EE2402 Notes Protection and Switchgear
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Protection and Switchgear...
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Department Of Electrical And Electronics Engineering
EE2402 PROTECTION AND SWITCHGEAR Question fand answers
For Two mark and 16,8,4 mark questions
UNIT I
INTRODUCTION
UNIT II
OPERATING PRINCIPLES AND RELAY CHARACTERISTICS
UNIT III
APPARATUS PROTECTION
UNIT IV
THEORY OF CIRCUIT INTERRUPTION
UNIT V
CIRCUIT BREAKERS
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t e n . y t i s r e v i n U i a n n e h C
www.chennaiuniversity.net EE2402 PROTECTION AND SWITCHGEAR LTPC 3003 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 conditions (faults, lightning and switching surges) of the apparatus and system. ii. To understand the characteristics 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 – terminology – definitions 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 – earthing – neutral neutral Earthing - basic ideas of insulation coordination.
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UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS 9 Electromagnetic relays – relays – over over current, directional and non-directional, distance, negative sequence, differential and under frequency relays – relays – Introduction 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 protection - zones zones of protection. protection. CTs and PTs and and their application applicationss in protection 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 – breakers – air air blast, air break, oil, SF6 and vacuum circuit breakers – comparative comparative merits of different circuit breakers – breakers – testing 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’, W iley 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, 110001, 2003.
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www.chennaiuniversity.net EE2402 PROTECTION AND SWITCHGEAR LTPC 3003 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 conditions (faults, lightning and switching surges) of the apparatus and system. ii. To understand the characteristics 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 – terminology – definitions 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 – earthing – neutral neutral Earthing - basic ideas of insulation coordination.
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UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS 9 Electromagnetic relays – relays – over over current, directional and non-directional, distance, negative sequence, differential and under frequency relays – relays – Introduction 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 protection - zones zones of protection. protection. CTs and PTs and and their application applicationss in protection 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 – breakers – air air blast, air break, oil, SF6 and vacuum circuit breakers – comparative comparative merits of different circuit breakers – breakers – testing 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’, W iley 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, 110001, 2003.
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www.chennaiuniversity.net DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING EE2402 PROTECTION AND SWITCHGEAR UNIT I
INTRODUCTION
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Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults and fault currents - relay terminology – terminology – definitions definitions - and essential qualities of protection. Protection against ov er voltages due to lightning and switching - arcing grounds - Peterson Coil - ground g round wires - surge absorber and diverters Power System earthing – earthing – neutral neutral Earthing - basic ideas of insulation coordin ation. Short Answers
2 Marks
1] List the types of faults in power system
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Active Faults Passive Faults Transient & Permanent Faults Symmetrical & Asymmetrical Faults A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, ax es, and represents a steady state condition. An asymmetrical fault displays a d.c. offset, transient in nature and d ecaying to the steady stead y state of the symmetrical fault after a period of time: Faults on a Three Phase System Types of Faults on a Three Phase System . (A) Phase-to-earth fault (B) Phase-to-phase fault (C) Phase-to-phase-to-earth fault (D) Three phase fault (E) Three phase-to-earth fault (F) Phase-to-pilot fault * (G) Pilot-to-earth fault * 2] What is the need for protection zones in the system?
Any fault occurring within the given zone will p rovide necessary tripping of relays or disconnecting or opening of circuit breakers and thus the healthy section is safe guarded. If a fault occurs in the overlapping zone z one in a proper protected scheme, more circuit breakers than the minimum necessary to isolate the faulty part of the system would trip. 3] what is surge absorber? How do they differ from surge diverter?
Surge Absorber: it is a protective device used to reduce the steepness steepness of the wave front of a surge surge and absorbs energy contained in the travelling wave. Surge Diverter
It will divert excess voltages from an electrical surge to earth. It measures the volts coming in and once it gets above a set amount (normally 260 volts), will divert the excess volts to earth.
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www.chennaiuniversity.net An Electrical Surge Diverter is a great way to ensure adequ ate lightning protection for your valuable electronic equipment. Unlike the more common Surge Protector Powerboards that simply switch off if there is spike in volts, a Surge Diverter will just divert the excess volts away. It is also installed on your main switchboard, thereby protecting all powerpoints. 4] Define the term “Insulation Coordinatio n”.
Insulation Coordination is the process of determining the proper insulation levels of various components in a power system as well as their arrangements. It is th e selection of an insulation structure that will withstand voltage stresses to which the system, or equipment will be subjected to, together with the proper surge arrester. The process is determined from the known characteristics of voltage surges and the characteristics of surge arresters. 5] Write any two functions of protective relaying?
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(i) The function of a protective relay is to detect and locate a fault and issue a command to the circuit breaker to disconnect the faulty element. (ii)It is a device which senses abnormal conditions on a power system by constantly monitoring electrical quantities of the system which differ under normal and abnormal conditions.
6] What are the desirable qualities of protective relaying? Or Mention the essential features of the power system protection. Or List the essential features of switchgear.
1. Selectivity
2. Speed & time
3. Sensitivity
4. Reliability
5. Simplicity
6. Economy
7] What is meant by switchgear?
The apparatus used for switching, controlling and protecting the electrical circuits and equipment is known as switchgear. 8] What are the functions of protection relaying?
The principal function of protective relaying is to cause the prompt removal from service of any element of the power system when it starts to operate in an abnormal manner or interface with the operation of rest of the system. 9] What are the causes of faults in power system?
(i) Internal causes.
(ii) Heavy short circuit current may cause damage to damage equipment or other element of the system of the system due to overheating and high mechanical forces set up due to heavy current. (iii) Arc associated with short circuits may cause fire hazards. Such fires resulting from arcing may destroy the fault element of the system. There is also possibility of firing spreading to the other devices if the fault is not isolated quickly. 4
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10] What are the different types of fault in power system transmission lines?
1. Symmetrical faults ═ 3 phase faults 2. Unsymmetrical faults ═ Single phase to ground, single phase to open circuit, Two phase to ground fault; phase to phase short circuit. 11] List out the types of faults in power system
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A].Single phase to ground B] phase to phase faults C] Two phase to ground fault and D] Three phase short circuit faults. 12] Explain the need for overlapping the zones of protection.
1. The circuit breakers are located in the connection to each power element. 2. This provision makes it possible to disconnect only the faulty element from the system.
13] Differentiate between primary and back – up protection.
No 1 2 3
Primary protection Back – Up Protection It is designed to protect the components It is second line of protection in case main of the power system. [main protection] protection fails. It is for instantaneous protection
It is designed to operate with enough time delay
Only faulty element will be removed.
Larger part of the power system is removed.
14] What are the causes of faults in power system?
1. Internal causes of the equipment. 2. Heavy short circuit current may cause s damage the equipment or other element of the system due to overheating and high mechanical forces set up due to heavy current. 3. Deterioration of insulation. 15] What are the functions of protective relays
To detect the fault and initiate the operation of the circuit breaker to isolate the d efective element from the rest of the system, thereby protecting the system from damages consequent to the fault. 16. Give the consequences of short circuit .
Whenever a short-circuit occurs, the current flowing through the co il increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing 5
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www.chennaiuniversity.net its contacts. The trip circuit is then closed , the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply. 17. Define protected zone.
Are those which are directly protected by a protective system such as relays, fuses or switchgears. If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone. 18. What are unit system and non-unit system?
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A unit protective system is one in which only faults occurring within its protected zone are isolated. Faults occurring elsewhere in the system have no influence on the operation of a unit system. A non-unit system is a protective system which is activated even when the faults are external to its protected zone. 19.What is primary protection?
Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker. It serves as the first line of defence. 20. What is back up protection?
Is the second line of defence, which operates if the primary protection fails to activate within a definite time delay. 21. Name the different kinds of over current relays .
Induction type non-directional over current relay, Induction type directional over current relay & current differential relay. 22. Define energizing quantity.
It refers to the current or voltage which is used to activate the relay into operation. 23. Define operating time of a relay.
It is defined as the time period extendind from the occurrence of the fault through the relay detecting the fault to the operation of the relay. 24. Define resetting time of a relay .
It is defined as the time taken b y the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset. 25. What are over and under current relays?
Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg: Induction type non-directional/directional overcurrent relay, differential overcurrent relay)whereas undercurrent 6
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www.chennaiuniversity.net relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)
26. Mention any two applications of differential relay.
Protection of generator & generator transformer unit; protection of large motors and busbars . 27. What is biased differential bus zone reduction?
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The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force. 16 Marks Questions
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 unsched uled shutdown on the first ground fault was p articularly 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 n o 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 grou nd. 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 h ow 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 Farthing: There are five methods for Neutral earthing. 1. Unearthed Neutral System 2. Solid Neutral Earthed System.
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www.chennaiuniversity.net 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: 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 b y 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; 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. con ditions. 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 no rmal voltage is present on all insulation in the system. This situation can often cause failures in older motors and transformers, due to insulation breakdown.
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Advantage: 1. After the first ground fault, assuming it remains as a single fault, the circuit ma y continue in operation, permitting continued production until a convenient shut down for maintenance can be scheduled. Disadvantages: 1. The interaction interaction between between the faulted faulted system system and its distribut distributed ed capacitance capacitance may cause cause transient transient overvoltages (several times normal) to appear from line to ground d uring normal switching of a circuit having a line-to ground fault (short). These over v oltages may cause insulation failures at points other than the original fault. 2. A second fault fault on another phase phase may occur before before the first first fault fault can be cleared. cleared. This can result result in very high line-to-line fault currents, equipment damage and disruption of both circuits. 3. The cost cost of equi equipme pment nt dam damage age.. 4. Complicate Complicate for locating locating fault(s) fault(s),, involving involving a tedious process process of trial and error: error: first isolatin isolating g the correct feeder, then the branch, and a nd finally, the equipment at fault. The result is unnecessarily lengthy and expensive down downtime. (2) Solidly Neutral Grounded Systems: Solidly grounded systems are usually used in low voltage applications at 600 volts or less.
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1.
1. 2. 3.
In solidly grounded system, the neutral point is conn ected to earth. Solidly Neutral Grounding slightly reduces the problem of transient over vo ltages found on the ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of th e sys ystem tem thr ee phase f aul t cur r ent nt.. 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 cond uit. Its purpose is to maintain very low impedance to ground faults so that a relatively high fault current will flow thus insuring that circuit breakers or fuses will clear the fault quickly and therefore minimize damage. It also greatly reduces the shock hazard to personnel
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If the system is not solidly grounded, the neutral p oint of the system would “float” with respect to ground as a function of load subjecting the line-to-neutral loads to voltage unbalances and instability. The single-phase earth fault current in a solidly earthed system may exceed the three phase fault current. The magnitude of the current depends on the fault location and the fault resistance. One way to reduce the earth fault current is to leave some of the transformer neutrals unearthed. Advantage: The main advantage advantage of solidly solidly earthed earthed systems systems is low over over voltages, voltages, which which makes the earthing earthing design design common at high voltage levels (HV). Disadvantage: This system system involves involves all the drawbacks drawbacks and and hazards of high high earth fault fault current: current: maximum damage damage and disturbances. There is no no service service continuity continuity on the the faulty faulty feeder. The danger for personnel personnel is is high during during the fault fault since since the touch touch voltages voltages created created are high. high.
Applications:
1. Distributed neutral conductor. 2. 3-phase + neutral distribution. 9
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www.chennaiuniversity.net 3. Use of the neutral conductor as a protective conductor with systematic earthing at each transmission pole. 4. Used when when the the short-ci short-circuit rcuit power power of the the source source is low. low.
