Three-Level Inverter Control for Zero-Cross Controllability
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Solution Overview
Problem
Conventional power conversion devices experience decreased controllability near zero cross of the voltage command value due to narrow PWM pulses being extinguished by dead time, affecting phase-to-phase voltage control in three-phase systems.
Innovation Solution
A power conversion device with a three-level inverter circuit and an inverter control circuit that generates corrected command values by setting a dead band around zero voltage command value, stabilizing the AC voltage command value and distributing the difference to other phases to maintain phase-to-phase voltage, thereby preventing controllability deterioration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dead time is provided to prevent arm short circuit in the inverter circuit, then reliability is improved, but PWM pulse width becomes too narrow and is extinguished near zero cross, causing controllability to deteriorate
Solution Approach 1:
The control device predicts the zero-cross timing of the voltage command value in advance and applies correction to the voltage command values before the actual zero-cross occurs. By preliminarily adjusting the command values based on predicted timing, the system ensures that PWM pulses maintain sufficient width even when dead time is applied, preventing pulse extinction while maintaining reliability.
2Manufacturing precision
If the width of triangular-wave carrier signals is reduced to improve PWM resolution, then manufacturing precision is improved, but PWM pulse width becomes narrower and is more easily extinguished by dead time
Solution Approach 1:
The control device dynamically changes the parameters of the voltage command values near zero-cross timing by applying correction based on predicted zero-cross timing. This parameter change ensures that even with narrow triangular-wave carrier signals for high PWM resolution, the resulting PWM pulses maintain sufficient width to overcome dead time effects and prevent extinction.
Data Source
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AI summary
The power conversion device includes a three-level inverter circuit and an inverter control circuit for driving the three-level inverter circuit in a double carrier modulation manner. The inverter control circuit includes a command value calculating part generating three-phase output voltage command values, a command value correcting part outputting corrected command values by correcting to the three-phase output voltage command value, and a gate signal generating part generating a gate signal based on the corrected command values. The command value correcting part is configured to output a predetermined voltage command value corresponding to an end of a dead band instead of one AC voltage command value, and distribute a difference between the one AC voltage command value and the predetermined voltage command value in the dead band passage period to other two AC voltage command values, during a dead band passage period in which the one AC voltage command value passes through the dead banc predetermined around a zero value of a voltage command value.