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7/29/2019 protection of HV motor
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E/CT 165 first issed, May 1995
J.Y. Blanc
He graduated as an engineer fromSUPELEC in 1979. He joined MerlinGerin in 1981 as Medium Voltagedielectric engineer. Still in theMV Division, he was then in turn asproject manager for the developmentof:
- Fluair range primary substationsand equipment,- SF set built-in protection circuit-breakers.He is currently in charge ofdevelopment of MV primarydistribution and switchgear.
n 165
control/monitoring and
protection ofHV motors
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Cahier Technique Merlin Gerin n 165 / p.2
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control/monitoring and protection ofHV motors
contents
1. Reminder of the various types of Asynchronous cage motors p. 4
Asynchronous slipring p. 5rotor motors
Synchronous motors p. 6
Sizing tolerances p. 6
Dielectric withstand and tests p. 7
2. Classical HV starting processes Direct stator starting on p. 8full voltage
Stator starting on reduced p. 8voltage
Startor starting with capacitors p. 9
Rotor starting p. 10
Choosing the starting mode p. 11
3. Control and monitoring equipment Electromechanical solutions p. 13
Electronic solutions p. 15
4. Protection of HV motors Main fault types p. 18Protection principles p. 18
Technological evolution p. 22
Appendix 1: determining motor Calculation hypotheses p. 23
Overall approach p. 23
Appendix 2: coordination of protection devices p. 25
Appendix 3: bibliography p. 26
Unit power of rotating machinesfrequently exceeds 100 kW in industryand large tertiary. In such cases and/orif the length of the supply line is
particularly long (voltage drops,losses), use of high voltage motors isadvantageous.The purpose of this Cahier Techniqueis to analyse and compare thesemotors, their starting systems and thevarious protection devices which maybe used, in order to simplify technicalchoices.
AC motors
starting mode
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current
torque
0
1
2
2
4
25 % 50 % 75 % 100 % (N/Ns)
1
25
4
6
3
2
1
Cn
In
current
torque
0 25 % 50 % 75 % 100 % (N/Ns)
Id
1. reminder of the various types of AC motors
Both high and low voltage AC motors,offer a large variety of electrical,dynamic and technologicalcharacteristics. However, except for asmall number of motors used for highlyspecific applications, they can bedivided into three families, namely:c asynchronous cage motors;c asynchronous slipring rotor motors;c synchronous motors.
They differ from each other in:c starting current and torque values;
c speed variation in normal operation;c power factor and efficiency values asa function of load.
HV motors are supplied with a voltagerarely exceeding 7.2 kV, their powerranges from 100 kW to over 10 MW,with an average of 800 kW.
asynchronous cage motorsThese HV motors fall into two maincategories according to their rotorcomposition which can be single ordouble cage.
This enables choice of starting currentand torque characteristics:c single cage rotors have:v a relatively low starting torque (0.6to 1 C
n),
v a maximum torque of around 2to 2.2 C
n,
v a starting current ranging from 4.5to 5.5 In,(Cn: rated torque, In: rated current);
c double cage or deep slot rotors have:v a slightly higher starting torque (0.8to 1.2 C
n),
v a maximum torque of around 2
to 2.2 Cn (slightly higher for deep slots),v a starting current ranging from 5to 6.5 In.
Figures 1 and 2 show the form of thesecurves as a function of speed (N/N
s).
Note that:c single cage motors have a minimumtorque (0.5 to 0.6 C
n), whereas the
torque curve, varying according to thespeed of the double cage or deep slotmotors, continues to increase up tomaximum torque.c these motors are ideal for intensive useand dangerous environments, due to:
fig. 2: curves C(N) andI(N) of an asynchronous double cage motor.
I
In
C
Cn
fig. 1: curves C (N) andI(N) of an asynchronous single cage motor.
C
Cn
I
In
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fig. 4: curves C (g) of an asynchronous slipring rotor motor.
v the simplicity of rotor design in
short-circuit providing them with an
excellent mechanical and electrical
robustness,
v absence of brushes.
These two features allow maintenance
to be reduced to a minimum.
The torque characteristics of
asynchronous cage motors are
especially suitable for machines such
as centrifugal pumps, compressors,
converter sets, machine-tools and fans.
However, all these motors have the
drawback of a relatively low power
factor, around 0.8 to 0.9 on full load,
and even less when they are running
on low load (see fig. 3).
If asynchronous motor installed power
is high, reactive power compensation is
required. This may either be global, for
each set of motors or for each motor
(large units).
asynchronous slipringrotor motorsThe rotor winding of these motors
connected to sliprings means the
resistance of this circuit can be
modified by introducing external
resistances.In the motor stability zone,
corresponding to the positive slope of
curve C = f (g) (see fig. 4), the slippage"g" is proportional to the rotor
resistance:
g = 1
ARr C
where g % = Ns N
Ns100
where:
Ns
: synchronous speed,
N : operating speed.
A = 3V2 p
M
L1 = constant
where:
V: phase to neutral supply voltage,p: number of pole pairs,
: pulsation of supply currents,M: reciprocal stator-rotor inductance,
L1: total stator choke,
(L1
= M + Ls)
Rr: rotor resistance = rotor inherent
resistance + external resistances,
C: motor torque.
cos
0 2/4 3/4 4/4
0.5
0.6
0.7
0.8
0.9
cos
(P/Pn)
fig. 3: efficiency curves (P) and power factor curves cos(P) of an asynchronous double
cage motor.
CM
C
Rr R'r > Rr R''r > R'r
CnCd
C'd
C''d
1
0.50
0.5
1 2 g
operation
on generator
operation
on motor
operation in
regenerative
braking
rotor stability zone
in short-circuit
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c the power factor which may be set bythe exciting current.Technologically, they are identical toAC generators.
In order to obtain asynchronous torqueand avoid oscillations, the rotors areequipped with a damping cage. Thiscage means synchronous motors canbe started with a low load torque insimilar fashion to asynchronous singlecage motors (they have practically thesame characteristic torque and currentcurves). To avoid surge voltages in theexciting circuit, this circuit is shuntedduring starting and on tripping by aresistance with a value chosenbetween 5 and 10 times the resistance
of the exciting circuit.In view of the fact that theasynchronous torque tends to zero onapproaching synchronous speed,coupling to the network when motorstarting is complete cannot take placeat synchronous speed as is the case forAC generators. The result is invariablya transient state varying according tothe speed acquired at the end ofstarting and motor power. To limit thistransient state:c either use a relay to monitor slippageby measuring frequency of the rotor
current passing through the startingresistance. This relay controls excitingcircuit supply when slippage is at itslowest.This device is practically indispensablewhen the synchronous motor accountsfor a large part of total installed power.c or apply the exciting current in twostages, automatically or manually.
The exciting sources can be either
separate:
c motor set, exciter;
c thyristor rectifier;
or placed at the end of the motor shaft:c reversed generator;
c rotating "diode rectifier and armature"reversed AC generator.
