Passive Filter for Harmonic Reduction in Multi-Level Converters
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Solution Overview
Problem
Existing multi-level converters face challenges in regulating circulating currents and removing zero sequence third harmonics without increasing the voltage rating of power semiconductor switches, while also minimizing the number of converter elements used.
Innovation Solution
A passive electronic filter is introduced between the voltage source and the AC terminal to reduce harmonics in circulating currents, specifically targeting components at twice the fundamental frequency to limit circulating current losses and remove zero-sequence third harmonics, thereby reducing the voltage rating requirements of power semiconductor switches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If circulating current regulation is implemented using additional inductors as described in WO 2008/067785, then circulating current limitation is achieved, but the voltage rating of power electronic switches must be increased to accommodate extra voltage needed for circulating current regulation
Solution Approach 1:
The patent extracts the circulating current regulation function from the main power conversion path by introducing a separate passive electronic filter connected in parallel. This filter specifically targets and removes harmful harmonics (particularly second and third order) from the circulating current without requiring the main power electronic switches to handle additional voltage stress, thus resolving the contradiction between achieving current regulation and maintaining switch voltage ratings
Solution Approach 2:
The passive electronic filter acts as an intermediary element that mediates between the converter bridge and the AC grid. It provides a dedicated path for circulating currents to flow through the filter components (inductors and capacitors) rather than forcing these currents through the main power switches, thereby protecting the switches from voltage rating increases while still achieving effective circulating current regulation
2Object-generated harmful factors
If zero sequence third harmonic removal is performed using a transformer, then harmonic removal is achieved, but device complexity and cost increase due to the additional transformer component
Solution Approach 1:
The patent merges the zero sequence third harmonic removal function with the circulating current filter into a single integrated passive electronic filter structure. The same inductors and capacitors that regulate circulating currents also provide the necessary impedance to block and remove zero sequence third harmonics, eliminating the need for a separate transformer while achieving both functions simultaneously
Solution Approach 2:
The passive electronic filter is designed with multi-functionality, serving dual purposes: (1) regulating circulating currents by filtering harmful harmonics, and (2) removing zero sequence third harmonics from the output. This universal filter structure replaces what would traditionally require separate dedicated components (inductors for circulating current and transformer for harmonic removal), thereby reducing overall device complexity
3Object-generated harmful factors
If a passive electronic filter is introduced to reduce harmonics in circulating current, then harmonic reduction is achieved, but the number of converter elements increases
Solution Approach 1:
The patent applies partial filtering action by designing the passive electronic filter to specifically target and address the most harmful harmonics (second and third order) rather than attempting to filter all frequency components. This selective approach achieves sufficient harmonic reduction to protect the power switches and improve performance without requiring an excessively complex filter structure with numerous elements, thus balancing harmonic reduction effectiveness with acceptable device complexity
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
The solution effectively regulates circulating currents and removes zero-sequence third harmonics without increasing the voltage rating of power semiconductor switches, reducing losses and system instability, and achieves these results with a limited number of converter elements, eliminating the need for a transformer.
Implementation Method 1
A passive electronic filter is introduced between the voltage source and the AC terminal to reduce harmonics in circulating currents, specifically targeting components at twice the fundamental frequency
Implementation Method 2
remove zero-sequence third harmonics, thereby reducing the voltage rating requirements of power semiconductor switches
Data Source
Figure 1~2
Figure 3~4
Figure 5~7
AI summary
A device for converting a DC voltage into an AC voltage comprises a passive electronic filter (18) having a first and a second energy storage element (9, 10), a third energy storage element (19) placed between the first and second energy storage elements, a fourth energy storage element (20) connected between a junction of the first energy storage element and the third energy storage element and an AC terminal and a fifth energy storage element (21) connected between a junction of the second energy storage element and the third energy storage element and the AC terminal. The energy storage elements are of two different types, capacitive and inductive, with values selected to provide reduction of frequency components at two times the fundamental frequency of the AC voltage and at three times the fundamental frequency of the AC voltage.