University of New Orleans ScholarWorks@UNO University of New Orleans Theses and Dissertations and Theses Dissertations Fall 12-16-2016 Fault Discrimination Algorithm for Busbar Differential Protection Relaying Using Partial Operating Current Characteristics Monir Hossain University of New Orleans, mhossai2@uno.edu Follow this and additional works at: https://scholarworks.edu/td Part of the Power and Energy Commons Recommended Citation Hossain, Monir, "Fault Discrimination Algorithm for Busbar Differential Protection Relaying Using Partial Operating Current Characteristics" (2016). University of New Orleans Theses and Dissertations.edu/td/2263 This Thesis is protected by copyright and/or related rights. It has been brought to you by ScholarWorks@UNO with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use.
For other uses you need to obtain permission from the rights- holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/or on the work itself. This Thesis has been accepted for inclusion in University of New Orleans Theses and Dissertations by an authorized administrator of ScholarWorks@UNO. For more information, please contact scholarworks@uno. Fault Discrimination Algorithm for Busbar Differential Protection Relaying Using Partial Operating Current Characteristics A Thesis Submitted to Graduate Faculty of the University of New Orleans in partial fulfillment of the requirements for the degree of Master of Science in Engineering Electrical Engineering By Monir Hossain University of New Orleans December, 2016 Acknowledgement First of all, I would like to express my utmost gratitude to my academic and research advisor Dr.
Parviz Rastgoufard for his guidance and constant support to conduct and accomplish this thesis. I would also like to express deepest appreciation to the members of the supervisory committee, Dr. Ittiphong Leevongwat and Dr. Ebrahim Amiri for their enormous support throughout my masters program to complete this work.
Finally, I like to wish acknowledge and thank to my parents, my wife and all of my friends for their encouragement and moral support to finish this work. ii Table of Contents List of Figures v List of Table viii Abstract ix 1 Introduction 1 1.1 Overview of Power System 1 1.2 Overview of Power System Protection 2 1.3 Overview of Busbar Protection 5 1.4 Low Impedance Differential Protection: CT Saturation Issues 10 1.5 Literature Review of Low Impedance Differential Protection 11 1.6 Current Transformer (CT) Saturation 16 1.7 Literature Review of CT Saturation Detection 18 1.8 Scope of Thesis 20 2 Mathematical Modeling 22 2.1 Differential Protection Principle 22 2.1 Basic of Differential Protection 22 2.2 Restrained Differential Protection 26 2.2 Mathematical Modeling of CT Saturation 28 2.3 Existing Methods to Discriminate Internal and External Faults 35 2.1 Phase Angle Comparison Method 35 2.2 Differential Rate of Change Method 37 3 Thesis Contribution 39 3.1 Problem Statement: Difficulties to Discriminate Faults 39 3.3 Mathematical Model of Partial Operating Current and Proposed Algorithm 40 3.1 Mathematical Model of Partial Operating Current 41 3.1 Data Processor 51 iii 3.2 CT Saturation Detection Algorithm 51 3.3 Trip Logic Unit 53 4 Test System 55 4.1 Description of the Test System 55 4.2 Transmission Line Data 55 4.4 Load Data 57 5 Simulation and Results 58 5.1 System Modeling at EMTP 58 5.2 Relay Modeling at Matlab 61 5.4 Results and Discussion 64 6 Concluding Remarks and Future Research 90 Bibliography 93 Vita 97 iv List of Figures Figure 2.1 Electrical node or junction 23 Figure 2.2 Two terminal zone under normal condition 24 Figure 2.3 Two terminal zone under fault condition 24 Figure 2.4 Multi terminal zone under normal condition 25 Figure 2.5 Multi terminal zone under fault condition 25 Figure 2.6 Characteristics curve of double slope restrained differential relay 27 Figure 2.7 CT circuit model 28 Figure 2.8 CT excitation curve 29 Figure 2.9 Method of determining the parameters Vs and S 29 Figure 2.10 Postulated instantaneous values saturation curve 30 Figure 2.11 Comparison of the rms/peak relationship for two wave shapes 32 Figure 2.12 Definition of per unit remanence 34 Figure 2.13 External Fault scenario 36 Figure 2.14 Internal Fault scenario 36 Figure 2.15 Trajectory of 𝐼𝑜𝑝 and 𝐼𝑟 37 Figure 2.16 Logic Diagram of Differential Rate of Change Method 38 Figure 3.1 Single phase representation of a typical multi terminal protection zone 41 Figure 3.2 Normal operating condition 42 Figure 3.3 Phasor diagram in normal operational condition 43 Figure 3.4 Internal fault condition 44 Figure 3.5 Phasor diagram during internal fault condition 45 Figure 3.6 External fault condition 46 Figure 3.7 Phasor diagram during external fault condition without CT saturation 46 Figure 3.8 Phasor diagram during external fault condition with CT saturation 47 Figure 3.9 Flow chart of proposed algorithm 49 Figure 3.10 Block diagram of proposed relay 50 Figure 3.11 CT saturation detection algorithm 52 Figure 3.12 Fast CT saturation detection algorithm 52 v Figure 3.13 Trajectory of operating and restrained current 53 Figure 3.14 Trip logic diagram 54 Figure 4.1 Three bus test system 55 Figure 5.1 EMTP model of three bus test system 59 Figure 5.2 EMTP current transformer (CT) model 60 Figure 5.3 CT secondary currents for LG internal fault 65 Figure 5.4 Responses from proposed method for LG internal fault 65 Figure 5.5 Comparative results for LG internal fault 66 Figure 5.6 CT secondary currents for LL internal fault 67 Figure 5.7 Responses from proposed method for LL internal fault 67 Figure 5.8 Comparative results for LL internal fault 68 Figure 5.9 CT secondary currents for LLG internal fault 69 Figure 5.10 