(3) Resistance earthed systems: Resistance grounding has been used in three-phase industrial applications for many years and it resolves many of the problems associated with solidly grounded and ungrounded systems. Resistance Grounding Systems limits the phase-to-ground fault currents. The reasons for limiting the Phase to ground Fault current by resistance grounding are:
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1. To reduce burning burning and melting effects effects in faulted faulted electrical electrical equipment equipment like switchgea switchgear, r, transformers, transformers, cables, and rotating machines. 2. To reduce mechanica mechanicall stresses stresses in circuits/ circuits/Equipm Equipments ents carrying carrying fault current currents. s. 3. To reduce reduce electrical electrical-shock -shock hazards hazards to personnel personnel caused caused by stray ground ground fault. fault. 4. To reduce reduce the arc arc blast blast or or flash flash hazar hazard. d. 5. To reduce reduce the the moment momentary ary line line-vo -volta ltage ge dip. dip. 6. To secure secure control control of the transien transientt over-voltag over-voltages es while while at the the same time. time. 7. To improve improve the the detection detection of the earth fault in a power power system. system.
Grounding Resistors are generally connected between ground and neutral of transformers, generators and grounding transformers to limi t maximum f ault cur rent as pe perr Oh ms L aw to a va valu lu e which wil l n ot damage the equi equi pme pment nt in the power system and allow sufficient flow of fault cu rrent to detect and operate Earth protective relays to clear the fault. Although it is possible to limit fault currents with high resistance Neutral grounding Resistors, earth short circuit currents can be extremely reduced. As a result of this fact, protection devices may not sense the fault. Therefore, it is the most common application to limit single phase fault currents with low resistance Neutral Grounding Resistors to approximately rated current of transformer transformer and / or generator. In addition, limiting fault currents to predetermined maximum values permits the designer to selectiv ely coordinate the operation of protective devices, wh ich minimizes system disruption and allows for quick location of the fault. There are two categories of resistance resistance grounding:(1) Low resistance Grounding. (2) High resistance Grounding.
Ground fault current flowing through either type of resistor when a single phase faults to ground will increase the phase-to-ground voltage of the remaining two phases. As a result, co conducto nductorr in sul ation and sur ge ar arrr estor r ati ngs must must be bas base ed on li ne-to-l in e voltage . This temporary increase in phase-toground voltage should also be considered when selecting two and three pole po le breakers installed on resistance grounded low voltage systems.
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Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated with phase-to-phase faults, but both systems significantly reduce or essentially eliminate the arc-flash hazards associated with phase-to-ground faults. Both types of grounding s ystems limit mechanical stresses and reduce thermal damage to electrical equipment, circuits, and apparatus carrying faulted current. The difference between Low Resistance Grounding and High Resistance Grounding is a matter of perception and, therefore, is not well defined. Generall y speakin g hi gh-r esistance groun din g r efers to a system i n wh ich the NGR l et-thr ough cur r ent i s less than 50 to 100 A. L ow resistance groun ding in dicates that NGR cur rent woul d be above 100 A.
A better distinction between the two levels might be alarm only and tripping. An alarm-only system continues to operate with a single ground fault on the system for an unspecified amount of time. In a tripping system a ground fault is automatically removed by p rotective relaying and circuit interrupting devices. Alarm-only systems usually limit NGR current to 10 A or less.
Rating of The Neutral grounding resistor:
1. 2. 3.
Voltage: Line-to-neutral voltage of the system to which it is c onnected. Initial Current: The initial current which will flow through the resistor with rated voltage applied. Time: The “on time” for which the resistor can operate without exceeding the allowable temperature rise.
(A).Low Resistance Grounded:
Low Resistance Grounding is used for large electrical s ystems where there is a high investment in capital equipment or prolonged loss of service of equipment has a significant economic impact and it is not commonly used in low voltage systems because the limited ground fault current is too low to reliably operate breaker trip units or fuses. This makes system selectivity hard to achiev e. Moreover, low resistance grounded systems are not suitable for 4-wire loads and hence have not been used in commercial market applications A resistor is connected from the system neutral point to ground and generally sized to permit only 200A to 1200 amps of ground fault current to flow. Enough current must flow such that protective devices can
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www.chennaiuniversity.net detect the faulted circuit and trip it off-line but n ot so much current as to create major damage at the
fault point.
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Since the grounding impedance is in the form of resistance, any transient over voltages a re quickly damped out and the whole transient overvoltage phenomena is no longer applicable. Although theoretically possible to be applied in low voltage systems (e.g. 480V),significant amount of the system voltage dropped across the grounding resistor, there is not enough voltage across the arc fo rcing current to flow, for the fault to be reliably detected. For this reason, low resistance grounding is not used for low voltage systems (under 1000 volts line to-line).
Advantages:
1. Limits phase-to-ground currents to 200-400A. 2. Reduces arcing current and, to some extent, limits arc-flash hazards associated with phase-to-ground arcing current conditions only. 3. May limit the mechanical damage and thermal damage to shorted transformer and rotating machinery windings. Disadvantages:
1. 2. 3. 4.
Does not prevent operation of over current devices. Does not require a ground fault detection system. May be utilized on medium or high voltage systems. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer. Used: Up to 400 amps for 10 sec are commonly found on medium voltage systems.
(B).High Resistance Grounded:
High resistance grounding is almost identical to low resistance grounding except that the ground fault current magnitude is typically limited to 10 amperes or less . High resistance grounding accomplishes two things. The first is that the ground f ault cur rent magnitude is suf fi cientl y low enough such that no appreciable damage is done at the fault point. This means that the faulted circuit need no t be tripped off-line when the fault first occurs. Means that once a fault does occur, we do not know where the fault is located. In this respect, it performs just like an ungrounded system. 12
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The second point is it can control the tr ansient overvoltage phenomenon present on ungrounded systems if engineered properly. Under earth fault conditions, the resistance must dominate over the system charging capacitance but not to the point of permitting excessive current to flow and thereby excluding continuous operation
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High Resistance Grounding (HRG) systems limit the fault current when one ph ase of the system shorts or arcs to ground, but at lower levels than low resistance systems. In the event that a ground fault condition exists, the HRG typically limits the current to 5-10A. HRG’s are continuous current rated, so the description of a particular unit does not include a time rating. Unlike NGR’s, ground fault current flowing through a HRG is usually not of significant magnitude to result in the operation of an over current device. Since the ground fault current is not interrupted, a ground fault detection system must be installed. These systems include a bypass contactor tapped across a portion of the resistor that pulses (periodically opens and closes). When the contactor is open, ground fault current flows through the entire resistor. When the contactor is closed a portion of the resistor is bypassed resulting in slightly lower resistance and slightly higher ground fault current. To avoid transient over-voltages, an HRG resistor must be sized so that the amount of ground fault current the unit will allow to flow exceeds the electrical system’s charging current. As a rule of thumb, charging current is estimated at 1A per 200 0KVA of system capacity for low voltage systems and 2A per 2000KVA of system capacity at 4.16kV. These estimated charging currents increase if surge suppressors are present. Each set o f suppressors installed on a low voltage system results in approx imately 0.5A of additional charging current and each set of suppressors installed on a 4.16kV system adds 1.5A of additional charging current. A system with 3000KVA of capacity at 480 volts would have an estimated charging current of 1.5A.Add one set of surge suppressors and the total charging current increases by 0.5A to 2.0A. A standard 5A resistor could be used on this system. Most resistor manufacturers publish detailed estimation tables that can be used to more closely estimate an electrical system’s charging current.
Advantages:
1. Enables high impedance fault detection in systems with weak capacitive connection to earth 2. Some phase-to-earth faults are self-cleared. 3. The neutral point resistance can be chosen to limit the possible over voltage transients to 2.5 times the fundamental frequency maximum voltage. 4. Limits phase-to-ground currents to 5-10A. 5. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase-to-ground arcing current conditions only. 6. Will eliminate the mechanical damage and may limit thermal damage to shorted transformer and rotating machinery windings. 7. Prevents operation of over current devices until the fault can be located (when only one phase faults to ground). 13
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www.chennaiuniversity.net 8. May be utilized on low voltage systems or medium voltage systems up to 5kV. IEEE Standard 141-1993 states that “high resistance grounding should be restricted to 5kV class or lower systems with charging currents of about 5.5A or less and should not be attempted on 15kV systems, unless proper grounding relaying is employed”. 9. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer. Disadvantages:
1. Generates extensive earth fault currents when combined with strong or moderate capacitive connection to earth Cost involved. 2. Requires a ground fault detection system to notify the facility engineer that a ground fault condition has occurred. (4) Resonant earthed system:
Adding inductive reactance from the system neutral point to ground is an easy method of limiting the available ground fault from something near the ma ximum 3 phase short circuit capacity (thousands of amperes) to a relatively low value (200 to 800 amperes). To limit the reactive part of the earth fault current in a power system a neutral point reactor can be connected between the transformer neutral and the station earthing system. A system in which at least one of the neutrals is connected to earth through an
1. Inductive reactance. 2. Petersen coil / Arc Suppression Coil / Earth Fault Neutralizer.
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The current generated by the reactance during an earth fault approximately compensates the capacitive component of the single phase earth fault current, is called a resonant earthed system. The system is hardly ever exactly tuned, i.e. the reactive current does not exactly equal the capacitive earth fault current of the system.
A system in which the inductive current is slightly larger than th e capacitive earth fault current is over compensated. A system in which the induced earth fault current is slightly smaller than the capacitiveearth fault current is under compensated However, experience indicated that this inductive reactance to ground resonates with the system shunt capacitance to ground under arcing ground fault conditions and creates very high transient over voltages on the system. To control the transient over voltages, the design must permit at least 60% of the 3 phase short circuit current to flow underground fault conditions. Example. A 6000 amp grounding reactor for a system having 10,000 amps 3 phase short circuit capacity available. Due to the high magnitude of ground fault current required to co ntrol transient over voltages, inductance grounding is r arely used wit hi n in dustr y.
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Petersen Coils:
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A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that the capacitive current in the eart h faul t i s compensated by an i ndu ctive cur rent passed by the Petersen Coil . A small residual current will remain, but this is so small that any arc b etween the faulted phase and earth will not be maintained and the fault will extinguish. Minor earth faults such as a broken pin insulator, could be held on the system without the supply being interrupted. Transient faults would not result in supply interruptions. Although the standard ‘Peterson coil’ does not compensate the entire earth fault current in a network due to the presence of resistive losses in the lines and coil, it is now possible to apply ‘residual current compensation’ by injecting an additional 180° out of phase current into the neutral via the Peterson coil. The fault current is thereby reduced to p ractically zero. Such systems are known as ‘Resonant e arthing with residual compensation’, and can be considered as a special case of reactive earthing. Resonant earthing can reduce EPR to a safe level. This is because the Petersen coil can often effectively act as a high impedance NER, which will substantially reduce any earth fault currents, and h ence also any corresponding EPR hazards (e.g. touch voltages, step voltages and transferred voltages, includin g any EPR hazards impressed onto nearby telecommunication networks).
Advantages:
1. Small reactive earth fault current independent of the phase to earth capacitance of the system. 2. Enables high impedance fault detection. Disadvantages:
1. Risk of extensive active earth fault losses. 2. High costs associated.
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www.chennaiuniversity.net (5) Earthing Transformers:
For cases where there is no neutral point available for Neutral Earthing (e.g. for a delta winding), an earthing transformer may be used to provide a return path for single phase fault currents
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In such cases the impedance of the earthing transformer may be sufficient to act as e ffective earthing impedance. Additional impedance can be added in series if required. A special ‘zig-zag’ transformer is sometimes used for earthing delta windings to provide a low zero-sequence impedance and high positive and negative sequence impedance to fault currents.
Conclusion:
Resistance Grounding Systems have many advantag es over solidly grounded systems including arc-flash hazard reduction, limiting mechanical and thermal damage associated with faults, and controlling transient over voltages.
