Motor Protection

Protection & Coordination

)he com.ination motor controller consists of an e=ternally opera.le circuit disconnecting means $a s*itch in series *ith orm 77 fuse for only short circuit protection or a circuit .reaer *ith magnetic element protection only% in series *ith a magnetic motor controller $contactor plus overload or a relay that matches the motor heating curve%( refer to setch on page 4# )he methods of starting a s8uirrel cage induction motor are: across the line $full voltage starting that provides the highest starting tor8ue as *ell as the highest starting current & accelerates the load to full speed in the shortest possi.le time%( auto" transformer( primary resistor( *ye"delta starting & part *inding#

Protection & Coordination

Protection & Coordination )he second to the fifth methods( inclusive are termed reduced voltage motor starting methods# )he selection of the reduced voltage method involves a co"ordination .et*een the re8uired starting tor8ue & allo*a.le starting current $this current overloads the circuit & may reduce the voltage " causing a sag " to other loads in the po*er distri.ution system%# Certain starting methods necessitate that the motors *ould .e .uilt in certain *ays# )he starting tor8ue varies *ith the s8uare of the starting current $*hen reduced voltage methods " applying less than full line voltage to the motor during starting " are used( the starting current & tor8ue *ill .e lo*er%# Contactors have to undergo certain tests to .e 8ualified or approved .y the .odies having Jurisdiction lie F'( CSA( 7EC# )he principle tests are: spacing .et*een live parts & ground( temperature rise of coils( contacts & terminals( operating voltage range( overload capa.ility( mechanicalelectrical endurance & dielectric strength tests# )he po*er $source connected% to the control circuit can .e classified into: common control( separate control or control circuit transformer# 7t can come directly from the po*er feeding the $po*er% terminals of the motor( from an e=ternal source $d#c#" .attery or a#c# station service transformer% or from a step"do*n control potential transformer connected to the supply line of the motor#

Protection & Coordination

Protection & Coordination Control panels can .e .uilt in different constructions depending on the environment and may include any or all of the follo*ing devices: contactors( .reaers( s*itches( relays( push".uttons( selector s*itches( meters( pilot lights $filament or 'E+%# )hese panels incorporate the logic $decision maing%( input & output devices# Motors are too sensitive to negative se8uence currents generated *hen the voltage supplied to the motor is un.alanced or *hen a single"phase condition occurs# A se8uence relay is used and is installed in the control panel# )his relay can .e of the negative se8uence voltage sensing or the current sensing type# )he type of the relay sensing techni8ue dictates the location of the relay i#e# upstream or do*nstream the fault $to .e sensed% for current sensing relays and do*nstream the fault for voltage sensing ones# )he fused s*itch( the moulded case current limiting circuit .reaer or the mcc. plus the current limiter is used *henever the availa.le short circuit current in the system is .eyond the circuit .reaer interrupting capacity# Hhen it is re8uired to limit the short circuit current $do*nstream% to a lo*er level than the availa.le in the system( a s*itchfuse com.ination is used *ith class G fuses $high rupture capacity current limiting fuses( 244 KA interrupting rating%# @ther fuse classes found in MCC line"ups are C & ># Programma.le logic controllers $P'C% are used often to control motors and mae decisions regarding starting( running & stopping the motors# )hey are usually in stalled in the MCC line"up#

Protection & Coordination Protection of medium voltage motors:

Protection & Coordination Medium voltage controllers: )hey consist of one or more contactors( each *ith  current limiting fuses( the dra*"out mechanism( instrument transformers( meters( protective lo* voltage devices $relays%( .us .ars( .arriers( shutters( operating coils( control transformers( au=iliary s*itches( activating levers & springs( the frame and the steel enclosure housing all these components# )he enclosures may .e front access $to .e .uilt .ac to .ac or to .e installed against the *all% only or front & .ac access# )ypical applications *ould include controlling & protecting the follo*ing e8uipment: s8uirrel cage induction motors $SC7M%( including non"reversing( reversing & multi"speed( reduced voltage starters including autotransformer & reactor starting types( synchronous motors including .rush"less & .rush types( *ound rotor motors( transformer & capacitor .ans# or decades( medium"voltage motors *ere s*itched e=clusively .y air".rea contactors# )he design of air".rea contactors has to allo* for the large contact gap re8uired $appro=imately 1# 6% & for the arc chutes $as the arc has to .e dra*n & forced into the chutes to get elongated( cooled( having its resistance increased & e=tinguished%# )he arc is( generally( maintained for several 12 cycles( it restries after a current /ero .ecause the dielectric strength of the .rea $gap% recovers relatively slo*ly# As the arc is driven into the chute( its voltage rises sharply & the current amplitude decreases# At the instant of arc e=tinction( the phase angle .et*een the current & voltage .ecomes /ero and the voltage stressing the .rea .ecomes less severe# )he second type of interrupting medium( used in motor controllers contactors( is vacuum# Hith vacuum contactors( the arc is confined to the

