Active Damping Control for AC-DC Converters with Unfolder Plus T-Bridge Series Resonant Converter

In single-stage three-phase unfolding-based ac-dc systems, the low-frequency operation of the Unfolder results in
LC oscillations due to the grid inductance and the soft-dc-link capacitors, which distort the converter’s waveforms, increase
grid current THD, and can lead to instability. In this work, an active damping control technique using current emulation
is developed for an unfolding-based ac-dc converter with a Tbridge series resonant converter (T-SRC) as the dc-dc stage. Using a feedforward term in the calculation of the duty cycles, the proposed active damping adds emulated current sources that
charge or discharge the soft-dc-link capacitors, thereby removing the need for an external passive damping filter and reducing cost
and size of the system. The presented analysis and design have been validated through simulation and experimental results of
an 18 kW ac-dc converter hardware prototype.