MMC Converter Control for Smaller Capacitors and Fault Ride-Through
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Solution Overview
Problem
MMC converters have a larger volume per output capacity compared to conventional three-level converters, making them unsuitable for applications with severe installation area and volume restrictions, such as pumped-storage power plants and offshore wind power plants. This is primarily due to the large capacitors used as energy storage elements, which occupy more than half of the arm volume.
Innovation Solution
The MMC converter is designed to adjust the active current component using the average voltage of capacitors, suppress the DC current command during system faults, and maintain operation during system fault propagation. This is achieved by configuring the host control device to detect positive-phase voltage drops and negative-phase voltage rises, and adjusting the DC current command accordingly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If large capacitors are used as energy storage elements in MMC converter, then the converter can maintain stable operation, but the volume and weight of the converter increase significantly
Solution Approach 1:
The patent changes the control parameters of the MMC converter, specifically using average voltage of capacitors as a control target instead of traditional methods. This allows for reduced capacitor sizes while maintaining stability through advanced control strategies that monitor and adjust based on voltage averages across multiple capacitors.
Solution Approach 2:
The patent implements feedback control mechanisms that detect positive-phase voltage drops and negative-phase voltage rises, and adjust the DC current command accordingly. This feedback system enables the converter to maintain stable operation with smaller capacitors by continuously monitoring and correcting voltage imbalances.
2Weight of stationary object
If capacitor size is reduced to decrease converter volume, then installation space is saved, but operation continuation performance during system faults deteriorates
Solution Approach 1:
The patent employs feedback control that detects system fault conditions through voltage monitoring and adjusts the DC current command to maintain operation continuation. The control system responds to positive-phase voltage drops and negative-phase voltage rises by modifying current commands, enabling smaller capacitors to support fault ride-through capability.
Solution Approach 2:
The patent prepares the control system in advance to handle faults by implementing detection mechanisms for voltage anomalies. The host control device is configured to recognize fault conditions and preemptively adjust operating parameters, allowing the system to maintain stability with reduced energy storage capacity.
3Loss of energy
If MMC converter is used instead of conventional three-level converter, then loss is reduced, but volume per output capacity increases
Solution Approach 1:
The patent modifies the control parameters and operating characteristics of the MMC converter to reduce its volume. By using average voltage control and adjusting modulation strategies, the system achieves lower losses while requiring smaller capacitor banks, thus reducing the overall converter volume while maintaining efficiency advantages.
Data Source
AI summary
A modular multilevel power converter includes: a capacitor voltage adjuster that calculates an active current component command to make an average voltage of capacitors match a command value; an active power detector that receives an AC current signal and an AC voltage signal and calculates an active power; an active power adjuster that calculates a DC current command value to make the active power match a command value; a first active power command suppressor that suppresses an absolute value of an active power command value; and a second active power command suppressor that adjusts an absolute value of an active power command value.