Technical Guide
Application Guide
MiCOM P220
2.
SETTING FUNCTIONS
2.1
PROTECTION Menu
2.1.1
Thermal overload [49]
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Overloads can result in excessive stator temperature rises in excess of the thermal
limit of the winding insulation. Whilst this may not cause the motor to burn out
immediately, it has been shown that the life of the motor can be shortened if these
overloads persist. The life of the motor is not purely dependant on the temperature of
the windings but on the time that it is exposed to these temperatures. Due to the
relatively high thermal storage capacity of induction motors, infrequent overloads of a
short duration may be tolerated without damage. Sustained overloads of a small
percentage may result in premature ageing and insulation failure.
In the same way, an unevenly distribution of load or a slight unbalance of the
network brings about the appearance of negative sequence currents which also
contribute to the heating of the rotor (for more details, see the negative overcurrent
protection function).
The motor temperature varies exponentially with the increase of the current. Similarly,
the temperature decreases in the same way. So to provide a close sustained overload
protection, the relay incorporates three thermal time constants, thanks to which the
thermal reproduction of the relay is paired narrowly with the protected motor during
heating and cooling conditions.
The thermal withstand capability of the motor is affected by heating in the winding
prior to the fault. The thermal replica is designed to take into account the extremes of
zero pre-fault current, known as the “cold” condition, and full rated pre-fault current,
known as the “hot” condition. With no pre fault current, the relay will be operating on
the “cold curve”. When the motor is , or has been, running at full load prior to a
fault, the windings will already be dissipating heat and the “hot curve “ is applicable.
Therefore, during normal operation, the relay will be operating within these two
limits, unless programmed to do otherwise.
However, it should be noted that the overload protection includes the monitoring of
both the stator and the rotor. This protection can be realised in various ways:
•
1: direct measurement through the use of temperature sensors (see the
corresponding paragraphs),
•
2: indirect measurement by the means of current measurement,
•
3: by a combination of the two preceding principles.
The P220 relay design combines all three principles listed. No.1 is detailed below in
the paragraph dealing with overload protection through the use of temperature
sensors. No.2 and No.3 are described in this paragraph.
P220/EN AP/B43
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In the case of minor overloads and of light-duty service conditions, stator current
measurement is sufficient to ensure protection. This control can be achieved using a
time-independent current threshold setting for a definite time overcurrent protection
or, still better, IDMT overcurrent protection. The thermal protection elements which
are overheated by a fraction of the main current, have a time-constant which is very
close to that of the motor. This makes it possible to obtain a real-time image of the
thermal status of insulation. This type of protection takes into account the fact that the
steady-state temperature of the motor is proportional to the square of the absorbed
current, the protection is also provided with a cold curve and a hot curve to ensure
that the relay takes into account the initial motor temperature.
The thermal protection described above makes use of current measurement to protect
the motor. Hence it will monitor balanced and unbalanced overloads. The thermal
time-constant is adjustable in order to match any type of motor. The positive (I1) and
negative (I2) components of the current are composed together in order to result in a
equivalent thermal current replica of the temperature of the motor.
2
2
This equivalent thermal current is given by the equation : Ieq = √(I1 + Kex I2 ),
where Ke is an adjustable parameter used to account for the effects of heating
produced by the negative component of the current when developing the thermal
image.
From this equivalent thermal current, the thermal state θ of the motor is calculated
every 5 cycles (every 100ms for a network of 50 Hz or 83.3ms for 60 Hz) by the relay
in accordance with the following formula.
θ i+1 = (Ieq/Iθ>)² . [1-e(-t/T) ] + θ i . e(-t/T)
θ i : is the initial thermal state.
If the absorbed current is less than the thermal overload threshold [Iθ>], thus typically
less
than
the
nominal
current
or
the
full
load
current,
then
the thermal state θ will be less than 100% , so no tripping occurs.
If the absorbed current is greater than the thermal overload threshold [Iθ>], in this
case the thermal state θ will be greater than 100% and so tripping will take place.
In the thermal model selected, the time of tripping depends on the initial state of the
motor. The equation used to calculate the tripping time for a thermal state of the
motor at 100 % is:
2
2
t = T x ln[ (K - θi) / (K -1)]
The equation is valid for currents whose value is constant over a certain period of
time, where:
−
the value of T, thermal time-constant which depends on the value of the ratio
Ieq / Iθ:
T = Te1
if 0 < Ieq ≤ 2*Iθ
(overload curve )
T = Te2
(start-up curve )
if Ieq > 2*Iθ
T = Tr
Ieq=0
(cooling curve –motor stopped)
if
NOTE:
Ieq = 0 is obtained through the logic input No.1 of the relay
which recovers the information «contactor position open».
