Motor Protection1

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EASUN REYROLLE LIMITED

POWER SYSTEM PROTECTION COURSE

MOTOR PROTECTION

EASUN REYROLLE LIMITED

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1



MOTOR PROTECTION

INDEX

1. INTRODUCTION

2. SUMMARY OF PROTECTION REQUIREMENTS

3. THERMAL OVERLOAD

4. STALL OR LOCKED ROTOR CONDITION

5. TOO MANY STARTS

6. UNBALANCED PHASE CURRENTS

7. SINGLE PHASING

. SHORT CIRCUIT BET!EEN PHASES

". SHORT CIRCUIT TO EARTH

1#. UNDERCURRENT OR UNDER PO!ER

11. UNDERVOLTAGE

12. REVERSE PHASE SEQUENCE

13. SPECIAL REQUIREMENTS FOR SYNCHRONOUS MOTORS

14. SOME APPLICATION CONSIDERATIONS

2



FIGURES

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3



MOTOR PROTECTION

3' Introdction

Electric motors are used extensively in Power stations and industrial complexes to drive mechanical plant

such as pumps, fans, blowers, compressors, conveyers, crushers, mixers and range in size from small k

!atings upto ratings in excess of 2"#$s%

&hese motors are generally 3 phase induction machines but, particularly at a higher rating, synchronous

machines are often employed%

'oltages employed to drive these motors vary, generally from 3("' upto 13%()' and may be controlled by

switching devices, either contactors *protected for short circuits by either #+$s or fuses-, .' motor

switching devices *protected by fuses- or circuit breakers%

Protection re/uirements depend on the rating, type of motor and controlling device*s-, the duty of the motor

and also cost considerations%

&he following provides information on the more significant protection re/uirements, the causes and effect

of each abnormality and how protection can be employed to detect the condition% 0inally some particular applications are considered%

Protection devices for motors are available in several forms e%g% direct measuring types and indirect types%irect types include thermistor devices embedded in the motor windings to detect overheating% ndirect

types include bi metallic strips *connected in the primary circuits- for thermal overload replica, or

magnetically operated oildashpots providing an idmtl overload characteristic% 0inally there are

multifunction relays monitoring current transformer secondary values which enable a wide range of faultconditions to be detected%

&he following concentrates on multifunction relays and makes only brief reference to other devices%

4' S""ary o$ protection Re<ire"ent!

&he main abnormalities that can occur can result in damage to the motor and for which protection can be

provided if re/uired are4

&hermal overloads

5tall or 6ocked !otor

5tart 6imitationPhase 7nbalance

5hort circuit between phases

5hort circuit to earth

7ndercurrent7ndervoltage

!everse phase se/uence

5ingle Phasing

8dditional for synchronous motors4

9ut of step

!otor earth fault7nder : over excitation

0ield 0ailure

6oss of source supply *underfre/uency or reverse power-

;



1' Ther"al O)erload

1'3 =eneral con!ideration!

n this section we are primarily considering 3phase balanced overload, e%g% an overload due to mechanical

output overload, re/uiring a tor/ue output beyond the motors design capability% &he resulting higher than

rated stator current results in excess heat within the motor and so reduces its life expectancy and eventually

results in failure in services%

&he system voltage being reduced can cause overheating of the stator winding% n order to maintain the

same output tor/ue, the stator current increases% &his relationship is maintained with voltage reductions

down to the order of ("<% &he resulting higher than rated stator current will cause overheating although theoutput load tor/ue is the same%

Protection can be achieved by direct measurement *e%g% use of thermistor based devices- or indirect

measurement employing a relay which monitors the stator current% 5uch relays create a thermal image of the stator winding temperature, employing a current time algorithm that mimics the heating and cooling

effect on the stator winding caused by variation of the stator current%

#odern microprocessor relays are very accurate for thermal monitoring and can be set to guarantee

operation for a very small excess stator current%

.eating and cooling replica characteristics can be selected to match the thermal time constants *if known-

of the specific motor design%

&he relay algorithms are designed to mimic the thermal response of the motor windings taking account of

the effect of magnetic core losses, copper losses, a winding configuration which may result in hot spots,

and the different effects of high starting currents and continuous low level overcurrents%

