Principal Investigator: Dr. Adel Nasiri
Solid-State Transformer (SST) which is a combination of the power converters and medium (or high) frequency transformer can provide a more compact and efficient solution than a low frequency design through the
implementation of improved control and monitoring functions. In order to enable high power grid connected systems, MV interfacing SSTs that have sufficient power throughput for multi-MW systems must be developed. The key to achieving compact SST solutions is the MV Medium Frequency MV-MF Transformer. The majority of MV SST implementations are based upon the connection of lower voltage rated modular isolated converters in series to divide the total system voltage stress across these modular converters. So far, the highest rated systems reported in the literature interface to a 10kV MVac system have a 0.5MVA rating 1. Implementations at these power levels are IGBT-based and utilize Input Series Output Parallel (ISOP) configurations of modular isolated dc-dc converters with a maximum MV-MF transformer throughput of 150kVA.
High voltage breakdown capability and development of integrate-able MV-rated half-bridge Silicon Carbide (SiC) MOSFETs with voltage ratings up to 10kV 2-11 reduces the number of levels required to implement MV
interfacing converters. Today, engineering samples of 15kV rated SiC MOSFETs and 24kV SiC IGBTs have been built and tested in the literature with the possibility of up to 30kV rated devices on the horizon. However, achievement of the same or higher power throughput in WBG-based SSTs, with higher rated Power Electronic Building Blocks (PEBBs), is limited by the complex design considerations and trade-offs within the medium frequency transformer design such as switching frequency, dv/dt, core losses, core saturation and inter-winding insulation, partial discharge, winding-to-core insulation and ground-wall insulation. Further complicating the drive towards higher power throughput is the coupling of thermal management into the ground-wall insulation requirements at the system level. The collective impact of these design considerations is limiting the reported 13.8kV-connected medium frequency transformer power throughput to levels below 50kVA 12. In this project, a 2MW SST is proposed for 13.8kV to 4.16kV MVAC systems, where the neutral on the 13.8kV side is grounded, or 8kV to 4.16kV MVAC systems having a floating ground on either side. This architecture is selected to directly connect to 13.8kV using 10kV SiC MOSFETS in a three-level converter topology. This effort will lead to a novel architecture for MVac-MVac conversion offering several improvements over the existing state-of-the-art including simultaneous compact size and modular structure, power and voltage scalability and high efficiency. An optimizing design methodology for MV-MF transformers will be developed and demonstrated through hardware design and test. This methodology takes into consideration performance parameters such as parasitic inductance, parasitic capacitance, partial discharge, insulation approach and optimizes core material selection against competing power density and efficiency objectives. The leakage inductance is one of the most important factors. It determines the di/dt value of the converter current and thus the power transfer ratio and switch losses. The topology is based upon the 3-level, Neutral Point Clamped (NPC) Dual Active Bridge (DAB) converters on each side. The design would be for a single-phase system rated at 0.666MVA. In the first year of the project, several tasks have been performed. These tasks include: (i) Designed a robust controller for the multi-level converter stage in the DAB structure, to minimize RMS current that reduce power losses both in switches and transformer, (ii) Designed a MV-MF high power transformer, pushing the achievable boundaries of power throughput to > 0.5MVA, with a target of 0.667MVA, (iii) Developed a co-simulation multiphysics model to analyze loss and performance, (iv) Studied packaging, creepage distance, and thermal management for the system, (iv) Formulated VPP to reach an optimal design for power density, efficiency, and cost. In the second year of the project, the transformer will be built and tested. The thermal management of the system packaging for creepage/clearance distances will be completed. In addition, the model of the system will be developed considering leakage and parasitic capacitances for packaging. Finally, the virtual prototyping method will be applied to optimize the entire system for size, efficiency, and cost.