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1[Type text]
Department Of Electrical And Electronics Engineering
EE2402 -PROTECTION AND SWITCHGEAR
UNIT I INTRODUCTION
UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS
UNIT III APPARATUS PROTECTION
UNIT IV THEORY OF CIRCUIT INTERRUPTION
UNIT V CIRCUIT BREAKERS
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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
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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
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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
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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,
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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.
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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
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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
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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.
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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. 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.