Predictive PWM for Multilevel Converter Harmonic Reduction
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Solution Overview
Problem
Multilevel converters generate significant lower order harmonics due to cell capacitor voltage imbalances and modulation schemes, which are difficult to correct with current controllers and can lead to increased system costs and resonance issues with passive filters.
Innovation Solution
A new pulse width modulation (PWM) technique predicts switching instants to minimize flux error at each half switching duration, reducing lower order harmonics by up to 0.2% at low pulse numbers, and allows additional switching to achieve zero or near-zero flux error, thereby reducing overall switching losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional PWM techniques are used, then the converter operates at low pulse numbers, but lower order harmonics are generated on the AC side
Solution Approach 1:
The control method performs preliminary calculations of switching instants based on predicted flux error before the actual switching occurs. By predicting the flux error at future switching instants and calculating optimal switching times in advance, the method proactively compensates for harmonic generation rather than reacting to it after the fact.
Solution Approach 2:
The method dynamically adjusts switching instants within each half switching duration based on real-time flux error predictions. The switching times are not fixed but are continuously optimized by calculating when the flux error will reach zero, allowing the system to adapt to changing operating conditions and minimize harmonics at each moment.
2Measurement precision
If current controller bandwidth is increased to correct harmonics, then harmonic correction improves, but system complexity and cost increase
Solution Approach 1:
The method replaces the need for high-bandwidth current controllers with a predictive flux-based control approach. Instead of using fast controllers to actively suppress harmonics, the system substitutes this with pre-calculated switching instants that proactively prevent harmonic generation, replacing complex control mechanics with predictive timing calculations.
3Object-generated harmful factors
If passive filters are added to reduce voltage distortion, then voltage quality improves, but system cost increases
Solution Approach 1:
The method converts the inherent flux error that would normally cause harmonics into a useful predictive signal. By calculating and using the flux error prediction to determine optimal switching instants, the system transforms what would be a harmful effect into the basis for harmonic-free operation, eliminating the need for external filtering.
4Loss of energy
If switching frequency is reduced to minimize losses, then switching losses decrease, but lower order harmonics increase
Solution Approach 1:
The method changes the critical parameter of switching instant timing while maintaining low switching frequency. Instead of increasing frequency to reduce harmonics, the system optimizes the timing parameters of each switch by calculating precise instants when flux error reaches zero, achieving harmonic-free operation at fundamental switching frequencies.
Data Source
Figure 1~2b
Figure 2c~2e
Figure 3
AI summary
The present disclosure relates to a method for reducing lower order harmonics of a multi-level power converter comprising at least one phase leg comprising a plurality of chain-link connected cells each comprising a capacitor. The method comprises, for each phase leg of the converter: obtaining a present reference voltage (uref) for use during a present half switching duration (Tsw /2); dividing the half switching duration into a plurality of time samples; and at the beginning of each time sample (tA, t1, t2): predicting the reference voltage waveform for the remainder of the half switching duration based on the obtained present reference voltage; 10 predicting the leg output voltage (uleg) waveform for the remainder of the present half switching duration for the case that no cell is inserted or bypassed in the leg during the time sample, and for the case that one cell is inserted or bypassed in the leg during the time sample; predicting the flux error (Ferror) at the end (tB) of the present half switching duration for each of the cases, based on the obtained present reference voltage, the predicted reference voltage waveform and the predicted leg output voltage waveforms; and determining whether to insert or bypass the cell, during the time sample, based on the predicted flux errors.