High resistance grounding systems may also be emplo yed to maintain service continuity and assist with locating the source of a fault. When designing a system with resistors, the design/consulting engineer must consider the specific requirements for conductor insulation ratings, surge arrestor ratings, breaker single-pole duty ratings, and method of serving phase-to-neutral loads.
2] Discuss the essential qualities of protective relaying .
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Protective Relaying (Part11) A protective relaying scheme should have certain important qualities. Such an essential qualities of protective relaying are, 1. Reliability 2. Selectivity and Discrimination 3. Speed and Time 4. Sensitivity 5. Stability 6. Adequateness 7. Simplicity and Economy 16
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www.chennaiuniversity.net 1.1 Reliability A protective relaying should be reliable is its basic quality. It indicates the ability of the relay system to operate under the predetermined conditions. There are various components which go into the operation before a relay operates. Therefore every component and circuit which is involved in the operation of a relay plays an important role. The reliability of a protection system depends on the reliability of various components like circuit breakers, relays, current transformers (C.T.s), potential transformers (P.T.s), cables, trip circuits etc. The proper maintenance also plays an important role in improving the reliable operation of the system. The reliability can not be expressed in the mathematical expressions but can be adjusted from the statistical data. The statistical survey and records give good idea about the reliability of the protective system. The inherent reliability is based on the design which is based on the long experience. This can be achieved by the factors like, i) Simplicity ii) Robustness iii) High contact pressure iv) Dust free enclosure iv) Good contact material vi) Good workmanship and vii) Careful Maintenance
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1.2 Selectivity and Discrimination The selectivity id the ability of the protective system to identify the faulty part correctly and disconnect that part without affecting the rest of the healthy part of system. The discrimination means to distinguish between. The discrimination quality of the protective system is the ability to distinguish between normal condition and abnormal condition and also between abnormal condition within protective zone and elsewhere. The protective system should operate only at the time of abnormal condition and not at the time of normal condition. Hence it must clearly discriminate between normal and abnormal condition. Thus the protective system should select the fault part and disconnect only the faulty part without disturbing the healthy part of the s ystem. The protective system should not operate for the faults beyond its protective zone. For example, consider the portion of a typical power system shown in the Fig. 1.
Fig. 1
It is clear from the Fig. 1 that if fault F 2 occurs on transmission line then the circuit breakers 2 and 3 should operate and disconnect the line from the remaining system. The protective system should be selective in selecting faulty transmission line only for the fault and it should isolate it without tripping the adjacent transmission line breakers or the transformer. If the protective system is not selective then it operates for the fault beyond its protective zones and unnecessary the large part of the system gets isolated. This causes a lot of inconvenience to the supplier and users. 1.3 Speed and Time a protective system must disconnect the faulty system as fast as possible. If the faulty system is not disconnect for a long time then, 1. The devices carrying fault currents may get damaged. 2. The failure leads to the reduction in system voltage. Such low voltage may affect the motors and generators running on the consumer sude. 17
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www.chennaiuniversity.net 3. If fault persists for long time, then subsequently other faults may get generated. The high speed protective system avoids the possibility of such undesirable effects. The total time required between the instant of fault and the instant of final arc interruption in the circuit breaker is called fault clearing time. It is the sum of relay time and circuit breaker time. The relay time is the time between the instant of fault occurrence and the instant of closure of relay contacts. The circuit breaker times is the time taken by the circuit breaker to operate to open the contacts and to extinguish the arc completely. The fault clearing time should be as small as possible to have high speed operation of the protective system. Though the small fault clearing time is preferred, in practice c ertain time lag is provided. This is because, 1. To have clear discrimination between primary and backup protection 2. To prevent unnecessary operation of relay under the conditions such as transient, starting inrush of current etc. Thus fast protective system is an important quality which minimises the damage and it improves the overall stability of the power system. 1.4 Sensitivity The protective system should be sufficiently sensitive so that it can operate reliably when required. The sensitivity of the system is the ability of the relay system to operate with low value of actuating quantity. It indicates the smallest value of the actuating quantity at which the protection starts operating in relation with the minimum value of the fault current in the protected zone. The relay sensitivity is the function of the volt-amperes input to the relay coil necessary to cause its operation. Smaller the value of volt-ampere input, more sensitive is the relay. Thus 1 VA input relay is more sensitive than the 5VA input relay. Mathematically the sensitivity is expressed by a factor called sensitivity factor . It is the ratio of minimum short circuit current in the protected zone to the minimum operating current required for the protection to start. K s = Is/Io where K s = sensitivity factor Is = minimum short circuit current in the zone Io= minimum operating current for the protection
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1.5 Stability The stability is the quality of the protective system due to which the system remains inoperative and stable under certain specified conditions such as transients, disturbance, through faults etc. For providing the stability, certain modifications are required in the system design. In most of the cases time delays, filter circuits, mechanical and electrical bias are provided to achieve stable operation during the disturbances. 1.6 Adequateness There are variety of faults and disturbance those may practically exists in a power system. It is impossible to provide protection against each and every abnormal condition which may exist in practice, due to economical reasons. But the protective system must provide adequate protection for any element of the system. The adequateness of the system can be assessed by considering following factors, 1. Ratings of various equipments 2. Cost of the equipments 3. Locations of the equipments 4. Probability of abnormal condition due to internal and external causes. 5. Discontinuity of supply due to the failure of the equipment
1.7 Simplicity and Economy In addition to all the important qualities, it is necessary that the cost of the system should be well within limits. In practice sometimes it is not necessary to use ideal protection scheme which is economically unjustified. In such cases compromise is done. As a rule, the protection cost should not be more than 5% of the total cost. But if the equipments to be protected are very important, the economic constrains can be relaxed. 18
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www.chennaiuniversity.net The protective system should be as simple as possible so that it can be easily maintained. The complex system are difficult from the maintenance point of view. The simplicity and reliability are closely related to each other. The simpler system are always more reliable.
3] Discuss the nature and causes of different faults in a power system.
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Nature and causes of Faults: Any faults in electrical apparatus are nothing but the defect in its electrical circuit which makes current path directed from its intended path. Normally due to breaking of conductors or failure of insulation, these faults occur. The other reasons for o ccurrence of fault include mechanical failure, accidents. Excessive internal and external stresses. The impedance of the path in the fault is low and the fault currents are comparatively large. The induction of insulation is not considered as a fault until it shows some effect sucj as excessive current flow or reduction of impedance between conductors or between conductors and earth.
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When a fault occurs on a system, the voltage of the three phases become unbalanced. As the fault currents are large, the apparatus may get damaged. The flow of power is diverted towards the fault which affects the supply to the neighboring zone.
A power system consists of generators, transformers, switchgear, transmission and distribution circuits. There is always a possibility in such a large network that some fault will occur in some part of the system. The maximum possibility of fault occurrence is on the transmission lines due to their greater lengths and exposure to atmospheric conditions. The faults cannot be classified according to the causes of their incidence. The breakdown may occur at normal voltage due to deterioration of insulation. The breakdown may also occur due to damage on account of unpredictable causes which include perching o f birds, accidental short circuiting by snakes, kite strings, three branches etc. The breakdown may occur at abnormal voltages due to switching surges or surges caused by lighting.sss
4] List the types of faults in power system Acti ve F aul ts
The “Active” fault is when actual current flows from one phase conductor to another (phase-to-phase) or alternatively from one phase conductor to ea rth (phase-to-earth). This type of fault can also be further classified into two areas, namely the “solid” fault and the “incipient” fault. Passive F aul ts
Passive faults are not real faults in the true sense of the word but are rather conditions that are stressing the system beyond its design capacity, so that ultimately active faults will occur. Typical examples are: Overloading - leading to overheating of insulation (deteriorating quality, reduced life and ultimate failure). 19
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www.chennaiuniversity.net Over voltage - stressing the insulation beyond its limits. under frequency - causing plant to behave incorrectly. Power swings - generators going out-of-step or synchronism with each other Tr ansient & Permanent F aults
Transient faults are faults which do not damage the insulation permanently and allow the circuit to be safely re-energized after a short p eriod of time. A typical example would be an insulator flashover following a lightning strike, which would be successfully cleared o n opening of the circuit breaker, which could then be automatically reclosed. Transient faults occur mainly on outdoor equipment where air is the main insulating medium. Permanent faults, as the name implies, are the result o f permanent damage to the insulation. In this case, the equipment has to be repaired and reclosing must not be entertained. Symmetri cal & Asymmetr ical F aul ts
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A symmetrical fault is a balanced fault with the sinusoidal waves bein g equal about their axes, and represents a steady state condition. An asymmetrical fault displays a d.c. offset, transient in nature and d ecaying to the steady state of the symmetrical fault after a period of time: Faults on a Th ree Phase System The types of faults that can occur on a three phase A.C. system are as follows: . Types of F aul ts on a T hr ee Phase System (A) Phase-to-earth fault (B) Phase-to-phase fault (C) Phase-to-phase-to-earth fault (D) Three phase fault (E) Three phase-to-earth fault (F) Phase-to-pilot fault * (G) Pilot-to-earth fault *
5] What is a surge absorber? Write a short note on Ferranti surge absorber. [8] Surge absorbers are protective devices used to ab sorb the complete surge i,e. due to lightening surge or any transient surge in the system..........unlike the lightening arrestor in which a non-linear resistor is provided which provides a low resistance path to the dangerously high voltages on the system to the earth...
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6. What are the causes of over voltage on a power system?
Overvoltage - Causes and Protection
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Over voltages occur in a system when the system voltage rises over 110% of the nominal rated voltage. Overvoltage can be caused by a number of reasons, sudden reduction in loads, switching of transient loads, lightning strikes, failure of control equipment such as voltage regulators, n eutral displacement,. Overvoltage can cause damage to components connected to the power supply and lead to insulation failure, damage to electronic components, heating, flashovers, etc. Overvoltage relays can be used to identify overvoltages and isolate equipment. These relays operate when the measured voltage exceeds a predetermined set-point. The voltage is usually measured using a Potential Transformers. The details of the ratio of the potential transformer are also entered into the relay. These relays are usually provided with a time delay. The time delay can be either instantaneous, fixed time or for IDMT (inverse definite minimum time) curves.
Generally, overvoltage relays are provided with sufficient time delay in order to avoid unwanted trippings due to transients (See article on Transients). These relays can be used to isolate feeders and other equipment connected to the network. In the case of 21
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www.chennaiuniversity.net generators, these relay also switch off the excitation system to the generators thereby preventing voltage buildup. 7. Describe the phenomenon of lightning.
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Lightning, an awesome and terrifying natural phen omenon, is really nothing more than an electrical discharge that happens to be at an enormous voltage. Lightning equalizes a difference of electrical potential, whether it is between a cloud and the ground, between two clouds, or between two different areas of the same cloud. Due to various mechanisms in storm cloud formation, such as the rising and falling of air currents carrying moisture and ice particles, the storm cloud generates an electrical charge at its base. Charged regions in a thundercloud create conductive ionization channels through the atmosphere called “stepped leaders,” while an opposite electrical charge or “shadow” accumulates on the ground below.
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These leaders jump and branch ever downward in a quest to connect with their oppositely charged image or “shadow” on the earth’s surface. As the fiercely charged electrical field builds, streamers rise from objects on the ground. When a stepped leader from the sky chooses to meet the strongest rising streamer from below, a lightning “circuit” is completed. The main lightning stroke erupts up the ionized path. Numerous additional strokes along the same channel are common, creating a mesmerizing strobe light effect. We are mo st interested in protecting our clients from the “shadow” and its resulting electrical discharge— a lightning strike. Whatever lightning protection method is implemented, the importance of the grounding system supporting it cannot be overemphasized. A w ell designed, correctly installed, low impedance and lo w resistance connection from the earth to the components of the lightning protection system is essential. With the addition of transient voltage surge suppression, a comprehensive facility protection approach addressing all concerns of safety and power quality finally is made possible. Only then may we watch in unflinching wonder as the sky explodes in its timeless electrical display.
