Hybrid Selective Harmonic Elimination for Direct AC/AC SST

Direct AC/AC solid-state transformer (SST) is a single-stage SST consisting of matrix converters and a high frequency transformer (HFT). It does not require any bulky reactive elements. It can achieve high power density by high frequency operation. However, the switching frequency is limited due to leakage inductance of the HFT. The limitation can be eliminated by adding filters in the output of the secondary side of the HFT and using modified selective harmonic elimination for direct AC/AC SST (MSHE-DAC) proposed in the literature. Selective Harmonic Elimination (SHE) is used to reduce or eliminate lower order harmonics from a switched waveform while maintaining a desired value for the fundamental component. This is achieved by introducing notches at certain time instants in the switched waveform. These time instants are also referred to as notch angles. By removing lower order harmonics, SHE allows reduction in filter size and unwanted resonance associated with the filter. However, the existing methods of implementation of SHE are for DC/AC converters and not suitable for implementing MSHE-DAC. MSHE-DAC requires variable voltage amplitude factor and calculation of notch angles of high frequency waveforms. Therefore, this paper proposes a hybrid implementation strategy for MSHE-DAC. The proposed strategy calculates variable voltage amplitude factor in real time and calculates notch angles offline, thus increasing calculation speed. It also proposes a selection strategy for notch angles for maintaining good precision of values of modulation indices. The proposed strategy is verified experimentally on hardware prototype.