Multilevel Converter Energy Storage Phase Balancing
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Solution Overview
Problem
Multilevel converters face increased costs and losses due to negative sequence conditions, which require higher silicon area and more cells to compensate for unbalanced phases, leading to higher current ratings and inefficiencies.
Innovation Solution
Incorporating energy storage elements, such as super-capacitors, batteries, or flywheels, in specific chain-link configurations within the converter's cells to balance DC link capacitor voltages without inducing extra net active power exchange with the AC network, thereby reducing the need for additional cells and minimizing losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the converter compensates for unbalanced phases by increasing current rating and number of cells, then the compensation capability is improved, but the cost and losses increase
Solution Approach 1:
The converter is divided into multiple phase legs, each with independent energy storage elements. This segmentation allows each phase leg to independently manage its own DC link capacitor voltage fluctuations, enabling targeted compensation without requiring the entire converter to be oversized for unbalanced conditions.
Solution Approach 2:
Energy storage elements are pre-integrated into each cell of the converter. These elements are prepared in advance to absorb or supply active power when DC link capacitor voltage deviations occur, enabling immediate response to unbalanced phase conditions without delaying for external compensation mechanisms.
2Reliability
If the converter compensates for unbalanced phases by increasing silicon area, then the compensation capability is improved, but the cost increases
Solution Approach 1:
The energy storage elements serve multiple functions: they balance DC link capacitor voltages, compensate for unbalanced phase conditions, and maintain converter operation during transient states. This multi-functionality eliminates the need for separate compensation devices, reducing overall system cost while maintaining compensation capability.
Solution Approach 2:
Each phase leg is equipped with its own energy storage element that automatically responds to DC link capacitor voltage deviations. The system compensates for unbalanced phases using its own internal resources rather than requiring external compensation equipment, thereby reducing total silicon area and cost.
3Stability of the object's composition
If energy storage elements are added to each cell, then DC link capacitor voltage balancing is improved, but the device complexity increases
Solution Approach 1:
The energy storage elements are merged with the existing DC link capacitors in each cell, forming a combined energy storage system. This integration allows the energy storage elements to work in conjunction with the capacitors rather than as separate additions, simplifying the overall structure while maintaining voltage balancing capability.
Solution Approach 2:
The energy storage elements act as intermediaries between the AC network and the DC link capacitors. They mediate the power flow to balance capacitor voltages during unbalanced conditions, providing a controlled interface that manages complexity rather than directly increasing it.
4Reliability
If the converter uses higher current ratings to compensate for unbalances, then the compensation capability is improved, but the losses increase
Solution Approach 1:
The energy storage elements enable dynamic response to unbalanced phase conditions by automatically absorbing or supplying active power based on real-time DC link capacitor voltage deviations. This dynamic compensation maintains stable operation without requiring sustained high current ratings, thereby reducing I²R losses compared to static oversizing approaches.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces the number of cells and silicon area required, lowering costs and losses by using energy storage elements to manage phase imbalances internally, maintaining the cascaded multilevel structure while supporting high voltage applications without substantial net active power exchange.
Implementation Method 1
an energy storage element configured for absorbing and supplying active power in the phase leg for converter internal DC link capacitor voltage balancing
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
The present disclosure relates to a multilevel power converter (1) connected to an AC network and a load. The converter comprises at least a first and a second phase leg (2), each phase leg comprising a plurality of cascaded chain link connected cells (3), each cell comprising a DC link capacitor (4). At least one cell in each phase leg comprises, in addition to the DC link capacitor, an energy storage element (5) configured for absorbing and supplying active power in the phase leg for converter internal DC link capacitor voltage balancing when compensating for unbalances between the phases of the network or the load without inducing any extra net active power exchange between the converter and the network.