MOTOR
PROTECTION
CONTENTS
INTRODUCTION.................................................................................................
CONTROL..............................................................................................................
PROTECTION - GENERAL..................................................................................
MECHANICAL OVERLOADING.......................................................................
SHORT CIRCUIT...................................................................................................
EARTH FAULTS....................................................................................................
STALLING.............................................................................................................
SINGLE PHASING................................................................................................
MOTOR WINDING TEMPERATURE PROTECTION.......................................
STARTING TIME...................................................................................................
OVERLOAD PROTECTION..............................................................................
INSULATION FAILURE..................................................................................
SETTINGS...........................................................................................................
DIFFERENTIAL PROTECTION....................................................................
LOSS OF SUPPLY..............................................................................................
SYNCHRONOUS MOTORS.............................................................................
INTRODUCTION
For many years a comprehensive system of
motor protection has been considered essential for vital services and
industrial processes to safeguard machines and their cables from damage caused
by overcurrents. Mechanical overload,
stalling, single-phasing and short-circuits are some of the potential
hazards. All of these result in
overcurrents and are usually detected by either time delayed or instantaneous
overcurrent relays. The vast majority of
electric motors, whether operating on high or low voltage, is of the squirrel-cage
induction type, and many except the very largest are direct-on-line started.
CONTROL
Most motors are switched on and off by
contactors, and in most cases it is necessary to provide back-up fuses to
protect the contactor itself, as well as the circuit, against high-level faults
such as short-circuits which are beyond the breaking capacity of the
contactor. The fuses have special
ratings both to ensure discrimination in relation to the tripping
characteristic of the contactor and to prevent their being blown by the
starting current of the motor. They do
not protect the motor against overloads; this is a function of the contactor.
PROTECTION - GENERAL
Like any other piece of electrical plant a
motor, whether HV or LV, must be protected against overcurrent and usually
against earth fault. In larger motors
over-temperature may also need to be monitored in both the cooling air and the
windings themselves.
Overcurrent in a motor can be caused by any
of five conditions:
·
mechanical
overloading
·
short-circuit
·
stalling
·
single-phasing
·
earth
fault or leakage.
MECHANICAL OVERLOADING
A motor may be overloaded mechanically by
either overloading the driven end (e.g. pump or compressor) beyond its rating
or by some internal mechanical malfunction such as a stiff bearing. Either may cause a rise of active current
above the full-load rated current of the motor.
Mechanical overloading is probably the commonest cause of overcurrent in
a motor.
The motor is protected by an inverse-time
overcurrent device which will cause the contactor to trip if the overcurrent is
sufficiently high and persists. The
device usually takes the form of a thermal element in each phase, either
directly or CT-operated. It has an
inverse-time characteristic which is more nearly matched to the thermal
behaviour of the motor itself than that of the inverse-time electromagnetic
overcurrent relay described earlier. It
must allow the large starting current (up to five times full-load current) to
flow during the run-up period without operating, but it must trip the motor if
even a small overcurrent persists for a longer period. A typical setting of such a device would be
110% full-load continuous current with the appropriate time setting. For short starting times the inverse-time
characteristic must be such that the starting current and run-up time are taken
into account. In this respect it must be
remembered that high-inertia loads such as a motor-generator set or a
compressor take much longer to run up than, say, a centrifugal fan.
For the majority of LV motors and a few HV
motors the inverse-time device is thermal.
For the smaller LV motors it is in series with the motor itself, but for
the others it is a separate relay operated through CTS. For most HV motors on the later platforms
however, the device is wholly electronic but with a similar characteristic; it
too is a separate relay, CT-operated (see Figure 12.1(b)). Where these relays are separate the
overcurrent device is combined with certain other features into a single ‘Motor
Protection Relay’ which is further discussed later.
A characteristic of inverse-time relays
which is particularly noticeable in thermal relays, and which has to be taken
into account in allowing for starting current, is ‘overshoot’ (or
‘overrun’). This means that if the relay
is energised with something more than its minimum operating current it may
close its contacts even after the current has subsequently fallen below the
operating level. For example, a motor
could be tripped after it had safely started and reached full speed, even
though the relay had not operated during the starting period. This can have a considerable effect on the
discrimination that can be achieved between starting and overload currents,
unless complications are added to the protection scheme.
