Switchgear and Protection lecture notes

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2015 SWITCHGEAR AND PROTECTIVE DEVICES Lecture Notes on SWITCH GEAR AND PROTECTIVE DEVICES TH 7 SEMESTER BTECH (EE) SUBJECT CODE: BEE 1701 VEER SURENDRA SAI UNIVERSITY OF TECHNOLOGY Department of Electrical Engineering Burla, Sambalpur, Odisha 768018 www.vssut.ac.in 1 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES DISCLAIMER This document does not claim any originality and cannot be used as a substitute for prescribed textbooks. The information presented here is merely a collection by the committee members for their respective teaching assignments. Various sources as mentioned at the end of the document as well as freely available material from internet were consulted for preparing this document. The ownership of the information lies with the respective authors or institutions. Further, this document is not intended to be used for commercial purpose and the committee members are not accountable for any issues, legal or otherwise, arising out of the use of this document. The committee members make no representations or warranties with respect to the accuracy or completeness of the contents of this document and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. The committee members shall not be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. 2 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES SYLLABUS SWITCHGEAR AND PROTECTIVE DEVICES (3-1-0) MODULE-I (10 HOURS) Protective Devices: Philosophy of protection, Nature, Causes and consequences of faults, Zone of protection, Requirements of a protective scheme, Basic terminology components of protection scheme. Relay classification, Principle of different types of electromagnetic relay. General equation of phase and magnitude comparators, Duality of comparators, Electromagnetic relays, over current relays Directional relays, Distance relay- impedance, Reactance and Mho type, Differential relays. MODULE-II (10 HOURS) Feeder: Protection: Over current and earth fault protection, Distance protection, Pilot wire protection, Carrier current protection. Generator Protection: Biased differential protection, restricted earth fault protection, Field suppression, Negative sequence protection, Earth fault detection in rotor circuit Power transformer Protection: Biased differential protections, restricted earth fault protection, Buchholz relay Protection of combined transformer and alternator. Bus Zone Protection: frame leakage and circulating current scheme-use of Translay relay. MODULE-III (10 HOURS) Circuit Breakers: Formation of arc during circuit breaking. Characteristics of electric arc. Theories of arc Interruption. Recovery and restriking voltage, interruption of capacitive and inductive currents. Current chopping. Principle of A.C. and D.C. circuit breaking requirements of good circuit breaker circuit breaker rating. Different types of circuit breakers. Air break and Air blast circuit breaker. Plain break and controlled break all circuit breakers. Minimum oil circuit breakers. Vacuum circuit breaker, SF6 circuit breaker. D.C. Circuit breaker. H.R.C. Fuse: Construction and characteristics MODULE-IV (10 HOURS) Static Relays : Development and classification of static relays, Different types of phase and amplitude capacitors, Basic static relays used in protective scheme, Elementary idea about digital & numerical protection. Testing and maintenance of protective gear, Protection against surge-surge absorber, Surge- diverter. Arrangement of Bus bar, Circuit breaker and isolator. Current limiting reactors in power system and their arrangement calculation of fault MVA for symmetrical short circuits. Circuit breaker capacity. 3 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES BOOKS 1. Van C Warrington, “Protective Relays” Vol.-I & II 2. Ravindranath, M.Chander, “Power System Protection and SwitchGear”, Wiley Eastern Ltd. New Delhi 3. T S Madhav Rao, “Power System Protection”, TMH Pulication 4. Sunil S.Rao, “Switch Gear and Protection”, Khanna Publication 4 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES MODULE-I Lesson Plan 1.1Fundamentals of Power System Protection 1.2 Consequences of occurrence of fault 1.3 Zones of Protection 1.4 Protection System Requirements and some basic terminologies used 1.5 Classification and principle of different types of electromagnetic relays 1.6 Over current relays 1.7 Directional relay 1.8 Distance relay 1.9 Differential relay 1.10 General equation of Comparators and the duality principle 5 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES 1.1 Fundamentals of Power System Protection The purpose of an Electric Power System is to generate and supply electrical energy to consumers. The power system should be designed and managed to deliver this energy to the utilization points with both reliability and economically The capital investment involved in power system for the generation, transmission and distribution is so great that the proper precautions must be taken to ensure that the equipment not only operates as nearly as possible to peak efficiency, but also must be protected from accidents The normal path of the electric current is from the power source through copper (or aluminium) conductors in generators, transformers and transmission lines to the load and it is confined to this path by insulation. The insulation, however, may break down, either by the effect of temperature and age or by a physical accident, so that the current then follows an abnormal path generally known as Short Circuit or Fault • Any abnormal operating state of a power system is known as FAULT. Faults in general consist of short circuits as well as open circuits. Open circuit faults are less frequent than short circuit faults, and often they are transformed in to short circuits by subsequent events. 