8 problem with thermal protection provided by current operated relays is that they do not take account of

overheating due to conditions such as fault cooling system *e%g% blocked ventilation ducts- or ambient

temperatures above the design maximum for the motor% !elays do not usually have automatic ambienttemperature compensation%

irect measurement devices overcome the problem outlined above but have other problems such as

establishing appropriate characteristics and thermal image, also establishing the appropriate siting of thedevices% n practice, for the motor ratings under consideration, it is normal for a current operated relay to be

employed% Embedded temperature detectors are only employed as backup protection and: or monitoring

devices, on very large machines, i%e% in addition to a relay%

1'4 Characteri!tic Operating Cr)e o$ a Ther"al Relay

n order to protect the motor from overtemperature at high load currents, i%e% beyond the #+! *maximum

continuos rated- up to stall current, the relay=s characteristic curve must be set within the boundary of themotor withstand current : time curve%

hen a motor has been running for a period of time it reaches a steady state temperature somewhat higher than ambient and so the period for which it can carry a given overload is less than for the initial conditionwhen the motor is at ambient temperature itself% &he motor has an infinite number of withstand

characteristics depending upon the steady state conditions applying before an overload occurs% &he two

extreme cases are $cold$ withstand curve *from initial ambient conditions- and the $hot$ withstand curve *a

reduced trip time for an overload occurring from initial steady state full load conditions-% 7nder multiplestarts, reaccelerations or cyclic overload conditions the motor temperature will rampup and the relay

thermal image must mimic this in order to provide appropriate protection% &ypical $hot$ and >cold= curves of

the inverse current time type are shown in 0igure 1%

?



&he most extreme form of balanced overload is the locked rotor or stall condition, which is discussed

separately below4

&he motor has several conditions to cope with during normal starting and operation i%e%

a- 5tarting from cold

b- +ontinuous running

c- 5tarting from hot or partially hotd- +old stall withstand until tripped

e- .ot stall withstand until tripped

ts withstand capabilities differ with the system voltage as outlined in section 3%1 paragraph 2%

8 protection relay therefore re/uires different characteristics for the $ hot$ and $cold$ conditions and also

needs to protect against the two stall conditions% 8t the same time however it must also allow a normal

startup where current e/ual to locked rotor value may flow for a very long time *e%g% fan drives-% n someinstances it is not possible within one relay, to protect for all conditions and allow a normal start% 8n

example of this is where a motor may have locked rotor withstand time of less than the runup time i%e%

because of the lack of cooling that would be available if the motor was accelerating its withstand time for

locked rotor current is less than the normal runup time% n these circumstances a speed switchmonitoring

device must be fitted alongside the rotor shaft%

8nother feature of the motor characteristics which can be mimicked by the relay is the $hot spot$

characteristic% 0or a homogenous type of plant, e%g% cable, there are no hot spots and the conductor temperature varies uniformly% .owever a motor is not homogenous and the temperature of different parts of

the winding does not vary uniformly% 8fter the motor is started up direct on line the hot spot temperatures

are initially higher than the rest of the copper:iron mass% 8fter normal running for some time all parts will

reach a more uniform temperature% &he design of the motor will determine this time% 5ome relay designshave an ad@ustable constant to take account of widely varying designs e%g% no hot spots or severe hot spots%

1'1 E$$ect! o$ Starting Method

irect on line starting or other starting methods *e%g% rotor resistance or variable fre/uency- do not have any

effect on the criterion described above provided that the relay thermal image takes account of the true !#5e/uivalent of the measured current% n practice, where variable fre/uency soft start systems are employed,

not all relays can take account of the fre/uency range concerned%

1'7 Ther"al O)erload Protection Setting!