The techniques most frequently used
are the thyristor rectifier and the"rotating diodes".
The latter does away with brushes,
removes the exciting cubicle and, in
addition, is often fitted with asynchronisation and recoupling
mechanism should synchronism fail.
These motors can supply reactiveenergy by increasing the excitingcurrent. This characteristic, whichenables compensation of network
reactive loads, is one of the mainreasons these motors are chosen.
The curves in figure 5 show statorcurrent variation as a function ofexciting current for a given constantload (Mordey curves). Use of this typeof motor for small powers is fairly rare.On the other hand, it is frequently usedabove 2,000 kW for its excellentefficiency and control of its powerfactor. In the case of highly regularmovements, synchronous motors are anecessity. However, the machines
being driven must have a relatively lowload torque during starting, and sizingof the dampening cage limits startingrate.
sizing tolerancesThe electromechanical characteristicsof motors are defined by standard
IEC 34-1. In the case of certain rated
characteristic values, the standard
defines the tolerances to be compliedwith by the manufacturer. It is useful tobe familiar with these tolerances since,
for certain characteristics, they directlyaffect the choice of motor and
equipment power and the setting of theprotection devices.
The table in figure 6 gives the
tolerances of the main characteristic
values.
By decreasing external resistance onstarting, the characteristic C (g) istranslated and the starting torqueadapted to the torque of the machine
being driven. Note that maximumtorque value does not depend on rotorresistance.
Moreover, for small slippages, rotorcurrent is inversely proportional to rotorresistance. Its amplitude is given by:
I2 = Bg
Rr
where
B = VM
L1.
Stator current follows the same law,except for the winding ratio and themagnetising current.
Consequently, the choice of initial rotorresistance solves virtually all theproblems concerning high startingtorque and inrush current on thenetwork and ensures these tworequirements are satisfied. The variouspossibilities for using asynchronousslipring rotor motors make themsuitable for driving machines with highstarting torque such as crushers,mixers, conveyors, etc.
Moreover, machines requiring highregenerative braking also use this typeof motor.
Just as for asynchronous cage motors,the power factor in normal operation isrelatively low: this characteristic and thepresence of sliprings and rotorresistances mean these motors areincreasingly replaced by double cage ordeep slot motors.
Figure 4 represents the characteristicC (g) curves, according to rotorresistance value and the stabilityzones. These curves show the
advantage of introducing a highresistance in the rotor circuit as ameans of obtaining efficientregenerative braking.
synchronous motorsThe main differences withasynchronous motors are:c their constant speed (synchronousspeed);c the rotor circuit supplied with DC;
fig. 5 synchronous motor: Mordey curves.
0 (i) excitation
(I) stator
stabilit
ylim
it full load1/2 load
1/4 load
cos AR cos AVcos = 1
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dielectric withstand andtestsMotors, just like all electrical network
components, are subjected to a varietyof surge voltages. They are particularlysensitive to steep front surge voltagesor high frequency since they are"jammed" by the first turns of the statorwindings.
Switching surge voltages
These are the result of transientphenomena occurring during changesin status in the supply network.The following phenomena, specific toinductive circuits and thus to motors,must be taken into consideration:c current pinch-off on current breaking;
c multiple re-ignitions on breaking andprearcing on current making if thebreaking device is capable of breakingthe high frequency currentscorresponding to these phenomena.
Steep front surge voltages
These are the result of direct or indirectlightning strokes. They spread onto thenetwork, creating a dielectric stresswhich, even when limited by the use ofsurge arresters, can be considerable.Surge voltage is studied in detail in"Cahier Technique" n 151"Overvoltages and insulation
coordination in MV and HV" andspecial motor sensitivity in"Cahier Technique" n 143 "Behaviourof the SF6 MV circuit-breakers Fluarcfor switching motor starting currents".To check the motors capacity towithstand to these various surgevoltages, the motors undergostandardized tests performed asdefined by IEC 34-1 standard.
The test voltage is applied between thewinding being tested and the body ofthe machine to which the magneticcircuits and all the other stator and rotor
windings are connected.
Two types of tests are stipulated in the
standards: standard frequency tests
and impulse withstand tests.
Standard frequency testWithstand to switching surge voltages
is checked in compliance with standard
IEC 71, by the standard frequency
withstand test. Testing commences
with a voltage of less than U/2 which is
gradually increased up to 2 U + 1,000V,
at which level it is applied for one
minute.
For the stator, U is the specified supplyvoltage. For the rotor, U is the voltage
which appears, with the rotor circuit
open, when the specified statorsupply voltage is applied with the
rotor locked in rotation. If the motor
is reversible (change of rotation
direction of motor already started), the
test voltage applied to the rotor will be4 U + 1,000 V.
Impulse withstand test
This test consists in applying animpulse voltage representative of
lightning:
c buildup time: 1.2 s;
c dropdown time at Upeak/2: 50 s;
c test voltage:
Upeak
= 4 U + 5,000 V.
The windings are subjected to a
number of positive and negative waves.
Impulse withstand tests are not
currently mandatory in standards, as
they can lead to early ageing of
armature and winding insulation.
More generally, the dielectric testsmust not be repeated; if a second testis performed, it will be carried out at
80 % of the voltages indicated above.
value tolerance
asynchronous motors
current with locked rotor + 20 % of current
and short-circuited (no lower limit)
torque with locked rotor - 15 % to + 25 % of torque guaranteed
minimum torque during starting - 15 % of torque guaranteed for cage motors Cdu at
a third of rated torque andi
at half of torque withlocked rotor, at full voltage
maximum torque - 10 % of torque guaranteed provided that once thistolerance is applied, the torque remains u 1.6 timesrated torque
synchronous motors
current with locked rotor + 20 % of value guaranteed
torque with locked rotor - 15 % to + 25 % of value guaranteed
pull-out torque - 10 % of value guaranteed provided that once thistolerance is applied, the torque remains u 1.35 timesrated torque (1.5 for synchronous motors with salientpoles)
fig. 6: tolerances on the main characteristic values as in standard IEC 34-1.
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0 0.2 0.4 0.6 0.8 1
0.2
0.4
0.6
0.8
1
2. classical HV starting processes
The main HV motor starting processesare as follows:c direct stator starting on full voltage;c stator starting on reduced voltage bystar-delta connection, by reactance orby autotransformer;c stator starting by capacitors;c rotor starting.
direct stator starting on fullvoltageThis starting mode is used forasynchronous motors with cage rotorand for synchronous motors.
Current peak on starting is around 4to 7 In, according to motorcharacteristics, and can last for roughly1 to 10 seconds depending on themoment of total inertia (motor +machine), motor torque and loadtorque.