6 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES 1.2 Consequences of occurrence of Faults Faults are of two type  Short circuit fault- current  Open circuit fault- voltage In terms of seriousness of consequences of a fault , short circuits are of far greater concern than open circuits, although some open circuits present some potential hazards to personnel Classification of short circuited Faults • Three phase faults (with or without earth connection) • Two phase faults (with or without earth connection) • Single phase to earth faults Classification of Open Circuit Faults • Single Phase open Circuit • Two phase open circuit • Three phase open circuit Consequences • Damage to the equipment due to abnormally large and unbalanced currents and low voltages produced by the short circuits • Explosions may occur in the equipments which have insulating oil, particularly during short circuits. This may result in fire and hazardous conditions to personnel and equipments • Individual generators with reduced voltage in a power station or a group of generators operating at low voltage may lead to loss of synchronism, subsequently resulting in islanding. 7 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES • Risk of synchronous motors in large industrial premises falling out of step and tripping out. The general layout of a protection system may be viewed as given in the following figure 1.3 Zones and types of Protection system 1.3.1 Zones of Protection system • An electric power system is divided into several zones of protection. Each zone of protection, contains one or more components of a power system in addition to two circuit breakers. • When a fault occurs within the boundary of a particular zone, then the protection system responsible for the protection of the zone acts to isolate (by tripping the Circuit Breakers) every equipment within that zone from the rest of the system. • The circuit Breakers are inserted between the component of the zone and the rest of the power system. Thus, the location of the circuit breaker helps to define the boundaries of the zones of protection. 8 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES • Different neighbouring zones of protection are made to overlap each other, which ensure that no part of the power system remains without protection. However, occurrence of the fault with in the overlapped region will initiate a tripping sequence of different circuit breakers so that the minimum necessary to disconnect the faulty element 1.3.2 Types of Protection (Primary and Back-up Protection) 9 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES 1.3.2.1 Primary Protection – The primary protection scheme ensures fast and selective clearing of any fault within the boundaries of the circuit element, that the zone is required to protect. Primary Protection as a rule is provided for each section of an electrical installation. However, the primary protection may fail. The primary cause of failure of the Primary Protection system are enumerated below. 1. Current or voltage supply to the relay. 2. D.C. tripping voltage supply 3. Protective relays 4. Tripping circuit 5. Circuit Breaker 1.3.2.2 Back-up Protection Back-up protection is the name given to a protection which backs the primary protection whenever the later fails in operation. The back-up protection by definition is slower than the primary protection system. The design of the back-up protection needs to be coordinated with the design of the primary protection and essentially it is the second line of defence after the primary protection system. 1.4 Protection System Requirements and some basic terminologies used • The fundamental requirements for a protection system are as follows: 1.4.1 Reliability: It is the ability of the protection system to operate correctly. The reliability feature has two basic elements, which are dependability and security. The dependability feature demands the certainty of a correct operation of the designed system, on occurrence of any fault. Similarly, the security feature can be defined as the ability of the designed system to avoid incorrect operation during faults. A comprehensive statistical 10 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES method based reliability study is required before the protection system may be commissioned. The factors which affect this feature of any protection system depends on some of the following few factors. a) Quality of Component used b) Maintenance schedule c) The supply and availability of spare parts and stocks d) The design principle e) Electrical and mechanical stress to which the protected part of the system is subjected to. 1.4.2 Speed: Minimum operating time to clear a fault in order to avoid damage to equipment. The speed of the protection system consists primarily of two time intervals of interest. a) The Relay Time : This is the time between the instant of occurrence of the fault to the instant at which the relay contacts open. b) The Breaker Time: This is the time between the instant of closing of relay contacts to the instant of final arc extinction inside the medium and removal of the fault. 1.4.3 Selectivity: This feature aims at maintaining the continuity of supply system by disconnecting the minimum section of the network necessary to isolate the fault. The property of selective tripping is also known as “discrimination”. This is the reason for which the entire system is divided into several protective zones so that minimum protion of network is isolated with accuracy. Two examples of utilization of this feature in a relaying scheme are as follows a) Time graded systems b) Unit systems 1.4.4. Sensitivity: The sensitivity of a relay refers to the smallest value of the actuating quantity at which the relay operates detecting any abnormal condition. In case of an overcurrent 11 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES relay, mathematically this can be defined as the ratio between the short circuit fault current (I ) and the relay operating current (I ). The value of I , should not be too small or large so s o o that the relay is either too sensitive or slow in responding. 