Performance evaluation of a new hybrid-modulation scheme for high-frequency-ac-link inverter: Applications for PV, wind, fuel-cell, and DER/storage applications

Advances in SiC (and upcoming GaN) based power semiconductor devices, high-permeability nanocrystalline high-frequency (HF) transformers, and advanced processing and sensing capabilities have paved the way for HF-ac-link (HFACL) based isolated inverter systems that have wide applications encompassing high-power (HP) renewable- and alternative-energy systems (e.g. photovoltaic, wind, fuel-cell energy systems), DG/DER applications, active filters, energy storage, and compact defense power-conversion modules. Unlike conventional Si-based low-frequency HP systems, the switches in these applications need to operate at frequencies 20-50 kHz instead of being limited by conventional HP devices (e.g. Si-based IGCTs, GTOs). Additionally, HFACL systems eliminate the intermediate dc-link filters, which are needed in fixed-dc-link (FDCL) approaches. However, one of the challenges of such HP and HF approaches is switching loss. In that regard, this paper demonstrates, under scaled-power condition, the efficacy of a recently-outlined and patented hybrid-modulation (HM) scheme, which can reduce the switching losses of the ac/ac converter of a HFACL inverter by up to 66%. This also leads to improved voltage utilization as compared to state-of-the-art space-vector-modulation (SVM) schemes and reduction in device current stress. We also demonstrate that, even though the HM based HFACL approach works without a dc-link capacitor, the output THD of the inverter is not worse than the SVM based FDCL inverter. The HM scheme is unlike all reported discontinuous-modulation (DM) schemes where the input to the final stage of the inverter is a dc and not a pulsating-dc; further, in the HM scheme, switches in 2 legs of the ac/ac converter do not change state in a 60° cycle and switches in any one leg do not change state for an overall 240°. In contrast, for a conventional DM scheme, at most switches of 1 leg of the ac/ac converter do not change stage in a 60° or 120° cycle.