Modular Multilevel Converter Control via Switching Tables
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Solution Overview
Problem
Modular multi-level converters face significant computational complexity in determining optimal switching states due to a vast number of possible configurations, leading to suboptimal performance over time as existing methods struggle to cope with the high number of degrees of freedom and dynamic changes in energy store discharging.
Innovation Solution
A method is introduced that divides the control into real-time and offline parts, where the real-time part allocates voltage levels and determines immediate switching states using a scheduler, while the offline part calculates an optimized switching table based on a cost function to minimize losses and ensure uniform energy store discharge, using an optimizer that continuously updates the switching table.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a scheduler determines switching states in real-time for each time step, then real-time control speed is maintained, but computational complexity increases due to the vast number of possible switching configurations
Solution Approach 1:
The patent pre-calculates and stores optimal switching states for all possible voltage level combinations in switching tables during an offline phase. During real-time operation, the controller simply looks up the appropriate switching state from the pre-computed tables based on current voltage levels, avoiding complex real-time calculations while maintaining optimal control performance
Solution Approach 2:
The control problem is divided into two distinct phases: an offline phase where switching tables are pre-computed using dynamic programming and optimization algorithms, and an online real-time phase where switching states are determined by simple table lookups. This segmentation separates the computationally intensive tasks from the time-critical tasks
2Ease of operation
If a fixed switching table is used for control, then real-time determination is simplified, but performance degrades over time as energy store discharging patterns change
Solution Approach 1:
The patent implements a dynamic switching table update mechanism where the switching tables are periodically regenerated based on current converter operating conditions, particularly the state of charge of energy stores. This allows the system to adapt to changing discharging patterns while maintaining the computational simplicity of table-based control
Solution Approach 2:
The system monitors the actual performance and energy store states, then uses this feedback information to determine when switching tables need updating. The switching tables are regenerated when performance degradation is detected or when energy store states change significantly, ensuring optimal performance throughout operation
3Loss of energy
If dynamic optimization is performed continuously to adapt to changing energy store states, then long-term efficiency is improved, but real-time control capability is compromised
Solution Approach 1:
Optimal switching strategies are pre-computed using dynamic programming algorithms during an offline phase, storing the results in switching tables. During real-time operation, the system simply retrieves pre-determined optimal switching states based on current voltage levels, achieving both optimality and real-time performance
Solution Approach 2:
The system performs comprehensive optimization calculations periodically or event-driven (when energy store states change significantly), rather than continuously. Between optimization events, the pre-computed switching tables are used, providing a balance between adaptation to changing conditions and maintaining real-time control capability
Data Source
AI summary
A method controls switching states of a multi-level converter with multiple modules. Each module has: terminals on a first and second side; controllable switches; and an energy store in series with a first switch in a first connection between the terminals. A second switch is arranged in a connection between the terminals. The control of the switching states is divided into a real-time and offline part. In the real-time part, for each time step: a voltage level is allocated to a voltage requirement; a total switching state is determined in a first switching table for the voltage level; and the total switching state is passed on as a control signal to the switches. In the offline part: a second switching table is calculated, resulting in accordance with a minimization of a cost function.