Whereas the contactor with its inverse-time
overcurrent device (thermal or electronic) provides overload protection for the
motor, such contactors cannot in general clear a fault of short-circuit
proportions. For this they must be
backed up by series HRC fuses.
When used with motors such fuses must have
special characteristics. They must have
a continuous rating which will allow them to pass the full-load current of the
motor continuously, and they must also allow the considerably greater starting
current to pass for the period of the run-up time without melting the fuses.
SHORT CIRCUIT
A short-circuit in a motor circuit will
cause a severe overcurrent. One of the
more vulnerable places to short-circuit in a motor is the incoming cable box
where a too-small radius or imperfect jointing could lead to weakness. As many process motors are located in
hazardous areas, it is essential to clear the circuit in the quickest possible
time under these conditions.
The overcurrent produced by a short-circuit
will operate the inverse-time device in a relatively short time, but in general
not short enough to cause a trip before the fuse blows. Indeed it is important that the contactor
should NOT operate as it is not rated to clear fault currents. Where HV or LV motor feeders are provided
with back-up fuses, it is these that will blow first and clear the fault very
quickly. Where very large motors are fed through circuit-breakers and there are
no back-up fuses, the inverse-time protection will be backed up by a high-set
instantaneous overcurrent relay element to trip the circuit-breaker
instantaneously without waiting for the inverse-time element to operate. Its setting will be well above the motor
overload and starting current levels.
EARTH FAULTS
Earth faults, especially those occurring in
solidly earthed systems, will also give rise to severe overcurrents in the
affected phases. They may be dealt with
by an earth-fault relay which trips the contactor. With small motor starters where the thermal
overcurrent devices are direct-connected, the relay is usually energised by a
core-balance CT through which the three cables pass. With larger motors where
there are CTS, these may be used to provide an earth-fault signal by their
spill current. The earth-fault relay may
be separate or may be part of the composite ‘Motor Protection Relay’.
The same consideration will apply to earth
faults in HV motors. Nearly all HV
systems are resistance-earthed, which limits earth-fault currents to a low
level that will not actuate the fuses or high-set instantaneous relays. Here also an earth-leakage relay operated
through CTS is necessary, but the fault current is then well within the
breaking capacity of the contactor.
STALLING
Stalling can occur if the motor becomes
heavily overloaded - for example by a mechanical seizure of a bearing or of the
driven element, or it may be unable to start against an excessive load. In all these cases the stalled motor draws
its ‘locked rotor’ current (that is, its maximum starting current) as long as
the supply remains connected, and severe overheating results. The condition is aggravated by the lack of
ventilation while the rotor is stationary.
There is also a temptation to make repeated attempts to start if
unsuccessful the first time.
More rarely, stalling can occur if the
whole power network goes unstable and begins to run down, causing all motors
throughout the system to lose speed. If
the system recovers while the run-down is proceeding, all the motors in the
system will find themselves running at a large slip and all taking nearly their
full starting currents. The combined
effect on the generators of all these simultaneous starting currents will be to
depress the system voltage to such a level that some of the larger motors may
not develop sufficient torque to recover against their loads. They will then continue to run down and
stall. This is sometimes called the
‘Patrickson Effect’.
The long period of drawing ‘starting
current’, though not actually starting, will appear as a normal overcurrent and
should, in principle, be dealt with by the motor thermal overcurrent
protection, but difficulty arises when this protection has to discriminate
between normal starting current (which is present during the run-up time but
then disappears) and the stalled motor current (which persists).
This problem is particularly difficult if
the run-up time of the motor is of the same order as the motor stall (or locked
rotor) withstand time. It is still worse
in the case of high-inertia loads, where the run-up time may well be longer
than the stall withstand time. For these situations ‘stall relays’ are
sometimes used, especially with HV motors.
Stall relays are of two types: one using
electric sensing of the motor starting current, and the other using detection
of actual rotation. The former uses a
current-sensitive element and a timer.
On a normal start the current-sensitive element energises, but the timer
prevents its causing a trip unless the normal run-up time has elapsed. On a genuine stall the timer will trip the
motor after run-up time has expired if the overcurrent is still present. This type of relay is often fitted with a
‘thermal memory’ which prevents a restart until sufficient cooling time has
elapsed. The stall relay is sometimes
included with other elements in a combined ‘Motor Protection Relay’.