&he criterion is to establish a setting that will protect the motor from excessive overload% &he motor willhave a declared maximum continuous rated *#+!- current% .owever the drive may be designed such that

the motor normally operates at a lower current than #+!%

8ccount must also be taken of consecutive start re/uirements and locked rotor withstands, referred to below%

Aormally the relay pickup current is set to the #+! current *especially if the pickup performance has nonegative tolerance so that it will never operate for a current magnitude less than the relay setting-% &hemotor time constant, starting time and stall withstand time are taken in to account of the protection and

control re/uirements%

0igure 2 shows a typical curve of motor current plotted against runup speed%

B



7' STALL OR LOC>ED ROTOR CONDITION

&his condition occurs when the motor experiences a severe overload and causes the rotor to lock or during

startup, to prevent acceleration% uring this condition, the motor draws the locked rotor current, which, for a direct on line start motor, is the starting current%

0or large motors a manufacturer may declare the locked rotor withstand times% &hese may be declared from

a cold start and from hot conditions and in both cases at 1""< voltage as well as may be ("< voltage% &helatter is significant because at this voltage a motor may not accelerate from switchon if loaded% #otors

depending on the design may have either the stator or the rotor as the part of the machine, which is the

critical item%

&he normal thermal overload characteristic of a relay can often be employed to protect the stall conditions

of a robust machine%

8lternatively, by monitoring the currenttime profile a relay algorithm can be employed to establish that amotor was running and has now stalled during a run condition% efinite time over current can be enabled

after the runup time to provide faster trip times compared with waiting for the thermal overload trip

function%

f the motor stall withstand time, either from hot or from cold, is less than the runup time the motor canonly be protected by means of speed monitoring% f, after switch on of the motor stator, the rotor does not

reach an appropriate speed within the stall withstand time it indicates that the motor is not accelerating

sufficiently% &he algorithm can be built into a modern numeric relay and, with an input from a speed switchto establish that a motor is accelerating or not, will provide the stall protection%

2' TOO MANY STARTS

hen a motor is started direct on line *i%e% no CsoftC start arrangements provided-, the motor thermal state

rapidly increases due to the high starting current% f the motor is stopped soon after *e%g% due to a drive

process re/uirement- the motor thermal state will reduce slowly since there is not likely to be any forced

cooling once the rotor has stopped turning% 8lthough the motor manufacturer sometimes specifies a heatingtime constant rarely do they specify a cooling time constantD .owever a thermal protection relay will have

a cooling characteristic and generally with numeric relay is ad@ustable%

f the motor is started again in a relatively short time, less than the cooling time constant, the thermal statewill ramp up higher% !ather than overstress thermally the motor it is often a re/uirement to provide a limit

to the number of starts allowable by using the relay% 8n output contact is employed in the motor control

device closing circuit, which locks out the close facility if the start condition is exceeded% &his could be on

a Cstart countC basis or by establishing via the relay algorithm that a further start would take the motor thermal state into a trip condition% 0igure 3 shows a typical thermal ramping due to consecutive starts%

0' UN?ALANCED P.ASE CURRENTS

&he motor being connected to a source supply, which is not exactly balanced, generally causes these, i%e%

there is an amount of negative phase se/uence voltage%

Aegative phase se/uence voltage results in negative phase se/uence *nps- current in the stator% &he npsimpedance is much less than the positive phase se/uence *pps- impedance and therefore the ratio of nps and

pps current is much higher than nps to pps voltage%

&hese stator 3phase currents generate an air gap flux, which is rotating in the opposite direction to that of the motor% &he flux induces e/uivalent nps voltages and currents in the rotor circuits, which have a

fre/uency of approximately twice the supply fre/uency%



.eat produced by the nps component in the rotor is significantly higher than the e/uivalent positive phase

se/uence *pps-% &his is due to the fact that the rotor current associated with pps stator current acts upon the

dc resistance of the rotor circuit whereas the double fre/uency nps current acts upon the motor ac resistance

increased due to skin effects% &his, along with the much higher speed of nps flux rotation relative to themotor results in a much higher heating effect on the motor particularly the rotor%

