Download CHAPTER 3 CAUSES AND EFFECTS OF ELECTRICAL FAULTS

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CHAPTER 3
CAUSES AND EFFECTS OF ELECTRICAL FAULTS
3.1
INTRODUCTION
A large number of asynchronous motors are used in industrial
processes even in sensitive applications. Consequently, a defect can induce
high losses in terms of cost and can be dangerous in terms of security and
safety. In the last few years, a great number of works relating to various
aspects of failures of the three phase asynchronous motors were carried out.
An IEEE transaction entitled, “Report of Large Motor Reliability
Survey of Industrial and Commercial Installation, Part I” included both the
results of an IEEE survey and the results of an EPRI survey. A summary of
the survey is given in Table 3.1. In spite of different approaches and criteria
both studies indicate very similar failure percentage associated with
mechanical and electrical related machine failures. Analyzing the data from
the Table 3.1, it is concluded that many positions are directly or indirectly
related to failures caused by extensive heating of the different motor parts
involved in motor operation.
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Table 3.1 Summary of IEEE and EPRI motor reliability surveys
IEEE Study
Failure contribution
EPRI Study
% Failed Component
Average
%
Persistent Overload
4.20 Stator ground
Insulation
23.00
Normal Deterioration
26.40 Turn Insulation
4.00
Bracing
3.00
Core
1.00
Cage
5.00
Electrical Related Total 30.60 Electrical Related
Total
36.00
High Vibration
15.50 Sleeve Bearings
16.00
Poor Lubrication
15.20 Antifriction Bearings
8.00
Electrical Related Failures
33%
Rotor shaft
5.00 Mechanical Related Failures
31%
2.00
Rotor Core
1.00
Trust Bearings
Mechanical Related
Total
%
30.70 Mechanical Related
Total
32.00
High Ambient Temp
3.0 Bearing Seals
6.00
Abnormal Moisture
5.8 Oil Leakage
3.00
Abnormal Voltage
1.5 Frame
1.00
Abnormal Frequency
0.6 Wedges
Abrasive Chemicals
4.2
Poor Ventilation cooling
3.9
Other Reasons
19.7 Other Components
1.00 Environmental, Maintenance
& Other Reasons Related
failures
36%
21.00
Environmental Related
Maintenance Related
& Other Reasons: Total 38.70 & Other Parts : Total 32.00
Normally electric motors do not fail suddenly. It happens over time
and regular inspection will detect a problem before a serious situation
develops. Faults in ACIMs can occur in any of the three main components of
the motor such as stator, rotor and bearings. Faults in the induction motor
drives can be classified into three different groups:
1.
Growing faults with only small effects on the operation.
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2.
Partial non-catastrophic faults with emergency operation
possible.
3.
Catastrophic faults with total drive breakdown.
The causes of faults can be categorized in to two groups:
i)
Mechanical Causes of Failure:
Misalignment
Mechanical unbalance
Soft foot
Bearing fatigue
Fractured rotor bars or end rings
Overheating
Loss of cooling
Improper lubrication
ii)
Electrical Causes of Failure:
Poor power quality
Resistance and Impedance unbalance
Insulation failure
Excessive loading and current
The abnormal operating conditions of induction motor are classified
as follows:
i)
Mechanical overloads : Sustained overloads, prolonged
starting, locked rotor and stalling.
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ii)
Abnormal supply conditions: Loss of supply voltage,
unbalanced supply voltage, phase sequence reversal of supply
voltage, over voltage, under voltage and under frequency.
iii)
Faults in supply: Interruptions in phases, blowing of fuse and
short circuit in supply cable.
iv)
Internal Faults in Motor itself (caused by 1, 2, 3 above): Phase
to phase faults, phase to earth faults, failure of phase (open
circuit) and mechanical failure.
It is preferable to find faults before complete motor failure. This is
called “incipient fault detection”. Often the motor can run with incipient
faults, but eventually it will lead to motor failure causing downtime and large
losses. Power quality problems that affect the induction motor behavior are
voltage sag (torque, power, speed and possible stall), harmonics (losses and
torque), unbalance (losses), short interruptions (mechanical shock and
possible stall), impulse surges (isolation damage), over voltage (expected life
shortening) and under voltage (over heating and low speed). Principally,
voltage distortions and amplitude variations cause the present power quality
problems.