Responses from proposed method for LLG internal fault 69 Figure 5.11 Comparative results for LLG internal fault 70 Figure 5.12 CT secondary currents for LLL internal fault 71 Figure 5.13 Responses from proposed method for LLL internal fault 71 Figure 5.14 Comparative results for LLL internal fault 72 Figure 5.15 CT secondary currents for LLLG internal fault 73 Figure 5.16 Responses from proposed method for LLLG internal fault 73 Figure 5.17 Comparative results for LLLG internal fault 74 Figure 5.18 CT secondary currents for LG high impedance internal fault 75 Figure 5.19 Responses from proposed method for LG high impedance internal fault 76 Figure 5.20 Comparative results for LG high impedance internal fault 76 Figure 5.21 CT secondary currents for LL high impedance internal fault 77 Figure 5.22 Responses from proposed method for LL high impedance internal fault 78 Figure 5.23 Comparative results for LL high impedance internal fault 78 Figure 5.24 CT secondary currents for LG external fault 80 Figure 5.25 Responses from proposed method for LG external fault 80 Figure 5.26 Comparative results for LG external fault 81 vi Figure 5.27 CT secondary currents for LL internal fault 82 Figure 5.28 Responses from proposed method for LL internal fault 82 Figure 5.29 Comparative results for LLG internal fault 83 Figure 5.30 CT secondary currents for LLG internal fault 84 Figure 5.31 Responses from proposed method for LLG internal fault 84 Figure 5.32 Comparative results for LLG internal fault 85 Figure 5.33 CT secondary currents for LLL internal fault 86 Figure 5.34 Responses from proposed method for LLL internal fault 86 Figure 5.35 Comparative results for LLL internal fault 87 Figure 5.36 CT secondary currents for LL internal fault 88 Figure 5.37 Responses from proposed method for LL internal fault 88 Figure 5.38 Comparative results for LL internal fault 89 vii List of Tables Table 3.1 The truth table for trip logic 53 Table 4.1 Transmission line data 56 Table 4.2 Transmission line tower configuration 56 Table 4.3 Generator data 57 Table 4.4 Load data 57 Table 5.1 CT parameters 60 Table 5.2 CT Ψ-I characteristic 61 Table 5.3 Settings for proposed relay 62 Table 5.4 List of bus faults 63 Table 6.1 Comparative summary of the results 91 viii Abstract 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. Keywords: Differential Protection; CT Saturation; Internal Fault; External Fault; Fault Discrimination; Relay. Introduction In Chapter 1 we provide a general idea about a power system and its protection, especially the bus protection.
Various differential protection schemes are used in modern power systems. Particularly, for bus protection, low impedance differential protection is very popular and effective [1]. However, current transformer (CT) saturation has a severe impact on the performance of low impedance differential protection. The overview of current transformer (CT) saturation and historical review of low impedance bus differential protection as well as current transformer (CT) saturation are presented.
After extensive historical review of existing methods, the outline of this thesis is provided in Section 1.1 Overview of Power System Modern power systems are the combination of various complex elements such as generators, transformers, transmission lines, loads and protection and control equipments. Generally, power systems are divided into three stages: generation, transmission and distribution. The most convenient method to generate electricity is to burn fossil fuels to convert water into steam which is used to rotate a turbine that is connected to the rotor shaft of an electric generator. Water is also used to turn generators in hydro-electric power plant.
In the last few decades, various new sources of electricity has been introduced which is called renewable energy such as solar, wind, geothermal and biomass etc. In all cases, the electricity generated at these facilities flows across the transmission system. Voltage at the generating stage is normally low, and hence, the generated voltage is raised by using step-up transformers 1 to transmit power over long distance to reduce the higher voltage level transmission loss by reducing current. At the end of transmission system, voltage is stepped down by using step down transformer for power flow through distribution system and for supplying to residential and commercial customers.
The primary goal of any electric power utility is to provide uninterrupted power to the end consumer, and to achieve the goal, electric utilities depend on protection systems to provide protection to power systems equipment and elements such as generators, transformers, bus bars, overhead transmission lines operating in abnormal or fault conditions. Most important criteria of power systems are the balance between generation and demand and to maintain the balance, utilities all over the world use various control systems such as supervisory control and data acquisition (SCADA) system and automatic generation control (AGC) system.2 Overview of Power System Protection The main purpose of a power system protection is to isolate a faulty section of the electrical power system from rest of the healthy systems so that the remaining live portion can function satisfactorily without any severe damage due to fault current [1]. Identification fault and isolating faulty part from the remaining healthy systems to secure the continuation of power supply are not straightforward. The elementary power system protective device is the fuse.
When the current through a fuse exceeds a certain threshold, the fuse element melts and produces an arc across the resulting gap that is then extinguished to interrupt the circuit [2].