9nly 1 < nps stator current will have a heating effect on the rotor e/uivalent to approximately B< of

normal pps current%

#otors are designed to operate at full #+! with, in addition, a small proportion of nps current

*e%g%resulting from a nps voltage of 2 <-% 8bove this value a motor rotor rapidly becomes overheated%

7nbalanced load or nps current protection is very important because any system voltage abnormality will

affect every motor connected to this source of supply and can result in mass failure%

n a modern relay design the nps current is filtered out of the phase currents and can then be employed invarious ways%

9ne method is to establish an idmtl characteristic based on nps only and with ad@ustable settings, e%g% both

nps pickup and operating time curve%

8lternatively the nps current heating effect can be added to the normal pps value as follows4

6 F *1-2 G ) *2 -

2

here 1 F pps current

2 F nps current

) F a constant, e%g% Bx

'ery rarely do motor manufacturers specify a nps withstand other than the simple statement included in the

relevant Aational and nternational 5tandards, e%g% temperature limits are not exceeded if the motor is

operated at full rated load with 1< nps voltage applied% &herefore it is not possible to select scientifically aspecific trip setting for the nps level% 8 continuous withstand of 1"< nps current can usually be tolerated%

.owever speed of operation for worst case conditions, *i%e% single phasing- re/uires a highspeed trip *refer to item below-%

0or smaller values of nps current it is necessary to introduce a time delay in order to allow transient

unbalance conditions to disappear first and allow the relay to be stable% &ransient unbalance will occur

during unbalanced faults on the system%

&o accommodate the above it is usual to have an idmtl trip characteristic%

8' SIN=LE P.ASIN=

&his refers to a 3 phase motor with voltage supply on two phases only, e%g% due to a primary current fuse

operating or an open circuit conductor or phase winding%

n this operating condition the nps and pps currents have the same value% f the motor is loaded then it islikely to stall, resulting in very high values of nps or pps%

&he effect on current drawn by the motor during single phasing is dependent on the winding configuration

e%g% star or delta and also the load on the motor% .owever when on load the current rises considerably andcould be as high as 2?"< of full load current% &he protection device employed must trip out the motor

/uickly, in a time shorter than the thermal overload function can%

(



,' S.ORT CIRCUITS ?ETWEEN P.ASES

Protection against short circuits is accompanied by two means depending upon what type of switching is

used to control the motor and also the design of the motor cable box%

here contactorswitching devices are employed for motor control it is necessary to provide fuses to cut

off high magnitude fault currents% 8 contactor cannot do this%

5ome designs of motor cable box will not withstand and internal shorts circuit current of high magnitude

fault current and would result in an explosion% n these circumstances a fuse is re/uired to limit the current%

0uses exhibit a cutoff value and the maximum peak is limited significantly i%e% depending on the fuserating%

here the switching medium is a circuit breaker and the cable box is ade/uately designed it is not

necessary to provide fuses% &he circuit is then protected by means of instantaneous overcurrent protection%.owever this must be set to a value which will not allow maloperation for switch on in rush current

*normally between ( and 1B times full load rating-% 8lso, to ensure operation, a setting of ≤ 2?< of the

minimum fault current level is recommended%

9n switchon of a directonline motor the inrush current may be upto 1Bx full load current but only for a

short time% E%g% 2? m%secs% &he current then decreases to the start level e%g% Bx full load current%

&o allow a reasonably low value of short circuit current setting and to therefore protect the motor from

severe short circuit current damage, a time delay of approximately ?"m%5ec can be introduced% &his allows

the initial inrush current to die down%

8lternatively numerical relays can be designed to double the pick up setting at start up% 5o, for the running

state the pickup level could be less than the steady state start current e%g%4

Aormal switch on current F Bx 065etting *normal running- F ;x 06

5etting *at start- F (x 06

0or high rated motors the current re/uired to cause operation will be very high *e%g% ( x 06 may be H2"""8-% &his becomes unacceptable because the fault damage is considerable before a trip is initiated% &he

cost of repair or replacement @ustifies the use of a more sensitive form of protection i%e%differential%