The other type of relay uses a shaft
rotation detector. This form is most
suitable when the motor safe stall withstand time is very close to the motor
run-up time, or even less as in the case of the very large gas re-injection
motors on certain offshore installations.
The rotation method is a more accurate indication of a stall
condition. However it does no
incorporate any ‘memory’ to protect against quick restarting, and it must be
used in conjunction with some form of lock-out protection.
SINGLE PHASING
A problem special to 3-phase motors is
single-phasing. Any such induction motor
needs three phases to produce its rotating field and to provide the necessary
starting torque, but once running, the removal of any one of the phases will
not necessarily stop it. It will however
reduce the driving torque and will also increase the current in the two
remaining phases. If the motor were
already well loaded it would eventually trip on sustained overcurrent. If, however, the mechanical loading on the
motor were not too high, there might still be sufficient torque to drive the
load. Also the current, although
increased, might still not be enough to actuate the inverse-time overcurrent
relay set, typically, to 110% full-load current. The situation could therefore pass unnoticed
except for a high-pitched whine from the motor, and no actual harm would
result.
There is a much greater risk, however, when
attempting to start. If the
single-phasing had occurred while the motor was running partially loaded and
had not been noticed, the motor would have been stopped normally when the
operation was complete, but still in its single-phased state. If later an attempt were made to restart it,
there would be excessive starting current but no starting torque, and it would
remain stationary. As the overcurrent
relay is set to allow adequate run-up time, this situation could persist for a
dangerously long time with no ventilation in the stationary motor. Still worse, the operator might make several
attempts to start, and each time the relay would reset and allow full run-up
time afresh. Eventually the motor would
probably burn out before the overcurrent relay disconnected it. Therefore if a motor fails to start after two
attempts, the operator must make no further attempts to start it until the
cause has been found and corrected.
A general 'Rule of Thumb' that can be
applied is that motors over 50kW are only suitable for two successive starts
(even if successful) when cold. Another
one-start attempt is allowable after a cooling period of 30 minutes at
standstill. No more than three starts
may be attempted in any one hour.
Damage to a motor by single-phasing is
caused by overheating of the windings due to the prolonged excessive currents
in two of the phases. It is normally
prevented by the protection system. In
the case of small motors provided with thermal overcurrent protection, the
three thermal elements are mounted in such a way that unequal heating produces
a differential movement which causes the contactor to trip.
With some HV and larger LV motors where
protection is through current transformers and where thermal overcurrent
protection is used, the single-phasing protection is provided by these same
overcurrent elements where unequal heating produces the differential movement.
With motors which are protected by
electronic relays the device includes a special element which detects the
single-phase condition, whether the motor is running or being started. It is
referred to as a ‘single-phasing’ or ‘Negative-Phase-Sequence’ (NPS) relay.
Single-phasing can be caused by the blowing
of one of the three back-up fuses, by the possible welding of contactor
contacts or, less probably, by the open-circuiting of one line due to damage or
vibration. It will not occur however
with HV fuses where they are fitted with external striker-pin contacts which
trip the contactor if any one of them blows.
MOTOR WINDING TEMPERATURE PROTECTION
Over-temperature protection is sometimes
used, in addition to the thermal overcurrent protection afforded by the Motor
Protection Relay, to safeguard the windings of a motor.
Three main types of temperature sensor are
used:
·
thermocouple
·
Resistance
Temperature Device (RTD)
·
thermistor
The sensing elements are normally embedded
in the winding insulation, usually in the overhang.
STARTING TIME
A major problem in motor design and
protection is to ensure that the starting current can flow for long enough to
accelerate the motor without bringing out the motor’s overcurrent protection,
while at the same time not impairing the close protection given to the motor
while it is running.
For this purpose a time-quantity ‘tE’
is considered. This is defined as the
time taken for the motor’s windings, while carrying the starting current IA
continuously, to be further heated from the maximum temperature reached in
rated service and in a maximum ambient temperature to the limiting allowable
temperature. The tE time of the motor should be given on
its nameplate.
The thermal relay for protecting a
squirrel-cage motor should be selected so that the tripping time read from the
thermal relay’s time/current characteristic using the IA/IN ratio (ratio of starting to normal
rated current) is not larger than the motor’s stated tE time.