−
Iθ = thermal current threshold setting
−
K = Ieq/ Iθ
−
θi = initial thermal state of the motor (ex.: thermal state of 50% ! θi=0.5)
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•
P220/EN AP/B43
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The heating time-constant Te1 can be estimated from the motor heating curve as
shown below.
Motor heating curve
Temperature
θm
0.632 θ m
Te
Time
P0170ENa
This curve corresponds to the following law:
θ(t)
= θm * (1 – e-t/Te) ,
Where:
θm
=
Te
t
=
=
maximum temperature after stabilisation of heat exchange,
in degrees °C
heating time-constant
time elapsed
The heating time-constant can be clearly defined. When a motor is absorbing its
rated current indefinitely, it reaches 63.2% of its steady-state temperature (θT =
63.2% θm) after one time-constant Te.
The cold curve of the motor is thus given by:
2
2
t = Tr x ln[ K / (K –1)]
Where the equation for the motor cooling temperature is given by.
θ = K² (1 – e –t/Tr ).
When the motor is stopped, the rotor fan cooling is stopped also, hence the motor
cooling down is few efficient. This causes the cooling time-constant to increase
considerably This constant is generally much longer than the heating time-constant.
In order to compensate for this phenomenon and to obtain a correct thermal replica,
the cooling time constant is used by the relay.
An adjustable cooling time-constant (Tr) is provided in order to take into account the
various modes of cooling.
The cooling time-constant Tr can be estimated from the motor cooling curve in the
following way:
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Motor cooling-down curve
Temperature
θm
0.368 θ m
Tr
Time
P0171ENa
This curve corresponds to the following law:
θ(t) = θm * e-t/Tr,
Where:
θm
Tr
t
=
=
=
maximum temperature when motor is stopped
cooling time-constant
time elapsed
The cooling time-constant can be clearly defined . When a motor is stopped, its
internal temperature decreases with time. This internal temperature reaches 36,8% of
the initial temperature (temperature at the time when the motor was turned off) at the
end of the period, which is equal to its time-constant Tr.
The P220 relay also has:
•
a thermal alarm to inform the user (when in operation mode) if the motor is likely
to become overloaded before a trip occurs. Remedial action can then be taken
before the motor is tripped.
•
Inhibition of a thermal tripping during starting
During the start-up stage (i.e. during the parametrically defined start-up delay time
tIstart), it is possible to inhibit thermal tripping. When thermal inhibition during start-up
is enabled, the calculation of the thermal state during the start-up delay time tIstart
remains effective but should this value exceed 90%, the value of the thermal state
would be retained at 90%. When the start-up delay time expires, the thermal
inhibition during start-up disappears. This function does not affect the operation of
the thermal alarm feature.
This inhibition during start-up can be useful for certain motors which can withstand a
locked rotor for a very short time but normally have very long start up times. This can
be the case of certain motors started using reduced voltage. The time-constant Te2 is
then set to take into account rapid heating which occurs if the rotor is locked,
whereas the motor would be thermally protected during the start-up stage by the
function «Start-up too long» and, as the case may be, by the temperature sensors.
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•
Thermal prohibition of restart
•
To a certain extent, the thermal overload protection can limit the number of startups by selecting a curve located just above the point of start-up. It is is actually
very difficult to satisfy the manufacturer’s recommendations, for the limit of the
number of starts by the thermal overload protection. This may allow the motor to
start and then to exceed the maximum temperature.
The purpose of the "Thermal prohibition of start-up "function is to avoid thermal
tripping during the start-up sequence of the motor. The motor will have to cool down
before it will be authorised to start.
The tripping time for the thermal replica protection is calculated in the following way:
ttrip = T * ln { [(Ieq / Iθ>)2 - θinitial] / [(Ieq / Iθ>)2 - 1] }
(1)
in order to avoid thermal tripping during the start-up stage, ttrip > td.
