Differential protection is the unit protection system which is applied to protect a particular unit of power systems. Unit is known as zone in protection terminology which is equivalent to simple electrical node. In recent time, low impedance current differential protection schemes based on percentage restrained characteristics are widely used in power systems to protect busbar systems. The main application issue of these schemes is mis-operation due to current transformer (CT) saturation during close-in external faults. Researchers have suggested various solution of this problem; however, individually they are not sufficient to puzzle out all mis-operational scenarios. This thesis presents a new bus differential algorithm by defining alternative partial operating current characteristics of a differential protection zone and investigating its performance for all practical bus faults. Mathematical model of partial operating current and operating principle of the proposed bus differential relay are described in details. A CT saturation detection algorithm which includes fast and late CT saturation detection techniques is incorporated in relay design to increase the sensitivity of partial operating current based internal-external fault discriminator for high impedance internal faults. Performance of the proposed relay is validated by an extensive test considering all possible fault scenarios.

Current differential protection principle is superior in terms of sensitivity and speed of operation in comparison with other protection principle used in power systems. From the last five decades, various current differential protection schemes are widely used to protect busbars, transformers, and short-transmission lines. The deployment of high capacity microwave and optical fiber technologies redefined the line protection systems by facilitating the use of current differential protection schemes for long transmission lines. The common application issue of these schemes is mis-operation due to current transformer (CT) saturation during close-in external faults. Moreover, transformer differential protection schemes face mis-trip due to inrush current during energization. The techniques presented in the literature to address those issues, de-sensitize protection function and increase the time of operation. A comprehensive fault discrimination algorithm and an inrush current detection algorithm are highly demanded for current differential protection schemes. The purpose of this dissertation is to optimize the performance of differential schemes applied to protect busbar, transformer and line. This research derives the mathematical model of saturated secondary current of CT and introduces the concept of Partial Operating Current (POC). Based on these mathematical developments, the characteristics of POC are identified for all three types of differential zones like busbar, transformer and line protection. A new inrush current blocking algorithm is developed for transformer differential protection. A new time-domain CT saturation detection algorithm is also proposed. Based on these new developments, three separate differential schemes are designed for busbar, transformer, and line protection, respectively. The proposed schemes provide complete immunity against the mis-operations due to CT saturation during close-in external faults and transformer inrush current without sacrificing the sensitivity for internal faults. The speed of operation is also improved. The model for each scheme is built in Matlab platform and the performance is validated using the test system simulated in Electro-Magnetic Transient Program (EMTP) for all possible fault scenarios. Documented results show the improved performance of the proposed schemes when compared to traditional differential schemes in terms of reliability, sensitivity, selectivity, and speed

The requirement for reliability and security in power system is increasing every passingday with the increase in complexity of the power system. To ensure highest level ofreliability, protection relays have to receive correct measurements. One of the most importantmeasurement that is needed by a relay as an input is current. However, currentmeasurements received from current transformer (CT) can become inaccurate due to aphenomenon called CT saturation. This Master's thesis objective is to build a novel algorithmfor the protection relays to detect CT saturation. The algorithm should be basedon a stand-alone method that is able to detect CT saturation within 1-2 ms for a samplingfrequency of 4 kHz.This project comprises a study of the background about CTs and CT saturation. Therelated work done to detect CT saturation is also studied. Later, major existing methodsused in the industry to deal with CT saturation are investigated in detail and modelledin Simulink. A novel method is proposed to exclusively detect CT saturation, keeping inmind the strict requirements, set in the beginning of the project. The proposed methodis implemented in Simulink and HiDraw(in-house software in ABB to create C code forprotection functions). Lastly, the proposed method and the existing methods are testedin Simulink for more than 1300 test cases based on dierent power system conditions,IEC-60255-187-1 and real current recordings from relays.The results of the tests showed that the proposed method successfully detect CT saturationand was better than the existing methods in terms of speed and accuracy. It was concludedthat the proposed method successfully detects CT saturation and hence, can be used withany protection function in a relay where there is a need to detect CT saturation. / Kraven pa tillforlitlighet och sakerhet i kraftsystem okar varje dag i samband med allthogre komplexitet i kraftsystemet. For att uppna den hogsta nivan av tillforlitlighetbehover relaskydden ta emot korrekta matvarden fran kraftsystemet. Ett av de mest viktigamatvardena som behovs for ett relaskydd ar strom. Emellertid, kan matvarden franen stromtransformator bli felaktiga pagrund av ett fenomen som kallas stromtransformatormattning. Malet for detta examensarbete ar att skapa en ny algoritm for relaskydd somdetekterar stromtransformatormattning. Algoritmen ska baseras pa en fristaende metodsom klarar av att detektera stromtransformatormattning inom 1-2 ms for en samplingsfrekvenspa 4 kHz.Detta projekt omfattar, forutom en studie av hur stromtransformatormattning kan detekteras,aven en bakgrundsstudie om stromtransformatorer och stromtransformatormattning.Viktigare existerande metoder, som anvands i industrin for att detektera stromtransformatormattning, undersoks i detalj och modelleras i Simulink. En ny metod foreslas foratt exklusivt detektera stromtransformatormattning med hansyn till de strikta krav somfaststalldes i borjan av projektet. Den foreslagna metoden ar implementerad i Simulinkoch i HiDraw (en intern ABB mjukvara for att skapa C-kod for skyddsfunktioner). Slutligenar den nya foreslagna metoden och de existerande metoderna testade i Simulinkmed er an 1300 testfall baserade paolika forhallanden i kraftsystemet, IEC-60255-187-1standarden och med verkliga inspelningar av strom gjorda av relaskydd i drift.Resultatet av testerna visar att den nya foreslagna metoden framgangsrikt detekterarstromtransformatormattning och gor det battre an existerande metoder med avseende pasnabbhet och noggrannhet. Det konstaterades att den nya foreslagna metoden framgangsriktdetekterar stromtransformatormattning och darfor kan anvandas for vilken relaskyddsfunktionsom helst i ett relaskydd dar behovet av att detektera stromtransformatormattningnns.