8. (a) What is necessity of protecting electrical equipment against traveling waves?
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www.chennaiuniversity.net 9. Describe the construction and principle of operation of (i) expulsion type lightning arrester, (8)
(ii) Value type lightning arrester. (8)
(i) expulsion type lightning arrester,
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10. Write short notes on the following . (i) klydonograph and magnetic link
(4)
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(ii) Rod gap (4) It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between as shown in Fig 8. One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators. The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly conducted to earth Limitations: (i) After the surge is over, the arc n the gap is maintained by the normal supply voltage, leading to short-circuit on the system.
(ii) The rods may melt or get damaged due to excessive heat produced by the arc. (iii) The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester. (iv) The polarity of the f the surge also affects the performance of this arrester.
Due to the above limitations, the rod gap arrester is only used as a back-up protection in case of main arresters. It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between as shown in Fig 8. One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators. The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and 29
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www.chennaiuniversity.net the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly conducted to earth
(iii) Arcing horns
(4)
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Transmission and other electrical equipment can be exposed to overvoltages. Overvoltages can be caused by a number of reasons such as lightning strikes, transient surges, sudden load fluctuation, etc. In the event of an overvoltage, the insulating equipment such as the insulators on a transmission line or bushings in a transformer can be exposed to high voltages whi ch may lead to their failure.
Arcing horns are protective devices that are constructed in the form of projections in the conducting materials on both sides of an insulator. Arcing horns are fitted in pairs. Thus in transmission lines they are found on the conducting line and the transmission tower across the insulators. In transmission lines, in the event of a lightning strike on the tower, the tower potential rises to dangerous levels and can result in flashovers across the insulators causing their failure. Arcing horns prevent this by conducting the arc across the air gap across them. Arcing horns function by bypassing the high voltage across the insulator using air as a conductive medium. The small gap between the horns ensures that the air between them breaks down resulting in a flashover and conducts the voltage surge rather than cause damage to the insulator.The horns are constructed in pair so that one horn is on the line side and the other is on the ground side.
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Arcing Horns are also used along with air insulated switchgear equipment. Air insulated switchgear are
vulnerable to damage due to arcing. Arcing horns serve to divert the arc towards themselves thus protecting the switching equipment. The arcing horns serve to move the arc away from the bushings or the insulators.
Figure shows the horn gap arrester . It consists of a horn shaped metal rods A and B separated by a small air
gap. The horns are so constructed that distance between them gradually increases towards the top as shown. The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and choke coil L while the other end is effectively grounded. The resistance R helps in limiting the follow current to
a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be
protected. The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the gap.
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Under normal conditions, the gap is non-conducting i.e. normal supply voltage is insufficient to initiate the arc
between the gap. On the occurrence of an over voltage, spark-over takes place across the small gap G. The heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves
progressively into positions 1,2 and 3. At some position of the arc (position 3), the d istance may be too great for the voltage to maintain the arc; consequently, the arc is extinguished. The excess charge on the line is thus conducted through the arrester to the ground.
(iv) Basic impulse insulation level
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BIL or basic impulse insulation level is the dielectric insulation gradient of a material tested to withstand the voltage stress at a voltage impressed between the material and a conductive surface beyond the BIL rating, an electric tracking starts to occur which will then result into an arcing flashover to the conductive surface. In 32
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www.chennaiuniversity.net addition it is the capacity of an equipment to withstand mechanical stress like lightning strike without causing any damage to the equipment...
11. What is Peterson coil? What protective functions are performed by this device?
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[Ref Q.No 1 IN 16 MARKS] ASSIGNMENT
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1. What are the requirements of a ground wire for protecting power conductors against direct lightning stroke? Explain how they are achieved in practice. (16) 2. Determine the inductance of Peterson coil to be connected between the neutral and ground to neutralize the charging current of overhead line having the line to ground capacitive of 0.15μf. If the supply frequency is 50Hz and the operating voltage is 132 KV, find the KVA rating of the coil. (16) 3. (a) Explain the term insulation coordination.
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(b) Describe the constn of volt-time curve and the terminology associated with impulse-testing. (8) 4. Explain the operation of various types of surge absorbers
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5. Why is it necessary to protect the lines and other equipment of the power system against over voltages? REF Q.No 6.Describe in brief the protective devices used for protection of equipment against such waves?
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7. What protective measures are taken against lightning over voltages?
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8.
(a) What is tower-footing resistance?
(4)
(b) Why is it required to have this resistance as low as economically possible? (c) What are the methods to reduce this resistance?
9. Describe the protection of stations and sub-stations against direct lightning stroke.
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(4)
(8)
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www.chennaiuniversity.net UNIT 2 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.
1] Draw the block diagram of a static differential relay.
2] What are the advantages of over current relays over electromagnetic relays? o Low power consumption as low as 1mW
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o No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts o No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc.
o A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks. This is not possible in electromagnetic relays o Static relay is compact o Superior operating characteristics and accuracy o Static relay can think , programmable operation is possible with static relay o Effect of vibration is nil, hence can be used in earthquake-prone areas
o Simplified testing and servicing. Can convert even non-electrical quantities to electrical in conjunction with transducers. 3] Compare static and electromagnetic relay. Electromagnetic relays are1st generation relays they use principle of electromagnetic principle. They depend upon gravitation also and the value changes to the surrounding magnetic fields also. Static relays are 2nd generation relays. In this relays transistors and IC's r been used. There v alue may vary with respect to temperature also. 34
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www.chennaiuniversity.net Numerical relays are present generation relays. They use Microprocessor. Within built software and predefined values if any values extend the predefined values then relay gets on by this relay we can find distance of fault also.
4] A relay is connected to 400/5 ratio current transformer with current setting of 150%. Calculate the Plug Setting Multiplier when circuit carries a fault current of 4000A.
5] What are the features of directional relay?
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• Dual polarized directional element capability • Zero sequence (Residual) voltage (3V0) • Zero sequence (Neutral) current (I0) • Directional element is insensitive to third or higher harmonics. • 12 Field selectable, inverse, definite time, and British Standard (BS142) time over current curves. • Independent time and instantaneous over current elements. • Optional independent directional and non-directional instantaneous over current functions. • Wide continuous setting ranges: • Current pickup accuracy +2% • Timing accuracy +5% • Time Over current Reset – Instantaneous • Optional Auxiliary Output Contact follows operation of user defined function (TOC, Dir INST, Non Dir INST) • Draw out construction, testable-in-case • Provision for trip circuit testing • Less than 0.1 Ohm burden for all sensing inputs. • Standard magnetically latched targets for each trip function. 6] What are the advantages and disadvantages of static relays?
a. b. c. d. e. f.
Low burden on CTs & PTs. No moving contacts Fast operation and long life.; Sensitivity is more. Fast reset & no overshoot. Size of the relay is small since measuring circuit needs very low current.
7] Classify the different types of over current relays based on inverse time characteristics?
a. Static instantaneous over current relay b. Directional static over current relay c. Inverse time over current relay 8] Mention any two applications of differential relay.
It is comparing miniature to heavy loads. 35
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Fault current can be easily detected.
Used for protecting generator, generator unit transformer unit, large motors and bus bars
9] Derive and draw the characteristics of an impedance relay.
Operating time Time of operation of relay
α
α Z α distance α distance
where V- voltage in volts I – current in amperes
Z – impedance in ohms
10] For what purpose distance relays used?
It is used in the main transmission lines or sub transmission lines of 33KV,66KV,& 132KV. It measures in terms of impedance, reactance etc., from relay to the point of fault.
11] Define the terms a] pick up value b]plug setting multiplier .
a] pick up value
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As the current or voltage on an un operated relay is increased, the value at or below which all contacts function b]Plug Setting Multiplier
The actual r.m.s current flowing in the relay expressed as a multiple of the setting current (pick u p current) is known as plug setting multiplier . mathematically , 36
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www.chennaiuniversity.net PSM= SECONDARY CURRENT/REL AY CURRENT SETTI NG or PSM= (primary curr ent durin g fault )/(relay current setting *C.T ratio)
PSM = PSM =
current through operating coil CT secondary current = plug setting current plug setting current line current CT ratio plug setting current
=
line current CT ratio ×plug setting current
12] What type of relay is best suited for long distance , very high voltage transmissions?
Impedance relay 13] What type of relay is best suited for generator protection?
Differential relay 14] Give any two applications of electromagnetic relays.
To protect the ac & dc equipments. Over/under current and over/under voltage protection of various ac & dc equipments.
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15. What is the need of relay coordination? The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system. 16. Mention the short comings of Merz Price scheme of protection applied to a power transformer. In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay under normal condition . 17. What are the various faults to which a turbo alternator is likely to be subjected? Failure of steam supply; failure of speed; overcurrent; over voltage; unbalan ced loading; stator winding fault . 18. What is an under frequency relay? An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value. 19. Define the term pilot with reference to power line protection. Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms. 20. Mention any two disadvantage of carrier current scheme for transmission line only. 37
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www.chennaiuniversity.net The program time (ie, the time taken b y the carrier to reach the other end-upto .1% mile); the response time of band pass filter; capacitance phase-shift of the transmission line 21. What are the features of directional relay? High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive.
22. What are the causes of over speed and how alternators are protected from it? Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.
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23. What are the main types of stator winding faults? Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same phase winding.
23. Give the limitations of Merz Price protection. Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral po int, the voltage may be insufficient to operate the rela y. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase d ifference between the primary and the seconda ry quantities and a possibility of current through the relay even whe n there is no fault. 24. What are the uses of Buchholz’s relay? Bucholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer and to connect the transformer from the supply in the event of severe internal faults. It is usually used in oil immersion transformers with a rating over 750KVA.
16 Mark questions 1] What are the different types of electromagnetic relays? Discuss their field of applications. [16]
· · · · · ·
Type of protection Over current. Directional over current. Distance. Over voltage. Differential. Reverse power.
Electromagnetic relays
Electromagnetic relays are constructed with electrical, magnetic and mechanical components, have an operating coil and various contacts and are very robust and reliable. The construction characteristics can be classified in three groups, as detailed below. Attraction relays 38
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www.chennaiuniversity.net Attraction relays can be supplied by AC o r DC, and operate by the movement of a piece of metal when it is attracted by the magnetic field produced by a coil. There are two main types of relay in this class. The attracted armature relay, which is shown in figure 1, consists of a bar or plate of metal which pivots when it is att racted towards the coil. The armature carries the moving part o f the cont act, which is closed or opened according to the design when the armature is attracted to the coil. The other type is the piston or solenoid relay, illustrated in Figure 2, in which α bar or piston is attracted axially within the field of the solenoid. In this case, the piston also carries the operating contacts. It can be shown that the force of attraction is equal to K 1I2 - K 2, where Κ 1 depends upon the number of turns on the operating solenoid, the air gap, the effective area and the reluctance of the magnetic circuit, among other factors. K2 is the restraining force, usually produced by a spring. When the relay is balanced, the resultant force is zero and therefore Κ 112 = K2, So that
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In order to control the value at which the relay starts to operate, the restraining tension of the spring or the resistance of the solenoid circuit can be v aried, thus modifying the restricting force. Attraction relays effectively have no time delay and, for that reason, are widely used when instantaneous operations are required. Relays with moveable 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 springs which also serve as connections to carry the current to the coil. The torque produced in the coil is given by: T = B.l.a.N.i
Where: T = torque B = flux density L =length of the coil a = diameter of the coil N = number of turns on the coil i = current flowing through the coil
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Figure 2 Solenoid-type relay
F igure 3 I nverse time characteri stic
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From the above equation it will be noted that the torque developed is proportional to the current. The speed of movement is controlled by the damping action, which is proportional to the torque. It thus follows that the relay has an inverse time characteristic similar to that illustrated in Figure 3. The relay can be designed so that the coil makes a large angular movement, for example 80º. Induction relays An induction relay works only with alternating current. It consists of an electromagnetic system which operates on a moving conductor, generally in the form of a disc or cup, and functions through the interaction of electromagnetic fluxes with the parasitic Fault currents which are induced i n the rotor by these fluxes. These two fluxes, which are mutually displaced both in angle and in position, produce a torque that can be expressed by
T= Κ 1.Φ1.Φ2 .sin θ,
Where Φ1 and Φ2 are the interacting fluxes and θ is the phase angle between Φ1 and Φ2. It should be noted that the torque is a maximum when the fluxes are out of phase by 90º, and zero when they are in phase.
