Soft-Switched Integrated AC–DC Bidirectional Converter With Natural Grid Harmonic Elimination

Abstract

Two-stage ac–dc converters have hard-switched ac–dc stage followed by soft-switched dc–dc stage. Since devices in the ac–dc stage are switched at low frequency (typically 10–20 kHz), bandwidth of current control loop of this stage is limited (typically less than 2 kHz). This low bandwidth current loop fails to reject current harmonic disturbances arising due to distorted grid voltages, thus necessitating multiple harmonic compensation loops or complex control algorithms for meeting IEEE Std 519-2014. Lack of soft-switching prohibits devices of the ac–dc stage from being switched at a frequency above 100 kHz and thereby attain a bandwidth of around 10 kHz for natural rejection of grid current harmonics. This article proposes an integrated three-phase bidirectional isolated ac–dc converter, where soft-switching operation is extended to devices in ac–dc stage as well. Thus, high bandwidth current loop can be achieved without reduction in efficiency. The proposed modulation scheme decouples ac–dc and dc–dc operations using <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">xor</small> logic. An iterative procedure for design of resonant tank is provided. The integration also leads to 25% fewer devices. Experiments carried out on a 5-kW prototype demonstrate that a low current total harmonic distortion, complying with IEEE standard, can be achieved with good efficiency, even under distorted grid voltages.