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Department Of Electrical And Electronics Engineering

EE2402 -PROTECTION AND SWITCHGEAR

UNIT I INTRODUCTION

UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS

UNIT III APPARATUS PROTECTION

UNIT IV THEORY OF CIRCUIT INTERRUPTION

UNIT V CIRCUIT BREAKERS

2AIM:To introduce the students to the various abnormal operating conditions in power system anddescribe the apparatus and system protection schemes. Also to describe the phenomena of current interruption to study thevarious switchgears.OBJECTIVES:i. To discuss the causes of abnormal operating conditions (faults, lightning and switchingsurges) of the apparatus and system.ii. To understand the characteristics and functions of relays and protection schemes.iii. To understand the problems associated with circuit interruption by a circuit breaker.

UNIT I INTRODUCTION 9Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults and faultcurrents - relay terminology definitions - and essential qualities of protection. Protection against over voltages due tolightning and switching - arcing grounds - Peterson Coil - ground wires - surge absorber and divertersPower System earthing neutral Earthing - basic ideas of insulation coordination.

UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS 9Electromagnetic relays over current, directional and non-directional, distance, negative sequence, differential and underfrequency relays Introduction to static relays.

UNIT III APPARATUS PROTECTION 9Main 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 protectionschemes.

UNIT IV THEORY OF CIRCUIT INTERRUPTION 9Physics 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 9Types of circuit breakers air blast, air break, oil, SF6 and vacuum circuit breakers comparative merits of differentcircuit breakers testing of circuit breakers.

TOTAL : 45 PERIODSTEXT BOOKS:1. M.L. Soni, P.V. Gupta, V.S. Bhatnagar, A. Chakrabarti, A Text Book on Power SystemEngineering, 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, FirstEdition Reprint 2007.

REFERENCES:1. Sunil S. Rao, Switchgear and Protection, Khanna publishers, New Delhi, 1986.2. C.L. Wadhwa, Electrical Power Systems, Newage International (P) Ltd., 2000.

3. B. Ravindranath, and N. Chander, Power System Protection & Switchgear, Wiley Eastern Ltd.,1977.4. Badri Ram, Vishwakarma, Power System Protection and Switchgear, Tata McGraw Hill, 2001.5. Y.G. Paithankar and S.R. Bhide, Fundamentals of Power System Protection, Prentice Hall of India Pvt. Ltd., NewDelhi110001, 2003.

UNIT I INTRODUCTION 9Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults andfault currents - relay terminology definitions - and essential qualities of protection. Protection against over

3voltages due to lightning and switching - arcing grounds - Peterson Coil - ground wires - surge absorber anddiverters Power System earthing neutral Earthing - basic ideas of insulation coordination.

1] Discuss and compare the various methods of neutral earthing.[Any two type may ask each carries 8marks or brief all the five divisions] [16]Types of Neutral Earthing in Power Distribution:

Introduction:In the early power systems were mainly Neutral ungrounded due to the fact that the first ground fault did

not require the tripping of the system. An unscheduled shutdown on the first ground fault was particularlyundesirable for continuous process industries. These power systems required ground detection systems, butlocating the fault often proved difficult. Although achieving the initial goal, the ungrounded system provided nocontrol 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 whensubjected to repetitive re-strikes of one phase to ground. This in turn, reduces insulation life resulting inpossible equipment failure.

Neutral grounding systems are similar to fuses in that they do nothing until something in the systemgoes wrong. Then, like fuses, they protect personnel and equipment from damage. Damage comes fromtwo factors, how long the fault lasts and how large the fault current is. Ground relays trip breakers and limithow 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. Theneutral points of transformers, generators and rotating machinery to the earth ground network provides areference point of zero volts. This protective measure offers many advantages over an ungrounded system, like,

1. Reduced magnitude of transient over voltages2. Simplified ground fault location3. Improved system and equipment fault protection4. Reduced maintenance time and expense5. Greater safety for personnel6. Improved lightning protection7. Reduction in frequency of faults.