Power supply variations may be classified into three categories:
i)
Frequency variation from rated
ii)
Unbalanced voltage between phases
iii)
Balanced phase voltage with voltage variation from rated
value.
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Causes of failures in induction motor are as follows:
a) Overheating of motor:
i)
Over load
ii)
Under voltage
iii)
Over voltage
iv)
Ventilation duct clogged
v)
Fuse blowing (single phase rotating)
vi)
Unbalanced three phase voltages
It can be seen that 44% of motor failure problems are related to heat.
Allowing a motor to reach and operate at a temperature 10oC above its
maximum temperature rating will reduce the motor’s expected life by 50%.
Operating at 10oC above this, the motor’s life will be reduced again by 50%.
The reduction in the expected life of the motor repeats itself for every 10 oC.
This is referred to as the “half life” rule. Although there is no industry
standard that defines the life of an electric motor, it is generally considered to
be 20 years. The term, “temperature rise” means that the heat produced in the
motor windings (copper losses), friction of the bearings, rotor and stator
losses (core losses) will continue to increase until the heat dissipation equals
the heat being generated.
b) Abnormal electromagnetic sound:
i)
Single phasing
ii)
Short circuit in windings
iii)
Unbalanced air gap resulted from serious bearing wear
iv)
Overheating of motor
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c) Speed fall:
i)
Voltage drop
ii)
Sudden overload
iii)
Single phase rotating
Motor life depends on electrical, thermal, mechanical and
environmental stresses. This thesis defines the loss of motor life as the
reduction in stator winding insulation life due to thermal stresses considering
single aging effect. Since the rotor is of squirrel cage type, there is no rotor
insulation and hence the rotor structure is more robust. Hence, motor life is
focused on stator winding insulation. This thesis examines the loss of motor
life when it is supplied by unbalanced voltages in combination with over and
under voltages, balanced under and over voltages, frequency variations, single
phasing, overload and ground fault.
Electrical faults are usually related to insulation failure. They are
generally known as phase to ground or phase to phase faults. It is believed
that these faults start as undetected turn to turn faults which finally grow and
culminate into major ones. Almost 30-40% of all reported induction motor
failures fall in this category.
3.2
OCCURRENCES OF ELECTRICAL FAULTS DURING
OPERATION
The squirrel cage induction motors are the most popular AC motor
used in agriculture firms and industrial applications due to their reliability,
low cost and high performance. However these motors experience several
types of electrical/incipient faults. The main faults considered in this analysis
are: Unbalanced supply voltage, Single phasing, Phase reversing, Over and
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under voltages, Over load, Earth fault and Power frequency variations. All the
above faults affect the motor performance and reduce the motor lifetime.
3.2.1
Unbalanced Supply Voltages
Since the three phase induction motors have been widely employed
in industrial, commercial and residential systems, the supplied three phase
voltage significantly affects their operating performance. When the supplied
three phase voltage is unbalanced, the startup transients, dynamic
performance and steady state characteristics of the three phase induction
motor will vary accordingly.
The positive sequence voltage is developed during normal balanced
operating condition of induction motor. When supply voltage is unbalanced,
the negative sequence voltage is developed. The positive sequence voltage
produces a positive torque, whereas the negative sequence voltage gives rise
to an air gap flux rotating against the forward rotating field, thus generating a
detrimental reversing torque. This negative sequence voltage produces a flux
in the air gap rotating opposite to the rotation of the rotor, tending to produce
high currents. When neglecting non-linearity, due to saturation, the motor
behaves like a superposition of two separate motors, one running at slip s with
terminal voltage Vp per phase and the other running with a slip of (2-s) with
terminal voltage of Vn. The result is that the net torque and speed are reduced
and torque pulsations are produced. Also, due to the low negative sequence
impedance (R 2 / (2-S)), the negative sequence voltage gives rise to large
negative sequence currents.