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Fi gure 4 El ectromagnetic ctromagnetic f orces orces in in duction r elays
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It can be shown that Φ1= Φ1sin ωt, and Φ2= Φ2 sin (ωt+ (ωt+ θ) , where θ is the angle by which Φ2 leads Φ1. Then:
And
Figure 4 shows the interrelatio interrelationship nship between the currents currents and the opp osing forces. fo rces. Thus: Th us: F = ( F 1 - F 2 ) α (Φ 2 i Φ1+ Φ1 i Φ2 ) F α Φ2 Φ1 sin θ α T
Induction relays can be grouped into three classes as set out below. Shaded-pole relay
In this case a portion of the electromagnetic section is short-circuited by means of a copper ring or coil. This creates a flux in the area influenced by the short circuited section (the so-called shaded section) which lags the flux in the nonshaded section, see Figure 5. Figure 5 Shaded- pole relay
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F igure 6 Wattmetric-type relay
In its more common form, this type of relay uses an arrangement of coils above and below the disc with the upper and lower coils fed by different values or, in some cases, with just one supply for the top coil, which induces an out-of phase phase f l ux in in the the lower lower coil coil because because of the air gap. gap. Figu Figure re 6 illu illust st r ates ates a typica typicall arran arrangem gement ent..
Cup-type relay
This type of relay has a cylinder similar to a cu which can rotate in the annular air gap between the poles of the coils, c oils, and has a fixed central core, see Figure 7. The operation of this relay is very similar to that
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Figure 7Cup-type relay Of an induction motor with salient poles for the win dings of the stator. Configurations with four or eight poles spaced symmetrically around the circumference of the cup are often used. The movement of the cylinder is limited to a small amount by the contact and a nd the stops. Α special spring provides the restraining torque.
The torque is a function of the product of the two currents through the coils and the cosine of the angle between them. The torque equation is T= ( KI 1 I I 2 cos (θ 12 – Φ) – K – Φ) 12 s ), s
Where K , .Κs and Φ are design constants, Ι1 and I2 are the currents through the two coils and θ 12 12 is the angle between I1 and I2.
In the first two types of relay mentioned above, which are provided with a disc, d isc, the inertia of the disc provides the time-delay characteristic. The time delay can be increased by the addition of a permanent magnet. The cup-type relay has a small inertia and is therefore principally used when high speed operation is required, for example in instantaneous units.
2.What are the various types of over current relays? Discuss their area of application. (16)
STATIC INSTANTANEOUS OVER-CURRENT RELAY The block diagram of an instantaneous over-current relay is shown in fig 21. The same construction may be used for under-voltage, over-voltage and earth fault relays too.
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The secondaries of the line CT’s are connected to a summation circuit (not shown in the fig). The output of this summation CT is fed to an auxiliary CT, whose output is rectified smoothened and supplied to the measuring unit (level detector). The measuring unit determines whether the quantity has attained the threshold value (set value) or not. When the input to measuring unit is less than the threshold value, the output of the level detector is zero. For an over-current relay For I input < I threshold; Ioutput = 0 For I input > I threshold; Ioutput = Present In an actual relay I threshold can be adjusted. After operation of the measuring unit, the amplifier amplifies the output. Amplified output is given to the output circuit to cause trip/alarm. If time-delay is desired, a timing circuit is introduced before the level detector. Smoothing circuit and filters are introduced in the output of the bridge rectifier. Static over-current relay is made in the form of a single unit in which diodes, transistors, resistors, capacitors etc., are arranged on printed board and are bolted with epoxy resin.
STATIC OVER-CURRENT TIME RELAY
The block diagram of static over current time relay is shown in fig
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www.chennaiuniversity.net The current from the line CT is reduced to 1/1000 th by an auxiliary CT, the auxiliary has taps on the primary for selecting the desired pick-up and current range and its rectified output is supplied to level detector I (overload level detector) and an R-C timing circuit. When the voltage on the timing capacitor Vc attains the threshold value of the level detector II, tripping occurs. Time delay given by the timing circuit shown in fig 22 b is given as Tc = RC log e E/(E – VT) . Where VT is the threshold value of the level detector II. By varying values of R and C the time can be varied without difficulties
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3.Describe the operating principle, constructional features and area of applications of reverse power or directional relay. (16) Directionalized relays are relays that use a polarizing circuit to determine which "direction" (in the zone of protection, or out of the zone protection) a fault is. There are many different types and different polarizing methods - ground polarizing, voltage polarizing, zero sequence voltage polarizing, negative sequence polarizing, etc.
The basic operation of this relay is just like any nondirectional relay, but with an added torque control - the directionalizing element. This element allows the relay to operate when it is satisfied that the fault is within the zone of protection (ie not behind where the relay is looking).
A reverse power relay is a directional power relay that is used to monitor the power from a generator running in parallel with another generator or the utility. The function of the reverse power relay is to prevent a reverse power condition in which power flows from the bus bar into the generator. This condition can occur when there is a failure in the prime mover such as an engine or a turbine which drives the generator.
Causes of Reverse Power
The failure can be caused to a starvation of fuel in the prime mover, a problem with the speed controller or an other breakdown. When the prime mover of a generator running in a synchronized condition fails. There is a condition known as motoring, where the generator draws power from the bus bar, runs as a motor and drives the prime mover. This happens as in a synchronized condition all the generators will have the same frequency. Any drop in frequency in one generator will cause the other power sources to p ump power into the generator. The flow of power in the reverse direction is known as the reverse power relay.
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Another cause of reverse power can occur during synchronization. If the frequency of the machine to be synchronized is slightly lesser than the bus bar frequenc y and the breaker is closed, power will flow from the bus bar to the machine. Hence, during synchronization(forward), frequency of the incoming machine is kept slight higher than that of the bus bar i.e. the synchroscope is made to rotate in the "Too fast" direction. This ensures that the machine takes on load as soon as the breaker is closed.
Setting the Reverse Power Relay The reverse power relay is usually set to 20% to 50% of the motoring power required by prime mover. By motoring power we mean the power required by the generator to drive the prime mover at the rated rpm. This is usually obtained from the manufacturer of the prime mover (turbine or engine). 46
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www.chennaiuniversity.net 4. Describe the construction and principle of operation of an induction type directional over current relay. (16) INDUCTION TYPE DIRECTIONAL OVER CURRENT RELAY
The directional power relay is not suitable under short circuit conditions because as short circuit occurs the system voltage falls to a low value resulting in insufficient torque to cause relay operations. This difficulty is overcome in the directional over current relay, which is independent of system voltage and power factor. Constructional details: –
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Figure shows the constructional details of a typical induction type directional over current relay. It consists of two relay elements mounted on a common case viz. (i) directional element and (ii) non-directional element.
(i) Directional element: It is essentially a directional power relay, which operates when power flows in a
specific direction. The potential of this element is connected through a potential transformer (PT.) to the system voltage. The current coil of the element is energized through a CT by the circuit current. This winding is carried
over the upper magnet of the non-directional element. The trip contacts (1 and 2) of the directional element are connected in series with secondary circuit of the over current element. The latter element cannot start to operate until its secondary circuit is completed. In other words, the directional element must first operate (ie. contacts 1 and 2 should close) in order to operate the over current element.
(ii) Non-directional element: – It is an over current element similar in all respects to a non-directional over
current relay. The spindle of the disc of this element carries a moving contact which closes the fixed contact after the operation of directional element. Plug setting bridge is provided for current setting. The tappings are provided on the upper magnet of over current element and are connected to the bridge. Operation:-
Under normal operating conditions, power flows in the normal direction in the circuit operated by the relay. Therefore, directional power relay does not operate, thereby keeping the (lower element) un-energized.
However, when a short circuit occurs, there is a tendency for the current or power to flow in the reverse
direction. The disc of the upper element rotates to bridge the fixed contacts 1 and 2. This completes the circuit for over current element. The disc of this element rotates and the moving contact attached to closes the trip circuit. This operates the circuit breaker which isolates the faulty se ction.
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5. Write a detailed note on differential relays.
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CURRENT DIFFERENTIAL PROTECTION
Differential protection is a very reliable method of protecting generators, transformers, buses, and transmission lines from the effects of internal faults.
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Figure: Differential Protection of a Generator
In a differential protection scheme in the above figure, currents on both sides of the equipment a re compared. The figure shows the connection only for one phase, but a similar connection is usually used in each phase of the protected equipment. Under normal conditions, o r for a fault outside of the protected zone, current I1 is equal to current I2 . Therefore the currents in the current transformers secondaries are also equal, i.e. i1 = i2 and no current flows through the current relay. If a fault develops inside of the protected zone, currents I1 and I2 are no longer equal, therefore i1 and i2 are not equal and there is a current flowing through the current relay.
Differential Protection of a Station Bus
The principle of the differential protection of a station bu s is the same as for generators. The sum of all currents entering and leaving the bus must be equal to zero under normal conditions or if the fault is outside of the protected zone. If there is a fault on the bus, there will be a net flow of current to the bus and the differential relay will operate.
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BIASED BEAM RELAY or PERCENTAGE DIFFERENTIAL RELAY
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The biased beam relay also called percentage differential relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected section. It’s called percentage differential relay because the ratio of differential operating current to average restraining current is a fixed percentage. It’s called bias relay because restraining known as biased coil produces the bias force. Fig 17 a, shows the schematic arrangements of biased beam relay. It is essentially an over current balanced beam type relay with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.
Under normal and through load conditions, the bias force due to restraining coil is greater than operating force.
Therefore, the relay remains inoperative. When an internal fault occurs, the operating force exceeds the bias force. Consequently the trip contacts are closed to open the circuit breaker. The bias force can be adjusted by varying the number of turns on the restraining coil.
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The equivalent circuit of a biased beam relay is shown in fig 17 b. The differential current in operating coil is proportional to i2 – i1 and the equivalent current in the restraining coil is proportional to (i 2 – i1)/2 since the operating coil is connected to the mid-point of the restraining coil. It is clear that greater the current flowing through the restraining coil, the higher the value of current required in the operating winding to trip the relay. Thus under heavy load, a greater differential current through the relay operating coil is required for operation, than under light load conditions VOLTAGE DIFFERENTIAL RELAY
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Fig. 18 shows the arrangement of voltage balance protection. In this scheme of protection, two similar current transformers are connected at either end of the element to be protected (e.g. an alternator winding) by means of pilot of wires. The secondaries of current transformers are connected in series with a relay in such a way that under normal conditions, their induced e.m.f’s are in opposition
Under healthy conditions, equal currents will flow in both primary windings. Therefore, the secondary voltages of the two transformers are balanced against each other and no current will flow through the relay-operating
coil. When a fault occurs in they protected zone, the currents in the two primaries will differ from one another and their secondary voltages will no longer be in balance. This voltage difference will cause a current to flow through the operating coil of the relay, which closes the trip circuit
Disadvantages The voltage balance system suffers from the following drawbacks (i) A multi-gap transformer construction is required to achieve the accurate balance between current transformer pairs.
