Fixed-Pulse Multilevel Converter Control for Switching-Loss Tradeoffs
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Solution Overview
Problem
Multi-level inverters face challenges in determining the optimal number of switching times to achieve maximum modulation ratio without increasing switching loss, as either insufficient switching times prevent maximum voltage level output or excessive times lead to increased device size and harmonic suppression issues.
Innovation Solution
A control system for multi-level power converters using a fixed pulse pattern that determines the minimum number of levels required for the target modulation ratio, compares this with the total number of voltage harmonic orders, and adjusts the number of switching times to optimize switching phases, using conditional expressions to derive the pulse pattern for efficient harmonic reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the number of switching times is increased to output maximum voltage level in high modulation ratio, then the maximum level output is achieved, but the switching loss increases and device size increases
Solution Approach 1:
The patent applies dynamics by making the number of switching times variable rather than fixed. The pulse pattern derivation unit dynamically determines the number of switching times based on the modulation ratio, using fewer switching times at low modulation ratios and more switching times at high modulation ratios. This dynamic adjustment resolves the contradiction by adapting the switching frequency to the actual operational requirements, reducing switching loss when maximum power is not needed while maintaining the ability to output maximum level when required.
2Object-generated harmful factors
If the number of switching times is reduced to suppress harmonics, then the switching loss decreases, but the maximum voltage level cannot be output in high modulation ratio
Solution Approach 1:
The patent resolves this contradiction through dynamic adjustment of the number of switching times based on modulation ratio. At low modulation ratios, fewer switching times are used to suppress harmonics effectively. At high modulation ratios, the system increases the number of switching times to ensure maximum voltage level output is achieved. This dynamic strategy allows the system to optimize between harmonic suppression and maximum power output depending on operational conditions.
3Loss of energy
If the number of pulses is fixed at minimum for harmonic suppression, then the switching loss is reduced, but the pulse width narrows at low modulation ratio
Solution Approach 1:
The patent applies dynamics by making the number of pulses variable based on modulation ratio. At low modulation ratios, the system uses more pulses to maintain adequate pulse width and avoid narrowing issues. At high modulation ratios, the system reduces the number of pulses to minimize switching loss. This dynamic adjustment resolves the contradiction between maintaining pulse width and reducing switching loss across different operational ranges.
Data Source
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AI summary
An object is to realize control of a fixed pulse pattern system at an optimum number of switching times according to a modulation ratio. The present invention has steps of determining a minimum number of levels (L_duty) required to output the target modulation ratio (d_ref) (S2), determining the total number (N_volt) of voltage orders to be controlled among a voltage fundamental wave and harmonics of the power converter (S3), comparing the determined required minimum number of levels (L_duty) and the determined total number (N_volt) of voltage orders to be controlled and fixing a larger one as the number of switching times N in a quarter cycle for the target modulation ratio (S4-S6); when (N_volt) is fixed, determining a shape of an output voltage (S7), and based on the target modulation ratio and the fixed number of switching times N in the quarter cycle, determining switching phases for N times, and deriving the pulse pattern for one cycle by which each output voltage level according to the target modulation ratio, the output voltage shape and the phase is determined (S8, S9).