Design of A Medium Voltage Solid-State Transformer based on Modular AC-AC Resonant Converter and an Input-Series-Output-Parallel Architecture

As an eventual replacement of the century-old line frequency transformer (LFT), the Solid-State Transformer (SST) has been investigated intensively in the past decade due to its ability to offer many smart functionalities such as voltage regulation and power flow control in the distribution power grid. For the medium voltage and high voltage SSTs, the availability and cost of high voltage power devices are the major limitations. One possible solution is to use modular SST architecture in an Input-Series-Output-Parallel (ISOP) configuration to reach high input voltage and high-power capacity. However, modular SST introduces additional challenges in control due to the need for many voltage and current sensors, multiple control loops to achieve cell to cell level voltage and power balancing. To address these problems, this paper introduces a novel modular SST solution utilizing single stage AC-AC resonant converter as the modular cell with distributed open loop control. The proposed modular SST is therefore capable of reaching medium and high voltages easily with simple and robust control. The proposed architecture eliminates the DC-DC stage hence the associated bulky DC link capacitors. Higher power density can therefore be achieved. 10 kW modular LLC converter prototype is developed based on 1.2kV SiC MOSFETs. Peak AC-AC efficiency up to 97.3% is achieved and demonstrated. Voltage and power balancing among modular cells are analyzed against cell to cell variations, including resonant frequency, switching frequency and losses. Simulation results show that the proposed modular SST has no performance degradation even considering those variations in a practical system. An ISOP architecture using two LLC converters is also demonstrated with preliminary experimental results, showing that with LLC converters working close to resonant frequency, the input voltage balancing can be achieved.