Method of Neutral Earthing: There are five methods for Neutral earthing.1. Unearthed Neutral System2. Solid Neutral Earthed System.3. Resistance Neutral Earthing System.

1. Low Resistance Earthing.2. High Resistance Earthing.

4. Resonant Neutral Earthing System.5. Earthing Transformer Earthing.

(1) Ungrounded Neutral Systems: In ungrounded system there is no internal connection between the conductors and earth. However, as

system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces.Consequently, the ungrounded system is, in reality, a capacitive grounded system by virtue of thedistributed capacitance.

Under normal operating conditions, this distributed capacitance causes no problems. In fact, it isbeneficial because it establishes, in effect, a neutral point for the system; As a result, the phaseconductors are stressed at only line-to-neutral voltage above ground.

But problems can rise in ground fault conditions. A ground fault on one line results in full line-to-linevoltage appearing throughout the system. Thus, a voltage 1.73 times the normal voltage is present on all

4insulation in the system. This situation can often cause failures in older motors and transformers, due toinsulation breakdown.

Advantage:1. After the first ground fault, assuming it remains as a single fault, the circuit may continue in operation,

permitting continued production until a convenient shut down for maintenance can be scheduled.Disadvantages:

1. The interaction between the faulted system and its distributed capacitance may cause transient over-voltages (several times normal) to appear from line to ground during normal switching of a circuithaving a line-to ground fault (short). These over voltages may cause insulation failures at points otherthan the original fault.

2. A second fault on another phase may occur before the first fault can be cleared. This can result in veryhigh line-to-line fault currents, equipment damage and disruption of both circuits.

3. The cost of equipment damage.4. Complicate for locating fault(s), involving a tedious process of trial and error: first isolating the correct

feeder, then the branch, and finally, the equipment at fault. The result is unnecessarily lengthy andexpensive down downtime.

(2) Solidly Neutral Grounded Systems: Solidly grounded systems are usually used in low voltage applications at 600 volts or less.

In solidly grounded system, the neutral point is connected to earth. Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the

ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of thesystem three phase fault current. However, if the reactance of the generator or transformer is too great,the problem of transient over voltages will not be solved.

While solidly grounded systems are an improvement over ungrounded systems, and speed up thelocation of faults, they lack the current limiting ability of resistance grounding and the extra protectionthis provides.

To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must beextend from the source to the furthest point of the system within the same raceway or conduit. Itspurpose is to maintain very low impedance to ground faults so that a relatively high fault current willflow thus insuring that circuit breakers or fuses will clear the fault quickly and therefore minimize

5damage. It also greatly reduces the shock hazard to personnel

If the system is not solidly grounded, the neutral point of the system would float with respect toground 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 reducethe earth fault current is to leave some of the transformer neutrals unearthed.

Advantage:1. The main advantage of solidly earthed systems is low over voltages, which makes the earthing design

common at high voltage levels (HV). Disadvantage:1. This system involves all the drawbacks and hazards of high earth fault current: maximum damage and

disturbances.2. There is no service continuity on the faulty feeder.3. The danger for personnel is high during the fault since the touch voltages created are high.

Applications:1. Distributed neutral conductor.2. 3-phase + neutral distribution.3. Used when the short-circuit power of the source is 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:

1. To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers,

6cables, and rotating machines.2. To reduce mechanical stresses in circuits/Equipments carrying fault currents.3. To reduce electrical-shock hazards to personnel caused by stray ground fault.4. To reduce the arc blast or flash hazard.5. To reduce the momentary line-voltage dip.6. To secure control of the transient over-voltages while at the same time.7. To improve the detection of the earth fault in a power system.

Grounding Resistors are generally connected between ground and neutral of transformers, generatorsand grounding transformers to limit maximum fault current as per Ohms Law to a value which will notdamage the equipment in the power system and allow sufficient flow of fault current to detect andoperate Earth protective relays to clear the fault.