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www.chennaiuniversity.net (i) The system is suitable for protection of cables of relatively short, lengths due to the capacitance of pilot wires. . 5. Explain the working principle of distance relays. Basic operation of Impedance relay
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Reactance relay: Here the operating torque is obtained by current while the restraining torque due to a current voltage directional relay. The over current elements develops the positive torque and directional unit produces negative torque. Thus the reactance relay is an overcurrent relay with the directional restraint. The directional element is so designed that the maximum torque angle is 900
Construction 51
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www.chennaiuniversity.net The str ucture used for reactance relay can be of induction cup type. It is a four pole structure. It has operating coil, polarizing coil and a restraining coil. The diagram is given above. The current I flows from pole 1, through iron core stacking to lower pole3. The winding on pole 4 is fed from voltage V. the operating torque is produced by interaction of fluxes due to the windings carrying current coils i.e., interaction of fluxes produced by poles 1,2 and 3. While restraining torque is developed due to interaction of fluxes due to poles 1,3 and 4. Hence operating torque is proportional to the square of the current [ I2] while the restraining torque is proportional to the product of V and I [V×I]. The desired maximum torque angle is obtained with the help of RC network. Torque equation:
The operating torque is proportional to the square of the current [ I2] while the restraining torque is proportional to the product of V and I [V×I]. Hence the net torque neglecting the effect of spring is given by
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T = K 1 I2 – K 2VI cos [Ø-τ] At the balance point net torque is zero
0 = K 1 I2 – K 2VI cos [Ø-τ] K 1 I2 = K 2VI cos [Ø-τ] K 1 = K 2
cos [Ø-τ]
K 1 = K 2 cos [Ø-τ]
Adding capacitor, the torque angle is adjusted as 900 K 1 = K 2 cos [Ø-900] K 1 = K 2 sinØ K1
sinØ = K2
Consider an impedance triangle
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= K2 = constant
Thus the relay operates on the reactance only. The constant X means a straight line parallel to X- axis on R-X diagram
ASSIGNMENT 7. Describe the realization of a directional over current relay using a microprocessor.
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(16)
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8. Derive a generalized mathematical model of distance relays for digital protection. 9. (a) How can digital distance relaying algorithm be implemented on the 8086 Micro processor? (b) It is possible to implement these algorithms on the 8085micro processor? 10. Explain with sketches and their R-X diagrams for the following distance relays.
(16)
(i) Impedance relay (ii) Mho relay (iii) Reactance relay 11. (a) Explain the applications of microprocessors in power system protection. (b) Explain microprocessor based inverse time over current relay.
(5) (5) (6) (8) (8)
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(8) (8)
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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.
CIRCUIT BREAKERS
1. What is circuit breaker? It is a piece of equipment used to break a circuit automatically under fault conditions. It breaks a circuit either manually or by remote control under normal conditions and under fault conditions. 2. Write the classification of circuit breakers based on the medium used for arc extinction? _ Air break circuit breaker _ Oil circuit breaker _ Minimum oil circuit breaker _ Air blast circuit breaker _ SF6 circuit breaker _ Vacuum circuit breaker
3. What is the main problem of the circuit breaker? When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which ma y cause damage to the system or to the breaker itself. This is the main problem. 4. What are demerits of MOCB? _ Short contact life _ Frequent maintenance _ Possibility of explosion _ Larger arcing time for small currents _ Prone to restricts 5. What are the advantages of oil as arc quenching medium? • It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties • It acts as an insulator and permits smaller clearance between line conductors and ea rthed components 55
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www.chennaiuniversity.net 6.What are the hazards imposed by oil when it is used as an arc quenching medium? There is a risk of fire since it is inflammable. It may form an ex plosive mixture with arc. so oil is preferred as an arc quenching medium 7. What are the disadvantages of MOCB over a bulk oil circuit breaker? The degree of carbonization is increased due to smaller quantity of oil o There is difficulty of removing the gases from the contact space in time o The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization. 8. What are the types of air blast circuit breaker? _ Arial-blast type _ Cross blast _ Radial-blast 9. What are the advantages of air blast circuit breaker over oil circuit breaker? o The risk of fire is diminished o The arcing time is very small due to rapid buildup of dielectric strength between contacts o The arcing products are completely removed by the blast whereas oil deteriorates with successive operations 10. What are the demerits of using oil as an arc quenching medium? • The air has relatively inferior arc quenching properties • The air blast circuit breakers are very sensitive to variations in the rate of rise of restriking voltage • Maintenance is required for the compression plant wh ich supplies the air blast
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11. What is meant by electro negativity of SF6 gas? SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas molecule, the electron is absorbed by the neutral gas molecule and negative ion is formed. This is called as electro negativity of SF6 gas.
12. What are the characteristic of SF6 gas? It has good dielectric strength and excellent arc quenching property. It is inert, non toxic, no inflammable and heavy. At atmospheric pressure, its dielectric strength is 2.5 times that of air. At three times atmospheric pressure, its dielectric strength is equal to that of the transformer oil. 13. Write the classifications of test conducted on circuit breakers. _ Type test _ Routine test _ Reliability test _ Commissioning test 14. What are the indirect methods of circuit breaker testing? o Unit test o Synthetic test o Substitution testing o Compensation testing o Capacitance testing 15. What are the advantages of synthetic testing methods? • The breaker can be tested for desired transient recovery voltage and RRRV. • Both test current and test voltage can be independently varied. This gives flexibility to the test 56
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www.chennaiuniversity.net • The method is simple • With this method a breaker capacity (MVA) of five time of that of the capacity of the test plant can be tested. 16. How does the over voltage surge affect the power system? The over voltage of the power system leads to insulation breakdown of the equipment’s. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line.
17. What are the advantages of MOCB over a bulk oil circuit breaker? • It requires lesser quantity of oil • It requires smaller space • There is a reduced risk of fire • Maintenance problem are reduced
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14. What is the need of relay coordination? The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently quick backup protection so that the rest of the system is protected. By coordina ting relays, faults can always be isolated quickly without serious disturbance to the rest of the s ystem.
15. Mention the short comings of Merz Price scheme of protection applied to a power transformer. In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay under normal condition. 16. What are the various faults to which a turbo alternator is likely to be subjected? Failure of steam supply; failure of speed; overcurrent; over voltage; unbalan ced loading; stator winding fault .
17. What is an under frequency relay? An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value. 18. Define the term pilot with reference to power line protection. Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.
19. Mention any two disadvantage of carrier current scheme for transmission line only. The program time (ie, the time taken b y the carrier to reach the other end-upto .1% mile); the response time of band pass filter; capacitance phase-shift of the transmission line 20. What are the features of directional relay? High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive. 57
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www.chennaiuniversity.net 21. What are the causes of over speed and how alternators are protected from it? Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs. 22. What are the main types of stator winding faults? Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same phase winding. 23. Give the limitations of Merz Price protection. Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral po int, the voltage may be insufficient to operate the rela y. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase d ifference between the primary and the secondary quantities and a possibility of current through the relay even whe n there is no fault.
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24. What are the uses of Buchholz’s relay? Bucholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer and to connect the transformer from the supply in the eve nt of severe internal faults. It is usually used in oil immersion transformers with a rating over 750KVA.
16 Mark Questions
1]Explain the function of Air Blast Circuit Breaker Air Blast Circuit Breaker Working In the air blast circuit breakers the arc interruption takes place to direct a blast of air, at high pressure and velocity, to the arc. Dry and fresh air of the air blast will replace the ionized hot gases within the arc zone and the arc length is considerably increased.Consequently the arc may be interrupted at the first natural current zero. In air blast circuit breakers, the contacts are surrounded by compressed air. When the contacts are opened the compressed air is released in forced blast through the arc to the atmosphere extinguishing the arc in the process.A compressor plant is necessary to maintain high air pressure in the receiver.
The air blast circuit breakers are especially suitable for railways and arc furnaces, where the breaker operates repeatedly. Air blast circuit breakers is used for interconnected lines and important lines where rapid operation is desired. In air blast circuit breaker (also called compressed air c ircuit breaker) high pressure air is forced on the arc through a nozzle at the instant of contact separation. The ionized medium between the contacts is blown away by the blast of the air. After the arc extinction the chamber is filled with high pressure air, which prevents restrike. In some low capacity circuit breakers, the isolator is an integral part of the circuit breaker . The circuit breaker opens and immediately after that the isolator opens, to provide addition gap.
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In the air reservoir there is a high pressure air stored between 20 to 30 kg/cm2. And that air is taken from compressed air system. On the reservoir there are three hollow insulator columns mou nted with valves at their base. On the top of the hollow insulator chambers there are double arc extinguishing chambers mounted . The current carrying parts connect the three a rc extinction chambers to each other in series and the pole to the neighboring equipment. since there exist a very high voltage between the conductor and the air reservoir, the entire arc extinction chamber assembly is mounted on insulators. Since there are three double arc ex tinction poles in series, there are six breakers per pole. Each arc extinction chamber consists of one twin fixed contact. There are two moving contacts. The moving contacts can move axially so as to open or close. Its opening or closing mechanism depends on spring pressure and air pressure.
The operation mechanism operates the rods when it gets a pneumatic or electrical signal. The valves open so as to send the high pressure air in the hollow of the insulator. The high pressure air rapidly enters the double arc extinction chamber. As the air enters into the arc extinction chamber the pressure on the moving contacts becomes more than spring pressure and i t causes the contacts to be open.The contacts travel through a short distance against the spring pressure. At the end of contacts travel the part for outgoing air is closed by the 59
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www.chennaiuniversity.net moving contacts and the entire arc extinction chamber is filled with high pressure air, as the air is not allowed to go out. However, during the arcing period the air goes out through the openings and takes away the ionized air of arc.
While closing, the valve is turned so as to close connection between the hollow of the insulator and the reservoir. The valve lets the air from the hollow insulator to the atmosphere. As a result the pressure of air in the arc extinction chamber is dropped down to the atmospheric pressure and the moving contacts close over the fixed contacts by virtue of the spring pressure. the opening is fast because the air takes a negligible time to travel from the reservoir to the moving contact. The arc is extinguished within a cycle. Therefore, air blast circuit breaker is very fast in breaking the current.Closing is also fast becau se the pressure in the arc extinction chamber drops immediately as the value operates and the contacts close by virtue of the spring pressure.
