Multilevel Converter Energy Storage Integration
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Solution Overview
Problem
Modular multilevel converters face challenges with high voltage DC links, leading to increased costs, losses, and reduced lifetime due to harmonic currents and variable battery voltages, necessitating the use of DC-DC converters for energy storage and harmonic filtering.
Innovation Solution
A multilevel power converter with cascaded chain-link connected cells forming mirrored cell-pairs, allowing energy storage to connect over one or both cell capacitors using switches, eliminating the need for DC-DC converters and enabling voltage control without de-rating, while filtering AC components and harmonics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If batteries are interfaced directly in parallel with the cell capacitors, then the converter structure is simplified, but the cell capacitors must vary their voltage accordingly, resulting in significant de-rating of the whole converter
Solution Approach 1:
The patent introduces a DC-DC converter as an intermediary device between the battery and the M2LC converter. This mediator allows the battery to be connected to the converter without requiring the cell capacitors to vary their voltage, thus avoiding de-rating while maintaining structural simplicity. The DC-DC converter handles the voltage matching and isolation, enabling the battery to operate independently at its own voltage level.
2Power
If DC-DC converters are used to interface batteries with the M2LC converter, then de-rating is avoided and harmonics are filtered, but the system cost and complexity increase
Solution Approach 1:
The patent designs the DC-DC converter to perform multiple functions simultaneously: it interfaces the battery with the M2LC converter, filters harmonic currents from the battery current, and provides voltage matching without requiring separate components. This multi-functional approach reduces the need for additional dedicated components, thereby limiting the increase in system complexity and cost while achieving the desired power rating maintenance and harmonic filtering.
3Adaptability or versatility
If the common DC link voltage is made very high to accommodate battery voltage variations, then the converter can handle a wider voltage range, but the cost for insulation, fault handling, and circuit breakers increases significantly
Solution Approach 1:
The patent segments the voltage handling function by introducing a DC-DC converter that operates at a lower, isolated voltage level between the battery and the high-voltage M2LC converter. This segmentation allows the high-voltage side to maintain its voltage range capability while the low-voltage side (battery interface) operates at manageable voltage levels, thus avoiding the need for expensive high-voltage insulation and fault handling equipment throughout the entire system.
4Reliability
If distributed energy storage devices are used within the arms of the converter, then protection costs are reduced and redundancy is improved, but the variable voltage of batteries still requires handling
Solution Approach 1:
The patent applies the intermediary principle at the distributed level by placing DC-DC converters at each arm or module where distributed energy storage devices are located. These local DC-DC converters act as mediators between the variable-voltage batteries and the fixed-voltage cell capacitors, allowing distributed energy storage to provide redundancy and reduced protection costs while maintaining voltage control through the intermediary converters.
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
The present disclosure relates to a multilevel power converter 1 comprising at least one phase leg. The phase leg comprises a plurality of cascaded chain link connected cells 2, each cell comprising a capacitor 3 and two semiconductor switches 5 in series, each with an anti-parallel connected diode 6. The plurality of cascaded chain link connected cells comprises first and second cells 2a and 2b which form a mirrored cell-pair such that the two semiconductor switches of each of the first and second cells are all connected in series with each other. The converter further comprises an energy storage 4 connected between the first and second cells.