Multi-Level Inverter Pulse Pattern Optimization
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Solution Overview
Problem
Medium voltage drives using multi-level inverters face high harmonic distortion at low switching frequencies, which is not effectively addressed by existing technologies, leading to increased switching losses and reduced efficiency.
Innovation Solution
A method for determining optimal pulse patterns for multi-level inverter systems that reduces switching frequency while minimizing harmonic distortion, using synchronous optimal modulation to calculate switching instants that minimize the total harmonic distortion of the output current, and incorporating constraints for continuity and hardware limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If space vector modulation (SVM) is used to achieve low harmonic content, then the switching frequency must be maintained around 1 kHz or higher, but this results in high switching losses
Solution Approach 1:
The patent changes the modulation parameters by determining optimal switching instants that minimize harmonic distortion while allowing operation at switching frequencies below 1 kHz. The objective function minimizes total harmonic distortion of machine currents, enabling frequencies as low as 200 Hz while maintaining acceptable harmonic content.
Solution Approach 2:
The patent introduces dynamic adjustment of switching instants based on the fundamental frequency and modulation index. The switching angles are continuously optimized to maintain low harmonic distortion across varying operating conditions, enabling flexible switching frequency selection without sacrificing waveform quality.
2Loss of energy
If the switching frequency is reduced below 1 kHz to minimize switching losses, then the harmonic distortion of machine currents becomes unacceptably high
Solution Approach 1:
The patent performs preliminary determination of optimal switching instants offline, storing them in lookup tables for online implementation. The optimal switching angles are pre-calculated for various fundamental frequencies and modulation indices, allowing the system to achieve low harmonic distortion at reduced switching frequencies without real-time computational complexity.
Solution Approach 2:
The patent replaces traditional SVM mechanical switching patterns with optimized pulse patterns derived from minimizing an objective function. This substitution enables flexible switching frequency selection while maintaining low harmonic distortion through mathematically optimized switching instants rather than fixed switching patterns.
3Object-generated harmful factors
If the number of switching instants N is increased to reduce harmonic distortion, then the number of possible pattern structures increases, leading to increased device complexity
Solution Approach 1:
The patent uses a sufficient but not excessive number of switching instants (typically 3-7 per half-cycle) to achieve the desired harmonic distortion reduction. This partial action approach balances waveform quality with computational and implementation complexity, avoiding the need to evaluate all possible pattern structures while still achieving low harmonic content.
Data Source
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AI summary
The present document relates to multi-level inverter systems. In particular, it relates to the control of switching instants of the switching devices of such multi-level inverter systems. A method for determining a pulse pattern of a multi-level inverter system for a motor drive is described. The multi-level inverter system comprises a set of switching devices providing L levels of output potentials and an output current. The pulse pattern comprises a set of N switching instants, at which switching of the multi-level inverter system to an adjacent level of output potential occurs. The method comprises the steps of determining a set of possible pattern structures, and of setting a set of fundamental frequencies the output current waveform. For a possible pattern structure in the set of possible pattern structures and for each fundamental frequency from the set of fundamental frequencies, the method comprises the further step of determining the N switching instants which provide a relative minimum value of an objective function which is associated with the total harmonic distortion of the waveform of the output current, thereby yielding a set of pulse patterns for the set of fundamental frequencies. Corresponding switching instants from the set of pulse patterns are continuous across the set of fundamental frequencies.