Protection & Coordination contact gap $*hich is only a fe* millimeters as vacuum has a dielectric strength of 4 K?mm%# Arcs in vacuum can .e classified into diffused $for currents less than 14 KA % & constricted $for higher currents%# Hhen the short circuit current e=ceeds 14 KA( a transition occurs from the diffused to the constricted form of arc in vacuum# @verheating $*hich is one of the side effects of constricted arcs% can lead to restries#

)he t*o most commonly used contact shapes( of vacuum interrupters( are the disc & the hollo* cylindrical ones# )he latter *ould have inclined slits $to divert the current flo*ing a*ay from the a=ial direction%# )he arc is driven along the circular end $contact end faces%# )he contact rings are .eveled to push the arc out*ards $to*ard the rim or edge of the

Protection & Coordination circular contact%# After contact separation( the metal vapour plasma fills the entire contact gap# Short recom.ination time of the charge carriers $in the contact gap /one% is one important property of the plasma# As the current decreases $on the short circuit current cycle heading to*ard /ero%( the carriers0 num.er decreases# After arc e=tinction( the remaining charge carriers recom.ine on the metal surface $contacts% and the gap $contacts .rea% de"ioni/es pretty fast# After current /ero( restriing is eliminated# )he current carrying contacts e=perience minimum erosion#
Protection & Coordination 1%7t should have the a.ility to interrupt all fault currents .eyond the capa.ility of the associated devices $the contactor% up to the ma=imum fault level of the system# 2%7t should have the highest possi.le current rating $for a given fuse si/e% to allo* for ma=imum utili/ation of space *ithin associated e8uipment# )he use of contactorfuse com.ination may lead to smaller si/e ca.les $than *ith circuit .reaers due to the fact of the limiting capa.ility for the let through energy of the fuses%# )he fuse elements $used in motor protection fuses% have a straight through element *ith stress relief points $rather than a spirally *ound filament on a core%# 7n general( surge arresters are not re8uired *ith vacuum contactors due to the fact that the ca.le distance .et*een the contactor & the terminal .o= of the motor is 8uite short and shot ca.le lengths are used# )hough( if multiple"reignition is e=pected( then surge arresters must .e used and located close to the motors# ?acuum contactors are not suita.le for use on direct current systems# +e"rating of the motor controller units may .e deemed necessary under a.normal operating conditions lie altitude( fre8uency of po*er supply( temperature( e=posure to fumes( dirt( humidity( etc# )he ta.le defining a medium voltage controller $contactor and fuse% *ill have the follo*ing data as a minimum: the contactor ma=imum continuous current( the interrupting capacity $at the specified K?%( the designation( the voltage rating and range( the interrupting capacity of the fuse $in KA and M?A  the rated voltage%( the ma=imum

Protection & Coordination voltage( reversing vs# non"reversing or reduced voltage( auto transformer vs# reactor type%# )he medium voltage fuses for controllers can .e defined *hen the follo*ing values are given: the motor loced rotor current( motor full load current & service factor( the ma=imum continuous current rating of the fuse inside the compartment( the fuse si/e( the pea current let through characteristics#

)he standard ratings for the medium voltage open $i#e# not installed in an enclosure% contactors are 544 & 944 amperes# )he contactor rating is derated *hen installed in its compartment in the cu.icle# or e=ample( *ith one"high controller cu.icle( the current ratings for the 944 & 544 amperes *ould .e D24 & 4 amperes#

Protection & Coordination

Protection & Coordination

Protection & Coordination Motor protection relays:  A relay offering comprehensive protection may have .ut not limited to the follo*ing set of features: a# thermal protection $5B% .# e=tended start protection monitoring $accelerating time% $59% c# stalling protection $mechanical Jam% d# num.er of starts limitation $% e# short circuit protection $4P% f# ground $earth% fault protection $413% g# *inding >)+ measurementtrip $9% h# negative se8uence current detection $5% i# undervoltage protection $2D%  J# loss"of"load protection or under current $D% # out"of"step protection $for synchronous motors only% $54% l# loss of supply protection $for synchronous motors only% $% m# au=iliary supply supervision $BK% Motor thermal protection (protection against overload conditions/overheating): )he maJority of *inding failures are either indirectly or directly caused .y overloading $either prolonged or intermittentcyclic%( operation on un.alanced supply voltage $causing negative se8uence current through the motor negative resistance%( or single phasing $has the same effect as un.alance source%( *hich all lead through e=cessive heating to the deterioration of the *inding insulation until an electrical fault occurs#