If this starting mode is used, thenetwork must be able to withstand theabove current overload without
disturbing the other loads, and themachine being driven must be able towithstand the mechanical impact due tothe motor torque. The simplicity of bothequipment and motor and the resultingsavings mean that this mode is verypopular and even recommendedprovided that voltage drop on thenetwork on start up is acceptable. Thedecisive factor lies in the motor power/short-circuit power ratio.
stator starting on reducedvoltage
Star-delta startingThis starting mode is used to reduce:c current in a ratio of e;c starting torque by a third.
It is used in LV and for low powers, butrarely in HV due to the high currentpeaks when moving to delta. In thiscase it is replaced by reactancestarting.
Voltage reduced by resistanceCommonly used in LV, it is rarely usedin HV due to the Joules to be dissipatedand resistance insulation problems.
Voltage reduced by reactance
This starting mode (refer to powerdiagram, figure n 8) is the one whichreduces current inrush on the networkin the simplest manner. Since motorstarting torque is low, the machinesbeing driven must have a relatively lowload torque during start up:compressors, centrifugal pumps,converter sets, etc.In point of fact, asynchronous motortorque varies according to square of the
supply voltage , whereas the absorbedcurrent remains proportional to thisvoltage.
C'd = Cd UdUn
2
where :C'
d: starting torque with reduced
voltage,C
d: starting torque with full voltage,
Ud: starting voltage,
Un: rated operating voltage.
I
'd = Id
Ud
Un
where:I'd: starting current with reducedvoltage,Id: starting current with full voltage.These relationships can also beexpressed using the ratedcharacteristics as follows:
I'dIn
= Id
In
UdUn
.
The curves in figure 7 give the ratiovariations as a function of
the ratio UdUn
.
Voltage at the motor terminalsincreases gradually during starting. Theresulting start up is smooth.c operation and schematic diagram:v first stage:operation on reduced voltage byclosing the line contactor C
L,
v second stage:normal operation by closing the short-circuit contactor C
C;
c determining a starting reactance(see fig. 9)
Starting voltage is determined by the
maximum current inrush l'd authorised
on the network:
Ud = Un I'dId
.
The phase-to-phase voltage drop in the
reactance has the following value:
Un
Ud
= j 3 L I'd
.
The diagram in figure 9 shows that this
relationship can be expressed in anarithmetic form for asynchronousmotors, since the power factor at theinitial moment start up moment is
initiated is virtually the same as the
power factor of the starting inductance.
Therefore:
L = Un Ud3 I'd
.
Knowledge of start up time and
operation rate is required to determinereactive power.
Voltage reduced by autotransformer
This starting mode sometimes makes it
possible to reconcile reduction of
current inrush on the network and
motor torque value. In point of fact, it
has the advantage of reducing current
inrush according to the square of the
winding ratio:
fig. 7: graphs showing start-up on reduced
voltage (by reactance or star-delta).
UdUn
I'dId
andC'dCd
C'd
Cd
I'dId
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I'd
d
Un
Ud
rating of the neutral point contactor tobe reduced;c there is an alternative version of thisconfiguration in which the neutral point
contactor is removed. This version isnot recommended since the move fromreduced to full voltage necessites themotor-network link be cut. In view of therelatively short switching time, and theresidual voltage at the motor terminals,a current inrush exceeding startingcurrent can ocur on transfer to normaloperation. This is unacceptable for boththe network and the motor and can leadto the tripping of the protection devices.
stator starting withcapacitorsThis process enables motor full voltage
starting characteristics to bemaintained. It is mainly used forkeeping the starting torque ofsynchronous motors constant duringstart up, for example in cement worksand crushing plants.
Capacitors, as well as motor, supplypart of the reactive energy during thestarting phase: the motor power factoris small at this stage. Power inrush on
I'dIn
= Id
In
UdUn
2
C'dCn
= CdCn
UdUn
2
where:
I'd: starting current on network side with
reduced voltage.
These relationships are used to
determine the value of reduced voltage
authorised on the network, as a function
of the ratio'dn
or of the ratioC'dCn
authorised by the machine being driven.
The curve in figure 10 gives the
variation ofUdUn
as a function
of'dn
orC'dCn
.
c operation and schematic diagram
(see fig. 11)C
L: line contactor,
CC: short-circuit contactor,
CPN
: HV neutral point formationcontactor,
AT: autotransformer
v first stageoperation on reduced voltage by
closing CPN
which causes CL
to close,
v second stageoperation in inductance by opening CPN,
v third stage
operation on full voltage by closing CC.
Remarks:
c in theory the second stage is short(around one second) since in mostcases it is a slowing-down time;Use of an autotransformer with air gapsconsiderably reduces this fault, butrequires knowledge of the value of thecurrent absorbed by the motor at theend of the first stage;
c the move to full voltage invariablyresults in a transient state whoseduration varies according to the speedacquired at the end of the first stageand the value of the current absorbed;
c the current flowing through theneutral point on start up is equal to thedifference between the motor currentand the line current, excluding themagnetising current of theautotransformer. This enables the
fig. 9: vectorial diagram to determine L.fig. 11: power diagram: starting by
autotransformer.
fig. 10: graphs showing start up byautotransformer on reduced voltage.
0 0.2 0.4 0.6 0.8 1
0.2
0.4
0.6
0.8
1
I'dId
andC'dCd
Ud
Un
fig. 8: power diagram: starting by reactance.
J L I'd
3
M
neutral point
AT
3L
CL CC CPN
3L
CC
CL
M
3L
L
j
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the network is reduced accordingly(see fig. 12).
This technique is tricky to implementand calls for a study of the motor and
capacitors to avoid resonance andsurge voltage (due to motor self-excitation), as well as mechanicaloscillations on the transmissionsystem.
Moreover, control equipment must bechosen especially for capacitorswitching.
rotor startingThis starting mode solves virtually allthe problems which may occur onstarting, namely:c reduction of current inrush on thenetwork with increase in motor torque;c adaptation of motor torque to loadtorque;c long, progressive starting (e.g. forloads with high inertia).
This mode can only be used forasynchronous slipring rotor motors andfor synchronous induction motors (its
use is increasingly rare in industry).It is particularly used for load starts.
Example of a rotor start in n time
This start up is illustrated in figures 13and 14. Motor torque varies betweentwo values at each notch. The lowervalue is taken as being equal to therated torque. At each notch, rotorresistance changes value and thetorque-speed characteristic evolves. Inthe last stage, rotor resistance is simplyequal to the rotor internal resistance.c first stagestator supply and starting on full rotorresistance by closing C
L;
c second stageshort-circuiting of the first section of therotor resistance by closing C
1;
c third stageshort-circuiting of the second section ofthe rotor resistance by closing C
2;
c Nth stageshort-circuiting of the n-1 section of therotor resistance by closing C
n-1.
The number of stages, or notches, n isalways greater by 1 than the number ofsections or contactors.
This number n is determinedapproximately by the formula:
n = log gn
logCnCp
or byCnCp
= gnn
where:C
p= peak torque,
gn
= rated slippage.
According to each case, n can bededuced from Cp, and vice-versa.