Therefore, according to (1),
td < Te2 * ln { [(Id / Iθ>)2 - θforbid start] / [(Id / Iθ>)2 - 1] }
Hence it follows that the setting of the threshold of prohibition of start-up θforbid start
must be lower than:
θforbid start < [(Id / Iθ>)2 * (1 – exp(td / Te2))] + exp (td / Te2)
Where:
Id
td
Te2
Iθ>
ttrip
•
=
=
=
=
=
actual start-up current,
actual start-up time,
thermal time-constant at the moment of start-up,
current threshold of thermal overload,
time of tripping for the thermal replica protection.
Thermal image influenced by the ambiant temperature
At the beginning of this paragraph we said that the overload protection afforded by
the P220 relay can also be ensured by a combination of a temperature sensor (direct
heating measurement), and a current measurement (indirect heating measurement).
In this case, it is possible to modify the calculated thermal image of the motor by
making use of information about the temperature outside of the motor. The
programmed thermal current threshold can be corrected using correction factors to
give a more precise representation of the thermal state of the motor.
This thermal current threshold correction factor is applied automatically by the relay
when calculating the thermal state of the motor – if this facility is set on.
The values of this factor is given below:
Ambient temperature
40°
45°
50°
55°
60°
65°
Thermal current threshold
correction factor (Coef)
1
0.95
0.90
0.85
0.80
0.75
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A typical setting of the thermal protection is :
−
Thermal overload current threshold [Iθ >] : between 105% et 108% (max.) of
the motor rated current (this threshold is typically equivalent to the full load
current).
NOTE:
The nominal current : is the current value for which the moteur
supplies his maximum efficiency.
The Full Load current : is the limit value of the thermal current
value of the motor with the time under its continuous duty rating
(This term is used in North of America)
−
Negative sequence current recognition factor : [Ke] = 3
−
Heating time-constants (Te1), during the start-up (Te2) and the cooling-down
time constants (Tr):
The manufacturer should be consulted for the heating and cooling time
constants.
⇒ Te1 must be set to be equal to, or even slightly lower than the motor
manufacturer’s value ( Stator thermal heating).
⇒ Te2 must be typically set to be lower than or equal to Te1. It is used to
modify the thermal curve of the motor during the start phase . In case of
a SOFT start, (Yye/Delta) for example, the current absorbed by the
motor after the start phase is 57% of the current controlled by the relay
(Delta connection) while durning the start phase ( Yye connection), the
current absorbed by the motor is equal to the current monitored by the
relay. For that, Te2 is used to reduce the operating time during the start
up. For application with Direct-on-line start up, adjust Te2=Te1, which
results in one thermal curve.
⇒ It is important to plot the thermal characteristics chosen to assure that
the “COLD” curve has no intersection area with the start up
charactersistics. In certain applications, the time constants could not be
available. However, a graphical presentation of these values could be
given. In this case, Te1 should be selected so once it is plotted, it will
match the cold motor curve.
⇒ For applications where neither constant time values nor thermal curves
are given, Te1 and Te2 should be chosen in such a way that they fall
above the start up characteristics but below the motor locked Rotor
current threshold. In this way, the thermal overload protection assure to
a certain degree the protection under locked rotor conditions.
⇒ The cooling-down time-constant Tr should ideally be set slightly higher
than the value provided by the manufacturer.
This element is important with motors having differents functionning
cycles because the precise information of the motor thermal state is
needed during heating and cooling phases. Il is usually a multiple of
Te1.
REMARK: IF HOWEVER THE MANUFACTURER’S DATA ARE NOT KNOWN, ONE SHOULD SET
THE FOLLOWING VALUES: TE1 = TE2 = 14MIN AND TR = 28MIN.
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•
2.1.2
P220/EN AP/B43
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Alarm threshold θALARM: Its setting is primarily related to the motor operation
modes and the concept of protection. A typical adjustment consists of setting the
threshold θALARM to be slightly higher than the ratio (Irated motor / Iθ>)2 , which
generally corresponds to a value of about 90%.
Short Circuit.[50/51]
A phase to phase short-circuit at the terminals of the motor or in the feeder cables,
draws very large currents capable of damaging the motor and its feeder cable This
also poses the threat of fire within the motor room.
In this case ,it is essential to detect the fault and to send the tripping command
rapidly to the breaking device. To attain these objectives, the P220 relay is provided
with an overcurrent element operating on fundamental component, with a settable
definite time delay.
