MMC Power Converter Capacitor Voltage Balancing Across AC Frequencies
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Solution Overview
Problem
Power conversion systems with MMC-type converters face challenges in maintaining voltage stability when connected to AC systems with different fundamental frequencies, leading to inefficient use of DC capacitors and increased system size due to the need for larger capacitance to suppress voltage fluctuations.
Innovation Solution
The system employs power converters with submodules that adjust average voltage values based on fundamental frequency and current requirements, allowing for the reduction in the number of submodules and system size while maintaining capacitor specifications, by using a difference in ripple voltages between capacitors to optimize capacitor utilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of repair
If the same DC capacitor is used in power converters connected to AC systems with different fundamental frequencies, then maintainability is improved, but voltage fluctuations in the lower frequency system cause larger capacitance requirements leading to increased system size
Solution Approach 1:
The patent applies parameter changes by adjusting the average voltage value of capacitors in submodules based on the fundamental frequency of the connected AC system. Power converters connected to systems with different fundamental frequencies operate with different capacitor voltage parameters, allowing the use of identical capacitor components while optimizing performance for each frequency condition and avoiding excessive capacitance requirements.
Solution Approach 2:
The patent implements dynamics by making the capacitor voltage operating point adjustable rather than fixed. The control system dynamically adapts the average voltage value of DC capacitors according to the fundamental frequency detection, enabling the same hardware to operate optimally across different frequency conditions without requiring oversized components for the lowest frequency case.
2Stability of the object's composition
If larger capacitance is used to suppress voltage fluctuations in the lower frequency system, then voltage stability is improved, but the system size and cost increase
Solution Approach 1:
The patent changes the operating voltage parameter of capacitors based on fundamental frequency. By detecting the fundamental frequency and adjusting the target average voltage value accordingly, the system achieves voltage stability appropriate for each frequency condition without requiring larger capacitance values, thus avoiding increased system size.
Solution Approach 2:
The patent employs feedback control by detecting the fundamental frequency of the connected AC system and using this information to adjust the capacitor voltage regulation target. The control system continuously monitors frequency conditions and adapts the voltage parameters to maintain stability while optimizing component utilization.
Data Source
AI summary
A power conversion system includes: a first power converter to perform power conversion between a first AC system and a DC circuit; and a second power converter to perform power conversion between a second AC system and the DC circuit. Each of the first power converter and the second power converter includes a plurality of submodules connected in series. Each of the plurality of submodules includes a plurality of switching elements and a capacitor. A first fundamental frequency of the first AC system is greater than a second fundamental frequency of the second AC system. A first average voltage value of a capacitor in a first submodule included in the first power converter is larger than a second average voltage value of a capacitor in a second submodule included in the second power converter.