Complete determination of rotor startingequipment calls for knowledge ofoperation (hourly rate and startingtime). Lack of standards for HV motorsmeans this equipment is determined ineach individual case by specialists.
RemarkA linear start is sometimes required.This calls for power electronics
enabling monitoring of rotor energy: forexample using a Gratz bridge and achopper to make a continually variableresistance (see fig. 15).
fig. 13: power diagram: rotor starting.
fig. 12: vectorial diagram for starting by
capacitor.
fig. 14: symmetrical rotor starting diagram.
power suppliedby the network
active power
power suppliedby capacitors
apparentmotor poweron starting
inductivereactivepower
M
neutral point
3L
CL
3L
C1
C2
Cn - 1
4th 3rd 2nd stage 1st stage
0 1 g
Cn
Cp
CM
C
gn g4 g3 g2 g1
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C
C
3
3
C
C
3
C
3 n
3 n
3 n
n
n
characteristics examples
constant torque piston compressor
parabolic torque pump:
valve opened
- - - - valve closed
high breakaway crusher
torque
Starting conditionsIn view of the required startingcharacteristics, it is necessary to checkthat starting can take place in correctly
as far as motor torque, current inrushand starting time are concerned, for thestarting techniques underconsideration:
c motor torque is always greater thanload torque (see fig. 17);c current inrush on the network and thecorresponding voltage drop are
acceptable to the network;c starting time is compatible with theequipment used.
Approximative calculation of startingtimeOperation of the motor-driven machineassembly is governed by the followingmechanical equation:
Cm Cr = Jddt
where:C
m: motor torque with Un voltage,
Cr: load torque,
J: inertia of rotating frames (motor andmachine driven),
ddt
: angular acceleration.
Angular velocity varies from 0 to nthroughout starting time t. Moreover, amean accelerating torque C
a, equal to
the mean difference between Cm
andC
rcan be defined:
This results in Ca
= (Cm
- Cr) mean
= J(n 0)
thence:
t = J nCa
.
Bearing in mind that real motor torquevaries according to the square of itssupply voltage:
C'mCm
= UrealUn
2
with real voltage.
Cm
= motor torque with real tension.Reducing this voltage will thus reduce
Ca, hence increasing starting time.
fig. 16: rreminder of load torque curves of machines to be driven (loads). fig. 17: case of non-starting (C'm < Cr).
choosing the starting modeChoice of starting mode is conditioned
by ensuring that the motor torque and
the load torque of the load are properlymatched.
Knowledge of the load torque is
required (see fig. 16).
torque
1.8
0.9
0speed
Cm
C'm
C'r
C
Cn
NI
I0
R1
I
N0
R2
H
L
--
+ +N
M
3L
tachometer dynamo
fig. 15: setting speed by rotor chopper.
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Starting mode selection table
The table in figure 18 summarises the
advantages and disadvantages of the
main starting modes for the various
applications.
For a given torque, the current
absorbed on the network is establishedin the following increasing order:
c rotor starting;
c starting by autotransformer;
c starting by stator impedance;c direct starting.
Choice of starting mode calls for good
communication between the electricalenergy supplier and the manufacturerof the motor and machine being driven.
The vital characteristics for making thischoice are:c supply network power and maximumauthorised current inrush;
c motor torque and current at fullvoltage as a function of rotation speed;c load torque of the machine driven(see fig. 16);
c moment of inertia of the rotatingframes.
If the supply network power/motor powerratio is less than 5, particular care mustbe paid to choice of starting mode andchoice of coordination of the protectiondevices (see appendices 1 and 2).
fig. 18: starting mode selection table for the most common applications.
application application starting mode controlled by advantagesneeds characteristics disadvantages
permanent or machines requiring direct 1 or 1 simplicity,quasi-permanent high start torque less investment.
process i 1/jour on starting:frequent starts > 1/day motors with low current direct chigh torque;
inrush or low power 1 chigh current inrush,;chigh mechanical stresses.
pumps, fans, machines starting stator 2 reduction of current inrushcompressors, with low torque by reactance on startingfrequent starts (possible adjustment).
optimisation of when starting current stator by 3 optimisation of torquestarting characteristics must be reduced, but autotransformer (reduced) and of current inrush
the necessary starting on startingtorque maintained (possible adjustment).
optimisation of the most difficult starts rotor generally small current inrush andhigh torque 3 high starting torque.starting characteristics
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3. control and monitoring equipment
The function of this equipment is
threefold:
c energising and stopping (control);
c disconnecting the motor should a
fault occur (protection);
c monitoring the motor (monitoring).
When we talk about monitoring, thisimplies the equipment is capable (or
not) of:
c initializing the start (starting sequenceautomation);
cacting on motor speed;c supplying information on motor
electrical status and contributing to
protection.
The monitoring function chiefly relies on
power electronics and low currents
(digital technology); it is currently being
fully developed. Protection of
HV motors will be dealt with in the next
chapter.
electromechanical
solutionsThe choice between the various
devices (switch, circuit-breaker or
contactor) depends on:
c operation rate;
c electrical endurance;
c motor power.
The main breaking device
characteristics are summarised in the
table in figure 19.
Fuse-switches
By their very design, the breaking
capacity, mechanical and electrical
endurance of switches is low. Thislimits their use to small powers
(In
= approx. 50 A - 5,500 V) andto rates of two to three operations a
day.
Moreover, the low breaking capacity ofthese devices makes choice ofprotection devices tricky.
Circuit-breakersCircuit-breakers are generally used forhigh motor powers of more than 300 A,with a small operation rate, and foroperating voltages of more than 6.6 kV.
Their use can naturally be extended tolower powers, operable by switch orcontactor.
Fuse-contactorsc operation rateTheir simple control mechanismcombined with the robustness and
simplicity of their contacts meancontactors have a high operation rate.This rate cannot be withstood by circuit-breakers, even special ones, and evenless so by switches.
Some installations use contactors withmechanical latching to do away withpermanent consumption of the closingelectromagnet. This may reduceendurance as a result of the greatercomplexity of the kinematic chain.
c network short-circuit powerThis factor does not really affectcontactors thanks to the presence of
fuses placed immediately after theisolating switch or next to thedisconnecting contacts on the busbarside. These fuses with their highbreaking capacity, limit the short-circuitcurrent.
This special feature means that,if network power is increased, themotor feeder cubicles can bemaintained. The busbar supports arestrengthened if required.
FusesFuse rating is determined according to:c rated current In;c the I
d/I
nratio (I
d= starting current);
c starting time determined using thechart in figure 20 page 14.
Finally, it should be pointed out thatfuses protect the motor againstovercurrents roughly five times greaterthan motor rated current and that theymust be combined with additionalprotection devices (thermal relays, etc.
see "protection device" chapter).Current transformersThe increasing use of digital protectiondevices means unconventional currentsensors can be used (e.g. Rogowskitoroids). These sensors have theadvantage of being linear and thusdelivering an accurate signalthroughout the useful current range.