The currents at normal operating speed with unbalanced voltages
will be greatly unbalanced in the order of approximately 6 to 10 times the
voltage unbalance. This introduces a complex problem in selecting the proper
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overload protective devices particularly. Because devices selected for one set
of unbalanced conditions may not be inadequate for a different set of
unbalanced voltages. Hence, increasing the size of the overload protective
device is not a solution. Figure 3.1 identifies the potential troubles which can
result in voltage unbalance. The effects of voltage unbalance are stator and
rotor heating with the ultimate failure mode being stator winding failure or
bearing failure due to bearing lubricant overheating or rotor vibration. Further
failure can only be prevented by derating the machine according to standards,
allowing it to operate within its thermal limitations.
Causes of unbalanced supply voltages are given below:
i)
Open delta transformers
ii)
Unbalanced loading
iii)
Unequal tap settings
iv)
High resistance connections
v)
Shunted single phase load
vi)
Unbalanced primary voltage
vii) Defective transformers
In general, following effects will occur due to voltage unbalance:
i)
Reduction in motor efficiency
ii)
Increase in stator and rotor copper losses
iii)
Rise in temperature
iv)
Serious reduction in starting torque
v)
Nuisance and overload tripping
vi)
Premature failure of motor winding
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vii)
Excessive and unbalanced full load current
viii)
Shaft vibration and noise
ix)
Speed and torque pulsation
x)
Reduction of motor output torque
POTENTIAL TROUBLES
EFFECTS
Utility Distribution System
Unbalanced
Income Line
Electrical Failures (33%)
Stator Winding Failure
Reduced usage of
Transpositions in
High Voltage
Transmission Lines
Primary Open
Delta Transformer
Bank
Stator
heating
Plant Distribution System
Large Single
Phase loads
Unbalanced
Three phase
Lighting Loads
Unequal conductor
Impedance
Blown Fuse on a
Three phase
Bank of P.F.
Correction Capacitors
Plant Expansions
Without additional
Distribution capacity
RESULTS
Voltage
Unbalance
(Negative
sequence
effects)
Rotor
heating
Rotor bar
heating
(Loose
bars)
Shaft/
Bearing
Lubricant
Over
heating
Stator
Induced
current
Bearing
failure
Rotor
Vibration
Mechanical Failures (31%)
Figure 3.1 Causes and effects of unbalanced voltages
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Unbalanced Supply voltage can further divided into two groups:
Under Voltage Unbalance: It is defined as unbalance due to the
positive sequence voltage lower than rated value.
Over Voltage Unbalance: It is defined as unbalance due to the
positive sequence voltage higher than the rated value.
Many unbalance voltage conditions are possible in electrical system.
They are:
i)
Single phase under voltage unbalance
ii)
Two phase under voltage unbalance
iii)
Three phase under voltage unbalance
iv)
Single phase over voltage unbalance
v)
Two phase over voltage unbalance
vi)
Three phase over voltage unbalance
In this thesis, all the above six types of unbalanced conditions are
considered for analysis.
3.2.2
Single phasing
The term “single phasing” means one of the phases is open. A single
phasing condition subjects an electric motor to the worst possible case of
voltage unbalance. If a three phase motor is running when the “single phase”
condition occurs, it will attempt to deliver its full horsepower, enough to drive
the load. The motor will continue to drive the load, until the motor burns out
or the properly sized overload elements take the motor off the line.
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When an induction motor loses one phase its slip increases but it is
does not stall unless the resulting single phase supply voltage is below normal
or the shaft load is more than 80% of full load. The losses increase
significantly when loaded near or above its rating. Single phasing is a
hazardous condition and steps should be taken to de-energize the motor. For
lightly loaded three phase motors, say 70% of normal full load amperes, the
phase current will increase by the square root of three under secondary single
phase conditions.
This will result the motor to draw a current of approximately 20%
more than the name plate full load current. If the overloads are sized at 125%
of the motor name plate, circulating currents can still damage the motor. That
is why it is recommended that motor overload protection should be based
upon the actual running current of the motor under its given loading, rather
than the name plate current rating. When the motor is single phased, the
current in the remaining two phases increase to 173% of normal current.
Causes of single phasing are given below:
i)
Open winding in motor
ii)
Any open circuit in any phase anywhere between the
secondary of the transformer and the motor
iii)
Primary fuse open.
