Current magnitude based compensation of dead time in DCM for grid-connected bridge inverters
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Date
2018
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University of New Brunswick
Abstract
Grid-connected inverters are becoming very common with the rise of distributed generation and residential photovoltaic systems. The most common type of inverter used in these applications is a full bridge voltage source inverter (VSI); which can be either single-phase or three-phase. One overlooked way to improve their performance is by reducing harmonics caused by dead time, by compensating discontinuous conduction mode (DCM) caused by dead time. While compensation methods have been developed for dead time during continuous conduction, limitations of these methods mean DCM is not addressed by most.
This thesis work details a method to modify the command signals sent to inverter switching elements in the zero-crossing regions where DCM is likely to occur. The modified signals are able to incrementally change the duty cycle of these switching elements so that there is no effect from DCM; while maintaining the dead time that is necessary to prevent a switching overlap. This method eliminates the need for current polarity dependent compensation which allows DCM to be addressed. The method requires input measurements of current and voltage that are easily measured and fed to a digital signal processor (DSP), where the command signal will be calculated.
The method was successfully simulated in PSim 11.0 for several switching schemes and tested experimentally on a 10kW single-phase wind and solar inverter for two switching schemes. Experimental results show that the proposed method works to reduce the output current total harmonic distortion (THD) in multiple different switching schemes by eliminating the discontinuity in the output current. Distortions that appear on the bridge voltage were also compensated, which is shown by simulated results.