Modular Converter Topology for Low-Frequency Voltage Shaping
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Solution Overview
Problem
Modular multilevel converters face issues with bulky structures due to large capacitors and inefficiency at low output frequencies, as capacitors in these converters are used for energy storage and not primarily for voltage shaping.
Innovation Solution
A converter circuit where multiple switching cells are arranged in arms, with capacitors used to shape the output voltage by connecting a desired number in series, and energy is transferred to a DC link capacitor during a brief blanking period, minimizing capacitor size and enabling operation at low frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If capacitors are used for energy storage in each switching cell, then the converter can operate independently without DC link, but the structure becomes bulky and capacitor size increases
Solution Approach 1:
The invention extracts the energy storage function from the switching cell capacitors and relocates it to the DC link capacitor. The switching cell capacitors are reduced to minimal size for voltage shaping only, while the DC link capacitor handles all energy storage requirements. This separation resolves the contradiction by maintaining independent operation capability while dramatically reducing overall capacitor size.
Solution Approach 2:
The DC link capacitor serves multiple functions: it acts as the primary energy storage device for the entire converter system and also provides voltage reference for the switching cells. This multi-functionality eliminates the need for large capacitors in each switching cell, resolving the contradiction between operational independence and compact size.
2Adaptability or versatility
If capacitors in switching cells are used for energy storage, then DC link is not needed, but operation at low output frequencies becomes inefficient
Solution Approach 1:
The invention extracts the energy storage function from switching cell capacitors and assigns it to the DC link capacitor. This allows the switching cells to operate efficiently at low frequencies by only performing voltage shaping, while the DC link capacitor handles energy storage and can operate efficiently across a wide frequency range including low frequencies.
Solution Approach 2:
The invention changes the functional parameters of the capacitors: switching cell capacitors are reduced to minimal capacitance for voltage shaping only, while the DC link capacitor is sized appropriately for energy storage. This parameter change enables efficient operation across different frequency ranges, particularly improving low-frequency performance.
3Manufacturing precision
If multiple voltage levels are used to improve THD, then output voltage quality increases, but the converter structure becomes more complex
Solution Approach 1:
The invention segments the voltage generation into discrete levels by connecting integer numbers of switching cell capacitors in series. Each switching cell capacitor contributes a fixed voltage level, and by selectively connecting different numbers of these cells, multiple output voltage levels are achieved. This segmentation approach improves THD through multiple voltage levels while keeping the structure relatively simple and modular.
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 size of capacitors and allows efficient operation at low frequencies by using the DC link capacitors for energy storage, achieving low total harmonic distortion and improved output voltage quality.
Implementation Method 1
Each switching cell comprises a capacitor and the voltage of the capacitor can either be by-passed or set in the series connection to provide a desired phase output
Implementation Method 2
resonance situation is obtained. The resonance is stopped with a diode allowing the energy transfer from the switching cell capacitors to the DC link capacitor
Data Source
AI summary
A converter and a method of operating a converter. The converter comprising a DC link capacitor, an upper arm (S1) and a lower arm (S4) with switching cells in series, an upper switch (S2) and a lower switch (S3). The upper switch (S2) and the lower switch (S3) are connected together and the connection point forming an output voltage terminal (a). The converter comprising further an upper valve component and a lower valve component, which are arranged such that the upper valve component allows current from the center point of the DC link capacitor towards the upper arm and the lower valve component allows current from the lower arm towards center point of the DC link capacitor. A current path through the upper valve component or the lower valve component comprises inductance to form a resonance circuit.