They present no saturation or thermalproblems, as is sometimes the casewith classical Current Transformers(see "Cahier Technique" n 112 "Thebreaking process with a Fluarc SF6
puffer-type circuit-breaker").
Special features due to use of fusesor circuit-breakers
c operation in single-phase due to fusemelt, with the striker not working.
Today striker reliability is such that therisk is slight. Their dependability maybe increased still further by use of anadditional protection device(undervoltage or unbalance relay).
fig. 19: breaking device application field.
device mean rate endurance acceptable motor
nb operations powerfuse-switch low: 2-3/day 2,000 low i 50 A
circuit-breaker low: 10/day 10,000 highu 7.2 kVA> 300 A
fuse-contactor high > 10/h > 100,000 average i 300 A
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fig. 20: fuse determination chart.
0.01
0.02
0.03
0.04
0.05
0.1
0.5
1
5
10
50
100
t seconds
100A 125A 160A 200A 250A
rating of 7.2 kV fuses
example:
motor In = 100 A
Id = 6 . Intd = 5 s
fuse rating = 200 A
motor
Amp In
10
20
30
40
50
60708090
100
150
200
I
Id=4
In Id=
5In
Id=6
In Id=7
In
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c discrimination with upstreamequipment.This can be hard to achieve when both:v the fuses used have a high rating
(200 A or 250 A),v and the feeder protected by thesefuses accounts for a large fraction ofthe power supplied by the main circuit-breaker (see fig. 21 and 22).
However, the high breaking capacity ofthe Rollarc contactors combined withthese fuses allows the use of slightlytime delayed overcurrent relays, thusensuring discrimination. Discriminationis easier to achieve if the motor feederis protected by circuit-breaker butsince, for high short-circuit currents thecurrent is not limited, there is an
increase in thermal stresses.c surge voltagesSome types of device, in particularvacuum breaking devices, generatesurge voltages on motor energising andstopping (due to their capacity to breakhigh frequency currents, resulting forexample from the current pinch-offphenomenon - see "Cahier Technique"n 143).In order to prevent these surgevoltages progressively damaging motorinsulation, manufacturers place ZnOtype surge voltage limiters in the
equipment if required.It can be concluded that today'selectromagnetic solutions are reliable,robust, economic and entirely suitablefor most applications.
electronic solutionsThese provide users with additionalpossibilities and advantages such as:c variable speed;c possibility of speed regulation;c high operation rate;c energy savings.
The electronic solution is rarely usedjust for starting.
Before dealing with standard cases ofelectronic devices, it should be pointedout that their use calls for a certainnumber of constructive precautions atmotor level:
fig. 21: protection diagram showing a high current motor feeder.
fig. 22: discrimination diagram for a high current motor feeder.
c safety margin in temperature rise dueto harmonics: a 15 % margin on currentis generally sufficient;c forced ventilation is recommended(motors can run at low speed);
c reinforced insulation between turnsdue to the high voltage gradientsgenerated by thyristor switching (whichmay reach the magnitude of impulsewithstand test ones).
t
I
contactor
breaking
time
contactor
overcurrent
relay
contactor
breaking
capacity
circuit-breaking
melting curve
remain circuit-breaker
overcurrent relay
discrimination
loss
M
overcurrent
relay
circuit-breaking
contactor
main
circuit breaker
overcurrent
relay
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The devices described below are theones most frequently used in mediumvoltage. Only the general principles arepresented.
The table in figure 23 gives an idea ofthe adequations between the type ofspeed controller, the type of motor andthe type of load driven.
Autonomous rectifiers/inverters
With their capacity to deliver variablevoltage and frequency, they guaranteecomplete control of motor speed andtorque.
A reminder for asynchronous motors:c torque is proportional to U2 if f and Nare constant;c torque is inversely proportional to f fora given voltage and speed.
There are three types of autonomousrectifiers/inverters.
c voltage rectifier/inverterv the thyristor rectifier sets voltage,v the thyristor inverter suppliesAC current at variable frequency.
This configuration is also used byUninterruptible Power Supplies (UPS)used in LV to supply computers. Theonly difference is that frequency andvoltage are fixed.
c rectifier/inverter with Pulse WidthModulation (PWM)
v the diode rectifier supplies theinverter,v the inverter generates voltage pulsesenabling reproduction of a sine wave
with variable period and amplitude (seefig. 24).c current rectifier/inverter (switch)v the thyristor rectifier combined with asmoothing choke acts like aDC generator,v the inverter switches current in turn in
the motor windings using capacitors.Motor frequency and thus speeddepend on switching speed.A few elements of comparisonbetween these three types ofrectifiers/inverters:c voltage inverterv suitable for high reactance motors,v often requires a filter between inverterand motor,
v enables regenerative braking if therectifier is reversible.c the PWM inverterv allows a wide range of speeds,v maximum speed limited by themaximum switching frequency
authorised by the inverter thyristors.Use of power transistors (IGBT) makesit possible to work at far higherfrequencies but at lower powers,v reversible operation possible (tworotation directions).c the current switchv suitable for low reactance motors,v enables operation in the fourquadrants.
motor loads speed variations power overall speed controllerefficiency type
asynchronous pumps, fans, 0 % to over 100 % a few 10 kW 0.85 to 0.90 autonomousor synchronous compressors, extruders to a few 100 kW rectifier/inverter
asynchronous same 60 % to 100 %* a few 100 kW 0.90 to 0.95 subsynchronousslipring to a few MW cascade
synchronous same 0 % 100 kW to a 0.90 to 0.95 selfcontrolledcentrifugal machines to several times 100 % few 10 MW rectifier/inverterTGV bogies(high speed)
asynchronous crushers, rolling 0 % to 33 % 100 kW 0.85 to 0.90 cyclo converteror synchronous mills, cement kilns to a few 10 MW
(low speed)
*: 100 % corresponds to relative speed at 50 Hz
fig. 23: application areas of electronic speed controllers for AC motors.
fig. 24: generation of variable voltages and frequency with PWM speed controllers.
U
U
0
0
t
t
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Subsynchronous cascadeAsynchronous slipring rotor motors arenormally supplied by the network.Speed is adjusted using the rotor
current by means of a rectifier/inverterset. The rectifier draws off energy from
the rotor circuit, thus increasing
slippage. The energy drawn off
depends on the conduction setting of
the thyristors of the inverter which
reinjects energy into the network (see
fig. 25). The subsynchronous cascade
enables continuous speed variation
with a maximum slippage of
around 40 %.Converter set power is low compared
with motor power, and the energy
regenerated results in outstandingoverall efficiency.
Note that the converter is put into
operation after starting by rotor
resistance.
This assembly can operate at speeds
higher than synchronous speed
(supersynchronous) in the case of
driving loads.
Selfcontrolled rectifier/inverter
The motor stator phases (in this case
synchronous) are supplied in turn just
as for the current switching
autonomous rectifier/inverter.