The present day tendency is to employ
motors to the limit of their thermal margins and to cater for this a relay with
an inverse-time characteristic, similar to the thermal time characteristic of
the motor, is used. The characteristic
must allow the motor starting current to flow for a time in excess of the motor
starting time.
It may be that a short resume' of the
operation of the three-phase induction motor would be useful at this
point. The three-phase voltage produces
current in the stator winding which sets up a rotating magnetic field. This field flux cuts the short-circuited
rotor conductors and induces a current in them.
The interaction of the current and flux produces a torque which causes
rotation. Figure 10.1 shows a torque-speed
curve for a typical motor and superimposed on this curve is the torque-speed
curve of a fan. Underneath is the
current-speed curve for the motor. As can be seen the current is at starting current level until about
80% speed is reached.
Figure
10.1 -
Current/Speed, Torque/Speed and Current/Speed Characteristics
The torque increases until it reaches a
maximum, in this case at 90% speed, and the value at this point is known as
pull-out torque. A further increase in
speed causes the torque to decrease until it would become zero if 100% speed
could be reached. At zero speed the
torque is in excess of that demanded by the fan and therefore the motor
accelerates. The speed increases
steadily as the excess torque is roughly the same value up to 30% speed.
After 40% speed there is a large excess of
torque and so the machine accelerates quickly until it delivers the amount of
torque required by the fan at A. If the
fan dampers were closed then the required torque is far less; the excess torque
and therefore the acceleration is greater and the machine runs up to speeds
quicker and delivers the amount of torque required by the fan at B. If vertical lines are dropped from points A
and B the load current will be indicated on the current-speed curve. The actual current is greater than that
indicated which does not include the magnetising current.
From the torque-speed curve it can be seen
that the risk of stalling is greatest up to 30% speed where the difference
between the motor and load torque is least.
If there was a reduction in motor torque which could happen if the
voltage was depressed to a level where it equalled the fan torque at that speed
then the motor would not accelerate and would draw starting current.
The speed at which pull-out torque occurs
depends on the ratio of rotor resistance to rotor reactance. Rotor reactance is proportional to the rotor
frequency which in turn depends on the difference between the speed of the
rotor and the speed of the rotating field which has been produced in the
stator. This difference is the
slip. Therefore rotor reactance is
proportional to slip frequency.
Pull-out torque occurs when Rotor
Resistance = Rotor Reactance when R2 = SX2 where S is the
slip.
In the case shown in the curve the X/R
ratio is 10/1 and therefore the pull-out torque will occur at
|
S =
= 0.1
With slip-ring motors it is possible to
introduce resistance into the rotor by connecting in a resistance bank. This will change the position of the pull-out
torque. For example, if resistance is
added so that the total resistance is equal to the reactance at 50 Hz then the
pull-out torque will occur at S = R/X = 1, i.e.
at motor standstill. This will produce
a relatively high torque to accelerate the rotor quickly but with the load
shown would run at only 82% speed.
If, when the motor achieved 30% speed,
the value of external resistance was reduced to just below half then the
pull-out torque would occur at N = 0.5, i.e.
half speed. Finally the external
resistor would be reduced to zero and the condition would be as shown in Figure
10.1. The torque-speed curves for the
three resistance steps are shown in Figure 10.2.
The above is an accepted method of
starting slip-ring motors but the change in external resistance values would be
carried out smoothly to give the best acceleration.
FIGURE
10.2 -
INDUCTION MOTOR TORQUE-SPEED CHARACTERISTICS WITH VARIOUS R/X RATIOS
OVERLOAD PROTECTION
One of the most widely used relay for a.c.
motor protection is the thermal relay which consists of three heaters supplied
by three current transformers measuring stator current. The heaters are in the proximity of
bimetallic strips which when heated produce a torque to move the relay contacts
towards a fixed contact. The deflection
is proportional to current squared and therefore a motor operating at full
load would move the contact three-quarters of the way towards a final contact
set at 115%. Hence the operating time
would only be a quarter of the time required to operate the relay if the motor
was running light. Whilst this is
obviously a desirable feature it should be remembered that the thermal time
constants of the bimetal and motor differ widely and if the motor load is
varying the bimetal will respond in seconds whereas the motor temperature
change will take minutes or even hours.
