Modular Multilevel Converter Voltage Balance Control
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Solution Overview
Problem
In three-phase modular multilevel converters, grid imbalances lead to unbalanced DC capacitor voltages due to active current flowing on the grid voltage, causing increased current through the converter and power conversion unit, especially during high imbalance rates.
Innovation Solution
An electric power conversion device with a delta-connected power conversion unit and a converter control unit that calculates phase arm current command values, negative sequence current command values, and zero sequence voltage command values to control circulating and negative sequence currents, thereby balancing DC capacitor voltages among phases, reducing the rated current and enabling downsizing of the power conversion unit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If circulating current is used to balance DC capacitor voltages during grid imbalance, then DC voltage balance is improved, but current through the converter increases
Solution Approach 1:
The patent introduces a reactor as an intermediary element connected in series with each arm of the delta-connected converter. This reactor mediates the relationship between the unbalanced DC capacitor voltages and the circulating current, allowing voltage balance to be achieved while limiting the increase in converter current during grid imbalance conditions.
Solution Approach 2:
The patent changes the electrical parameters (inductance values) of the reactors connected in series with each arm to optimize the balance between DC capacitor voltages. By adjusting these parameter values, the system can control the circulating current characteristics and minimize the current increase while maintaining voltage balance during unbalanced grid conditions.
2Strength
If transformer multiplexing is used for high-voltage conversion, then voltage withstanding capability is improved, but transformer configuration complexity and cost increase
Solution Approach 1:
The patent extracts the transformer from the system by implementing a transformerless multilevel converter topology. The converter directly connects to the grid through reactors, eliminating the need for complex multiplexed transformer configurations while achieving the required voltage withstanding capability through the series connection of multiple unit cells with DC capacitors.
Solution Approach 2:
The patent replaces the mechanical/electromagnetic transformation system (transformers) with an electronic power conversion system (multilevel converter with semiconductor switches and DC capacitors). This substitution achieves voltage transformation and isolation through electronic switching and capacitor voltage summation, avoiding the complexity of transformer multiplexing.
Data Source
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AI summary
This electric power conversion device (1) includes a power conversion unit (6) and a converter control unit (7). The power conversion unit (6) includes three arms (4) connected to an AC circuit. The converter control unit (7) includes a phase DC voltage control unit (60), a negative sequence current command value calculation unit (80), an output current control unit (90), a circulating current control unit (100), a voltage command value calculation unit (110), and a gate signal generation unit (120), and imbalance of cell DC capacitor voltages among phases due to grid imbalance is controlled by circulating current and negative sequence current.