Switching from one stator phase to thenext is selfcontrolled by motor speed bymeans of a "notched disk" sensor.
There is thus a correspondence
between the excitation flux and the
armature flux, as for DC machines, and
the pull-out risk is zero.
Since switching presents problems both
on starting and at low speed, the con-
verter control system must be modified.
This solution is ideal for synchronous
motors.
Cyclo converter
Each motor phase is "supplied" by athree-phase double bridge.The first bridge is used to draw off
current throughout the positive cycle oneach of the network phases accordingto the required frequency.
The second bridge is used for reversecurrent during negative alternation toone of the phases.
The cyclo converter produces a three-
phase pseudo mains requiring filteringand with a frequency varying between0 % and a third of network frequency.
fig. 25: subsynchronous cascade power diagram.
M
P
Pr
3L
3L
3L
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"Motor protection" groups all theprotection devices used to preventserious damage due to abnormaloperating conditions at supply, motor orprocess level.
The protection devices to be installedare chosen according to the followingcriteria:c operating conditions;c importance of the operationperformed by the motor;c the degree of dependability required;
c the relative cost of the protectiondevice with respect to the motor;c the likelihood of the faults consideredoccurring.
As well as:c the type of load driven;c disturbances which could occur onthe network;c the type of motor protected.
The faults listed below may thereforerequire use of a protection device.
main fault types
Asynchronous motors
c overloads;c short-circuits;c phase breaking, reversal andunbalance;c insulation fault between turns;c stator frame;c under and overvoltage;c incomplete starting.
Synchronous motors
All the above faults, plus:c loss of synchronism;c loss of excitation;
c rotor frame;c prolonged operation in asynchronousmode on starting;c overloads and short-circuits in theexciting winding;c reverse power (operation onAC generator).
Other faults linked to the process orload
c over frequent starts;c locked rotor;c underpower or undercurrent.
The detection/protection processes forthe main fault types are studied in theparagraph below.
protection principles
Overloads
Overloads can be detected by reverse
time overcurrent relays, thermal imagerelays or heat sensors.
The relays process the information
"current absorbed by the motor" whichis generally detected by current
transformers.
The heat sensors are inserted in the
live parts of the motor.
c reverse time overcurrent relaysTheir use requires:
v either an operating curve I(t) allowing
starting, or a device blocking the relay
during starting,v an operating threshold I0 close to therated current In of the motor
I0 1.10 In .
These relays do not memorise theoverloads.
c thermal image relays
These relays are certainly the mostsuitable since they ensure the greatest
possible advantage is derived frommotor overload possibilities without
damage.
The operating curve I(t) of the relay
must enable the starting current to flowwithout tripping and be approved by the
motor manufacturer.
c heat sensors
These are resistors, the ohmic value ofwhich varies with temperature.
In theory these devices are not used bythemselves, but rather back up therelays using the current absorbed as ameasuring means.
Overloads due to temperature rise of abearing are, in theory, insufficient fordetection by the overload relays.
Bearings must therefore be protectedby thermostats or heat sensors.
Short-circuits
Short-circuits on circuit-breakerequipment are detected byinstantaneous operation overcurrentrelays, set above the starting current.
On fuse-contactors, short-circuits arecleared by the fuses.
However, an useful solution is to addslightly time delayed overcurrent relaysto the fuses. This means that thecontactor can be used right up to itsbreaking capacity.
Phase breaking, reversal andunbalance
These faults are detected by a filterwhich highlights the negative phasesequence components.
It is vital to monitor phase breaking andunbalance since these faults give riseto:c increased current, in the stator,;c additional temperature rise by Jouleeffect, due to the fact that all out-of-balance states result in the appearanceof reverse currents flowing through the
rotor at twice supply frequency in therotor.
Phase reversal is detected either bycurrents or voltages:c by currents: this reversal is detectedafter contactor closing: the drivenmachine receives the fault;c by voltages: this means contactorclosing can be prohibited, if necessary,if phase order is not the normal networkone.
Insulation fault in winding
Stator windings may have faultsbetween turns on the same phase or
between windings of different phases.As a result of its electrical position, thefault may not be detected quicklyenough by the overload protectiondevice, thus causing serious damage.These faults are normally detected bycurrent comparison.
c longitudinal earth leakage protectionProvides protection against faultsbetween windings of different phases.For this, the ends of the motor windingsmust be accessible on the neutral side.
4. protection of HV motors
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Faults are shown up by comparing theinput and output currents of the samephase (see fig. 26).If there are no faults, these currents are
identical and the protection relay is nottripped. It trips when the differencebetween these currents reaches avalue set by the relay setting.
c transverse earth leakage protectionProvides protection against faultsbetween turns of the same phase.It applies to machines with dividedphases, i.e. with two windings perphase.The operating principle is the same asabove, that is the currents of eachwinding are compared (see fig. 27).
Stator frame
This protection device is vital to complywith the statement of the 14.11.1988 onworkers' protection. It is chosenaccording to the earthing system of thenetwork supplying the motor.
c motor protection supplied by networkwith earthed or impedance-earthedneutral.The fault is detected by measuring thezero sequence current formed betweenthe faulty phase and the network frame.Low threshold overcurrent relays areused for measuring.The zero sequence current is deliveredby three parallel-connected currenttransformers or, better still, by a toroid(see fig. 28).Use of a toroid prevents theappearance of a false zero sequencecomponent due to unequal saturation ofcurrent transformers on motor starting,and allows a relatively low operatingthreshold.
These relays must operate for a faultcurrent value such that frame potentialcompared with earth is never raised tomore than 24 V in conductiveenvironments, with frames
interconnected, or 50 V in otherinstallation cases.
Knowledge of earth connection valueand of the frame interconnectionconfiguration is therefore necessary todetermine this setting point.If the frames are not interconnected,the value of the operating threshold isgiven by:
IF i 24 or 50 V
RTM.
fig. 28: diagram showing a stator frame zero sequence protection with toroid sensor or CT +
earthed or impedance-earthed neutral.
NM
protection
relay
fig. 26: diagram showing a longitudinal earth leakage protection.
M N
protection
relay
zero
sequence
current relay
toroid
CT
zero
sequence
current relay
eventually toroid
fig. 27: diagram showing a transverse earth leakage protection.
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RTM
is the value of the earth resistance
of the frame in question.
Note that the lower the threshold, the
earlier the detection, and the smaller
the risk of damage to the magnetic
circuits.
c unearthed neutral
The fault is detected by continuous
measurement of global network
insulation compared to earth by means
of DC injection devices such as
continuous insulation monitors (see
diagram 1 in figure 29) or by zero
sequence overvoltage relays where this
overvoltage is delivered by three
voltage transformers with the
secondary winding in open delta (see
diagram 2 in figure 29).
Under and overvoltage
(see fig. 30)
c undervoltage
This relatively common protection
prevents the motor from working on
overload and waiting for tripping by the
overload protection device. Moreover, if
the contactor coil is supplied by an
auxiliary LV source not from the
network, the latch undervoltage
protection device is vital to prevent
untimely startup on restoration of
power.
