Modular Multilevel Converter Voltage Equalization Control
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Solution Overview
Problem
Current methods for voltage equalization in modular multilevel converters (MMCs) are complex and costly, especially when dealing with a large number of sub-modules of different voltage levels, as they require hardware circuits or complicated control loops, and existing software methods only support sub-modules of the same voltage level.
Innovation Solution
A control method and system that detects and calculates actual capacitor voltages, divides sub-modules into groups based on reference voltages, sorts and normalizes voltages, and determines which sub-modules to switch on or off to maintain a consistent bridge arm level, using a modulation algorithm that adjusts with a preset value step.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If hardware voltage equalization circuits are adopted, then voltage equalization can be achieved, but system complexity and cost increase significantly when the number of series connected sub-modules increases
Solution Approach 1:
The patent replaces hardware voltage equalization circuits with a software-based control method. The control system detects capacitor voltages of sub-modules, classifies them by voltage level, and selectively switches sub-modules to achieve equalization through control algorithms rather than physical circuits, thereby reducing system complexity while maintaining equalization capability
Solution Approach 2:
The patent extracts the voltage equalization function from hardware circuits and implements it through software control. By separating the equalization logic from physical hardware, the system achieves voltage balance through detection, classification, and control algorithms without requiring additional hardware components for each sub-module
2Device complexity
If classification control strategy is used for sub-modules, then the number of sorting operations is reduced, but voltage equalization can only be achieved for sub-modules of the same voltage level
Solution Approach 1:
The patent creates a universal control strategy that handles sub-modules of different voltage levels through a unified classification and control framework. The system detects voltage levels, classifies sub-modules accordingly, and applies appropriate switching control to achieve equalization across mixed voltage levels, making the control method adaptable to various sub-module configurations
Solution Approach 2:
The patent segments sub-modules by voltage level and applies differentiated control strategies to each segment. By dividing the control process into detection, classification by voltage level, and selective switching, the system efficiently manages sub-modules of different voltages without requiring complex sorting operations for each individual sub-module
3Reliability
If individual voltage equalization control loops are implemented for each sub-module, then voltage equalization can be achieved, but control complexity becomes extremely high with a large number of sub-modules
Solution Approach 1:
The patent merges individual sub-module control into a unified group-level control strategy. Instead of implementing separate control loops for each sub-module, the system detects voltages of multiple sub-modules, classifies them by voltage level, and applies coordinated switching control to groups, significantly reducing control complexity while maintaining equalization precision
Solution Approach 2:
The patent changes the control parameter from individual sub-module voltage to group-level voltage classification. By detecting capacitor voltages and classifying sub-modules into voltage level groups, the system controls voltage equalization through parameter aggregation and group-based switching strategies rather than individual sub-module control
Data Source
AI summary
Disclosed are a control method and control system for a modular multilevel converter and a power transmission system. The control method includes calculating an actual capacitor voltage of the sub-module; calculating a reference capacitor voltage of the sub-module; dividing the plurality of sub-modules into a plurality of modules, reference capacitor voltages of the sub-modules in the same module are the same, and reference capacitor voltages of the sub-modules from different modules are different; sorting in the module to obtain a first voltage sequence; sorting among different modules to obtain a second voltage sequence; and determining the sub-modules to be switched on or switched off according to charging and discharging states of the sub-module, the first voltage sequence and the second voltage sequence, until an actual level of the bridge arm is consistent with a desired level, wherein the desired level changes using a first preset value as a step.