Circulating Current Suppression in Multi-cell Series-parallel Converter for Cost-effective Medium-voltage Solid-state transformer

This work presents a cost-effective and compact medium-voltage (MV) solid-state transformer (SST). The pro-posed SST is rated at 5kW, and provides a low voltage dc output serving dc loads and sources such as batteries, photovoltaics, and household dc loads. The proposed multi-cell topology features an internal capacitor voltage balancing and requires only one dc-link voltage sensor. A major challenge of the proposed topology is the unbalanced circulating currents when using the conventional interleaved pulse-width modulated (PWM) scheme. The unbalanced circulating currents cause uneven component stress, and may require different current rating among the switch devices. By analyzing non-ideal equivalent circuits of the converter, we propose a new PWM scheme that minimizes and balances the circulating currents, thus increasing system efficiency. The proposed modulation scheme is verified on a full scale prototype.