Protection & Coordination )he generally accepted rule is that insulation life is halved for each 14 C rise in temperature a.ove the rated value( modified .y the length of time spent at the higher temperature# As an electrical machine has a relatively large heat storage capacity( it follo*s that infre8uent overloads of short duration may not adversely affect the machine# )+ .iasing of the )hermal Model to adapt to real"time temperature measurements O Compensation for
Protection & Coordination )he relay function protects the motor from overheating in cases of a.normal loading during motor starts# )he motor can .e tripped if the motor does not reach a running condition *ithin the programma.le Motor Acceleration time# Stalling protection (mechanical jam): )his function senses increased loading associated *ith process or load related faults causing the motor to Jam or stall# A programma.le delay setting can .e used to allo* the process to attempt clear itself .efore issuing a trip# Num.er of starts limitation $start inhi.it%: )his function prevents starting of a motor *hen the motor is too hot and does not have a sufficient amount thermal capacity availa.le to allo* a start *ithout .eing re"tripped# Short Circuit Protection: Motor short"circuit protection is often provided to cater for maJor stator *inding faults and terminal flashovers# Short Circuit @vercurrent protection protects damage to the motor during a loced rotor condition# )he relay provide for instantaneous overcurrent protection( the elements can .e configured for protection( alarming( and control during loced rotor conditions# or motors fed .y circuit .reaers( a single definite time overcurrent relay element is all that is re8uired for this purpose( set to a.out 12! of motor starting current# )he time delay is re8uired to prevent spurious operation due to C) spill currents( and is typically set at 144ms# Ground ault protection: @ne of the most common faults to occur on a motor is a stator *inding fault# Hhatever the initial form of the fault $phase"phase( etc#% or the cause $cyclic overheating( etc#%(

Protection & Coordination the presence of the surrounding metallic frame and casing *ill ensure that it rapidly develops into a fault involving earth# )herefore( provision of earth fault protection is very important# )he type and sensitivity of protection provided depends largely on the system earthing# 7t is common( ho*ever( to provide .oth instantaneous and time" delayed relay elements to cater for maJor and slo*ly developing faults# !vertemperature protection: )he crelay ould support >)+ inputs that allo* for configuring the Alarming and )ripping )emperature of each >)+( detecting >)+ shorting conditions( and selecting >)+ voting that re8uires more than one >)+ to detect an over"temperature condition .efore it *ill issue a )rip command# )he programma.le >)+ inputs can .e used in monitoring the Stator( earing and the am.ient temperatures# Each >)+ input has 2 operational levels: alarm and trip# )he relay could support >)+ trip voting and provide open >)+ failure alarming "egative se#uence current detection: Fn.alanced currents give rise to a negative se8uence component in the stator current# )he negative se8uence current produces an additional flu= *hich rotates at synchronous speed in the opposite direction of the rotor# )he eddy currents *hich are induced in the rotor parts *ill have the dou.le net*or fre8uency# +uring such sustained conditions( the temperature of the rotor may reaches high levels *hich accelerate the ageing of the insulation and cause mechanical stress on the rotating components# )he relay calculates negative se8uence current

Protection & Coordination and compared it *ith the set operate value for the alarm and the trip function# $ndervoltage protection: Motors may overheat or even stall *hen su.Jected to prolonged undervoltage conditions# )ransient undervoltages *ill generally allo* a motor to recover *hen the voltage is restored( unless the supply is *ea# Motors fed .y contactors have inherent undervoltage protection( unless a latched contactor is used# Hhere a specific undervoltage trip is re8uired( a definite time undervoltage element is used# 7f t*o elements are provided( alarm and trip settings can .e used# An interloc *ith the motor starter is re8uired to .loc relay operation *hen the starting device is open( other*ise a start *ill never .e permitted# )he voltage and time delay settings *ill .e system and motor dependent# )hey must allo* for all voltage dips liely to occur on the system during transient faults( starting of motors( etc# to avoid spurious trips# As motor starting can result in a voltage depression to 94! of nominal( the voltage setting is liely to .e .elo* this value# >e" acceleration is normally possi.le for voltage dips lasting .et*een 4#"2 seconds( depending on system( motor and drive characteristics( and therefore the time delay *ill .e set .earing these factors in mind# %ossoload protection or under current: )he Fndercurrent Protection element provides the a.ility of tripping the motor due to e=ternal conditions that can cause the load .eing pulled .y the motor to drop .elo* a pre"set level# )his element is useful *hen the loss of the load results in a loss of cooling *hich *ill cause the asset to overheat# 'oss"of"load protection has a num.er of possi.le functions# 7t can .e used to protect a pump against .ecoming unprimed( or to stop a motor in case of a failure in a mechanical transmission $e#g# conveyor .elt%

Protection & Coordination !utostep protection (or s'nchronous motors onl'):  A synchronous motor may decelerate and lose synchronism $fall out"of"step% if a mechanical overload e=ceeding the pea motor tor8ue occurs# @ther conditions that may cause this condition are a fall in the applied voltage to stator or field *indings# Such a fall may not need to .e prolonged( a voltage dip of a fe* seconds may .e all that is re8uired# An out"of"step condition causes the motor to dra* e=cessive current and generate a pulsating tor8ue# Even if the cause is removed promptly( the motor *ill pro.a.ly not recover synchronism( .ut eventually stall#