1.4.5 Stability: It is the quality of any protection system to remain stable within a set of defined operating scenarios and procedures. For example the biased differential scheme of differential protection is more stable towards switching transients compared to the more simple and basic Merz Price scheme in differential protection 1.4.6 Adequacy: It is economically unviable to have a 100% protection of the entire system in concern. Therefore, the cost of the designed protection system varies with the criticality and importance of the protected zone. The protection system for more critical portions is generally costly, as all the features of a good protection system is maximized here. But a small motor can be protected by a simple thermally operated relay, which is simple and cheap. Therefore, the cost of the protection system should be adequate in its cost. 1.4.7 Some basic terminologies used in protection system Some basic terminologies commonly used in the protection system are enlisted below. i) Measuring Relay ii) Fault Clearing Time iii) Auxilliary relay iv) Relay Time v) Pick up value vi) Reset Value vii) Drop out viii) Reach ( under and over reaches) ix) Relay Burden x) Unit/ Non unit protection xi) All or Nothing relay 1.5 Classification and construction of relays 1.5.1 Classification Protection relays can be primarily classified in accordance with their construction, the actuating signal and application and function 1.5.1.1 According to the Construction principle 12 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES Depending upon the principle of construction, the following four brad categories are found. • Electromechanical • Solid State • Microprocessor • Numerical 1.5.1.2 According to the actuating signals The actuating signal may be any of the following signals including a numbers of different combinations of these signals depending upon whether the designed relay require a single or multiple inputs for its realization. • Current • Voltage • Power • Frequency • Temperature • Pressure • Speed • Others 1.5.1.3 Function The functions for which the protection system is designed classify the relays in the following few categories. • Directional Over current • Distance 13 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES • Over voltage • Differential • Reverse Power • Others It is important to notice that the same set of input actuating signals may be utilized to design to relays having different function or application. For example, the voltage and current input relays can be designed both as a Distance and/ or a Reverse Power relay. Electromechanical relays These relays are constructed with electrical, magnetic & mechanical components & have an operating coil & various contacts,& are very robust & reliable. Based on the construction, characteristics, these are classified in three groups. Attraction relays Attraction relays can be AC & DC and operate by the movement of a piece of iron when it is attracted by the magnetic field produced by a coil. There are two main types of relays: 1. The attracted armature type 2. Solenoid type relay Attracted armature relays • Consists of a bar or plate (made of iron) that pivots when it is attracted towards the coil. • The armature carries the moving part of the contact ,which is closed or opened, according to the design, when the armature is attracted to the coil. 14 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES Solenoid type relays In this a plunger or a piston is attracted axially within the field of the solenoid. In this case, the piston carries the moving contacts. 2 KIK 12 The force of attraction = 15 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES Where, K depends on 1 • The number of turns of the coil • The air gap • The effective area • The reluctance of the magnetic circuit K is the restraining force, usually produced by spring 2 For threshold or balanced condition, the resultant force is zero. 2 KIK 12 In order to control the value of current at which relay operates, the parameters K and K may 1 2 adjusted. Attraction relays effectively have no time delay and are widely used when instantaneous operation is required. Relays with movable coils This type of relay consists of a rotating movement with a small coil suspended or pivoted with the freedom to rotate between the poles of a permanent magnet. The coil is restrained by two special springs which also serve as connections to carry the current to the coil. 16 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES The torque produced in the coil is TBlaNi Where, T= Torque B= flux density l= length of the coil a= distance between the two sides of the coil i=current flowing through the coil N=number of turns in the coil • The relay has inverse type characteristic Induction relays • An induction relay works only with AC • It consists of an electromagnetic system Which operates on a moving conductor, generally in the form of a DISC or CUP Production of actuating torque 17 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES Various quantities are shown at instant when • Both fluxes are directed downward • Are increasing in magnitude Let (tt )sin () 11 m It may be assumed with negligible error that the paths in which rotor current flow have negligible self inductance. F F F 21 Since sinusoidal flux waves are assumed, we may substitute the rms values of the fluxes for the crest values in the above equation. F sin mm 12 • It may be noted that the net force is same at every instant. • The net force is directed from the point where the leading flux process the rotor towards the point where the lagging flux pierces the rotor. • Actuating force is produced in the presence of out of phase fluxes. 18 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES o • Maximum force is produced when θ=90 Classification of induction relays 1. Shaded pole relay 2. Watthour- meter type relay 3. Cup type relay The air gap flux produced by the current flowing in a single coil is split into two out of phase components by a so called „Shading Ring‟ generally of copper, that encircles part of the pole face of each pole at the air gap. • The shading ring may be replaced by coils if control of operation of the shaded pole relay is desired. • The inertia of the disc provides the time delay characteristics. 19 P a g e 2015 SWITCHGEAR AND PROTECTIVE DEVICES Watt hour –meter structure • This structure gets its name from the fact that it is used in watt hour meters. • As shown in the top figure below, it contains two separate coils on two different magnetic circuit, each of which produces one of two necessary fluxes for driving the rotor, which is also a disc 20 P a g e

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