MMC Submodule Selection Algorithm for HVDC Loss Reduction
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Solution Overview
Problem
High voltage direct current (HVDC) transmission systems using modular multilevel converters (MMCs) face significant losses due to high commutation and conduction losses in submodule switching, which are not effectively reduced by existing voltage balancing and cell selection algorithms.
Innovation Solution
A novel switching algorithm that adjusts the selection of submodules based on current polarity and voltage thresholds to minimize the need for submodule insertion and bypassing, thereby reducing switching frequency and associated losses, by using a histogram-based approach to select submodules that were previously active, and applying dynamic or modified voltage shift values to alter their ranking and likelihood of re-selection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If existing voltage balancing and cell selection algorithms are used, then submodule selection is performed, but commutation and conduction losses are not effectively reduced
Solution Approach 1:
The algorithm performs preliminary action by identifying and prioritizing submodules that were previously active in the current polarity direction before making selection decisions. This preliminary identification allows the system to preferentially select these submodules, reducing the need for commutation and thereby reducing switching losses while maintaining effective power transmission.
2Power
If submodule insertion and bypassing is performed frequently, then desired valve output voltage is generated, but switching frequency increases leading to higher losses
Solution Approach 1:
The algorithm incorporates feedback by using the current polarity direction and the selection history of submodules to inform future selection decisions. This feedback mechanism allows the system to maintain the desired valve output voltage while reducing unnecessary commutation events, thereby lowering switching losses without compromising power transmission capability.
3Reliability
If voltage balancing algorithms are applied, then voltage balance is maintained, but commutation losses are not reduced
Solution Approach 1:
The algorithm changes the selection parameter by introducing a preference criterion based on previous activity state and current polarity direction. Instead of using traditional voltage balancing metrics alone, the system modifies the selection parameter to include the historical activity information, which enables voltage balance maintenance while simultaneously reducing commutation losses through more intelligent submodule selection.
Data Source
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AI summary
A method is provided for selecting submodules of a valve of modular multilevel converter in order to generate a desired valve output voltage. The method comprises: obtaining (410) a voltage value for each submodule of the valve indicative of a voltage across an energy storage element of the submodule; for each submodule of the valve: if the submodule was previously selected: (420) adding or subtracting a first shift voltage value to the obtained voltage value for the submodule based on a sign of a current in the valve to generate a modified voltage value for the submodule; and adding (430) the submodule to a histogram according to the modified voltage value for the submodule; if the submodule was not previously selected: adding (430) the submodule to a histogram according to the voltage value for the submodule. No submodules, one submodule or more than one submodule, are selected (440), based on their ranking in the histogram, to activate so as to generate the desired valve output voltage