The potential troubles which can lead to single phasing are shown in
Figure 3.2. The effects of single phasing on three phase motors vary with
service conditions and motor thermal capacities. The effects are similar to
those identified for unbalance voltages, since single phasing represents the
worst case of an unbalance voltage condition. An additional effect is the
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remaining phase windings experience excessive over heating, there by
creating a greater potential for stator winding failure.
POTENTIAL TROUBLES
EFFECTS
RESULTS
Utility Distribution System
Transmission line
Out on one phase
Electrical Failures (33%)
Utility fuse blown
In one phase
Stator
Winding
Failure
Transmission line
Fault on one phase
Plant Distribution System
Unbalanced voltage
Source
Single phased
Motor control
Blown motor
Winding lead
Blown Fuse in
one phase
Disconnect switch
Blade Malfunction
in one phase
Cable fault in
One phase
Cable disconnected
In one phase
Single
Phasing
(Maximum
Voltage
Unbalance,
Negative
sequence
effects)
Remaining
Phase
windings
experience
Excessive
over heating
Stator
Induced
current
Rotor
bar
heating
(Loose
bars)
Bearing
failure
Rotor
vibration
Rotor
heating
Shaft/
Bearing
Lubricant
Over
heating
Mechanical Failures (31%)
Over heated/Loose
Electrical connection
In one phase
Figure 3.2 Causes and effects of single phasing
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3.2.3
Phase reversal
The phase reversal occurs when two of the three phases (RYB) of
supply line reverses. Most of motor will react very badly to such a
situation. Motor suddenly in the wrong direction, causing major collateral
damage.
3.2.4
Over and under voltages
Over and under voltages are caused by load variations on the
system. Over loaded circuits result in under voltages. The under voltage fault
occurs when the motor is supplied by reduced voltage with rated mechanical
load. An under voltage is a decrease in the RMS ac voltage to less than 90%
at power frequency for duration longer than 1 min. Examples include load
switching, capacitor bank switching off, Overloaded circuits, etc., Under
voltage faults result in increased currents, excess heating of machine and
increased stator and rotor losses.
If any one of the line voltage in the motor is greater than 110% of
rated value, over voltage fault occurs. An over voltage is an increase in the
RMS voltage greater than 110% at power frequency for duration longer than 1
min. Examples include load switching, incorrect tap settings on transformers,
etc., Over voltage faults result in harmful effects on machine insulation.
3.2.5
Overload
When there is increase in mechanical load on the motor beyond the
rated value, overload situation occurs. Due to high load torque, motor begins
to draw more current. Over load condition results in increased phase currents,
overheating of machine and high stator and rotor copper losses. Figure 3.3
illustrates the potential troubles which can result in over loading effects.
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POTENTIAL TROUBLES
EFFECTS
RESULTS
Utility Distribution System
Voltage low on
Incoming line
Plant Distribution System
Excessive cycling
Or pulsating load
Electrical Failures (33%)
Continuous
Overloading
Turn to
turn short
Prolonged
Acceleration
Stator
Winding
Overheating
Time
Motor load
Mechanical
Troubles
Insulation
Deterioration
Cracking
Over heating
Tracking
Pulverizing
Low voltage
Supply
Repeated starting
Repeated stalling
Mechanical
stresses on
winding end
turns and
individual
coils
Stator
Winding
Failure &
Potential
Iron core
Damage
Coil to
coil short
Phase to
phase
short
Coil to
ground
short
Iron
damage
Mechanical Failures (31%)
Figure 3.3 Causes and effects of over loading
The majority of troubles only generate stator winding over heating.
Whereas two troubles: repeated starting and repeated stalling
generate
mechanical stresses on winding end turns and individual coils. These
mechanical forces, generated via the production process (starting, stopping,
etc.,) result in physical stress on the “end turns” of the copper winding,
thereby increasing the potential for cracking and conductive paths. Another
possibility is a substantial increase in insulation temperature as a result of
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motor jogging operations or repeated starts within a short time period. This
can be the result of normal production, start up of the process line or the
implementation of new production equipment.
