Analysis and Improvement of Harmonic Content in Multi-level Three-phase DAB Converters with Different Transformer Windings Connections

The transformer of a dual-active-bridge (DAB) converter is usually designed to operate with fundamental frequency ranging from a few kHz to hundreds of kHz, depending on the application of the converter. Since the waveforms of the ac-link are non-sinusoidal, the transformer is subjected to harmonics of the fundamental frequency. The higher the harmonic content, the higher the losses in the transformer. This paper studies the harmonic content of the phase voltages and currents for a three-phase multi-level DAB converter. The effect of the windings connection of the transformer-ΔY, and YY-on the harmonic content is studied in detail. It is shown that the share of harmonics in the phase currents is independent of the power level for the YY type. However, for the ΔY type, the lower the load level, the higher the share of harmonics in the waveform. This results in high no-load currents in the transformer. A control strategy is proposed to reduce the harmonic content of the phase currents of the ΔY type in partial loads. It is shown that the proposed control method effectively reduces the low-order harmonics. As an example, the amplitudes of the 5
^th
and the 7th harmonics reduce by 78% and 70%, respectively, at a 5% load level.