The "voltage" information is provided by
a voltage transformer and processed by
a time-delayable threshold device.
c overvoltage
This protection is necessary if there is a
risk of strong fluctuations occurring on
the supply network. It means it is no
longer necessary to wait for the
overload relays to trip, since
overvoltage results in motor overcurrent
and increased motor torque which
could be detrimental for the machine
being driven.
The fault is detected by time delayed
overvoltage measuring relays.
Incomplete or overlong starting
This protection is justified for starting in
several stages.
It is provided by a time delay relay put
into operation at the beginning of
starting and removed at the end. The
value monitored may be speed or
current.
Prolonged use of the starting system,calculated to run for a set time, is thusavoided.
Loss of synchronismThis protection is vital for synchronousmotors.
In point of fact, the dampening cageof a synchronous motor is relativelyfragile. If the motor jams, inducedcurrents will flow through this cage andcould destroy it if the motor is notdisconnected.Jamming may occur following amechanical overload, undervoltage, aloss of or drop in excitation.
This fault is detected by underimpedance or power factor relayssupplied by voltage transformersand current transformers (see fig. 31).
fig. 29: insulation fault monitoring diagram with continuous insulation monitor or zero sequence
voltage relay - unearthed neutral
fig. 30: diagram showing under and
overvoltage protection. In most cases, two
VTs supply the relay phase-to-phase
voltages.
voltage
relay
M
zero sequence
voltage relay
DC injection
insulation
monitorsurge
voltage
limiterC
R = load resistance
R R R
diagram 1
diagram 2
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directional power
relay
M
under
impedance
or cos relay
M
Loss of excitationThis loss due, for example, to a break inrotor winding, causes the motor to jam.It can be detected by:
c the "loss of synchronism" protectiondescribed above;c or an exciting undercurrent orundervoltage relay.
Rotor frame of a synchronous motor
This protection is determined accordingto the supply configuration and theDC production mode.
If the entire DC exciting circuit isunearthed, an insulation fault will notaffect motor operation.However, should a second fault occur,it may give rise to an overload or short-
circuit with all its consequences. Therelays used to detect this type of faultare generally devices injecting lowfrequency 10 Hz or 20 Hz AC current(see fig. 32).
The 50 Hz frequency is also used,which requires that no 50 Hzcomponents must be present in theexciting circuit.
Prolonged operation in theasynchronous mode on starting
Overlong starting on synchronousmotors causes excessive temperaturerise of the damping cage.
Either the "incomplete starting"protection described above is used or athermal device adapted to the rotorthermal time constant, placed in series
with the inductance during starting (seefig. 32 element b).
Overload and short-circuit in theexciting windingThese protection devices preventdamage due to temperature rise of theexciting winding and its power supply.The fault is detected by an excitingovercurrent relay. Moreover an excitingundervoltage relay is normally used,tripping on voltage drop, caused forexample by a short-circuit.
On starting, it is used as an excitingvoltage presence relay and authorisesclosing of the exciting contactor atthe end of starting in the
asynchronous mode (see fig. 32element g).
Reverse power
This protection particularly applies tosynchronous motors.
When the supply circuit-breaker trips, itprevents energy from being sent backto the loads connected on the samebusbar. It also prevents a fault on thisbusbar from being supplied by themotor.
The protection device has to detect areversal in current or power direction.
Its function is therefore performed by adirectional power relay (see fig. 33).
Frequent starts
Too many starts over a given period oftime may cause motor damage if themotor is not designed for such anoperation.
This protection is provided by a relayperforming metering and time delayfunctions, which automatically limits:c either the number of starts over agiven period of time;c or the time interval between start upsover a given period of time.
fig. 31: protection against loss of
synchronism.
fig. 32: rotor protection devices on startup
and in operation.
a. exciting windingb. thermal protection: prolonged operation inasynchronous modec. starting resistanced. exciting contactore. rotor frame protectionf. overcurrent protectiong. undervoltage protectionh. to tripping of motor feeder contactors. continuous source
fig. 33: protection against reverse power.
ab
c
d
e
f
g
h
s
Z
U
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Locked rotor
Jamming of a motor for mechanicalreasons causes an overcurrent roughlyequivalent to starting current. The
resulting temperature rise isconsiderably greater since rotor lossesare maintained at their highest valuethroughout jamming and ventilation isno longer present if connected to rotorrotation. As a result, when there is arisk of jamming, the "locked rotor"protection is necessary since overloadrelays sometimes have an excessivelylong response time.
This fault is detected by an overcurrentrelay set at a value less than thestarting current. This value is validated
after a time delay initiated when themotor is energised. This time delay isset at a value greater than or equal to
normal starting time.
Undercurrent or underpower
Pumps may be damaged when
unpriming occurs. Unpriming could also
result in a drop in the active power
absorbed by the motor. Protection
against this fault is provided by an
undercurrent relay.
technological evolutionThe term "relay" is often used in the
above description of the various
protection devices. This is a thetraditionally used term (relay = a type ofprotection device) which goes back tothe time when motor protection
required the use of separate "relays"each with a single protection function.
In the seventies, manufacturers broughtout RACKS able to house a number ofdifferent protection devices in order tomeet needs more closely.
In the eighties digital technologyincreased adaptation possibilities stillfurther. Thus a single programmabledevice performs the various protectionand control/monitoring functionsrequired in each specific case.
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overall approachThe approach chosen by the designeris to look for the best technico-economic choice. For this, first try andvalidate the easiest and mosteconomical solution. Then, if this doesnot work, follow the order shown in thetable in figure 34.
Direct starting
Motor apparent power at the beginningof starting:
Sm = Pn
cos Id
In
Sm = 1,500
0.845 = 8,925kVA
where, with power factor on starting:cos
d= 0.15, i.e. :
d= 81.
This power is vectorially added to thepower delivered by the transformer onthe other feeders (see fig. 35).
The total value of the apparent powerrequired by the transformer isgraphically deduced from the following:
S 9,580 kVA.The maximum inrush authorised is6,000 kVA: direct starting is thus notpossible.
Starting by reactance
Introduction of a reactance reduces theapparent power absorbed by the motor.The available starting power isgraphically determined (see fig. 35).
When the motor starts, the presence ofthe reactance means that the powerfactor approaches zero. Thus
d 90.
appendix 1: determining motor starting mode
The purpose of the example below isnot to fully deal with a problem butrather to illustrate, through an example,an approach wich helps to choose theappropriate starting mode for a givenapplication.
calculation hypothesesAsynchronous motor with:c rated power P
n= 1,500 kW;
c rated voltage Un
= 5,500 V;
c efficiency x power factor: x cos = 0.84;c starting torque/rated torque ratio onfull voltage:
CdCn
= 0.8
c starting current/rated current ratio onfull voltage:
Id
In
= 5 ;
c breakaway torque of the drivenmachine: 0.2 C
n;
c main supply transformer power:Pt = 3 MVA;c maximum apparent power inrushauthorised by the transformer network:S
t= 6 MVA.