Because the bimetallic strips are heated
indirectly, i.e. by heat radiated from the heaters as opposed to heating by
passing current through the bimetal, there is a time delay in its
response. This means that the moving
contact will continue to move towards tripping even after the motor-starting
current has disappeared. To avoid
operation the relay operating time must be at least twice the motor starting
time.
These relays often have a special contact
arrangement which operates if the current in any phase differs from the current
in the other phases by more than 12%.
The reason that unbalanced phase currents
require disconnection of the machine is that any unbalance in the current
results in a negative phase sequence component which produces a rotating field
in the opposite direction to the rotating field produced by the applied system
voltage. This counter-rotating field
will cause induced currents in the rotor of almost twice normal system
frequency resulting in overheating and possible damage.
Apart from the condition where one phase of
the supply is missing completely, for example, owing to a blown fuse it may be
thought that any unbalance in the system is small. This may be true in terms of voltage but as
the negative phase sequence voltage will be applied to the standstill impedance
of the motor the current will be substantial.
If the negative phase sequence voltage was 5% then, if the starting
current is 6 times full-load current, the negative phase sequence current will
be 3%. In the case where one phase is
missing completely the positive and negative sequence currents will be the
same.
Other conditions which can cause unbalanced
voltages are heavy single-phase loading or a blown fuse in a power factor
correction capacitor circuit.
Overload and unbalanced load are conditions
associated with the situation external to the motor. Overload is caused by an increase in the
mechanical load whereas unbalanced currents are caused by the supply.
If tripping has been initiated by either of
these conditions, indicated by operation of the flag relay marked
"Thermal", it is usually in order to restart the motor. Only one restart should be attempted and
during the starting period the relay should be used to diagnose the type of
fault, overload or unbalanced current if it is still present.
As shown in Figure 10.1 the starting
current of a direct-on-line motor is practically constant at short-circuit
level during most of the run-up period and there is, therefore, no means of
detecting a stalled condition by current level alone. The thermal relay will trip the motor
eventually, but because the time is long it may be too slow to prevent
damage. In this case a single element
stalling relay is used. This relay has a
directly heated bimetal which gives a low overrun and therefore the operating
time can be set close to the maximum run-up time. In some cases the run-up time is greater than
the allowable stall time this means that the condition can only be resolved by
the addition of a speed-measuring relay.
INSULATION FAILURE
Protection against short-circuits is
accomplished by instantaneous overcurrent relays but as these must have a
setting greater than the starting current of the motor only a limited amount of
the stator winding is protected. For
example, if a short-circuit occurs at the motor terminals the current is the
full system fault current whereas if the short-circuit was at the star point no
fault current would flow as the star point itself is a short-circuit. Therefore along the length of the winding
there is a reduction of fault current from maximum to zero. The decrease is not linear but roughly
proportional to the square of the distance from the star point. Figure 10.3 shows this and also that an instantaneous
relay which is set to avoid operation under starting conditions will only
operate for faults just beyond the motor terminals. In fact these overcurrent relays can only be
regarded as protection for the cable and the motor terminal box.
In a delta-connected motor there is, of
course, no point in the winding where a short-circuit would produce zero
current. Nevertheless there is a large
reduction in fault current for short-circuits away from the motor terminals.
FIGURE
. 10.3
REDUCTION OF FAULT LEVEL IN A STAR WINDING
The detection of short-circuits between
windings is accomplished by the earth-fault relay. This is residually connected and has a
setting of 10% which means that practically all of a star-connected winding
will be protected. It is assumed that
any insulation failure will result in an earth fault which is a reasonable
assumption in that all the stator windings are in the close proximity to the
stator iron.
The relay itself should have a voltage
setting with a stabilising resistor but in some cases this is not required as
stability is required only to the level of the motor starting current.
Operation of either the instantaneous
overcurrent or the earth- fault relays operates a flag indicator marked
'Instantaneous". When the motor has
been tripped by these relays no attempt must be made to restart until an
insulation test has been carried out.
SETTINGS
A typical motor protection relay is
available with different time ranges, namely 14 20 and 30 minutes. These do not describe the relay in a very
precise manner as they refer to points on the curves which are asymptotic. The times would have more meaning if they
were quoted at a higher multiple of setting current. The 15- and 20-minute relays are the
standard. A 30-minute relay would be
used where the starting period is long or the machine is started frequently.
