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Presented By:
Sourabh KanthaliyaB.E. Final YearElectrical Engineering
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INTRODUCTION
GENERAL CAUSES
INTRODUCTION TO VOLTAGE SAGS IN
INDUCTION MOTOR NON RECTANGULAR VOLTAGE SAG
CLASSIFICATION
STUDY & ANALYSIS
OUTPUT
CONCLUSION
2
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Voltage sags are short-duration reductions in the r.m.s.voltage magnitude
Sag magnitude range from 10% to 90% of nominalvoltage
Sag duration ranges from cycle-1 min
One of the major power quality problem
A voltage sag is not a complete interruption of power
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ILLUSTRATION OF VOLTAGE SAG
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GENERAL CAUSES OF VOLTAGE SAG5
Voltage Sag due to Faul ts
The sag magnitude due to faultsdepends on:
Distance to the fault
Fault impedance
Type of fault
Presag voltage level
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The sag magnitude depends upon:
Characteristics of the induction
motor
Strength of the system at the point
where motor is connected
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Voltage Sags due to M otor Starting
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The causes for voltage sags due to
transformer energizing are:
Normal system operation, which
includes manual energizing of a
transformer.
Reclosing actions
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Voltage Sags due to Transformer Energizing
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VOLTAGE SAG IN
INDUCTION MOTOR
In induction motors it has been noticed that voltagesags cause speed loss, peak currents and torquesthat appear with voltage recovery. However, thereare also non-rectangular voltage sags which arebasically caused by starting high power motors thatdemand high currents that lower as speed increases
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NON RECTANGULAR VOLTAGE SAGS
Non-rectangular voltagesags are the result of therecuperation of a motor
steady-state after a faultin the bus at which it isconnected. Theirdifference withrectangular sags is a
reacceleration time thatlasts from the end of thefault that caused thevoltage sag and the
steady state voltage.
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CLASS A
CLASS B
CLASS C
CLASS D
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CLASS A
Voltage sags Type A
are caused mainly
by three-phasefaults, producing
equal voltage drop
in three phases
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CLASS B
Type B sags are
caused by single
phase faults
producing voltage
drop only in the
faulted phase.
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CLASS C
Type C are due to
phase-to-phase
faults which causevoltage drop and
phase-angle
movement in the
faulted phases
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CLASS D
Type D are also due tophase-to-phase faultswhich cause voltage
drop and phase-anglemovement in thefaulted phases. Theseare slightly different
than type C fault i.e.magnitude of thirdphase voltage drops.
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FIRST STEP :
The first stage is togenerate the non
rectangular voltagesag signals. for thissimulation letsmanipulate a
sinusoidal waveparameters in termsof the amplitude,sag duration andrecovery time.
Study and Analysis
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SECOND STEP :
The second stage is to
select a representativemotor. It is convenient tochoose a motor withmiddle-low power inorder to see the effects.
Power 3746 VA
Nominal Voltage 180V
Frequency 60 Hz
Poles 2
Inertia coefficient 0.089 J
Slip 3%
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THIRD STEP :
Finally, different types (A, B, C and D) of sags are applied to
the machine considering the non rectangular voltage sagmodel.
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SPEED
TORQUE
CURRENT
Outputs
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It can be seen that Non rectangular voltage sags type A an D cause a
huge speed loss considering that the duration of the sag is only 0.4seconds. Instead, Type B sags are not as severe as the others in terms ofspeed loss; this is an expected result because in this type of sags thereis only one phase involved during the event.
Sag Type Loss of
speed
A 6.46%
B 2.44%
C 4.01%
D 7.50%
SPEED
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It can be seen from that in alltype of sags there is a counter-torque at the moment that thesag starts. After that, this torquerecuperation occurs until the endof the sag. It can be noticed thatduring this recuperation there isoscillation due to the linearcharacteristic of the sagreacceleration. The oscillationsare more severe in sags type Cprobably because of the
combined effect of voltage dropand phase angle change. Onthe other hand, sags type Acause little oscillation. This isbecause of the remainingequilibrium in voltage amplitude
and phase angle.
tORQUE
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PHASE A
PHASE B
PHASE C
current
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PHASE A
PHASE B
PHASE C
current
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PHASE A
PHASE B
PHASE C
current
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It can be noticed thatphase A is the mostaffected having veryhigh current peak,especially in sags typeA and D. On the otherhand, phase c is theless affected; yet, sag
type A has a strongimpact. In terms ofoscillations, sag type Bcause little effect.
Sag
Type
Ratio
peak
current
to
nominalcurrent
phase
A
Ratio
peak
current
to
nominalcurrent
phase B
Ratio
peak
current
to
nominalcurrent
phase
C
A 4.375 3.375 2.20B 2.875 3.25 2.01
C 2.975 2.125 3.25
D 4.875 4.125 3.00
current
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CONCLUSION
Sags type A and D cause an important loss ofspeed that could become harmful for themechanical work of the machine if sags last more.
At the moment of occurrence of sag there is ahuge counter-torque that results in a high demandof currents. These current peaks cause windingdamage and, as a consequence, a loss of usefullife of the machine.
Sags type A and D also cause high peak currentswhich make them a phenomenon of special care.
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