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Switch Gear and Protection

by Deepak Saini
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Deepak Saini
Deepak Saini
BHARATHIDASAN ENGG COLLEGE, NATARAMPALLI COURSE MATERIAL (LECTURE NOTES) [Type text] Department of Electrical And Electronics Engineering EE2402 -PROTECTION AND SWITCHGEAR UNIT I INTRODUCTION UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS UNIT III APPARATUS PROTECTION THEORY OF CIRCUIT INTERRUPTION UNIT V CIRCUIT BREAKERS UN IT IV EE 2402 PROTECTION & SWITCHGEAR UNIT-I Page 1
BHARATHIDASAN ENGG COLLEGE, NATARAMPALLI COURSE MATERIAL (LECTURE NOTES) 1 AIM: To introduce the students to the various abnormal operating conditions in power system and describe the apparatus and system protection schemes. Also to describe the phenomena of current interruption to study the various switchgears. OBJECTIVES: i. To discuss the causes of abnormal operating conditions (faults, lightning and switching surges) of the apparatus and system. ii. To understand the characteristics and functions of relays and protection schemes. iii. To understand the problems associated with circuit interruption by a circuit breaker. UNIT I INTRODUCTION 9 Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults and fault currents - relay terminology – definitions - and essential qualities of protection. Protection against over voltages due to lightning and switching - arcing grounds - Peterson Coil - ground wires - surge absorber and diverters Power System earthing – neutral Earthing - basic ideas of insulation coordination. UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS 9 Electromagnetic relays – over current, directional and non-directional, distance, negative sequence, differential and under frequency relays – Introduction to static relays. UNIT III APPARATUS PROTECTION 9 Main considerations in apparatus protection - transformer, generator and motor protection protection of bus bars. Transmission line protection - zones of protection. CTs and PTs and their applications in protection schemes. UNIT IV THEORY OF CIRCUIT INTERRUPTION 9 Physics of arc phenomena and arc interruption. DC and AC circuit breaking - restriking voltage and recovery voltage rate of rise of recovery voltage - resistance switching - current chopping - interruption of capacitive current. UNIT V CIRCUIT BREAKERS 9 Types of circuit breakers – air blast, air break, oil, SF6 and vacuum circuit breakers – comparative merits of different circuit breakers – testing of circuit breakers. TOTAL : 45 PERIODS TEXT BOOKS: 1. M.L. Soni, P.V. Gupta, V.S. Bhatnagar, A. Chakrabarti, ‘A Text Book on Power System Engineering’, Dhanpat Rai & Co., 1998. (For All Chapters 1, 2, 3, 4 and 5). 2. R.K.Rajput, A Tex book of Power System Engineering. Laxmi Publications, First Edition Reprint 2007. REFERENCES: 1. Sunil S. Rao, ‘Switchgear and Protection’, Khanna publishers, New Delhi, 1986. 2. C.L. Wadhwa, ‘Electrical Power Systems’, Newage International (P) Ltd., 2000. 3. B. Ravindranath, and N. Chander, ‘Power System Protection & Switchgear’, Wiley Eastern Ltd.,1977. 4. Badri Ram, Vishwakarma, ‘Power System Protection and Switchgear’, Tata McGraw Hill, 2001. 5. Y.G. Paithankar and S.R. Bhide, ‘Fundamentals of Power System Protection’, Prentice Hall of India Pvt. Ltd., New Delhi–110001, 2003. EE 2402 PROTECTION & SWITCHGEAR UNIT-I Page 2
BHARATHIDASAN ENGG COLLEGE, NATARAMPALLI COURSE MATERIAL (LECTURE NOTES) UNIT I INTRODUCTION 9 Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults and fault currents - relay terminology – definitions - and essential qualities of protection. Protection against over 2 voltages due to lightning and switching - arcing grounds - Peterson Coil - ground wires - surge absorber and diverters Power System earthing – neutral Earthing - basic ideas of insulation coordination. 1] Discuss and compare the various methods of neutral earthing. [Any two type may ask each carries 8marks or brief all the five divisions] [16] Types of Neutral Earthing in Power Distribution: Introduction: In the early power systems were mainly Neutral ungrounded due to the fact that the first ground fault did not require the tripping of the system. An unscheduled shutdown on the first ground fault was particularly undesirable for continuous process industries. These power systems required ground detection systems, but locating the fault often proved difficult. Although achieving the initial goal, the ungrounded system provided no control of transient over-voltages. A capacitive coupling exists between the system conductors and ground in a typical distribution system. As a result, this series resonant L-C circuit can create over-voltages well in excess of line-to-line voltage when subjected to repetitive re-strikes of one phase to ground. This in turn, reduces insulation life resulting in possible equipment failure. Neutral grounding systems are similar to fuses in that they do nothing until something in the system goes wrong. Then, like fuses, they protect personnel and equipment from damage. “Damage comes from two factors, how long the fault lasts and how large the fault current is. Ground relays trip breakers and limit how long a fault lasts and Neutral grounding resistors limit how large the fault current is”. Importance of Neutral Grounding: [seven points alone 2marks] There are many neutral grounding options available for both Low and Medium voltage power systems. The neutral points of transformers, generators and rotating machinery to the earth ground network provides a reference point of zero volts. This protective measure offers many advantages over an ungrounded system, like, 1. Reduced magnitude of transient over voltages 2. Simplified ground fault location 3. Improved system and equipment fault protection 4. Reduced maintenance time and expense 5. Greater safety for personnel 6. Improved lightning protection 7. Reduction in frequency of faults. Method of Neutral Earthing: There are five methods for Neutral earthing. 1. Unearthed Neutral System 2. Solid Neutral Earthed System. 3. Resistance Neutral Earthing System. 1. Low Resistance Earthing. 2. High Resistance Earthing. 4. Resonant Neutral Earthing System. 5. Earthing Transformer Earthing. (1) Ungrounded Neutral Systems: EE 2402 PROTECTION & SWITCHGEAR UNIT-I Page 3
BHARATHIDASAN ENGG COLLEGE, NATARAMPALLI COURSE MATERIAL (LECTURE NOTES) • • • In ungrounded system there is no internal connection between the conductors and earth. However, as system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces. Consequently, the “ungrounded system” is, in reality, a “capacitive grounded system” by virtue of the distributed capacitance. Under normal operating conditions, this distributed capacitance causes no problems. In fact, it is beneficial because it establishes, in effect, a neutral point for the system; As a result, the phase conductors are stressed at only line-to-neutral voltage above ground. But problems can rise in ground fault conditions. A ground fault on one line results in full line-to-line voltage appearing throughout the system. Thus, a voltage 1.73 times the normal voltage is present on all 3 insulation in the system. This situation can often cause failures in older motors and transformers, due to (2) Solidly Neutral Grounded Systems: • Solidly grounded systems are usually used in low voltage applications at 600 volts or less. • • • • In solidly grounded system, the neutral point is connected to earth. Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of the system three phase fault current. However, if the reactance of the generator or transformer is too great, the problem of transient over voltages will not be solved. While solidly grounded systems are an improvement over ungrounded systems, and speed up the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides. To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must be extend from the source to the furthest point of the system within the same raceway or conduit. Its EE 2402 PROTECTION & SWITCHGEAR UNIT-I Page 4

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