Therefore, it is the most common application to limit single phase fault currents with low resistanceNeutral Grounding Resistors to approximately rated current of transformer and / or generator.

In addition, limiting fault currents to predetermined maximum values permits the designer to selectivelycoordinate the operation of protective devices, which minimizes system disruption and allows for quicklocation of the fault.

There are two categories of 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 willincrease the phase-to-ground voltage of the remaining two phases. As a result, conductor insulation andsurge arrestor ratings must be based on line-to-line voltage. This temporary increase in phase-to-ground voltage should also be considered when selecting two and three pole breakers installed on

resistance grounded low voltage systems.

Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated withphase-to-phase faults, but both systems significantly reduce or essentially eliminate the arc-flash hazardsassociated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses andreduce 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. Generally speaking high-resistance grounding refers to asystem in which the NGR let-through current is less than 50 to 100 A. Low resistance groundingindicates that NGR current would be above 100 A.

A better distinction between the two levels might be alarm only and tripping. An alarm-only systemcontinues to operate with a single ground fault on the system for an unspecified amount of time. In atripping system a ground fault is automatically removed by protective relaying and circuit interruptingdevices. Alarm-only systems usually limit NGR current to 10 A or less.

7Rating of The Neutral grounding resistor:

1. Voltage: Line-to-neutral voltage of the system to which it is connected.2. Initial Current: The initial current which will flow through the resistor with rated voltage applied.3. 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 systems where there is a high investment incapital equipment or prolonged loss of service of equipment has a significant economic impact and it isnot commonly used in low voltage systems because the limited ground fault current is too low toreliably operate breaker trip units or fuses. This makes system selectivity hard to achieve. Moreover, lowresistance grounded systems are not suitable for 4-wire loads and hence have not been used incommercial market applications

A resistor is connected from the system neutral point to ground and generally sized to permit only 200Ato 1200 amps of ground fault current to flow. Enough current must flow such that protective devices canfault point.

Since the grounding impedance is in the form of resistance, any transient over voltages are quicklydamped out and the whole transient overvoltage phenomena is no longer applicable. Althoughtheoretically possible to be applied in low voltage systems (e.g. 480V),significant amount of the systemvoltage dropped across the grounding resistor, there is not enough voltage across the arc forcing currentto flow, for the fault to be reliably detected. For this reason, low resistance grounding is not used forlow 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. Does not prevent operation of over current devices.2. Does not require a ground fault detection system.3. May be utilized on medium or high voltage systems.4. 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 faultcurrent magnitude is typically limited to 10 amperes or less. High resistance grounding accomplishes

8two things. The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable

damage is done at the fault point. This means that the faulted circuit need not be tripped off-line whenthe fault first occurs. Means that once a fault does occur, we do not know where the fault is located. Inthis respect, it performs just like an ungrounded system.

The second point is it can control the transient overvoltage phenomenon present on ungroundedsystems if engineered properly.

Under earth fault conditions, the resistance must dominate over the system charging capacitance but notto the point of permitting excessive current to flow and thereby excluding continuous operation

High Resistance Grounding (HRG) systems limit the fault current when one phase of the system shortsor 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. HRGs are continuous current rated, so the description of a particular unit does not include a time rating.

Unlike NGRs, ground fault current flowing through a HRG is usually not of significant magnitude toresult in the operation of an over current device. Since the ground fault current is not interrupted, aground fault detection system must be installed.

These systems include a bypass contactor tapped across a portion of the resistor that pulses (periodicallyopens 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 resistanceand slightly higher ground fault current.

To avoid transient over-voltages, an HRG resistor must be sized so that the amount of groundfault current the unit will allow to flow exceeds the electrical systems charging current. As a rule ofthumb, charging current is estimated at 1A per 2000KVA of system capacity for low voltage systemsand 2A per 2000KVA of system capacity at 4.16kV.