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Advantages: How air blast circuit breaker is better than oil circuit breaker: 1. The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very small. this reduces the size of device. 2. The risk of fire is eliminated. 3. Due to lesser arc energy, air blast circuit breakers are v ery suitable for conditions where frequent operation is required. 4. The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations; the expense of regular oil is replacement is avoided. 5. The energy supplied for arc extinction is obtained from high pressure air and is independent of the current to be interrupted. 6. The arcing time is very small due to the rapid build up of dielectric strength between contacts. Therefore, the arc energy is only a fraction that in oil circuit breakers, thus resulting in less burning of contacts. Disadvantages: 1. Considerable maintenance is required for the compressor plant which supplies the air blast. 2. Air blast circuit breakers are very sensitive to the variations in the rate of restriking voltage. 3. Air blast circuit breakers are finding wide applications in high voltage installations.Majority of circuit breakers for voltages beyond 110 kV are of this type. 2] Describe the operating principle of air vast circuit breakers. Two types of air vast circuit breakers are i. Axial air blast circuit breaker ii. Cross Blast air circuit breaker I] Axial-blast air circuit breaker
breaker. The fixed and moving contacts are held in closed position by spring pressure under normal con ditions. The air reservoir is connected to the arcing chamber through an air valve. This valve remains closed under normal conditions but
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tripping impulse causes the opening of the air valve which connects the circuit breaker reservoir to the arcing chamber.The high pressure air entering the arcing chamber pushes away the moving contact against spring pressure.
rated and an arc is struck.At the same time,high pressure air blast flows along the arc and takes away the ionised gases along with it.Consequently,the arc is extinguished and current flow is interrupted. e contact separation required for interruption is generally small about 1.75 cm. Such a small gap may constitute inadequate clearance for the normal service voltage.Therefore,an isolating switch is incorporated as part of this type of circuit breaker.This switch opens immediately after fault interruption to provide necessary clearance for insulation. II] Cross Blast air circuit breaker
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In this type of circuit breaker,an air blast is directed at right a ngles to the arc.The cross-blast lengthens and forces the arc into a suitable chute for arc extinction.Figure below shows the parts of a typical cross blast air circuit breaker When the moving contact is withdrawn,am arc is struck between the fixed an d moving contacts.the high pressure cross-blast forces into a chute consisting of an arc splitters and baffles.The splitters serve to increase the length of the arc and baffles give improved cooling.The result is that arc is e xtinguished and flow of current is interrupted.Since the blast pressure is same for all currents,the inefficiency at low currents is eliminated.The final gap for interruption is great enough to give normal insulation clearance so that series isolating switch is not necessary.
3]With the neat sketch explain the construction, principle of operation and application of Air break circuit breaker
Air break circuit breaker These circuit breakers employ high resistance interruption principle. The arc is rapidl y lengthened by means of the arc runners and arc chutes and the resistance of the arc is increased by cooling, lengthening and spilitting the arc. The arc resistance increases to such an extent that the voltage drop across the arc becomes more than the supply voltage and the arc extinguished. Air breaker circuit breakers are used in d.c circuits and a.c circuits upto 12 kV. Magnetic field is utilized for lengthening the arc i n high voltage air break circuit breaker. The arc resistance is increased to such an extent that the system voltage cannot maintain the arc and the arc gets extinguished. There are two set of contacts: Main contacts (1) and Arching contacts (2). Main contacts conduct the current in closed position of the breaker. They have low contact resistance and are silver plated. The arching contacts (2) are hard, heat resistance and usually made of copper alloy. While opening the contact, the main contacts dislodge first. The current is shifted to the arching contacts. The arching contacts dislodge later and arc is drawn between the m (3). This arc is forced upwards by the electromagnetic force and 62
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www.chennaiuniversity.net thermal action. The arc ends travel along the Arc Runner (Arcing horns). The arc moves upwards and is split by arc splitter plates (5). The arc is extinguished by l engthening, cooling, splitting etc. In some breakers the arc is drawn in the direction of the splitter by magn etic field. Operating Mechanisms for Air Break Circuit Breakers The operating mechanisms are generally operating spring. The closing force is obtained from the following means: a. Solenoid b. Spring charged manually or by motor c. Pneumatic
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The solenoid mechanisms drive power from battery suppl y or rectifiers. The solenoid energized by the direct current gives the necessary force for the closing of the circuit breaker.
The springs used for closing operation can b e charged either manually or by motor driven gears. At the time of closing operation the energy stored in the spring is released by unlatching of the spring and is utilized in closing of the circuit breaker.
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4] I] What are the properties of SF 6 gas that makes it more useful for circuit breaking? II] With the neat sketch explain the construction, principle of operation and application of SF 6 circuit breaker.
At this point we are aware that the medium in which arc extinction of the circuit breaker takes place greatly influences the important characteristics and life of the circuit breaker. In the last article the working of a vacuum circuit breaker was illustrated. We already know that the use of vacuum circuit breaker is mainly restricted to system voltage below 38 kV. The characteristics of vacuum as medium and cost of the vacuum CB does not makes it suitable for voltage exceeding 38 kV. In the past for higher transmission voltage Oil Circuit Breaker (OCB) and Air Blast Circuit Breaker (ABCB) were used. These days for hi gher transmission voltage levels SF6 Circuit Breakers are largely used. OCB and ABCB have almost become obsolete. In fact in many installations SF6 CB is used for lower voltages like 11 kV, 6 kV etc.. 66
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www.chennaiuniversity.net i] Sulphur Hexafluoride symbolically written as SF6 is a gas which satisfy the requirements of an id eal arc interrupting medium. So SF6 is extensively used these days as an arc interrupting medium in circuit breakers ranging from 3 kv upto 765 kv class. In addition to this SF6 is used in many electrical equipments for insulation. Here first we discuss in brief, some of the essential properties of SF6 which is the reason of it's extensive use in circuit breakers
SF6 gas has high dielectric strength which is the most important quality of a material for use in electrical equipments and in particular for breaker it is one of the most desired properties. Moreover it has high Rate of Rise of dielectric strength after arc extinction. Th is characteristics is very much sought for a circuit breaker to avoid restriking. SF6 is colour less, odour less and non toxic gas. SF6 is an inert gas. So in normal operating condition the metallic parts in contact with the gas are not corroded. This ensures the life of the breaker and reduces the need for maintenance. SF6 has high thermal conductivity which means the heat dissipation capacity is more. This implies greater current carrying capacity when surrounded by SF6 . The gas is quite stable. However it disintegrates to other fluorides of Sulphur in the presence of arc. but after the extinction of the arc the SF6 gas is reformed from the decomposition. SF6 being non-flammable so there is no risk of fire hazard and explosion.
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A sulfur hexafluoride circuit breaker uses contacts surrounded b y sulfur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and ma y be incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or de -rating of the circuit breakers may be required due to liquefaction of the SF6 gas.
Advantages :
Due to superior arc quenching property of sf6 , such breakers have very short arcing time Dielectric strength of sf6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents. Gives noiseless operation due to its closed gas circuit Closed gas enclosure keeps the interior dry so th at there is no moisture problem There is no risk of fire as sf6 is non-inflammable There are no carbon deposits Low maintenance cost, light foundation requirements and minimum auxiliary equipment sf6 breakers are totally enclosed and sealed from atmosphere, they are particularly suitable where explosion hazard exists
Disadvantages:
sf6 breakers are costly due to high cost of sf6 sf6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose
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www.chennaiuniversity.net CONSTRUCTION, PRINCIPLE OF OPERATION The construction and working principles of SF6 circuit breaker varies from manufacturer to manufacturer. In the past double pressure type of SF6 breakers were used. Now these are obsolete. Another type of SF6 breaker design is the self blast type, which i s usually used for medium transmission voltage. The Puffer type SF6 breakers of single pressure type are the most favoured types prevalent in power industry. Here the working principle of Puffer type breaker is illustrated (Fig-A).
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www.chennaiuniversity.net As illustrated in the figure the breaker has a c ylinder and piston arrangement. Here the piston is fixed but the cylinder is movable. The cylinder is tied to the moving contact so that for opening the breaker the cylinder along with the moving contact moves away from the fixed contact (Fig-A(b)). But due to the presence of fixed piston the SF6 gas inside the cylinder is compressed. The compressed SF6 gas flows through the nozzle and over the electric arc in axial direction. Due to heat convection and radiation the arc radius reduces gradually and the arc is finally extinguished at current z ero. The dielectric strength of the medium between the separated contacts increases rapidly and restored quickly as fresh SF6 gas fills the space. While arc quenching, small quantity of SF6 gas is broken down to some other fluorides of sulphur which mostly recombine to form SF6 again. A filter is also suitably placed in the interrupter to absorb the remaining decomposed byproduct. The gas pressure inside the cylinder is maintained at around 5 kgf per sq. cm. At higher pressure the dielectric strength of the gas increases. But at higher pressure the SF6 gas liquify at higher temperature which is undesired. So heater is required to be arranged for automatic control of the temperature for circuit breakers where higher pressure is utilised. If the SF6 gas will liquify then it loses the ability to qu ench the arc.
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Like vacuum breaker, SF6 breakers are also available in modular design form so that two modules connected in series can be used for higher voltage levels. SF6 breakers are available as both live tank and dead tank types. In Fig-B above a live tank outdoor type 400 kV SF6 breaker is shown.
5] Explain the construction and operation vacuum circuit breaker with neat diagram. Vacuum Circuit Breakers( VCB )
In this breaker, vacuum is being used as the arc quenching medium. Vacuum offers highest insulating strength, it has far superior arc quenching properties than any other medium. When contacts of a breaker are opened in vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times that obtained with other circuit breakers.
Principle: When the contacts of the breaker are opened in vacuum (10 -7 to 10 -5 torr), an arc is produced between the contacts by the ionization of metal vapours of contacts. The arc is quickly extinguished because the metallic vapours, electrons, and ions produced during arc condense quickly on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength. As soon as the arc is produced in vacuum, it is
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Construction:
Fig shows the parts of a typical vacuum circuit breaker. It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows .This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak .A glass vessel or ceramic vessel is used as the outer insulating bod y. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic va pours falling on the inside surface of the outer insulating cover.
Working: When the breaker operates the moving contacts separates from the fixed contacts and an arc is struck between the contacts. The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts. The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in short time and seized by the surfaces of moving and fixed members and shields. Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation.
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Advantages: a. They are compact, reliable and have longer life. b. There are no fire hazards c. There is no generation of gas during and after operation d. They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy fault current perfectly just before the contacts reach the definite open position. e. They require little maintenance and are quiet in operation f. Can withstand lightning surges g. Low arc energy h. Low inertia and hence require smaller power for control mechanism.
Applications: For outdoor applications ranging from 22 kV to 66 kV. Suitable for majority of applications in rural area.
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6] Explain the construction and operation bulk oil circuit breaker with neat diagram. Types Of Oil Circuit Breakers Oil circuit breakers can be classified into following types: 1)Bulk oil circuit breakers which use a large quantity of oil. In this circuit breaker the oil serves two purposes. Firstly it extinguishes the arc during opening of contacts and secondly it insulates the current conducting parts from one another and from the earthed tank. Such circuit breakers are classified into: a)Plain oil circuit breakers b)Arc control circuit breakers In the former type no means is available for controlling the arc and the contacts are exposed to the whole of the
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oil in the tank. In the latter special arc control devices are employed to get the beneficial action of the arc as efficiently as possible
2)Low oil circuit breakers, which use minimum amount of oil. In such circuit breakers oil is used only for arc extinction, the current conducting parts are insulated b y air or porcelain or organic insulating material.
Construction
There are two chambers in a low oil circuit breaker,the oil in each chamber is separated from each other.The main advantage of this is that low oil is required and oil in second chamber wont get polluted.Upper chamber is called the circuit breaker chamber and lower one is called the supporting chamber.Circuit breaking chamber consists of moving contact and fixed contact.Moving contact is connected with a piston its just for the movement of the contact and no pressure build due to its motion.There are two vents on fixed contact they are axial vent for small current produced in oil due to heating of arc and radial vents for large currents.The whole device is covered using Bakelite paper and porcelain for protection.Vents are placed in a turbulator. Operation Under normal operating conditions,the moving contacts remain engaged with the upper fixed contact.When a fault occurs,the moving contact is pulled down by the tripping springs and an arc is struck.The arc vapourises oil and produces gases under high pressure.This action constrains the oil to pass through a c entral hole in the moving contact and results in forcing series of oil through the respective passages of the turbulator.The process of turbulation is orderly one,in which the sections of arc are successively quenched by the effectof separate streams of oil ,moving across each section in turn and bearing away its gases
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Advantages A low oil circuit breaker has following advantages co mpared to bulk oil circuit breaker 1. It requires lesser quantity of oil 2. It requires smaller space 3. There is reduced risk of fire 73
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www.chennaiuniversity.net 4. Maintenance problems are reduced Disadvantages A low oil circuit breaker has following disadvantages co mpared to bulk oil circuit breaker 1. Due to smaller quantity of oil,the degree of carbonisation is increased 2. There is a difficulty of removing the gases from the contact space in time 3. The dielectric strength of oil deteriorates rapidly due to high degree of carbonisation.
