Motor Protection Siemens
Motor Protection Siemens
Motor Protection Siemens
Protection of a
Motor up to 200 kW
n 1. Introduction
Drive motors often play a decisive role in the
functioning of a production process. Motor dam-
age and breakdowns not infrequently lead also to
consequential damage and production shutdowns,
the cost of which significantly exceeds the cost of
repairing the motor. Optimum design of the mo-
tor protection ensures that damage following
thermal overload is prevented, meaning that there
is no reduction to the normal service life. Second-
ary faults are minimized in the event of short-
circuits, earth faults and winding faults.
The spectrum extends from small low-voltage
motors with an output of a few kW to high-
LSP2731.tif
voltage motors with outputs measured in MW.
Protection system design must be based on the
rating of the motor, the importance of the drive Fig. 1 SIPROTEC 7SJ602 multifunction protection
for the technological process, the operating condi-
tions and the requirements of the motor manufac- Such overloading cannot and should not be de-
turer. tected by short-circuit protection since any poten-
The setting of a SIPROTEC protection relay for tial delay must be very short on these occasions.
motor protection is described below taking a Overload protection prevents thermal overloading
high-voltage motor (10 kV) as the example. of the motor to be protected. The 7SJ602 relay de-
tects stresses either before overload occurs (over-
n 2. The tasks of motor protection load protection with complete memory = thermal
Motors have some striking features in their oper- replica) or only after exceeding a preset start-up
ating conditions. These are important for under- current (overload protection without memory
standing the various possible causes of failure and function).
must be taken into account when designing pro- n Overload protection without memory
tection systems. If overload protection without memory is cho-
2.1 Protection of the stator against thermal sen, the tripping time is calculated according to
overload a simple formula. Pre-stressing is not taken into
The power drawn by the motor from the supply account because currents are only recorded if
system during operation is supplied to the shaft as they are greater than 1.1 times the set value.
mechanical power for the production machine. 35
t= ⋅ t 6IB for I > 1.1 IB
The power lost to the winding during this energy (I / I B )2 − 1
conversion is the decisive factor for the arising t Tripping time
motor temperatures. The loss of heat is propor- I Overload current
tional to the square of the current. The motor IB Set threshold
heating time characteristic is determined by its t6IB Set time factor
heat storage capability and heat transfer proper- (t6-time = tripping time when applying 6 times
the actual set value IB)
ties, and characterized by the thermal time con-
stant [τ]. Electrical machines are at particular risk
from long-term overload. Thermal overloading of
the motor leads to damage to the insulation and
therefore to secondary faults or to a reduction in
the total service life of the motor.
n Overload protection with memory permitted start-ups from cold (nc) and operat-
The relay calculates the temperature rise in ac- ing temperature (nW) conditions. The relay cal-
cordance with a thermal homogenous-body culates from this the value of the thermal rotor
model and a thermal differential equation. In replica and gives a blocking command until the
this way the previous load, with all load cycles, thermal replica of the rotor reaches a value be-
can be recorded and evaluated correctly by the low the restart limit and therefore permits a new
relay. Such a thermal replica can be optimally start-up. As long as a blocking command pre-
adapted to the overload capacity of the pro- vails, switching on by the relay’s integrated
tected equipment. switch control is prevented. In this case, it is not
necessary to allocate the restart inhibit’s block-
2.2 Protection of the rotor from thermal overload ing command to a command relay or an exter-
Among the many causes of excessive temperatures nal link with the switch control. If however the
caused by currents in motors is an unacceptably motor can be switched on from another posi-
long start-up time or, in limit cases, blocking of tion, an output relay must be allocated to the
the rotor. Such conditions are caused by an exces- blocking command and its contact looped into
sive mechanical load torque, such as can occur in the starting circuit.
overfilled mills and breakers or overloaded centri-
fuges, etc. 2.3 Negative-sequence protection
In protection of the motor, negative sequence
n Start-up time monitoring
(unbalanced load) protection assumes particular
The protection relay has start-time monitoring,
importance. Unbalanced loads produce a reverse
which represents a meaningful addition to over-
field in motors which drives the rotor at twice the
load protection for electrical machines. The trip
frequency. Eddy currents are induced on the sur-
time depends on the current. This enables even
face of the rotor, leading to local temperature rises
extended start-up times to be correctly evalu-
in the rotor. If the motor is protected by fuses, a
ated when the start-up current is reduced be-
phase voltage failure is a frequent fault in practice.
cause of voltage sags when the engine is started.
