Modular Multilevel Converter Variable DC Voltage Control
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Solution Overview
Problem
Conventional AC/DC/AC power converters with constant DC voltage are unsuitable for variable speed drive applications due to excessive voltage oscillations in capacitors at low machine frequencies, which cannot be maintained within acceptable ranges passively.
Innovation Solution
The power converter operates with a variable bipolar DC voltage and unipolar DC current, using a reduced full-bridge submodule topology and a controller to manage the DC voltage and current for effective power flow between network and machine-side converter units, allowing for controllable submodule output states and reduced voltage oscillations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional AC/DC/AC power converters with constant DC voltage are used, then the power converter structure is simple and easy to control, but excessive voltage oscillations occur in capacitors at low machine frequencies
Solution Approach 1:
The patent applies the dynamics principle by transitioning from a static constant DC voltage system to a dynamic variable DC voltage system. The DC voltage is actively adjusted based on machine frequency to maintain stable capacitor voltages across the entire operating range, particularly at low frequencies where conventional constant voltage systems fail.
Solution Approach 2:
The patent implements parameter changes by varying the DC voltage parameter dynamically. Instead of maintaining a fixed DC voltage, the system changes the DC voltage parameter in response to machine frequency variations, thereby preventing excessive capacitor voltage oscillations and enabling stable operation at low frequencies.
2Device complexity
If constant DC voltage is maintained, then the power flow control is simplified, but the converter cannot support variable speed drive applications at low frequencies
Solution Approach 1:
The system employs dynamic adjustment of DC voltage to adapt to varying operating conditions. This dynamic behavior enables the converter to support variable speed drive applications across the full frequency range, including low frequencies, while maintaining manageable control complexity through systematic voltage adjustment strategies.
Solution Approach 2:
By changing the DC voltage parameter dynamically, the converter achieves adaptability for variable speed drive applications. The parameter change approach allows the system to maintain stable operation at low frequencies where constant voltage systems would fail, thereby expanding the converter's operational versatility.
3Stability of the object's composition
If the DC voltage is reduced at low machine frequencies, then voltage oscillations in capacitors are limited, but the power flow management becomes more complex
Solution Approach 1:
The patent systematically changes the DC voltage parameter in response to machine frequency to limit capacitor voltage oscillations. This parameter change strategy effectively controls voltage stability while the associated power flow management complexity is handled through coordinated control of both converter units.
Solution Approach 2:
The system employs feedback mechanisms where the DC voltage is adjusted based on machine frequency and capacitor voltage conditions. This feedback approach enables effective oscillation control while managing power flow through adaptive voltage regulation, balancing stability requirements with control complexity.
Data Source
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AI summary
A power converter (1) configured as a variable speed drive (VSD) is described. The power converter (1) includes a network-side converter unit (2) with one or more AC terminals electrically connectable to an AC power network (8) and a machine-side converter unit (4) with one or more AC terminals electrically connectable to an electrical machine (e.g., a motor). The DC terminals (6a, 6b) of the network-side converter unit (2) are electrically connected to the DC terminals (6c, 6d) of the machine-side converter unit (4). The network-side and machine-side converter units are configured as modular multilevel converters (MMCs). The power converter (1) is adapted to carry a variable bipolar DC voltage and a unipolar DC current. A controller is used to control one of the network-side and machine-side converter units (2, 4). The controller is adapted to control the magnitude and polarity of the intermediate DC voltage to achieve a desired magnitude and direction of power flow between the network-side and machine-side converter units (2, 4).