ifferential protection involved positioning current transformers *+&s- on the neutral end side of the motor winding as well as on the line end connections% t is therefore necessary to provide end connections from

the motor% &here is therefore a significant extra cost concerned with provision of this function, e%g% for

motor neutral and connections, for the +&5 and for the pilot wires between the neutral end of the motor and

the supply end circuit breaker%

&wo designs of differential schemes are normally considered4

i- .igh impedanceii- iased

.igh impedance schemes can be slightly faster and they have a setting, which is more sensitive than biased

differential scheme designs%

&he two schemes are shown in 0igure ; and ? and a typical relay performance characteristics for a biased

differential relay is shown in 0igure B%

+' S.ORT CIRCUIT TO EART.

I



Earth faults are relatively more common than phasetophase faults due to the design of the motor% Earth

faults usually develop as insulation fails and, depending on the method of earthing the supply system

neutral connection may develop into a short circuit which results in considerable damage to the motor%

t is desirable therefore to provide a sensitive means of detecting an earth fault at the beginning of its

development and to isolate the fault very /uickly% t may therefore be possible to carryout a repair in situ

rather than carry out a ma@or repair at the manufacturer works%

Earth fault detection is generally achieved in two ways%

a- by measuring the residual current from the three phase mounted protection +&s see 0igure a% b- by measuring the current from a core balance +&, see 0igure b%

&he preferred method is *b-% f the 3phase residual method is employed, due to some residual appearing in

the relay circuit at switchon of a direct online start motor the sensitivity must be limited to approximately2"< of the circuit rating% t may also be necessary to introduce a time delay of approximately "%3second to

allow any transient to die away% &he transient current is due to the three phase mounted +&s being driven

into different states of saturation%

ith a core balance +&% ratio is not dependant on the circuit rating and therefore with a low +&% !atios of say ?":1 amp or lower, a primary operating current of less than 1 amp is often achievable%

8 lower limit of setting is established by the motor circuit capacitance% 8ny system fault on neighboringcircuits causes zero se/uence capacitance current to flow towards the fault from the motor circuit% &he relay

will detect this current and, because it is a nondirectional relay, it would operate if the relay earth fault

setting were set too low%

ndustrial plant power system supply neutrals are usually earthed as follows4

a- 5olid #aximum fault levels at least e/ual to a three phase fault

or

b- !esistance #aximum fault levels around 1"" to 12"" amp% or

c- solated #aximum fault current from total system zero

se/uence capacitance, typically 2 to 1" amps

n the case of isolated neutral systems the motor circuit relay setting needs to be carefully considered toachieve a correctly graded system, especially if a nondirectional earth fault measuring function is

employed% &he nondirectional relay must be set above its own circuit capacitance current *to prevent a

maltrip for ad@acent circuit faults- and below say 2?< of the total system zero se/uence fault current to

ensure operation for faults on the motor circuit being protected%

f the ratio of circuit zero se/uence current to total zero se/uence current does not exceed 1:? it is not

possible to achieve the above criterion% f a discriminating zone scheme is still re/uired it is necessary to provide a sensitive directional earth fault relay on each feeder circuit% &hese can then be set below theindividual circuit capacitance current because they will not operate for a reverse fault%

&he two differential systems described above will also operate for an earth fault but only where significant

earth fault current is able to flow e%g% H 2"< circuit rating%

0igure ( shows a directional earth fault scheme%

39' UNDERCURRENT OR UNDER POWER

1"



5ome drive applications are such that there is a possibility that the load on the motor may suddenly be

removed e%g% a drive coupling shaft may shear or in pump applications, the fluid being pumped may not be

available and the drive is left spinning in air%

n some instances this leads to a dangerous situation either for the plant associated with the drive or for the

motor itself%

&o detect this condition usually an undercurrent measuring function is sufficient% .owever in some

instances the no load excitation characteristic of the motor circuit results in a condition where the current is

not much different in magnitude than load current% n these circumstances it is necessary to employ an

under power relay%

8n example of this protection re/uirement is for submersible pump applications where the pumping fluid

acts as a cooling medium as well% &he motor design is based on this criteria and if the motor is not

submersed it will /uickly overheat% 7sually submersible pump motors have a large no load current of low power factor and therefore an underpower relay is re/uired%