During this breakdown, the line-to-line voltage is
The start-up time monitoring begins when a set
fed to the stator winding by the remaining work-
current level is exceeded. The trip time depends
ing phases. Depending on the load, a more or less
on the actual measured start-up current. If the
circular rotating field is maintained by the motor,
permissible locked rotor time is shorter than the
so that it can develop sufficient torque with in-
start-up time, the rotational speed (motor sta-
creased current input. There is also the risk of
tionary or rotating) must also be requested via a
thermal overload if the system voltage is unbal-
binary input.
anced. Even small voltage unbalances can lead to
n Restart inhibit large negative sequence currents because of the
Restart inhibit prevents the motor restarting if, small negative sequence reactance.
during this start-up, the permissible rotor heat-
ing is expected to be exceeded.
The rotor temperature of a motor generally lies
well below its permitted temperature limit both
during normal operation and with increased
load currents. On the other hand, during
start-up, with the associated high start-up cur-
rents, the rotor is at a higher risk of thermal
damage than the stator because of its smaller
thermal time constant. The motor must be pre-
vented from switching on if the permissible ro-
tor heating is expected to be exceeded during
this start-up. This is the task of the restart in-
hibit. Because the rotor current is not directly
measurable, stator currents must be relied upon
from which the rotor temperature is indirectly
calculated. It is therefore assumed that the ther-
mal limit values for the rotor winding in the
data provided by the motor manufacturer for
the rated start-up current equal the maximum
permitted start-up time and the number of the
Fig. 4 Connection of 3 CTs and 1 VT with measurement of the earth current and one
phase voltage
Earth-fault protection detects earth faults in the dimensioning the setting in order to be independ-
stator winding of three-phase machines. Because ent of the circuit state in the power supply system.
motors are usually connected directly by a busbar With machines in compensated power supply
to a power supply system (directly connected to a systems a load device and measurement of the dis-
busbar) it is important to recognize whether the placement voltage are always recommended so
earth fault is in the machine feeder or on another that a safe earth-fault decision can be made.
feeder of the busbar.
2.5 Short-circuit protection
With earthed systems, this can usually be clearly
recognized from the magnitude of the earth cur- The task of the short-circuit protection when a
rent. When a fault occurs in the machine, the full short-circuit occurs is both to prevent increased
earth-fault current driven by the power supply damage to the motor (destruction of the iron
system flows via the protection measuring point. core, etc.) by quickly switching off the motor and
The machine must be isolated from the power to minimize the effect on the power supply system
supply system as quickly as possible to prevent with its connected loads (voltage unbalance, volt-
more damage. When there is a power system age sags, etc.).
earth-fault the recorded earth current is essentially The overcurrent-time protection in the 7SJ602
determined by the machine capabilities and can take the form both of definite-time overcur-
therefore considerably smaller. There must be no rent-time protection and of inverse-time over-
tripping. current-time protection. For the latter, a range of
In compensated, isolated and low-resistance characteristics defined in IEC 60255---3 or in
earthed systems, a design with sensitive earth-cur- ANSI standards is available. A high-current stage
rent input and sensitive earth-fault detection I>>, which always works with definite tripping
should be chosen. The high-resistance earth-fault time, can be superimposed on the selected over-
detection then replaces the earth-current stage of current characteristics. An instantaneous tripping
the overcurrent-time protection. Because of its stage I>>> can also be superimposed on the phase
high sensitivity it is not suitable for detecting earth branches. In this way the tripping characteristics
faults with large earth currents (more than around can be optimally adapted to the motor's start-up
1.6 ⋅ IN on the terminals for sensitive earth-cur- characteristics.
rent connection). In order to be able to switch off during high cur-
Overcurrent-time protection for earth currents rent faults in the machine, the 7SJ602 has a special
must be used here. instantaneous tripping stage. The I>>> stage must
Should the magnitude of the earth current be suf- be set safely above the motor’s inrush current, so
ficient to determine the earth fault, no voltage in- that switching on the motor does not lead to trip-
put is needed. The 7SJ602 has a two-stage current/ ping. Experience has shown that the inrush cur-
time characteristic which works with earth-cur- rents can be around 1.5 to 1.6 times the start-up
rent values. They are appropriate where the mag- current.
nitude of the earth current enables the location of
the earth fault to be defined. This can, for exam- The I>> stage should be set above the motor start-
ple, happen with machines on low-resistance up current to prevent tripping. With the time de-
earthed systems (with earth-current limiting). lay TI>> the period of the inrush current must be
taken into account. Because the inrush current
With machines directly connected to a busbar to lasts only a few ms, the TI>> can be selected at
isolated power systems, it is essential that the ca- around 50 ms.
pacity of the upstream power system delivers a
sufficiently large earth current but that the earth
current at the relay location is comparably small
in the case of earth faults on the power system
side. The magnitude of the earth current is used to
reach a decision on the position of the fault
location.