7. I) What are the advantages and disadvantages of air blast circuit breakers? II) Discuss the synthetic testing of circuit breakers.
[8] [8]
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I]Advantages: 1. The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very small. this reduces the size of device. . The risk of fire is eliminated. 3. Due to lesser arc energy, air blast circuit breakers are v ery suitable for conditions where frequent operation is required. 4. The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations; the expense of regular oil is replacement is avoided. 5. The energy supplied for arc extinction is obtained from high pressure air and is independ ent of the current to be interrupted. 6. The arcing time is very small due to the rapid build up of dielectric strength between contacts. Therefore, the arc energy is only a fraction that in oil circuit breakers, thus resulting in less burning of contacts.
Disadvantages: 1. Considerable maintenance is required for the compressor plant which supplies the air blast. 2. Air blast circuit breakers are very sensitive to the variations in the rate of restriking voltage. 3. Air blast circuit breakers are finding wide applications in high voltage installations.Majority of circuit breakers for voltages beyond 110 kV are of this type.
II]Synthetic testing The principle of synthetic testing is given below. The current source provides short circuit current. The voltage source gives restriking and recovery voltage. The test observations are proceeded by L,R & C. the circuit current is feed by closing the switch S1 [IG]. final current becomes zero when switch S2 is closed and voltage contains transient as it contains I and C . Advantages The breakers can be tested for desaired TRV and RRRV The shor circuit generator has to supply currents at less voltage. It is flexible because of independent voltage test and current test It is very simple & it can applied to unit test also. Upto five times of plant capacity can be tested.
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Synthetic testing Types of Synthetic test circuits.
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a.Parallel current injection b.Series current injection
a.Parallel current injection method.
This method is used for testing circuit breakers. It gives high frequenc y voltage as given by the standards. It is given by the graph. Here the voltage circuit is effectively connected in parallel with current circuit and test breaker before main IG in test breaker current is properly simulated.
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b. Series current injection method
Here the voltage circuit [2] is connected to current circuit in series before main current zero. Due to this IG and IH are in opposition. The stresses produced in synthetic test and those in actual ne twork must be same but it is not the actual case because of several factors like high current, high voltage , instant of applying voltage etc.,
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ASSIGNMENT
1.
(a) Enumerate various types of ratings of a circuit breaker.
(4)
(b) Discuss symmetrical and asymmetrical breaking capacity,
(4)
(c) Making capacity
(4)
(d) Short-time current capacity.
(4)
2. What are the different methods of testing of circuit breakers? Discuss their merits and demerits.
(16)
3. What is the difficulty in the development of HVDC circuit breaker? Describe its construction and operating principle.
(16)
4. (a) What are the physical chemical and dielectric properties of SF6 Gas (b) Define switchgear. What are the essential features of switchgear
(8) s
(8)
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5. A 3-phase alternator has the line voltage of 11kV. The generator is connected to a circuit breaker. The inductive reactance upto the circuit breaker is 5_/phase. the distributed capacitance upto circuit br eaker between phase and neutral is 0.001 μF. Determine peak restriking voltage across the CB, frequency of restriking voltage, average rate of restriking voltage upto peak restriking voltage, maximum RRRV. (16) 77
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www.chennaiuniversity.net 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.
1. What are the types of graded used in line of radial relay feeder? Definite time relay and inverse-definite time relay. 2. What are the various faults that would affect an alternator? (a) Stator faults 1, Phase to phase faults 2, Phase to earth faults 3, Inter turn faults (b) 1, Earth faults 2, Fault between turns 3, Loss of excitation due to fuel failure (c) 1, Over speed 2, Loss of drive 3, Vacuum failure resulting in condenser pressure rise, resulting in shattering of the turbine low pressure casing (d) 1, Fault on lines 2, Fault on busbars 3. Why neutral resistor is added between neutral and earth of an alternator? In order to limit the flow of current through neutral and earth a resistor is introduced between them. 4. What is the backup protection available for an alternator? Overcurrent and earth fault protection is the backup protections. 5. What are faults associated with an alternator? (a) External fault or through fault (b) Internal fault 1, Short circuit in transformer winding and connection 2, Incipient or slow developing faults 6. What are the main safety devices available with transformer? Oil level guage, sudden pressure delay, oil temperature indicator, winding temperature indicator . 7. What are the limitations of Buchholz relay? (a) Only fault below the oil level are detected. (b) Mercury switch setting should be very accurate, otherwise even for vibration, there can be a false operation. (c) The relay is of slow operating type, which is unsatisfactory
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8. What are the problems arising in differential protection in power transformer and how are they overcome? 1. Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires. 2. Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil. 3. Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.
4. Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.
9. What is REF relay? It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the 78
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www.chennaiuniversity.net relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay. 10. What is over fluxing protection in transformer? If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing. 11. Why busbar protection is needed? (a) Fault level at busbar is high b) The stability of the system is affected by the faults in the bus zone. (c) A fault in the bus bar causes interruption of supply to a large portion of the system network. 12. What are the merits of carrier current protection? Fast operation, auto re-closing possible, easy discrimination of simultaneous faults . 13. What are the errors in CT? (a) Ratio error Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100 The value of transformation ratio is not equal to the turns ratio. (b) Phase angle error: Phase angle _ =180/_[(ImCos _-I1Sin _)/nIs]
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14. What is field suppression? When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself. Hence the field circuit breaker is opened and stored
energy in the field winding is discharged through another resistor. This method is known as field suppression. 15. What are the causes of bus zone faults? _ Failure of support insulator resulting in earth fault _ Flashover across support insulator during over voltage _ Heavily polluted insulator causing flashover _ Earthquake, mechanical damage etc.
16. What are the problems in bus zone differential protection? _ Large number of circuits, different current levels for different circuits for external faults. _ Saturation of CT cores due to dc component and ac component in short circuit currents. The saturation introduces ratio error. _ Sectionalizing of the bus makes circuit complicated. _ Setting of relays need a change with large load changes. 17. What is static relay? It is a relay in which measurement or comparison of electrical quantities is made in a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device. 18. What is power swing? During switching of lines or wrong synchronization surges of real and reactive power flowing in transmission line causes severe oscillations in the voltage and current vecto rs. It is represented by curves originating in load regions and traveling towards relay ch aracteristics.
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www.chennaiuniversity.net 19. What is a programmable relay? A static relay may have one or more programmable units such as microprocessors or microcomputers in its circuit. 20. What is CPMC? It is combined protection, monitoring and control s ystem incorporated in the static system. 21. What are the advantages of static relay over electromagnetic relay? Low power consumption as low as 1mW No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc. A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks. This is not possible in electromagnetic relays Static relay is compact Superior operating characteristics and accuracy Static relay can think , programmable operation is possible with static relay Effect of vibration is nil, hence can be used in earthquake-prone areas Simplified testing and servicing. Can convert even non-electrical quantities to electrical in conjunction with transducers.
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16 arks Questions 1.What are the various faults that may occur in an alternator. Give the diagram for circulating current protection in alternator. [8] The fault occurs in an alternator are classified as follows A] stator faults B] rotor faults 1. stator inter-turn faults 1. Field ground fault 2. stator over heating 2. Loss of exitation 3. stator external faults 3. Rotor over heating because of unbalanced 3Q stator Current. ALTERNATOR WITH PERCENTAGE DIFFERENTIAL PROTECTION
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www.chennaiuniversity.net The diagram show the alternator stator protection with circulating protection scheme. During the normal condition or external fault, the current flowing through the two CT’s are same. So no current flows through the restraining coil and operating coil so relay cannot operate. During the inter turn fault, the winding of stator will be grounded and hence the current will flow to the ground. So differential current will flow through the two CT’s and hence the resultant current will flow through the operating coil. Due to the current flowing the operating coil, the relay operates. The relay senses second and gives signal to the CB and CB gets opened and breaks the current flow to the alternator.
II] A 5000KVA, 6600v star connected alternator has a synchronous reactance of 2 Ω per phase and 0.5Ω resistance. It is protected by Merz prize balanced current system which operates when the out of balanced current exceeds 30% of the load current. Determine what proportion of the alternator winding is unprotected if the star point is
earthed through a resistor of 6.5Ω? Given: Rating 5000 KVA Voltage 6600V Xs = 2 Ω /ph R = 0.5 Ω Operating current = 30% of load current Earth resistor [R h] = 6.5Ω Solution Primary earth fault current at which the relay operates
Fault current [or] operating current =
5000×103 30 × √3×6600 100
= 131.22 Amps. Percentage of winding unprotected
√ 3 = × 100
=
√3×131.22×6.5 6600
% of winding unprotected = 22.38% Therefore the % of winding protected is = 100 – 22.38 = 77.62%
1. Explain the time graded over current for radial feeders. 1. Enumerate the relaying schemes which are employed for the protection of a modern alternator. 2. (a) What is transverse or split phase protection of an alternator? (b) What type of fault is this scheme of protection employed? (c) With a neat sketch discuss the working principle of this scheme. 3. What type of a protective device is used for the protection of an alternator against Overheating of its (i) stator (ii) rotor? Discuss them in brief. 4. What type of a protective scheme is employed for the protection of the field winding of the alternator against ground faults? 5. Discuss the protection employed against loss of excitation of an alternator. 6. (a) What do you understand by field suppression of an alternator? (b) How is it achieved? 7. Briefly discuss the protection of an alternator against. (i) Vibration of distortion of motor (ii) Bearing overheating (iii) Auxiliary failure 81
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(16) (4) (4) (8) (8+8) (16) (16) (8) (8) (4) (4) (4)
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www.chennaiuniversity.net (iv) Voltage regulator failure 8. What type of pilot protection is used for EHV and UHV transmission lines. 9. (a) What is carrier protection? (b) For what voltage range is it used for the protection of transmission line? (c) What are its merits and demerits? 10. (a) What is carrier aided distance protection. (b) What are its different types? (c) Discuss the permissive under-reach transfer tripping scheme of protection. 11. (a) Draw and explain the merz-price protection of alternator stator winding. (b) A generator is protected by restricted earth fault protection. The generator ratings are 13.2kv, 10MVA. The percentage of winding protected against phase to ground fault is 85%. The relay setting is such that it trips for 20% out of balance calculate the resistance to be added in the neutral to ground connection.
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(4) (16) (4) (4) (8) (4) (4) (8) (10)
(6)
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FRAME LEAKAGE PROTECTION
EXTERNAL FAULT
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Phase and earth fault protection
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Merz-prize protection for star-star power transformer
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Magnetizing Inrush Current Wave Forms
Magnetizing Current Compensation
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FRAME LEAKAGE PROTECTION
BASIC ARRANGEMENT OF BUCH-HOLZ RELAY
CONSTRUCTION OF BUCH-HOLOZ RELAY
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Phase fault protection
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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 IV THEORY OF CIRCUIT INTERRUPTION
1. What is resistance switching? It is the method of connecting a resistance in parallel with the contact space(arc). The resistance reduces the restriking voltage frequency and it diverts part of the arc current. It assists the circ uit breaker in interruptingthemagnetizing current and capacity current. 2. What do you mean by current chopping? When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping. 3. What are the methods of capacitive switching? • Opening of single capacitor bank • Closing of one capacitor bank against another
4. What is an arc? Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition. 5. Give the two methods of arc interruption? High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished _ Current zero method:-The arc is interrupted at current zero position that occurs100 times a second in case of 50Hz power system frequency in ac. 92
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