Other data necessary for calculation:c torque-speed C (N) characteristic ofthe motor;c load torque-speed C
r(N);
characteristic of the driven machine
c transformer delivery on feeders otherthan the motor one: 1,200 kVA undercos ' = 0.87.
OA
= 1,200 kVA : transformer power
used on other feeders.
OB
= St = 6,000kVA: maximumapparent power authorised. Theapparent power available for starting(motor + reactance) is graphicallydeduced.
AB
= Sd = 5,300kVA.Power reduction to be caused by the
reactance:SdSm
= 5,3008,925
0.6 .
fig. 34: decisive criteria for choosing motor starting mode.
starting solutions main acceptance criteria
direct power inrush compatible with network
reactance c starting torque greater than breakawaytorquec current peak (for full voltageresumption) acceptable by network
autotransformer the same
fig. 35: starting power diagram.
S = 9,580*
B
d= 81
' = 29.51,200*
St= 6,000*
Sd
= 5,300*
Sm
= 8,925*
A
active power*: in kVA
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(a)
(c)
(d)
(b)
(e)
0 0.1 0.2 0.5 0.8 1
0.2
0.8
5
1
1
Cn
In
point of balance
at reduced
voltage
current peak
on move
to full voltage
(N/Ns)
I'd is the new starting current value.
Sd = Un I'd 3
Sm
= Un
Id
3
thus: I'd = 0.6 Id.
Moroever:I'd
Id
= UdUn
= 0.6 .
The value of voltage at the motorterminals is U
d= 0.6 U
n. Once the
electrical problem has been solved, itremains to be seen if this solution isvalid from the mechanical standpoint. Inthe case of direct starting, the startingtorque equals : C
d = 8 Cn (see fig. 36).
In the case of starting by reactance, thestarting torque C'
dequals:
C'd = 0.8 Cn UdUn
2
= 0.288 Cn
This value is compatible with thebreakaway torque of the machine beingdriven.
One last point remains to be checked:if the point of mechanical balanceC
m= C
ris placed at an excessively low
speed, a current peak may occur on themove to full voltage. If this peak is toohigh for the network, the starting modewill have to be reconsidered andstarting by autotransformer, for
example, chosen (see fig. 36).Remark 1Let us assume that the value of thebreakaway torque of the drivenmachine is 0.35 C
ninstead of 0.2 C
n.
Starting by reactance is thenincompatible with the breakawaytorque.The solution of starting byautotransformer must then beenvisaged.
The apparent power available remainsS
d= 5,300 kVA. From this value the
magnetising force of the auto-
transformer Smg must be deducedwhich, at the initial starting moment, isarithmetically added to motor apparentpower. S
mgis around 0.2 to 0.4 times
motor apparent rated power.
i.e. with the factor 0.4:
Smg = 0.4Pn
cos
Smg = 0.41,500
0.84 = 720 kVA .
The power reduction factor is then:
Sd SmgSm
= 5,300 7208,925
= 0.513
At constant voltage Un, the current
inrush on the network side is therefore:0.513 Id.
Determining the reduced startingvoltage U
d.
Equality of primary and secondaryautotransformer powers results in thefollowing:
0.513 Id Un = I"d UdI"
dis the reduced voltage starting
current on the motor side
0.513 Id Un = Id UdUn
Ud
hence:UdUn
2
= 0.513
i.e. Ud = 0.718 Un
New starting torque:
0.8 Cn UdUn
2
= 0.41 Cn .
This value is sufficient for starting totake place.
Remark 2
For a breakaway torque greater than0.41 C
n, stator starting with this motor
is no longer possible.
Either a slipring rotor motor with rotor
starting must be used, or a motor with aspecial cage possessing a high startingtorque.
fig. 36: torque and current curves for starting by reactance.
a: curve C (N) on full voltage
b: curve C (N) on reduced voltage (0.6 Un)
c: curve I (N) on full voltage
d: curve I (N) on reduced voltage (0.6 Un)
e: curve Cr (N)
C
Cn
I
In
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appendix 2: coordination of protection devices
Once the protection devices have beenchosen according to operatingrequirements, they must be coordinatedto obtain the maximum advantage fromtheir possibilities. A balance must besought between untimely tripping anddelay on fault removal. The problem ofprotection device coordination is solvedby studying the curves t (I) of therelays, of the circuit-breaker and of thebreaking capacity of the contactor. Thecharacteristics of the motor shown inthe figure are as follows:
c Pn
= 550 kW;c U
n= 3150 V;
cIn
= 130 A;cId = 5 In.
The contactor is of the Rollarc fuse type.
Type of protection devices
c thermal relay with indirect tripping,set at In = 130 A for overloads;c positive phase sequence componentrelay set at 6 I
n, time delayed at 0.05 s
for balanced faults;c negative phase sequence componentrelay set at 0.3 or 0.4 In, time delayed
at 0.6 s.In the case of networks with quasi-permanent unbalances, a two-thresholdrelay is used:c a lower time delayed threshold, setjust above the permanently acceptednegative phase sequence componentratios;c an upper instantaneous threshold forphase break protection.
Instantaneous tripping by the positivephase sequence component relayenables best use to be made of thecontactors breaking capacity and
avoids fuse melting. Analysis of thecurves in figure 37 shows that themotor and network are protectedagainst:c unbalances of approximately 0.3 I
nto 10 I
n,
c balanced faults from 6 In to 28 In.
The fuses only take action beyond 15 Infor unbalanced faults and 25 I
nfor
balanced faults.
The maximum current the contactormay have to break is 28 In = 3,640 A.This value is considerably lower than itsbreaking capacity of 10 kA.
fig. 37: minutes thermal relay operating zone.
1 2 5 10 20 50 100
130 260 650 1,300 2,600 6,500
0.01
0.03
0.05
0.1
1
5
10
20
40
1
2
34
68
10
20
30
40
60
cold
hot
fuse
phase sequence
component:
negative
positive
contactor
breaking
time
minutes
seconds
Rollarc contactor
breaking capacity
thermal relayoperating zone
3,640 A
k InIn (A)
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Cahier Technique Merlin Gerin n 165 / p.26
appendix 3: bibliography
Standards
c IEC 34-1: Rotating electricalmachines.
c IEC 71: Insulation coordination.
Merlin Gerin's Cahier Technique
c The breaking process with a FluarcSF6 puffer-type circuit-breaker,Cahier Technique n 112 -J. HENNEBERT
c Behaviour of the SF6 MV circuit-breakers Fluarc for switching motor
starting currents,Cahier Technique n 143 -J. HENNEBERT and D. GIBBS
c Overvoltages and insulationcoordination in MV and HV,Cahier Technique n 151 -D. FULCHIRON
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Cahier Technique Merlin Gerin n 165 / p.27
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