command value and control characteristics. This parameter separation allows the circulating current control to respond quickly without being constrained by the DC circuit impedance, while still ensuring voltage stability.
Data Source
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AI summary
A power conversion device includes: a DC current calculation unit (50) for calculating a circulating current component (izuc, izvc, izwc) for each phase which circulates between the phases through first arms (18) and/or second arms (20) not via an AC power supply (14) and a DC power supply (16); and a circulating current control unit (52) for controlling the circulating current component (izuc, izvc, izwc) for each phase so as to follow a predetermined circulating current command value (izuc*, izvc*, izwc*), thereby reliably suppressing variation in voltages of DC capacitors of unit cells (10) among the phases even in such a case where an impedance (15) is additionally inserted in a DC circuit.