33' UNDER@OLTA=E

&here are two separate conditions concerning an undervoltage of supply source to which motors areconnected% oth result from the fact that the available output tor/ue of the motor is proportional to the

s/uare of the terminal voltage%

& F '2 &m &6

'n

& F excess tor/ue ' F terminal voltage 's

'n F rated terminal voltage &m F motor tor/ue

&6 F load tor/ue

&he two conditions are4

a- 7ndervoltage during motor running and

b- 7ndervoltage at switchon

a- 7ndervoltage uring !unning

f the voltage falls during motor running the motor, current will rise by an amount depending upon thedegree to which the speed of that particular motor must fall to regain a tor/ue balance% &his general

increase in the current drawn by the motors and in particular the increasing proportion of reactive current

tends to reduce the voltage further e%g% due to source impedance voltage drop% 8s the voltage falls certain

motors *those must heavily loaded and those driving constant tor/ue loads- will eventually stall% n short,depending upon the level and type of motor loading, the system voltages will tend to fall further and current

increase together with a complex mixture of motor and supply transformer overloading and motor stalling%

&he motor shall be able to operate at its rating with a supply variation of ± ?< and allows an increase in

temperature above its classification of 1"°+ for motors upto 1"") and ?° + above the rating%

7ndervoltage is a problem for individual motors *and motor starters- and undervoltages releases should

drop out between ?< and 1"< 'n with slowly falling volts% &he best they can be said for these devices isthat they will fall out when the supply volts collapse% 8lthough a time delay is not specified for these

undervoltage releases they are normally slugged *time delayed- to prevent maloperation due to transient

voltage dips% &his time delay is generally very short and may not coordinate with the system$s overcurrent

protection%

11



7ndervoltage relays can be used to provide a much more accurate and selective response to undervoltage

conditions% f the voltage falls to ("< it is more than likely that the motors will need to be tripped to

prevent a system voltage collapse as motors start to stall% t does depend on what percentage of the total

load of the supply transformer is motor load%

7ndervoltage if considered necessary, can be provided on a per busbar basis rather than per motor circuit%

b- 7ndervoltage at 5tartup

8 given motor may be incapable of accelerating when the supply voltage is low% 8s we have seen under

these conditions thermal or stall protections will trip in several seconds whereas the fact that an

undervoltage is present is indicative from the outset that the motor would not start% &he undervoltagereleases should allow starting if the voltage is greater than (?< 'n but block should it be less than ?<%

8gain this performance can be significantly improved upon and matched to a given motor$s capability by

applying undervoltage relays which have an ad@ustable setting range%

c- 9ther considerations

hen considering motor installations, continuity of production is generally very important and it becomes

necessary to consider the systems response to voltage interruptions% 8synchronous motor act as alternators

upon loss of supply and the stored electromagnetic energy try to maintain the terminal voltage% &heterminal voltage decays with time at a rate depending upon connected load and the amount of stored

electromagnetic energy% f the supply returns /uickly *e%g% high speed autoreclose schemes- it is probable

that it will be out of phase with this regenerated voltage and can result in current surges upto twice thestarting current, and result in sever mechanical stressing%

!eacceleration schemes involve understanding the type of supply interruption which is possible on a given

system *e%g% fast or slow autoreclose, automatic changeover etc%-, determining which loads are essential for reaccelerating and deciding upon what criteria to base the decision to reconnect%

34' RE@ERSE P.ASE SEUENCE

Phase se/uence determines the direction of rotation and therefore a phase se/uence in reverse to the normal

direction will cause the mechanical drive to be driven in the opposite direction% n some applications it isessential that protection is provided to detect this condition immediately%