In setting the thermal section of the relay
there are two adjustments which allow the correct setting to be made. A rough adjustment by changing the turns
ratio on the auxiliary current transformer and a fine adjustment by alteration
of the fixed contact position.
The possible settings on the auxiliary CT
are 80% 90% or 1000/0 and a setting corresponding to the ratio of full load
current to line CT primary current should be chosen.
Examples
45kW motor FL
current = 84 A, CT ratio 100/1,
|
= 0.84A set to 80%.
55 kW motor FL
current = 98 A, CT ratio 100/1,
|
= 0.98A set to 100%.
75
kW motor FL current t36 A, CT ratio
150/1,
|
= 0.91 set to 90%.
75
kW motor FL current = 136 A, CT ratio 200/1,
|
= 0.68 set to 80%.
The auxiliary CT, which incidentally
utilises the magnetic circuit of the instantaneous overcurrent relay as a core,
changes the overall ratio of the current transformer circuit. The l50/1 CT with the auxiliary CT on a 90%
tap gives an overall ratio of
0.9 x 150/1 = 135/1.
The adjustment of the fixed contact depends
on the duty of the motor. If the motor
load is fairly constant, say a fan or a pump, then the contact can be set
fairly close to the full-load current value, say at 110%. If the load fluctuates, for example a
conveyor then a wider setting may be needed, a setting of 115% or even more.
This does not mean the fixed contact will
be set at 110% or 115% although in most cases it will be fairly close to these
values.
The contact should be set, for a 110%
setting to:
|
110% x
in the case of
the 37 kW motors
|
110% x = 1.16 set, to 116%
or the other
motors
|
110% x = 1.08 set, to 108%
|
110% x = 1.11 set, to 111%
|
110% x = 0.94 set, to 94%
or for a 115%
setting
|
115% x = 0.98 set, to 98%
The setting of the instantaneous
overcurrent relay must be about l½ times the motor starting current in order to
avoid operation during initial peak of the starting current which can be more
than twice the steady short-circuit current but has a duration of less than one
cycle.
It would be more correct to say that the
relay setting should be about 1l3 times the motor short-circuit current. This is the same as starting current in the
case of direct-on-line motors but not when the motor is started by a method
which limits the starting current. The
fact is that an induction motor will contribute current to a system fault at a
level equal to the short-circuit current.
This is the initial current at the moment of fault but quickly dies
away.
Therefore even though the current has been
limited during starting to, say twice full-load current an external fault will
cause a current of 6 to 8 times full-load current to flow from the motor. The instantaneous overcurrent relay must be
set so that it does not operate under these circumstances. The actual setting is the times full-load
figure on the scale multiplied by the tap setting on the auxiliary CT.
The success of this type of relay is
undoubtedly due to its simplicity in setting and the ability to check its
performance whilst in service. The
moving contact arrangement carries a pointer which indicates the percentage
load. From observation of the panel
ammeter and a knowledge of the overall CT ratio the correct operation of the
protection can be verified. The overall
CT ratio is the line CT ratio, which can usually be deduced from the ammeter
range multiplied b~ the auxiliary CT tap which is indicated on the relay
nameplate.
For example,
the 45 kW motor, CT ratio 100/1 auxiliary CT tap 80%. If the panel ammeter was indicating 68 A, on
the percentage load scale
|
x 100% = 85%
should be
indicated.
There are a number of electronic relays
which protect the motor in the same way as the thermal relay which are capable
of matching the motor characteristic more accurately. In addition to adjustments for current level
the operating time can be adjusted as well as the settings for unbalanced
current and earth fault.
FIGURE
10.4 - COMPOSITE CHARACTERISTIC THERMAL
RELAY AND 200A FUSE
The type of protection described would only
be applied where the motor is supplied via a circuit-breaker. In many cases the switching of the motor is
by a contactor which, although capable of making fault current, cannot
interrupt fault current. Therefore fuses
are used to clear the fault instead of the instantaneous overcurrent
relays. The earth-fault relay is used as
it will detect low-level faults and trip the contactor hut for high-fault
levels the fuse 'would operate faster than the earth-fault relay. It may be that a time delay is to be
introduced into the earth-fault relay circuit to ensure that the fuse operates
faster. Figure 10.4 shows a typical
composite characteristic where 160 A fuse is used in conjunction with a relay
thermal element and earth-fault relay to protect a 550kW motor with a full-load
current of 120 A. From 132A, 110% full
load current to 700A the motor is protected by the thermal relay which would
trip the contactor. Above 700A the 200A
fuse would clear the fault. Similarly
earth fault from 12A to 3000A would be cleared via the contactor and above
3000A by the fuse. The contactor will
never be called upon to break a fault current beyond its capability.