These estimated charging currents increase if surge suppressors are present. Each set of suppressorsinstalled on a low voltage system results in approximately 0.5A of additional charging current and eachset 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 of1.5A.Add one set of surge suppressors and the total charging current increases by 0.5A to 2.0A. Astandard 5A resistor could be used on this system. Most resistor manufacturers publish detailedestimation tables that can be used to more closely estimate an electrical systems charging current.

Advantages:

1. Enables high impedance fault detection in systems with weak capacitive connection to earth2. 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

9fundamental 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).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 chargingcurrents of about 5.5A or less and should not be attempted on 15kV systems, unless proper groundingrelaying is employed.

9. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutralloads must be served through an isolation transformer.

Disadvantages:

1. Generates extensive earth fault currents when combined with strong or moderate capacitive connectionto earth Cost involved.

2. Requires a ground fault detection system to notify the facility engineer that a ground fault condition hasoccurred.

(4) Resonant earthed system:

Adding inductive reactance from the system neutral point to ground is an easy method of limiting theavailable ground fault from something near the maximum 3 phase short circuit capacity (thousands ofamperes) 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 beconnected 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.

The current generated by the reactance during an earth fault approximately compensates the capacitivecomponent 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 capacitiveearth fault current of the system.

A system in which the inductive current is slightly larger than the capacitive earth fault current is overcompensated. A system in which the induced earth fault current is slightly smaller than thecapacitiveearth fault current is under compensated

However, experience indicated that this inductive reactance to ground resonates with the system shuntcapacitance to ground under arcing ground fault conditions and creates very high transient over voltageson the system.

To control the transient over voltages, the design must permit at least 60% of the 3 phase short circuitcurrent to flow underground fault conditions.

Example. A 6000 amp grounding reactor for a system having 10,000 amps 3 phase short circuit capacityavailable. Due to the high magnitude of ground fault current required to control transient over voltages,inductance grounding is rarely used within industry.

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Petersen Coils:

A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that thecapacitive current in the earth fault is compensated by an inductive current passed by the PetersenCoil. A small residual current will remain, but this is so small that any arc between the faulted phase andearth will not be maintained and the fault will extinguish. Minor earth faults such as a broken pininsulator, could be held on the system without the supply being interrupted. Transient faults would notresult in supply interruptions.

Although the standard Peterson coil does not compensate the entire earth fault current in a network dueto the presence of resistive losses in the lines and coil, it is now possible to apply residual currentcompensation by injecting an additional 180 out of phase current into the neutral via the Peterson coil.The fault current is thereby reduced to practically zero. Such systems are known as Resonant earthingwith 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 effectivelyact as a high impedance NER, which will substantially reduce any earth fault currents, and hence alsoany corresponding EPR hazards (e.g. touch voltages, step voltages and transferred voltages, includingany 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.

(5) Earthing Transformers: For cases where there is no neutral point available for Neutral Earthing (e.g. for a delta winding), an earthingtransformer 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 effective earthingimpedance. Additional impedance can be added in series if required. A special zig-zag transformer issometimes used for earthing delta windings to provide a low zero-sequence impedance and high positiveand negative sequence impedance to fault currents.

Conclusion:

Resistance Grounding Systems have many advantages over solidly grounded systems including arc-flash hazardreduction, limiting mechanical and thermal damage associated with faults, and controlling transient overvoltages.

High resistance grounding systems may also be employed to maintain service continuity and assist withlocating the source of a fault.

When designing a system with resistors, the design/consulting engineer must consider the specificrequirements 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. 16

Protective Relaying (Part11)A protective relaying scheme should have certain important qualities. Such an essential qualities of

protective relaying are,1. Reliability2. Selectivity and Discrimination3. Speed and Time4. Sensitivity5. Stability6. Adequateness7. Simplicity and Economy

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1.1 ReliabilityA 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 arelay operates. Therefore every component and circuit which is involved in the operation of a relay plays animportant role. The reliability of a protection system depends on the reliability of various components likecircuit breakers, relays, current transformers (C.T.s), potential transformers (P.T.s), cables, trip circuits etc. Theproper maintenance also plays an important role in improving the reliable operation of the system. Thereliability 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 inherentreliability is based on the design which is based on the long experience. This can be achieved by the factors like,i) Simplicity ii) Robustnessiii) High contact pressure iv) Dust free enclosureiv) Good contact material vi) Good workmanship andvii) Careful Maintenance