&he protection re/uirement is more generally specified for low voltage system applications e%g% 3("volt% n

these applications a voltage operate se/uence detector is employed%

0or other applications normal for the negative phase se/uence overcurrent function to be employed%!everse phase se/uence means that 1""< of the load current is negative phase se/uence%

31' SPECIAL REUIREMENTS :OR SYNC.RONOUS MOTORS

&he re/uirements for synchronous motors include all protection functions considered for asynchronous

motors% 8dditionally the following abnormalities : fault conditions must be considered%

31'3 Ot o$ Step Protection

&he response of a synchronous motor to an excessive load re/uirement or severe voltage depression is that

it will probably fall out of step% nstead of maintaining synchronous speed it falls below this and is referred

to as pole slipping because the field generated by the field supply is slipping relative to the field generated by the stator supply%

&his phenomenon causes severe current fluctuations in the stator and rotor circuits%

12



&he condition causing pole slip may only be transient *e%g% fault clearance on the associated power

network, transient overload on the mechanical output-% epending on various factors such as the insertion

constant of the motor and the electrical strength of the power supply source and many others, the transient

pole slipping may die away and the motor eventually pull back into step with the network supply%

8lternatively the pole slip fre/uency may increase and may never be recoverable even though the initial

condition causing pole slip no longer exists%

&he out of step condition can be detected by an impedance measuring relay, taking account of the

impedance magnitude, vector position and rate of change of position%

!elays designed specifically for this condition are sometimes difficult to set, scientific calculation beingdifficult to establish% 8dditionally, testing such a relay is also difficult% .owever it is an important feature,

especially for large synchronous drives, due to the costly damage to the motor and the possible effect on the

stability of the power system%

31'4 E-citation Sy!te" :alt!

&he excitation system of a synchronous drive can be compared to that of a generator and therefore the

following protection functions must be considered%

a- !otor Earth 0ault

b- 7nder : 9ver Excitation

c- 0ield 0ailure

31'1 Under)oltage $or Synchrono! Motor!

Just as for asynchronous motors the synchronous type re/uires protection against under voltage and alsorestoration of voltage outofphase following loss of source supply% .owever because of the synchronous

motor excitation system the phenomena is different and therefore basic under voltage relays are not

sufficient%

f the supply source fails the synchronous motor is connected to the supply busbar and, depending on what

size of electrical load is still connected to the same busbar, there will be either a step rise in voltage or a power reversal or perhaps even both% 8lso the fre/uency of this supply, generated by the synchronous

motor excitation and the motor inertia, will also fall from the original source fre/uency% &his will occur rapidly if the busbars are heavily loaded%

&he protection functions, which can be employed therefore to detect /uickly the loss of grid supply, are

a- 9ver voltage

b- !everse power

c- 7nder fre/uency

7nless power reversal is a possible normal condition *e%g% with combined motor : generator sets as with

pumped storage schemes- the reverse power would be the protection function to employ% 8lternatively a

combination of over voltage and under fre/uency protection which can be applied /uickly, without timedelay, in order to isolate the motor before any autoreclose takes place%

!everse power needs to be time delayed to allow for normal power swings during start or run conditions%

37' SOME APPLICATION CONSIDERATION

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37'3 Co!t Con!ideration!

8s with most items of electrical plant the protection finally chosen take into account the cost of the

protection and also the cost of not having a particular protection system%

8 main consideration is if the protection can reduce damage to the motor in the event of a fault or

abnormality and also if it can improve the security of the power system%

8nother consideration is the duty of the motor% 0or emergency operation of a system *e%g% a motor needed

for safe shutdown-% t is not allowed that overload protection be applied% &he motor is expendable in that

respect% n other circumstances a small rated relatively low cost motor may have a duty that warrants

sensitive protection to stop the process as soon as there is any abnormality%

Kenerally speaking however the cost of the drive, which is related to the rating, dictates the protection that

can be @ustified% 0or very small motors only a fuse, following through simple overloadsL low precision : low

cost : limited function relaysL to the more expensive multifunction numerical relays% &he latter arenormally always employed on motors connected to #' or .' supplies% 0or 6' motors, depending on the

duty, precision multifunction relays would generally be considered for all ratings above ?k%