DIFFERENTIAL PROTECTION
Differential protection on a phase by phase
basis is shown in Figure 10.5. This type
of protection is eminently suitable and will detect faults on practically the
whole of the winding but is generally only used on large motors. The leads between the current transformers in
the motor neutral terminal box and the relay which is associated with the
switchgear, may be long and therefore could have a high resistance.
FIGURE
10.5 -
DIFFERENTIAL PROTECTION
However. as the most onerous condition under which
stability is required is the motor starting current a fairly low relay
setting. and reasonably small current
transformers can he used
LOSS OF SUPPLY
When the supply is removed from an
induction motor its back e.m.f. will
decay exponentially and virtually disappear in a seconds. During that time there will also be a slight
decrease in speed so that the phase of the back e.m.f. moves away from the position which it
occupied before the removal of the supply.
The result is that the locus of the back e.m.f. traces a spiral as shown in Figure 10.6. The figures are arbitrary and are not meant
to represent any particular motor.
If the voltage was restored before 0.4 s
then the voltage applied to the motor would be less than system voltage because
of the back e.m.f. and the current would
be less than short-circuit current.
After 0.4 s the voltage between the applied voltage and the hack
e.m.f. is greater than the applied
voltage and the short-circuit current would be correspondingly greater. If the voltage was restored after 0.8s the
short-circuit current would be 1½-, times normal. This means that the mechanical forces exerted
on the rotor would be over twice the normal starting forces and could cause
damage to the rotor structure.
FIGURE
10.6 -
LOCI OF BACK EMF OF MOTOR DURING LOSS OF SUPPLY
For this reason undervoltage relays are
used on large machines to ensure that the machines are disconnected if the loss
of voltage exceeds say 0.3 s. The relay
used is either an attracted armature relay with a time-delay relay or an
induction relay
During a system fault there is a loss of
supply to all motors connected to the system until the fault is cleared by unit
protection. The loss of supply will be
of the of the order of 120 to 250ms, the protection-operating time plus the
circuit-breaker opening time. Even if
the fault persisted for a longer time there is not much danger of the high
short-circuit current. This is because
the motor will be contributing current to the fault and consequently- the decay
of the back e.m.f. is far more
rapid. It will in all probability have
disappeared in less than 0.5 s.
SYNCHRONOUS MOTORS
The protection for synchronous motors is
the same as that for induction motors but with the addition of a relay to
detect loss of synchronism and loss of supply.
For loss of synchronism an out-of-step
relay is applied to motors which could he subjected to sudden overloads. The motor could pull out of step because of
an increase in mechanical load or if there is a reduction in supply
voltage. When pole slipping occurs the
stator current increases and the power factor changes to a very low value and
it is this condition that the out-of-step relay detects and trips out the motor
during the first slip cycle.
The relay coil is connected to a bridge
circuit which compares the current from one phase with the voltage from the
other two phases. The relay is energised
by the voltage and is in the operated position at all times when the current is
zero or under normal load condition.
When the motor pulls out of step the current is such that it subtracts
from the current produced by the voltage to such an extent that the net current
falls below relay drop-off level. A
non-linear resistor increases the overall tripping area of the characteristic
to ensure correct operation. This type
of relay will also detect the loss of field.
FIGURE
10.7 -
OUT OF STEP PROTECTION
If the supply to a synchronous motor is
interrupted for more than say, 0.3s, then there is a danger that if the supply
is restored the motor may be out of step and therefore an undervoltage relay is
required to trip the machine. This relay
will also prevent starting and running under abnormally low voltage conditions. Other protection devices are underpower or
reverse power relays which are usually induction relays and are identical
except that the former closes its contacts when the forward power is less than
say, 3% and the latter closes its contacts when the reverse power exceeds
3%. The reverse power relay should
always be preferred as it is more stable to momentary swings of power but it
depends for operation on the protected motor generating to other loads
connected to the same busbar. If there
are no other loads then an underpower relay must be used.
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