1.2 Selectivity and DiscriminationThe 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 andabnormal condition and also between abnormal condition within protective zone and elsewhere. The protectivesystem should operate only at the time of abnormal condition and not at the time of normal condition. Hence itmust clearly discriminate between normal and abnormal condition. Thus the protective system should select thefault part and disconnect only the faulty part without disturbing the healthy part of the system.

The protective system should not operate for the faults beyond its protective zone. For example, considerthe portion of a typical power system shown in the Fig. 1.

Fig. 1

It is clear from the Fig. 1 that if fault F2 occurs on transmission line then the circuit breakers 2 and 3 shouldoperate and disconnect the line from the remaining system. The protective system should be selective inselecting faulty transmission line only for the fault and it should isolate it without tripping the adjacenttransmission line breakers or the transformer.

If the protective system is not selective then it operates for the fault beyond its protective zones andunnecessary the large part of the system gets isolated. This causes a lot of inconvenience to the supplier andusers.1.3 Speed and Time

a protective system must disconnect the faulty system as fast as possible. If the faulty system is notdisconnect 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 generatorsrunning on the consumer sude.

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The total time required between the instant of fault and the instant of final arc interruption in the circuitbreaker is called fault clearing time. It is the sum of relay time and circuit breaker time. The relay time is thetime between the instant of fault occurrence and the instant of closure of relay contacts. The circuit breakertimes is the time taken by the circuit breaker to operate to open the contacts and to extinguish the arccompletely. The fault clearing time should be as small as possible to have high speed operation of the protectivesystem.

Though the small fault clearing time is preferred, in practice certain time lag is provided. This is because,1. To have clear discrimination between primary and backup protection2. To prevent unnecessary operation of relay under the conditions such as transient, starting inrush of currentetc.

Thus fast protective system is an important quality which minimises the damage and it improves the overallstability of the power system.1.4 Sensitivity

The protective system should be sufficiently sensitive so that it can operate reliably when required. Thesensitivity 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 relationwith 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 itsoperation. Smaller the value of volt-ampere input, more sensitive is the relay. Thus 1 VA input relay is moresensitive than the 5VA input relay.

Mathematically the sensitivity is expressed by a factor called sensitivity factor . It is the ratio of minimumshort circuit current in the protected zone to the minimum operating current required for the protection to start.

Ks = Is/Iowhere Ks = sensitivity factor

Is = minimum short circuit current in the zoneIo= minimum operating current for the protection

1.5 StabilityThe 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 impossibleto provide protection against each and every abnormal condition which may exist in practice, due to economicalreasons. But the protective system must provide adequate protection for any element of the system. Theadequateness of the system can be assessed by considering following factors,1. Ratings of various equipments2. Cost of the equipments3. Locations of the equipments4. 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 EconomyIn 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 economicallyunjustified. In such cases compromise is done. As a rule, the protection cost should not be more than 5% of thetotal cost. But if the equipments to be protected are very important, the economic constrains can be relaxed.

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The protective system should be as simple as possible so that it can be easilymaintained. The complex system are difficult from the maintenance point of view.The simplicity and reliability are closely related to each other. The simpler system arealways more reliable.

3] Discuss the nature and causes of different faults in a power system.

(16) Nature and causes of Faults: Any faults in electrical apparatus are nothing

but the defect in its electricalcircuit which makes current path directed from its intended path. Normally due tobreaking of conductors or failure of insulation, these faults occur. The other reasons foroccurrence of fault include mechanical failure, accidents. Excessive internal andexternal stresses. The impedance of the path in the fault is low and the fault currents arecomparatively large. The induction of insulation is not considered as a fault until itshows some effect sucj as excessive current flow or reduction of impedance betweenconductors or between conductors and earth.