If this is not the case an additional earth-current
production device must be installed on the
busbar. This produces a defined earth current
during an earth fault. The connected displacement
voltage is then used to make a direction decision.
Should a load device (earth-current production
device) be installed, it should only be used when
In the overcurrent-time protection function an value applies for all three phases. If the set current
inverse-time characteristic must be chosen since value in one phase is exceeded, the circuit-breaker
this can be better adapted to the motor’s opera- is considered closed. In machines the value se-
tional performance. lected must be smaller than the machine’s mini-
mum no-load current.
The inverse-time short-circuit protection Ip> pro-
tects the motor from short-circuits during opera- Motor data is generally related to the rated motor
tion in transient condition (after ramping up). current. A matching factor must be communi-
The higher the short-circuit current the quicker cated in the system data to the 7SJ602 so that the
the tripping. The extreme inverse characteristic settings for motor protection functions can be
must be selected as tripping characteristic. provided directly as a reference quantity.
Example:
Current transformer 100 A / 1 A
Rated motor current INM = 74 A
n Ratio of rated motor current to rated trans-
former current
Im = INM/INTRANSF = 0.74 [from transformer
data]
The motor’s start-up current is likewise preset
in the 7SJ602's system data.
The start-up current is specified as a value re-
lated to the rated motor current (INM).
It depends on the size and nature of the motor
Ip> Short-circuit protection, inverse-time stage and in a normal load-free start-up is approxi-
I>> Short-circuit protection, definite-time stage mately 5 ⋅ INM.
I>>> High-set instantaneous tripping stage,
definite-time stage n Motor start-up current referred to rated motor
Fig. 5 Current characteristic of motor start-up
current
Ia = 5 [from motor data sheet]
In the 7SJ602, the motor’s start-up time is pre-
n 3. Adjustments set in the system data. After this time the start-
Calculation examples are oriented towards the fol- up current must be safely undershot.
lowing motor data: n Motor start-up time
Motor/system data tSTART-UP = 4.3 [from motor data sheet]
Current transformer phase
INPRIM/INSEC 100 A / 1 A 3.1 Overload protection
Current transformer earth For overload protection the load must be taken
(60/1) IEE/IPH 0.6 (core- into account before the overload occurs; i.e. the
balance CT) overload function must be used with full memory.
Voltage transformer 10 kV / 100 V
The relay calculates the temperature rise in accor-
Motor rated current INM 74 A
dance with a thermal homogenous-body model
Max. permissible unbalanced load 10 %
and a thermal differential equation:
Permissible unbalanced load period 15 s
Permissible continuous thermal dΘ 1 1 2
+ ⋅Θ = ⋅I
current IMax 1.1 ⋅ INM dt τ τη τ th
Thermal stator time constant τth 40 min
Standstill transient factor kτ 5 Θ Present temperature rise referred to the final
temperature with maximum permissible line
Start-up current IA 5 ⋅ INM current k · IN
τth Thermal time constant for heating of the object
Data of the system and equipment to be protected to be protected
is input. Some data which particularly involves I Present effective current referred to the maximum
motor protection functions is worthy of mention permissible current Imax =k · IN
here.
