Coordinated differential protection for multimode operation in a future spherical tokamak fusion power plant

Tools

Yu, Shiyao and Zhang, Min and Hong, Qiteng and Sato, Eduardo and Antoniou, Ioannis (2026) Coordinated differential protection for multimode operation in a future spherical tokamak fusion power plant. IEEE Transactions on Plasma Science, 54 (6). pp. 2691-2697. ISSN 1939-9375 (https://doi.org/10.1109/tps.2026.3693938)

[thumbnail of Yu-etal-2026-Coordinated-differential-protection-for-multimode-operation-in-a-future-spherical-tokamak-fusion-power-plant]
Preview
 Text. Filename: Yu-etal-2026-Coordinated-differential-protection-for-multimode-operation-in-a-future-spherical-tokamak-fusion-power-plant.pdf
Accepted Author Manuscript
License: Creative Commons Attribution 4.0 logo
Download (1MB)| Preview

Abstract

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.

ORCID iDs

Yu, Shiyao, Zhang, Min ORCID logoORCID: https://orcid.org/0000-0003-4296-7730, Hong, Qiteng ORCID logoORCID: https://orcid.org/0000-0001-9122-1981, Sato, Eduardo and Antoniou, Ioannis;
CORE (COnnecting REpositories)