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 isdiverted towards the fault which affects the supply to the neighboring zone.

A power system consists of generators, transformers, switchgear, transmission anddistribution circuits. There is always a possibility in such a large network that some faultwill occur in some part of the system. The maximum possibility of fault occurrence is onthe transmission lines due to their greater lengths and exposure to atmosphericconditions.

The faults cannot be classified according to the causes of their incidence. Thebreakdown may occur at normal voltage due to deterioration of insulation. Thebreakdown may also occur due to damage on account of unpredictable causes whichinclude perching of birds, accidental short circuiting by snakes, kite strings, threebranches etc. The breakdown may occur at abnormal voltages due to switching surgesor surges caused by lighting.

4] List the types of faults in power systemActive Faults

The Active fault is when actual current flows from one phase conductor to another(phase-to-phase) or alternatively from one phase conductor toearth (phase-to-earth). This type of fault can also be furtherclassified into two areas, namely the solid fault and theincipient fault.

Passive FaultsPassive faults are not real faults in the true sense of the word but are rather conditionsthat are stressing the system beyond its design

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capacity, so that ultimatelyactive faults will occur.

Typical examples are:

Overloading - leading to overheating ofinsulation (deteriorating quality,reduced life and ultimate failure).

under frequency - causing plant to behaveincorrectly.

Power swings - generators going out-of-step or synchronism with each other

Transient & Permanent FaultsTransient faults are faults which do not damage the insulation permanently and allowthe circuit to be safely re-energized after a short period of time. A typical examplewould be an insulator flashover following a lightning strike, which would besuccessfully cleared on opening of the circuit breaker, which could then beautomatically reclosed.Transient faults occur mainly on outdoor equipment where air is the main insulatingmedium.Permanent faults, as the name implies, are the result of permanent damage to theinsulation. In this case, the equipment has to be repaired and reclosing must not beentertained.Symmetrical & Asymmetrical FaultsA symmetrical fault is a balanced fault with the sinusoidal waves being equal about theiraxes, and represents asteady state condition.An asymmetrical fault displays a d.c. offset, transient in nature and decayingto the steady state of the symmetrical fault after a period of time:

Faults on a Three Phase System

The types of faults that can occur on a three phase A.C. system are as follows:Types of Faults

on a Three PhaseSystem. (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

5] What is a surge absorber? Write a short note on Ferranti surge absorber.[8] Surge absorbers are protective devices used to absorb the complete surge i,e.due to lightening surge or any transient surge in the system..........unlike thelightening arrestor in which a non-linear resistor is provided which provides alow resistance path to the dangerously high voltages on the system to theearth...Ferranti surge absorber

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6. What are the causes of over voltage on a power system?(8)

Overvoltage - Causes and Protection

Over voltages occur in a system when the system voltage rises over 110% of thenominal rated voltage. Overvoltage can be caused by a number of reasons, suddenreduction in loads, switching of transient loads, lightning strikes, failure of controlequipment such as voltage regulators, neutral displacement,. Overvoltage can causedamage 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

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ratio of the potential transformer are also entered into the relay. These relays areusually provided with a time delay. The time delay can be either instantaneous, fixedtime or for IDMT (inverse definite minimum time) curves.

Generally, overvoltage relays are provided with sufficient time delay in order toavoid 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

generators, these relay also switch off the excitation system to the generators therebypreventing voltage build- up.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 inbetween as shown in Fig 8. One rod is connected to the line circuit and the otherrod 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 theinsulator and damage it. Generally, the gap length is so adjusted that breakdown shouldoccur at 80% of spark-voltage in order to avoid cascading of very steep wave frontsacross the insulators. The string of insulators for an overhead line on the bushing oftransformer has frequently a rod gap across it. Fig 8 shows the rod gap across thebushing 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 sparksover and the surge current is conducted to earth. In this way excess charge on the linedue 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 theperformance of rod gap arrester. (iv) The polarity of the f the surge also affects theperformance of this arrester.

