Contents lists available at ScienceDirect Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr Design of an unsaturated core-based fault current limiter to tackle unsymmetrical faults Alireza Pirhadi ⁎ , Hossein Shayan, Mohammad Tavakoli Bina Faculty of electrical engineering, K.N. Toosi University of technology, Tehran, Iran ARTICLEINFO Keywords: Fault current limiter Unsaturated core Unsymmetrical faults Thyristor controlled braking resistor Over-voltage reduction Sequence networks ABSTRACT Duetoincreasingdemandforelectricalpowerinnowadayspowergrids,inevitably,theshortcircuitcurrentwill be increased. The power system faults are categorized in two groups: frst, the symmetrical three-phase faults which generally caused by the carelessness of operating personnel and second, unsymmetrical faults which are verycontingentintransmissionsystem.Thelatteristheprincipalaimofthispaper.FaultCurrentLimiter(FCL) isadevicetolimitthefaultcurrent.Conventionalcore-basedFCLshavesomedrawbackssuchas:(1)corelosses and heat generation caused by permanent operation in saturation region under normal condition, (2) requiring two cores per phase that makes FCL bulky and (3) extra cost, losses as well as reducing reliability by using externalDCsource.Thispaperproposesanovelmethodforlimitingunsymmetricalfaultscurrent.Thesuggested designemploysacommonunsaturatedcoreforallthreephasesinordertolimitofthe frstcycleoffaultcurrent automatically and without fast detection systems; employment of thyristor controlled braking resistor (TCBR) is considered for eliminating the imposed over-voltages on faultless lines and further limitation of fault current. Simulations and design aspects of implementing practical issues show the validity of the proposal. 1. Introduction Power grids, especially high voltage (HV) power systems, usually introduce high short circuit current (i.e. fault current) which is mainly due to unsymmetrical faults. Such a high short circuit current has de- structive thermal efects depending on RMS value of the fault current andmechanicalefectsonequipmentdependingonthepeakofthefault current which includes both DC and AC components. Short circuits can also afect power system stability [1]. Moreover, due to high short circuitcurrentandequivalentinductanceoftransmissionlines,opening circuit breakers (CB) induces high L di dt (i.e. transient voltage due to current changes) on the CBs under these condition, capable of de- stroying equipment too [2]. The fault current limiter (FCL) is the main well-known solution to limit the short circuit currents and reduce de- structiveconsequences.TheFCLisanapparatuswhichremainsinactive during normal condition (i.e. having no efect under the normal op- eration of power grids) while inserts a series impedance to the line for limiting the fault current in the presence of short circuit faults. This basic idea of FCLs have been developed through signifcant research work [3–8]. Typical main ideas of researches on the FCL include the saturated core FCLs [4,9], high temperature superconducting FCLs (HTS-FCL) [6,10], solid state FCLs (SSFCL) [3], hybrid FCLs [7] among many others. FCLs are mainly classifed in three groups; passive FCLs, solid-state FCLs and hybrid FCLs [5]. Among three aforementioned groups, saturated core FCLs, as part of passive group, have attracted many attentions. Diferent magnetic confguration in the literatures have been suggested [4,11]. Conventional saturated core FCLs use two magnetic core per phase which both of them are saturated under normal conditions by using DC current source. This introduces low impedancetothetransmissionline [4].Underfaultconditions,positive cycle of fault current will run one of the cores to the deep saturation region(i.e.thiscoredoesnotinsertanyimpedancetotheline)andthe other core to the linear region (i.e. this core inserts considerable im- pedance into the line). Negative cycle of fault current substitute the core saturation (linear) mode with linear (saturation) mode. A three phase conventional FCL usually consists of three single- phase FCL units. The main advantages of passive FCLs in comparison withothertypesarenoneedtofaultdetectionandcapableofhandling several consecutive faults. However, they show certain drawbacks such as large amount of material and related high cost [5], the induced overvoltageacrosstheDCcurrentsourceduringafaultinterval [5],and low reliability due to DC current source interruption (i.e. DC source interruptionforcestheFCLtobeoperatedinlinearregionofB-Hcurve and it means the FCL insert a high impedance to the line under normal conditions). Power system faults are divided into symmetrical and https://doi.org/10.1016/j.epsr.2020.106482 Received 30 December 2019; Received in revised form 7 June 2020; Accepted 16 June 2020 ⁎ Corresponding author. E-mail addresses: pirhadi@email.kntu.ac.ir (A. Pirhadi), shayan@email.kntu.ac.ir (H. Shayan), tavakoli@eetd.kntu.ac.ir (M.T. Bina). Electric Power Systems Research 187 (2020) 106482 Available online 07 July 2020 0378-7796/ © 2020 Elsevier B.V. All rights reserved. T