37'4 :!e protection Contactor! $or Motor Control

&he contactor or motor switching device is different from a fully fault rated circuit breaker% &he contactor is

designed primarily for a high number of operations with relatively low fault breaking capacity whereas a

circuit breaker has full fault breaking capacity%

here contactors are employed they are protected by fuses, at 6%'% sometimes a #++ is employed% n

either case the short circuit current breaking capacity of this protection device is employed to prevent the

contactor from being opened at fault currents above its breaking capacity%

a- &he two devices, e%g% contactor and fuse, are selected such that they are coordinated i%e% they are

chosen to ensure the re/uired degree of protection is afforded%

here relays are employed for motor protection the fast acting functions must be prevented from opening

the contactor if they are likely to negate the coordination achieved by the contactor : fuse selection%

0or these circuits is normal to omit highspeed high set short circuit protection and rely on the fuse% f thefault is unbalanced, i%e% phasephase, the negative phase se/uence protection will operate% .owever this is

normally time delayed and should be sufficient to ensure the fuse operates first for current magnitudes

higher than the breaking capacity of the contactor%

0or earth faults there are other considerations% ecause the fault may be well into the winding of the motor,

or because the system maximum earth fault is limited *e%g% neutral earthing resistor- a sensitive setting is

re/uired and therefore this short circuit function cannot be omitted%

&he difficulty is establishing the difference between a single phase to earth fault and a fault involving earth

and more than one phase% f only the lowlevel earth fault is present is possible to trip immediately but not

if more than one phase is involved%

9ne solution is to delay tripping of an earth fault% 0or "%3 second to allow fuses to operate should there be

phasephase fault at the same time% 8nother solution is to employ a blocking feature, i%e% when the current is

high block the earth fault trip and wait for a fuse to operate%

here it is known that there is likely to be a problem the coordination must be plotted on grading curves

plotting the fuse operating characteristic, the breaking capacity of the contactor and the operating

characteristic, of the protection% 0rom this it can be seen if the contactor is likely to be opened at the wrongtime%

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37'1 Delta Connected Motor!

&he 3phase winding may be connected in star or delta depending on the design of the motor% #ore usually,for .' motors the connection is in star with the neutral ends not available unless specified for use with

differential protection as discussed above%

0or 6' motors, often the start delta method of starting is employed% &he motor has all six ends of the threewindings brought out and the control system employed two contactors, one for startup and one for the

normal running condition% n start up mode the three windings are connected in star and for normal running

mode in delta%

0or the delta connected motor or startdelta staring application the position of the +&s must be considered

and its effect on the protection performance% &he choice is between connecting them in the line *or phase-

connections or the winding circuit connections% 0or star connection it is the same connection, for delta

connection the line current is 1%32 the phase current and this must be taken into account in setting the relayor overload device%

&he main consideration is for single phasing, particularly as a result of loss of a line phase *e%g% fuse

operation- rather than an open circuit winding%

f single phasing occurs, the motor draws a higher current from the two healthy phases of the supply% 0or

start connection this condition overloads two windings e/ually and there is no current in the third winding%

8n overload relay connects in the line connections performs correctly i%e% its current image is the same asthat of the windings%

0or delta connection the relay has a current image which is different from that of the windings, i%e% because

of the delta connection, resulting in a series : parallel circuit presented to the supply, two windings have lesscurrent than two lines carrying current and one winding has more current than the third line, which is open

circuit%

f the overload function is to provide protection for single phasing then the conditions described above must be considered to make sure that the motor is protected% .owever, for the case of multifunction relays these

always have separate unbalance load protection, either by means of sensitive unbalance detection or negative phase se/uence detection% &his feature will always be sufficiently sensitive and fast in operation to

ensure a trip is initiated before an over temperature occurs due to the singlephase overcurrent condition%

37'7 Typical Protection Sche"e!

5ome typical schemes for asynchronous and synchronous motors are shown in 0igures I, 1" and 11%

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Documents

Motor Protection1