Emerging and Disruptive Applications of Solid-State-Transformers

Abstract

The power distribution system is going through a brisk transformation with a higher penetration of renewable energy sources, battery energy storage and DC loads like electric vehicles and data-centres. This has necessitated increasing power electronics based control of power flow within the distribution grid. Solid-state transformer (SST) is an emerging power electronic technology, also referred to as power electronic transformers, energy routers, etc., which is a cascaded combination of power converter cells with medium-frequency isolation. SSTs are used to interface a medium-voltage (MV) network (e.g. the utility distribution grid) with a low-voltage (LV) network (e.g. a microgrid). It has been shown in recent literature that for MVAC-LVDC applications (e.g. where solar energy or battery integration is required), SSTs have proven to be a competitive technology, in terms of efficiency, power-density and cost, compared to the on-load tap-changer (OLTC) transformers. Advancements in wide band-gap semiconductor technologies and passive components have led to further improvements in SSTs, making them endeavour in newer applications in power conversion. Mission critical applications such as subsea and space power distribution systems had traditionally avoided SSTs due to reliability concerns. However, with better semiconductor devices and prognostic health monitoring, even such applications are slowly opening up to SSTs. This tutorial will focus on the emerging applications of SSTs and the state-of-the-art research and developments in these domains. The tutorial will start off with a brief introduction of the SST technology and its potential in future smart-grid paradigm, as well as provide a succinct overview of the various SST topologies. Secondly, the discussion will shift to modelling, control aspects and reliability of SSTs for grid-connected applications. The practical implementation aspect of a grid-connected SST, based on industrial design and product requirements, will also be discussed in detail. Thirdly, an emerging application of SST for public electric-vehicle (EV) charging station and its implementation aspect would be discussed, so that the expectations of mimicking an existing public refuelling station can be met. In continuation to this, a multi-objective optimal design strategy for the grid-connected SST technology will be discussed along with the merits of such a design optimization technique. Subsequently, the prospective of SST technology for mission critical applications like power distribution systems for subsea applications and on lunar surface will be discussed. Here, the merits of high efficiency and power density for SSTs based on wide-bandgap semiconductor devices will be highlighted that can be leveraged to provide competitive solutions in comparison with the conventional low-frequency transformer based solutions. Lastly, the tutorial will conclude with the main takeaway message and the potential areas where SST technology has the capability to dominate in the near future.