Modular Multilevel Converter Voltage Control
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Solution Overview
Problem
Current methods for voltage equalization in modular multilevel converters (MMCs) with mixed sub-modules of different voltage levels are inefficient, leading to low sub-module utilization and high Total Harmonic Distortion (THD) at the AC side.
Innovation Solution
A control method that detects actual capacitor voltages, calculates reference voltages, divides sub-modules into groups with equal reference voltages, sorts and normalizes voltages, and determines which sub-modules to switch on or off based on charging and discharging states to maintain a consistent bridge arm level, using a changing step selected from a combination of elements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If mixed sub-modules with different voltage levels are used to reduce hardware complexity, then the number of hardware drive circuits and communication lines is reduced, but large level jumping and high Total Harmonic Distortion (THD) occur
Solution Approach 1:
The patent segments sub-modules into different voltage level groups (first voltage level and second voltage level) and applies distinct control strategies to each group. By dividing the mixed sub-modules into manageable segments with different control parameters, the system maintains low hardware complexity while controlling THD through differentiated management of each voltage level group.
Solution Approach 2:
The patent applies local quality by implementing voltage equalization control specifically targeted at sub-modules of the same voltage level. The control method adjusts switching strategies and equalization priorities locally for each voltage level group, ensuring that sub-modules with identical voltage characteristics are managed uniformly, thereby reducing THD without requiring complex global control of all mixed sub-modules.
2Reliability
If traditional voltage equalization control is applied to mixed sub-modules, then voltage equalization is attempted, but the control becomes complicated requiring each sub-module to have corresponding voltage equalization control loop
Solution Approach 1:
The patent creates a universal voltage equalization control framework that handles mixed voltage level sub-modules through a single integrated control architecture. Instead of requiring separate control loops for each sub-module, the control method universally applies voltage equalization principles to all sub-modules by grouping them into voltage level categories, thereby achieving reliable voltage equalization without proportional increase in control complexity.
Solution Approach 2:
The patent changes the control parameters based on voltage level groups rather than individual sub-modules. By adjusting switching frequencies, equalization priorities, and control weights as parameters specific to each voltage level group, the system achieves effective voltage equalization for mixed sub-modules without requiring complex individualized control loops for each sub-module.
3Reliability
If half-voltage sub-modules and full-voltage sub-modules are used for voltage equalization, then voltage equalization is achieved, but sub-module utilization is low and voltages are limited to two electrical levels
Solution Approach 1:
The patent implements dynamic sub-module selection and switching strategies that adapt to real-time voltage conditions. Instead of statically assigning sub-modules to fixed voltage levels, the control method dynamically adjusts which sub-modules are activated and at what voltage level, based on current system demands and capacitor voltage states. This dynamic approach maximizes sub-module utilization while maintaining voltage equalization and enabling operation at multiple voltage levels beyond just two.
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 comprises: calculating an actual capacitor voltage and a reference capacitor voltage of the sub-module; dividing the plurality of sub-modules into a plurality of modules, wherein reference capacitor voltages of the sub-modules in the same module are the same, and reference capacitor voltages of the sub-modules among different modules are different; obtaining a first voltage sequence and 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-modules, 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 taking an insert value selected from a combination of one or more elements in a collection {INTERk} as a step.