Ioannis Antoniou

Spherical tokamak for energy production (STEP) is an ambitious program to design and build a prototype fusion energy plant in West Burton (Nottinghamshire) that can demonstrate the delivery of net electricity to the electrical grid. Its ultimate goal is to pave the way for the commercial viability of fusion technology (Waldonetal., 2024). Fusion power plants, like STEP, have an array of large parasitic electrical loads, which are necessary to initiate and sustain the fusion reactions. These loads are typically much larger and more dynamic than the ones found in conventional power plants. STEP is planning to supply these loads using power sourced from the electricity transmission grid and/or some form of an on-site electrical energy storage system (ESS). The exact split between the two power sources will be dependent on the future electrical characteristics of the 400-kV grid connection at the West Burton site. STEP’s electrical distribution network is expected to experience periods in which rapid electrical loading and unloading will occur. These power dynamics are caused by the large number of power converters embedded within the electrical plant, which support plasma control within the tokamak. The majority of these fast power transients are expected to be managed by an electrical ESS. This is necessary in order not to breach the limits and constraints imposed upon STEP by the respective industry codes (e.g., grid code). In the future, it may be beneficial to dedicate some ESS to maintaining plant voltage levels within acceptable limits and providing reactive power support across a wide range of load dynamics. Furthermore, like in conventional power stations, the ESS could provide back-up power during emergency events. This work presents STEP’s electrical energy storage requirements and explores the current and future technology landscapes, considering the solutions that are best suited for the support of STEP’s future operations.

Commercial deployment of the Spherical Tokamak for Energy Production (STEP) requires a reliable protection philosophy for its prototype power plant to prevent system faults from damaging reactor-related components. However, relay protection for the STEP prototype power plant (SPP) microgrid has not yet been specifically addressed. This article develops a coordinated differential protection scheme tailored to the SPP’s multimode operation and hybrid sources. The scheme augments slope-biased current differential protection with a phase-angle-superimposed term derived from two-terminal phasors and employs current restraint for inrush-related disturbances and time synchronization mismatch. Mode-dependent bias curves and pickup thresholds are used to accommodate bidirectional power flow and changing fault levels. The scheme is evaluated on an RTDS/RSCAD model based on U.K. Fusion Energy (UKFE) SPP data, U.K. Grid Code provisions, and IAEA Safety Standards No. SSG-34 credible fault scenarios, covering multiple fault types, locations, impedances, and operating modes. Compared with conventional slope-biased differential protection, the proposed scheme improves the detection of high-impedance internal faults while maintaining external-fault restraint. These results address a protection gap for fusion power plants and support the grid-facing viability of STEP.

The Spherical Tokamak for Energy Production (STEP) prototype powerplant (SPP) will be a first-of-a-kind powerplant—its prime objective is to export electrical power, to the national power transmission system (‘grid’), above 100 MWe. As part of a wider issue, addressing the STEP concept design, this article seeks to explore how electrical power will be generated from a spherical tokamak heat source. Accordingly, the following key functions of the SPP power infrastructure are reviewed. Cooling the tokamak : cooling the tokamak while extracting useful thermal energy. Generating power : conversion of thermal energy to electrical energy (power generation). Managing energy : management of the site-wide distribution, storage and energy export. In each of these areas, the design scope, challenges and solution spaces have been discussed. This has shaped the design of the SPP power infrastructure, which in turn has ensured a powerplant design focused on operability and performance. Furthermore, it has been demonstrated that the SPP will achieve its prime objective in generating net power, which is enabled by a unique power infrastructure. Confidence in the ability to generate net power will be refined as the design matures. Finally, this article recommends key opportunities that STEP could use to improve power generation and reduce the parasitic load of the SPP. This article is part of the theme issue ‘Delivering Fusion Energy – The Spherical Tokamak for Energy Production (STEP)’.