3.2.6
Ground fault
Ground fault is more prevalent in motors than other power system
devices, because of the violent manner and frequency with which they are
started. These faults are detected by observing zero sequence current. The
ground fault is monitored and detected by measuring leakage current. Almost
80% of the electrical faults in low voltage distribution systems are line to
ground faults. Monitoring leakage to ground is a predictor of an impending
ground fault. Most faults start with leakage to ground. When these leakage
currents begin to increase, this eventually will lead to a ground fault. Early
detection keeps the damage at the fault point very low. Therefore the cost and
time required to repair the equipment is also significantly reduced and the
fault damage will be limited. The fault can be easily located and repairs can
be scheduled.
Periodic monitoring of ground leakage current is a predictor of
incipient failure and can be used effectively in preventive maintenance
program. Ground fault sensing is normally applied on the line side of the
drive. Aging and thermal cycling cause a decrease in dielectric strength of the
insulation in the stator winding. This can produce a low impedance path from
the supply to ground resulting in ground fault currents which can be quite
high in solidly grounded systems.
In resistance grounded systems, there is a resistance is series with
the supply source to limit ground fault current and allow the system to
continue operating for a short time under fault conditions. The fault should be
located and corrected as soon as possible, however, since a second fault on
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another phase would result in a very high current flow. In addition to
damaging the motor, a ground fault can place the motor casing above ground
potential thus presenting a safety hazard to personnel.
On the occurrence of a ground fault caused by insulation break
down, an unprotected motor will commonly suffer severe structural damage
and have to be replaced. The fault could also shut down the power supply bus
to which the faulty motor is connected. Ground faults occur in good motors
because of environmental conditions. Moisture or conductive dust, which are
often present in mines, can provide an electrical path to ground thus allowing
ground fault current to flow. In this case, ground fault protection should shut
down the motor immediately so that it can be dried or cleaned before being
restarted.
The amount of current flow due to fault depends on the location of
fault in the motor winding. A high current flow will result if a short to ground
occurs near the end of the stator winding nearest the terminal voltage. A low
ground fault current will flow if a fault occurs at the neutral end of the
winding, since this end should be a virtual ground. This low level of ground
fault pickup is desirable to protect as much of the stator winding as possible
and to prevent the motor casing from shock hazard.
The two types of insulation failures occurred in a motor are: turn to
turn and turn to ground. The amount of phase current unbalance is a very
good indication of the turn-turn insulation conditions. Turn-turn insulation
failure is a prelude to most insulation failure in motors and normally occurs
before a fault propagating to a turn to ground failure. Therefore, in the
proposed system current unbalance protection is included to avoid turn-turn
insulation failure.
Ground fault current leads to insulation failure in motor. Therefore,
a considerable amount of attention is given to the ground current levels
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available in the system. Very low level ground fault currents between 100mA
to 2A can be used only as a possible indication of fault since capacitive
charge currents can also indicate a false ground fault. A ground fault that
propagates to a threshold level of 5A or more is imminent indication of a
motor failure. Ground fault results in hazards for human safety, thermal stress
due to fault current, voltage stress, interference with telecommunication and
interruption of power supply.
3.2.7
Power frequency variations
Power frequency variations are defined as deviation of the power
system fundamental frequency from its specified nominal value. No Load,
locked rotor and full load currents vary inversely with a change in applied
frequency. Locked rotor, minimum pull up and break down torques vary
inversely as the square of the frequency change. An increase in motor torque
and /or speed with decrease in power frequency may damage the driven
machine. A decrease in motor torque and/or speed with increase in power
frequency may cause a reduction in output of the driven machine and slight
increase in power factor.
3.3
CONCLUSION
Poor power quality (unbalanced supply voltage, single phasing, over
and under voltages), over load and earth fault will cause the motor to imbibe
more current. Due to the above operating conditions, there is increase in the
stator and rotor losses, reduction in efficiency, rise in temperature, shaft
vibration and noises, etc., in induction motor. Phase reversal will cause a
major collateral damage to the motor driven system. Over load condition
leads to heavy temperature rise in the stator winding. Motor currents and
torques vary inversely with frequency. All the above electrical faults affect
the life of the stator winding insulation.