For some protection functions it is important to
recognize whether the circuit-breaker is closed or
open. As a criterion for this overshooting or un-
dershooting a current threshold is applied. The set
The following parameters must be set: The following parameters must be set:
n Set value of the k factor = Imax/INTransf n Start-up current threshold Ia> for start-up time
Imax = maximum permissible continuous monitoring, referred to rated motor current INM
thermal current = 1.1 ⋅ INM = 81.4 A with Ia = 5 ⋅ INM motor data entered with system
k = 0.82 data
n Set value of the thermal time constant τth in Ia> = 0.5 Ia = 2.5 INM
minutes Should the start-up time exceed the tripping time
th = 40 min [from motor data sheet] of the overcurrent time protection, said protec-
For motors the t6 time, i.e. the permissible time tion is blocked during start-up after 70 ms.
for the six-fold permissible continuous current, is n Blocking the I> / Ip stages during start-up
often specified instead of the time constant. NO
As a result the τth is calculated as follows:
If the permissible locked rotor time is less than the
t6 start-up time, the rotational speed (engine stands
t
Set value τ th[min] = s ⋅ 36 = 0.6 ⋅ 6 or rotates) must be additionally requested via a bi-
60 s nary input.
n Transient factor kτ between time constant
(during standstill) and running of the motor 3.3 Restart inhibit
k = 5 according to motor data Rotor temperature simulation plays a decisive role
n Alarm temperature rise as a percentage of the in the restart limit. The parameters required for
operating temperature rise ΘALARM/ΘTRIP this such as start-up current, rated motor current
ALARM = 90 % [preset]
and maximum permissible start-up time are con-
figured with the system data.
The 7SJ602 also provides the option to connect an
external thermobox to the relay. This affords an The following parameters must also be set in
opportunity to connect the coolant temperature addition during restart inhibit:
or ambient temperature of the protected object n Temperature equalization time of the rotor
into the relay using the serial interface and include As the thermal time constant of the rotor is
it in the overload calculation. considerably smaller than that for the stator, a
value of 1 min. at most (preset) is practicable
3.2 Start-up time monitoring for the temperature equalization time of the ro-
The start-up time monitoring interprets over- tor.
shooting the current value Ia> as a motor start-up. tEQUAL = 1min [empirical value]
Consequently this value must be chosen so that it n Number of maximum permissible warm
is safely exceeded during motor start-up in all load start-ups
and voltage conditions by the actual start-up cur- nW = 2 [empirical value]
rent but is not reached during permissible, short-
term overload. It must also be configured above If no specifications are available from the motor
the maximum load current. The set value is re- data sheet, empirical value 2 is set.
lated to the rated motor current. Half the value of n Difference between the number of maximum
the rated start-up current is customary. If the permissible cold start-ups and the maximum
start-up current is 5 ⋅ rated motor current (INM), number of permissible warm start-ups
Ia> is set at 2.5 ⋅ rated motor current. nc – nW= 1 [empirical value]
The tripping time is calculated quadratically
according to the magnitude of the current: If no specifications are available from the mo-
tor data sheet, empirical value 1 is set.
2
I
t TRIP = t START -UP ⋅ a with I > Ia> n Factor for the thermal cooling-down time of the
I rotor when the machine is at standstill.
tTRIP Actual tripping time for flowing current I The reduced cooling (when the motor is at
tSTART-UP Max. start-up time standstill) in motors with self-ventilation is
I Actual flowing current (measured quantity) taken into account by the factor kτSTI (related to
Ia Rated motor start-up current the time constant during no-load operation).
Undershooting the current threshold set in the
system data as LS I> is considered as criterion
for the motor’s standstill.
In our example it is assumed that the supplying If the earth current is insufficient for fault loca-
10 kV power system has a corresponding size and, tion, an earth-fault direction determination is
during an earth fault, a capacitive earth-fault cur- configured. In this case a voltage input (Uen) is
rent ICE of approximately 20 A flows to the fault obligatory.
location. Information about the magnitude of the
With sensitive earth-fault direction determination
capacitive earth-fault current must be requested
it is not the magnitude of the current that counts
from the power system operator. Of course, the
but rather the component of the current vertical
motor feeder also delivers an earth-fault current.
to a settable directional characteristic (symmetry
The motor feeder must be connected via a 100 m
axis). A precondition for direction determination
long 10 kV cable. The motor feeder earth-fault
is exceeding of the displacement voltage stage UE
current is calculated as follows:
and a likewise parameterizable current compo-
ICEcable = I'CE ⋅ l nent that determines the direction (active [cos ϕ]
or reactive component [sin ϕ]).
I'CE = 1.8 A/km (from the cable data sheet)
l = 0.1 km In electrical machines directly connected to
busbar on the isolated power system, cos ϕ and a
ICEcable ≈ 0.20A
correction angle of around +45° can be set for the
The following earth-fault current distribution is measuring mode, because the earth-fault current
the result. often consists of a superimposition of the capaci-
tive earth-fault current from the power system
and the resistive current of a load resistor.