Abstract
In a two-stage inverter, the instantaneous output power pulsates at twice the output frequency (2fo), generating the second harmonic current (SHC), which will propagate into the front-end dc-dc converter. This article aims for suppressing the SHC in the front-end dc-dc converter with dc transformer (DCX)-LLC resonant converter. The small-signal model of the DCX-LLC resonant converter is proposed, and based on which, the unified small-signal model of the preregulator+LLC converter is built. Then, the basic ideas for reducing the SHC are proposed, including the determination of the filter capacitors in the preregulator+LLC converter and the control approach. After that, from a perspective of output impedance, the control schemes for reducing the SHC in the front-end preregulator+LLC converter are proposed. By inserting a notch filter in the voltage loop and/or introducing a virtual impedance in series with the output rectifier of the LLC resonant converter, the output impedance of the preregulator+LLC converter at 2fo is increased and, thus, the SHC can be reduced. Finally, a 6-kVA two-stage three-phase inverter with boost+LLC converter as the front-end dc-dc converter was fabricated and tested. The experimental results are provided to verify the proposed control schemes.
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