A scalable and plug-and-play Solid-State Transformer based on 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 higher power density and many smart functionalities, such as output voltage regulation, grid reactive power support and power flow control in the distribution grid. As more and more distributed energy resources (DERs) are connected to the grid to achieve the high penetration of renewable energies, the SST serves as an Energy Router, the intelligent interface between the DERs and the distribution grid. To make the deployment of the SST simple and efficient, SSTs should also provide power and communication plug-and-play (PnP) feature. The power PnP feature enables instant power exchange between the medium voltage distribution grid and the DERs and loads, and the communication PnP enables the instant management and many SSTs as a collective Energy Routers. So far, the investigation on the SST with the PnP feature is limited. This thesis’s main contribution is to investigate the capability of a proposed PnP SST, which can achieve immediate power flow once the connection between SST and the medium voltage grid is established. The proposed PnP SST has an input-series-output-parallel (ISOP) modular configuration using a number of SST cells. This greatly improves the scalability and reliability of the PnP SST for medium voltage application. To better adopt the PnP feature, a distributed controller solution is proposed to control the SST cell. To further improve the power density and control robustness, a single stage direct AC/AC power conversion topology is used for the PnP SST. The AC/AC converter employs the soft switching LLC resonant converter to improve the power efficiency. To enable the PnP start-up of the SST, a distributed Flyback auxiliary power supply is proposed to facilitate automatic control power generation. For the modular PnP SST, the input voltage balance is a major concern and is investigated in this thesis. A major contribution from this research is that the LLC based modular PnP SST solution can achieve input voltage balance even with an open loop control, eliminating complex centralized controller needed in traditional SST. The voltage balance performance is capable of tolerating component parameter variations within a reasonably large range. This paves the way for the PnP to achieve very high input voltage needed for medium voltage grid connection. The modular PnP SST also features very high input impedance hence very low in-rush current. Direct PnP to the medium voltage grid can be applied to simplify the installation and the need for a large in-rush current limiter is eliminated. The salient smart functions of the proposed PnP SST is also investigated. The investigation indicates that the proposed PnP SST has the capability of achieving fast dynamic voltage regulation, bi-directional power flow, and reactive power processing. A SiC based PnP SST prototype was developed as part of this thesis. Experiments verified that the SST cell achieves 97.88% peak efficiency. Each cell has a 11 kW power capability and a 550Vrms input voltage. The natural input voltage balance is verified by the PnP SST prototype with two SST cells.