Multilevel Converter Voltage Balancing via Circulating Currents
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Solution Overview
Problem
Multilevel converters with star- or wye-connected phase legs face challenges in balancing voltage levels of energy storage elements without affecting power transmission networks, as existing solutions either require larger number of levels or introduce harmonics when trying to balance voltages.
Innovation Solution
A multilevel converter design with serially connected switching cells and energy storage elements in a star- or wye-configuration, featuring two parallel branches per phase leg to create a closed circuit, allowing for voltage balancing through circulating currents without impacting power transmission, using controllers to monitor and adjust switching to maintain balanced voltage levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If voltage balancing is achieved by adjusting switching timing in star- or wye-connected multilevel converters, then voltage levels of energy storage elements can be balanced, but harmonics are introduced into the power transmission network
Solution Approach 1:
The converter is divided into multiple independent switching cells, each with its own energy storage element. This segmentation allows independent control of each cell's switching timing to balance voltages without collectively introducing harmonics to the grid, as each cell can be controlled individually to maintain sinusoidal output.
Solution Approach 2:
The invention changes the control parameter from uniform switching timing adjustment to cell-specific timing adjustment. By monitoring individual capacitor voltages and adjusting only the switching timing of cells with deviating voltages, the system achieves voltage balancing while maintaining overall sinusoidal waveforms and avoiding harmonic introduction.
2Manufacturing precision
If more switching levels are added to achieve voltage balancing in star- or wye-connected converters, then voltage control precision improves, but device complexity increases
Solution Approach 1:
The invention implements dynamic voltage balancing by continuously monitoring capacitor voltages and adjusting switching timing in real-time. This dynamic control allows the system to maintain precise voltage control with a moderate number of levels, as the precision comes from adaptive control rather than simply increasing the number of fixed switching levels.
Solution Approach 2:
The system incorporates feedback control by monitoring the voltage levels of energy storage elements and using this information to adjust switching timing. This feedback mechanism enables precise voltage control without requiring an excessive number of switching levels, as the control precision is achieved through closed-loop regulation.
3Reliability
If switching timing is adjusted to balance capacitor voltages, then voltage balancing is achieved, but power transmission to the network is affected
Solution Approach 1:
The invention extracts the voltage balancing function from the main power transmission function. By dedicating specific switching cells to voltage balancing tasks while maintaining their primary power transmission role, the system achieves voltage balancing without significantly impacting overall power transmission to the network.
Solution Approach 2:
The switching cells are designed to perform multiple functions: both power transmission to the network and voltage balancing of energy storage elements. This multi-functionality allows the same hardware to achieve voltage balancing through timing adjustments without requiring separate dedicated balancing circuits that would consume additional power.
Data Source
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AI summary
A multilevel converter and a method for controlling a multilevel converter is provided. The multilevel converter is a single phase converter with one phase leg (1), or a three phase converter with three phase legs (1A-C), the phase legs of the three phase converter are interconnected in a star-configuration. The (1), or each (1A-C), phase leg comprises switching cells (11, 21),and each switching cell (11, 21) comprises semi-conductor switches (41, 51) arranged to selectively provide a connection to a corresponding energy storage element (42, 52). The converter also includes a controller (31), which is provided to monitor the DC voltage (VDC) of the energy storage elements (42, 52), and the controller (31) is provided to control the switching of each switching cell (11, 21). The phase leg (1) of the single phase converter, or each phase leg (1A-C) of the three phase converter, comprises two parallel branches (10, 20) of switching cells (11, 21), the branches (10, 20) being configured in a closed circuit. The method includes monitoring (105) the voltage levels of each of the energy storage elements, and balancing (107) the voltages of the energy storage elements, wherein the balancing includes circulating a current (104, 107) within the two branches of the (1), or each (1A-C), phase leg of the multilevel converter.