Coordinated overcurrent protection in distribution grids incorporating an active superconducting fault current limiter

 Corresponding author: Ali Taher Saber. Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. Coordinated overcurrent protection in distribution grids incorporating an active superconducting fault current limiter Ali Taher Saber 1, * and Aland Faraidoon Ismael 2 1 Human Resources Section, College of Education for Humanities, University of Kirkuk, Kirkuk, Iraq. 2 Independent Researcher in Electrical Engineering, Kirkuk, Iraq. Global Journal of Engineering and Technology Advances, 2025, 24(02), 175-180 Publication history: Received on 06 July 2025; revised on 12 August 2025; accepted on 15 August 2025 Article DOI: https://doi.org/10.30574/gjeta.2025.24.2.0244 Abstract The active saturated iron-core superconducting fault current limiter (SISFCL) is a promising technology for mitigating excessive fault currents in modern power distribution systems. This device is particularly valuable for integration into evolving smart grids, as it effectively suppresses fault currents during short-circuit events throughout the network. This paper offers a concise explanation of the SISFCL's working principles and its impedance behavior under fault conditions. The impact of integrating such a limiter on conventional inverse and fixed time-delay overcurrent protection schemes is examined. To ensure dependable protection performance in systems using SISFCLs, a method for coordinating protection settings is introduced. A model based on an actual 35 kV power distribution system with an integrated SISFCL was developed and evaluated using EMTDC/PSCAD software. The simulation outcomes confirm the soundness of the analytical framework. Index Terms—Active SFCL, saturated core, power distribution network, overcurrent relay, coordination, inverse-time protection. Keywords: Active SISFCL; Saturated Iron-Core; Power Distribution Network; Overcurrent Relay Coordination; Inverse-Time Protection 1. Introduction The rapid expansion in energy demand, along with the growing penetration of distributed energy resources within modern power networks, is driving the evolution of power distribution systems toward smarter, more integrated grid architectures [1]. One major challenge that arises is the elevated magnitude of fault currents, which threatens the security and reliability of these networks. Conventional techniques for limiting fault current typically introduce impedance during both normal and fault conditions, which may negatively affect voltage quality and system efficiency [2]. As an alternative, superconducting fault current limiters (SFCLs) have emerged as effective means to curb excessive fault current magnitudes [3]. The saturated iron-core SFCL (SISFCL), developed in the early 1980s, was among the earliest devices of this kind [4]. Unlike resistive SFCLs, SISFCLs do not rely on superconductor quenching, which simplifies their cryogenic requirements and reduces operational costs. Via the implementation of high-temperature superconducting (HTS) systems, SISFCLs can be made more compact and cost-effective [5]. Further advancements led to the development of the active SISFCL by incorporating control logic into the DC magnetization circuit [6], [7]. A fully functional active SISFCL unit has since been installed and tested within a live distribution grid [8]–[10]. Additionally, hybrid configurations that pair superconductors with traditional limiting reactors have been deployed in utility settings, where their effects on protective relays have been successfully validated [11], [12]. Despite these efforts, thorough coordination of protective relays—especially under conditions with active SISFCL integration—remains underexplored. In this study, we evaluate how inverse-time overcurrent protection schemes Global Journal of Engineering and Technology Advances, 2025, 24(02), 175-180 176 operate in the presence of an active SISFCL, and propose suitable coordination strategies to maintain reliable system operation. 2. Material and methods 2.1. Current-Limiting Behavior of the SISFCL The saturated iron-core superconducting fault current limiter (SISFCL) incorporates two magnetic cores as its primary magnetic components, alternating current (AC) windings, and a superconducting direct current (DC) coil connected to a magnetization control circuit. The structural schematic is illustrated in Figure 1. By regulating the DC bias current in the superconducting coil, the impedance of the AC windings exhibits a nonlinear behavior. To overcome several design challenges, including heavy structure, magnetic coupling issues between the AC and DC paths, and insufficient limiting capability, recent SISFCL designs feature optimized iron-core geometry and highspeed electronic switches. These improvements collectively define the “active SISFCL.” Figure 1 Schematic electrical circuit of an SISFCL Under normal operation, under the influence of the applied direct current, both magnetic cores are forced into deep magnetic saturation, which forces the magnetic fields close to their respective DC bias points. As a result, the inductances are significantly reduced, rendering the SISFCL nearly “invisible” to the power system, as shown in Figure 2(a). During a short-circuit event, deactivation of the high-speed switch in the direct current circuit occurs, eliminating the bias magnetic field and causing a sharp increase in the inductances, as illustrated in Figure 2(b). Accordingly, the SISFCL imposes a high impedance into the circuit, which inversely depends on the fault current magnitude, as presented in Figure 2(c). Global Journal of Engineering and Technology Advances, 2025, 24(02), 175-180 