Due to the above limitations, the rod gap arrester is only used as a back-upprotection 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 theline circuit and the other rod is connected to earth. The distance between gap andinsulator (i.e. distance P) must not be less than one third of the gap length so that thearc may not reach the insulator and damage it. Generally, the gap length is so adjustedthat breakdown should occur at 80% of spark-voltage in order to avoid cascading ofvery steep wave fronts across the insulators. The string of insulators for an overheadline on the bushing of transformer has frequently a rod gap across it. Fig 8 shows the

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rod gap across the bushing of a transformer. Under normal operating conditions, thegap 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 tothe conducted to earth surge is harmlessly

(iii) Arcing horns(4)

Transmission and other electrical equipment can be exposed to overvoltages.Overvoltages can be caused by a number of reasons such as lightning strikes, transientsurges, sudden load fluctuation, etc. In the event of an overvoltage, the insulatingequipment such as the insulators on a transmission line or bushings in a transformercan be exposed to high voltages which may lead to their failure.

Arcing horns are protective devices that are constructed in the form ofprojections in the conducting materials on both sides of an insulator. Arcing hornsare fitted in pairs. Thus in transmission lines they are found on the conducting lineand the transmission tower across the insulators. In transmission lines, in theevent of a lightning strike on the tower, the tower potential rises to dangerous levels andcan result in flashovers across the insulators causing their failure. Arcing horns preventthis by conducting the arc across the air gap across them.

Arcing horns function by bypassing the high voltage across the insulator usingair as a conductive medium. The small gap between the horns ensures that the airbetween them breaks down resulting in a flashover and conducts the voltage surgerather than cause damage to the insulator.The horns are constructed in pair so that onehorn 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. Airinsulated switchgear are vulnerable to damage due to arcing. Arcing horns serve todivert the arc towards themselves thus protecting the switching equipment. The arcinghorns 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 Bseparated by a small air gap. The horns are so constructed that distance between themgradually increases towards the top as shown. The horns are mounted on porcelaininsulators. One end of horn is connected to the line through a resistance and choke coil Lwhile the other end is effectively grounded. The resistance R helps in limiting the followcurrent to a small value. The choke coil is so designed that it offers small reactance atnormal power frequency but a very high reactance at transient frequency. Thus the chokedoes not allow the transients to enter the apparatus to be protected. The gap between thehorns is so adjusted that normal supply voltage is not enough to cause an arc across the

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gap.

Under normal conditions, the gap is non-conducting i.e. normal supply voltage isinsufficient 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 themagnetic 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 distance may be toogreat for the voltage to maintain the arc; consequently, the arc is extinguished. Theexcess charge on the line is thus conducted through the arrester to the ground.

(iv) Basic impulse insulation level(4)

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BIL or basic impulse insulation level is the dielectric insulation gradient of a materialtested to withstand the voltage stress at a voltage impressed between the material anda conductive surface beyond the BIL rating, an electric tracking starts to occur whichwill then result into an arcing flashover to the conductive surface. In

addition it is the capacity of an equipment to withstand mechanical stress likelightning strike without causing any damage to the equipment...

11. What is Peterson coil? What protective functions are performed by this

device? (16) [Ref Q.No 1 IN 16 MARKS]

Types of Faults ona Three PhaseSystem. (A)Phase-to-earthfault(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 tem?Any fault occurring within the given zone will provide necessary tripping of relays ordisconnecting or opening of circuit breakers and thus the healthy section is safeguarded.

If a fault occurs in the overlapping zone in a proper protected scheme, more circuitbreakers than the minimum necessary to isolate the faulty part of the system wouldtrip.

3] what is surge absorber? How do they differ from surge diverter?Surge Absorber: it is a protective device used to reduce the steepness of the

wave front of a surge and absorbs energy contained in the travelling wave.

