Multilevel Converter Submodule Energy Balancing
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Solution Overview
Problem
Existing methods for multilevel converters, such as those used in high-voltage direct current transmission, result in high load on power semiconductors due to frequent switching, leading to complex maintenance and potential failures.
Innovation Solution
The method involves determining a switching time based on the energy difference between a phase module branch setpoint and the total actual energy storage value, selecting the next energy store to switch on or off based on energy change states, and using a switching threshold to minimize frequent switching, thereby reducing the load on power semiconductors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If energy stores are switched on and off in kilohertz cycles to maintain voltage balance, then voltage distribution across submodules is improved, but the load on power semiconductor circuit increases leading to frequent failures and complex maintenance
Solution Approach 1:
The invention changes the switching frequency parameter from kilohertz cycles to a much lower frequency by using energy storage values as the switching criterion. This parameter change maintains voltage balance while significantly reducing the switching frequency and associated semiconductor stress
Solution Approach 2:
The invention introduces dynamic selection of submodules based on their real-time energy storage values. Instead of fixed-frequency switching, the system dynamically determines which submodules to switch based on current energy levels, creating an adaptive switching strategy that reduces overall switching frequency
2Manufacturing precision
If energy stores are switched at high clock rates to maintain voltage levels, then voltage balance is achieved, but device complexity and maintenance requirements increase
Solution Approach 1:
The invention changes the control parameter from time-based (fixed frequency) to energy-based (storage value threshold). This fundamental parameter change simplifies the control logic while maintaining voltage balance, as submodules are switched based on their energy state rather than external timing signals
Data Source
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AI summary
Method for charging and/or discharging energy stores in a multilevel converter having at least one phase module branch (6p1, 6p2, 6p3, 6n1, 6n2, 6n3), which has a series circuit formed by sub-modules (7) which each have at least one power semiconductor circuit (T1, T2) for connection or disconnection of an energy store (8) in a parallel circuit to the power semiconductor circuit and have a sub-module sensor for detection of an energy store actual value Uc, in which: with an energy change state being obtained a determination is made as to whether connected energy stores in a phase module branch can be charged or discharged and the next energy store to be switched in each phase module branch is selected by means of predetermined logic as a function of the energy change state, by means of which the energy stored in the energy stores is kept approximately at the same level, with a high clock rate at the same time being avoided for connection and disconnection of the selected energy store.