177 Figure 2 Illustrative comparison of SISFCL performance (a) passive configuration, (b) active configuration, and (c) impedance profile of a 35 kV SISFCL during current limitation 2.2. Coordinating Overcurrent Protection with SISFCL Integration Figure 3 outlines a typical radial power distribution layout. Fault current amplitudes tend to decrease with increasing distance from the power source. In such settings, inverse-time overcurrent relays are preferred, and their timing behavior follows a characteristic curve. Figure 3 Power disturbance typical configuration Coordination is structured as follows: for a fault at point F3, relay R3 responds first, causing circuit breaker CB3 to open. Upstream relay R2, guarding CB2, incorporates a coordination delay (commonly 0.3 to 0.5 seconds) to allow downstream tripping. Similarly, R1 delays its action behind R2 by another coordination interval. Figure 4 compares fault current profiles with and without SISFCL deployment. If the limiter is placed near the beginning of the feeder, the farther the fault, the more impedance is introduced by the SISFCL. The extremes of SISFCL impedance are denoted as minimum and maximum impedance values. The corresponding fault current profile is shown as a dashed line in Figure 4. Global Journal of Engineering and Technology Advances, 2025, 24(02), 175-180 178 From this, we can conclude • The relay pickup current is not altered by the SISFCL in standard operation, since the device presents minimal impedance. • When the SISFCL engages, it lowers fault currents and thus extends the relay tripping times, but still maintains time coordination among relays. • Coordination between the upstream substation relay and feeder protection (R1) must be reviewed against the altered fault current curve. • In practice, the altered curve may be hard to derive. As a workaround, engineers may adopt constant limiting values to retune relays, ensuring system-wide coordination. Figure 4 Three-phase fault current profiles at different bus locations B1, B2, and B3 2.3. Instantaneous and Definite-Time Overcurrent Settings Inverse-time schemes introduce longer delays for lower currents—hence, closer faults result in quicker operation. When a feeder is long and fault current drops with distance, instantaneous protection becomes effective, provided its pickup threshold is set above the highest fault level at the feeder end. With the SISFCL in place, fault magnitudes are curtailed, reducing the effective zone of instantaneous protection and maintaining selectivity. Unlike inverse-time relays, definite time-delay relays depend solely on fault magnitude. This may cause them to activate beyond their intended zone, particularly in the presence of an SISFCL. As a result, these settings must be re-evaluated to maintain correct operation. 3. Results and discussion The SISFCL effectively lowered fault levels across all locations. However, for definite time-delay protection, this broader impedance may extend the relay’s protected zone, requiring longer trip delays for proper coordination. The time adjustment (scaling) factors for each relay were determined. Relay operation times for faults at points F3 and F2 confirm that time margins between R2–R3 and R1–R2 exceed 0.3 seconds with the SISFCL installed, thus preserving coordination. Extensive simulation scenarios were conducted. The results show that inverse-time protection remains robust and is better suited than definite time-delay settings in systems utilizing active SISFCLs. Global Journal of Engineering and Technology Advances, 2025, 24(02), 175-180 179 Table 1 Operational Time Delays of Overcurrent Relays in Scenarios Without the Proposed SISFCL Inverse Time-delay Types Δt1-2 (Without SISFCL Δt2-3 (Without SISFCL) Δt1-2 (With SISFCL) Δt2-3 (With SISFCL) Normal Inverse Time 0.3 s 0.3 s 0.358 s 0.355 s Very Inverse Time 0.3 s 0.3 s 0.455 s 0.428 s Extreme Inverse Time 0.3 s 0.3 s 0.632 s 0.552 s Table 2 Operational Time Delays of Overcurrent Relays in Scenarios with the Proposed SISFCL Inverse Time-delay Types Fault Point t1 t2 t3 Normal Inverse Time-delay f1 0.488 s - - f2 0.638 s 0.338 s - f3 0.755 s 0.400 s 0.100 s Very Inverse Time-delay f1 0.271 s - - f2 0.569 s 0.269 s - f3 0.846 s 0.400 s 0.100 s Extreme Inverse Time-delay f1 0.124 s - - f2 0.497 s 0.197 s - f3 1.010 s 0.400 s 0.100 s Table 3 Coordination Times of Inverse Time-Delay Overcurrent Relays on the Feeder Inverse Time-delay Types Fault Point t1 t2 t3 Normal Inverse Time-delay f1 0.604 s - - f2 0.762 s 0.404 s - f3 0.893 s 0.473 s 0.118 s Very Inverse Time-delay f1 0.494 s - - f2 0.864 s 0.408 s - f3 1.208 s 0.571 s 0.143 s Extreme Inverse Time-delay f1 0.384 s - - f2 1.046 s 0.414 s - f3 1.860 s 0.737 s 0.184 s 4. Conclusion The active Superconducting Fault Current Limiter (SISFCL) offers a dynamic and effective solution for managing fault currents in contemporary distribution grids. Its impedance response, which diminishes with increasing fault current, ensures seamless integration without compromising normal system operation. Though it alters fault levels, our investigation confirms that inverse-time overcurrent relay coordination remains intact. Only the relay linking the substation and feeder may require recalibration. For definite time-delay relays, careful analysis of actual fault currents is essential to preserve sensitivity and selectivity when SISFCLs are deployed. Compliance with ethical standards. Global Journal of Engineering and Technology Advances, 2025, 24(02), 175-180 180 Compliance with ethical standards Acknowledgments The authors would like to thank all individuals who contributed to the completion of this research. 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