Surge Diverter

It will divert excess voltages from an electrical surge to earth. It measures thevolts coming in and once it gets above a set amount (normally 260 volts), will divertthe excess volts to earth equipment. Unlike the more common Surge ProtectorPowerboards that simply switch off if there is spike in volts, a Surge Diverter willjust divert the excess volts away. It is also installed on your main switchboard,thereby protecting all powerpoints.

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4] Define the term Insulation Coordination.

Insulation Coordination is the process of determining the proper insulationlevels of various components in a power system as well as their arrangements. It is theselection of an insulation structure that will withstand voltage stresses to which thesystem, or equipment will be subjected to, together with the proper surge arrester. Theprocess is determined from the known characteristics of voltage surges and thecharacteristics of surge arresters.

5] Write any two functions of protective relaying?

(i) The function of a protective relay is to detect and locate a fault and issuea command to the circuit breaker to disconnect the faulty element.

(ii)It is a device which senses abnormal conditions on a power systemby constantly monitoring electrical quantities of the system which differ undernormal and abnormal conditions.

6] What are the desirable qualities of protective relaying? Or Mention theessential features of the power system protection. Or List the essential features ofswitchgear.

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 electricalcircuits 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 removalfrom service of any element of the power system when it starts to operate in anabnormal manner or interface with the operation of rest ofthe system.

9] What are the causes of faults in power system?

(ii) Heavy short circuit current may cause damage to damage equipment orother element of the system of the system due to overheating and high mechanicalforces set up due to heavy current.

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(iii) Arc associated with short circuits may cause fire hazards. Such firesresulting from arcing may destroy the fault element of the system. There is alsopossibility of firing spreading to the other devices if the fault is not isolated quickly.

10] What are the different types of fault in power system transmission lines?

1. Symmetrical faults 3 phase faults2. Unsymmetrical faults Single phase to ground, single phase to open circuit,Two phase to groundfault; phase to phase short circuit.

11] List out the types of faults in power system

A].Single phase to ground B] phase to phase faults C] Two phase to groundfault 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 thesystem.

13] Differentiate between primary and back up protection.

No Primary protection Back Up Protection

1It is designed to protect the componentsof the power system. [main protection]

It is second line of protection in case mainprotection fails.

2It is for instantaneous protection It is designed to operate with enough time

delay

3 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 updue 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 isolatethe defective element from the rest of the system, thereby protecting the system from

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damages consequent to the fault.

16. Give the consequences of short circuit.

Whenever a short-circuit occurs, the current flowing through the coil increasesto an enormous value. If protective relays are present , a heavy current also flowsthrough the relay coil, causing it to operate by closing its contacts. The trip circuit isthen closed , the circuit breaker opens and the fault is isolated from the rest of thesystem. Also, a low voltage may be created which may damage systems connected tothe supply.

17. Define protected zone.

Are those which are directly protected by a protective system such as relays, fusesor switchgears. If a fault occurring in a zone can be immediately detected and orisolated by a protection scheme dedicated to that particular zone.

18. What are unit system and non-unit system?

A unit protective system is one in which only faults occurring within itsprotected zone are isolated. Faults occurring elsewhere in the system have noinfluence on the operation of a unit system.

A non-unit system is a protective system which is activated even whenthe 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 byits 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 toactivate within a definite time delay.

21. Name the different kinds of over current relays.

Induction type non-directional over current relay, Induction type directionalover 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.

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23. Define operating time of a relay.

It is defined as the time period extendind from the occurrence of the faultthrough 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 by the relay from the instant of isolating the faultto 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 apredetermined value. (eg: Induction type non-directional/directional overcurrent relay,differential overcurrent relay)whereas undercurrent relays are those which operatewhenever the current in a circuit/line drops below a predeterminedvalue.(eg:differential over-voltage relay)

26. Mention any two applications of differential relay.

Protection of generator & generator transformer unit; protection of large motors andbusbars .

27. What is biased differential bus zone reduction?

The biased beam relay is designed to respond to the differential current in termsof its fractional relation to the current flowing through the protected zone. It isessentially 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 operatingforce.