The current threshold must be set as low as possible, without tripping due to
−
the start-up current of the motor
−
the contribution of the motor to an external fault as well as
−
the re-acceleration current due to voltage drops.
In order to achieve this, the direct on-line start-up current must always be taken into
account in the calculation of the setting even if the motor started under reduced
voltage (soft start). Thus the short-circuit current threshold must be set higher than the
direct on-line start-up current value.
Taking into account aperiodic current components, the typical settings are:
−
[I>>] = 130% x kstart x Inmotor
and
[tI>>] = 100ms
−
[I>>] = 180% x kstart x Inmotor
and
[tI>>] = 0 ms
where kstart : start-up current of the motor in per unit.
It should then be checked that the threshold [I>>] is lower than :
−
90% of the limiting saturation current of the CTs used, and
−
1/3 of the minimum three-phase fault current at the motor terminals.
IMPORTANT:
2.1.3
IF A FUSED CONTACTOR IS USED TO CONTROL THE MOTOR , THE SHORT
CIRCUIT PROTECTION MUST NOT TRIP THE CONTACTOR. THE SHORT
CIRCUIT PROTECTION MUST BE DISABLED AND THE FUSE SHOULD
INTERRUPT THE FAULT CURRENT. IF THE CONTACTOR IS ALLOWED TO
INTERRUPT FAULT CURRENT, SERIOUS DAMAGE COULD BE CAUSED DUE
TO EXCESSIVE ARCING AT THE CONTACTS.
Earth fault [50N/51N]
Overheating of the stator windings is likely to lead to insulation deterioration. Since
the windings are surrounded by an earthed metal case, stator faults usually manifest
themselves as earth faults.
To protect against this, the P220 relay is provided with two independent earth fault
overcurrent elements with settable definite time delays. This function reacts only to the
fundamental component of the earth fault current, and thus remains insensitive to the
disturbances of the higher-order harmonics (equal to or higher than 2).
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The earth fault protection function may be provided either by residual connection of
the 3 phase current transformers (CTs), or by the use of a core-balance current
transformer.
It is preferable to use a core balance current transformer as this is more stable and is
more sensitive. If residually connected CTs are used, the tripping setting would have
to be increased by as much as 10 % higher than the rated current of the CT. This is
highly undesirable because of the resulting increase in the earth fault current setting.
Incorrect tripping can result from the saturation of one or more CTs during motor
starting. Increased stability can be achieved in two ways :
•
increasing the current threshold,
•
insertion of a stabilising resistance in series with the P220 relay.
The value of stabilising resistor can be found from the following equation.
Rstab > (Id / Is) * (RCT + 2*Rf + RRE),
where:
Id
=
start-up current magnitude brought to the secondary
Is
=
earth fault setting in Amps (threshold Io> or Io>>)
RCT
=
dc resistance of CT secondary windings.
Rf
=
resistance of single lead from CT to relay
RRE
=
other resistances connected in series to the CT (relays etc.)
The following earthing systems may be employed.
2.1.3.1 Neutral earthed through an impedance
The earth fault current is mainly comprising active current component resulting from
the resistance of neutral point, the capacitive zero sequence (residual) contribution
from the cables being of much lower value, even negligible.
Typical settings are :
•
•
in the case of a residual connection to three phase CTs:
−
[Io>>] is higher than 10% of the CT rated primary current, and
−
2 times higher than the capacitive residual current resulting from the motor
feeder cables in case of external fault, and
−
lower than the residual current resulting from the resistance of neutral point,
and
−
[t Io>>] = 100 ms
in the case of a core balance transformer.
−
[Io>>] is 2 times higher than the capacitive residual current resulting from
the motor feeder cables in case of external fault, and
−
lower than the residual current resulting from the resistance of neutral point,
−
[tIo>>] = 100 ms
Note that current settings lower than 1 Amps are usually not applied.
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2.1.3.2 Insulated neutral :
A core balanced transformer is used as the fault current is due to the cable capacitive
leakage current.
A single earth fault will not cause the relay to trip but the fault should be localised .
Typical settings are :
−
[ Io>> ] is 2 times higher than the capacitive residual current resulting from
the motor feeder cables in case of external fault, and
−
lower than the capacitive residual current resulting from the other cables,
−
[tIo>>] = 100 ms.
Note that current settings lower than 1 Amps are usually not applied.
If these settings are not compatible with the maximum value of the earth fault current,
it is then necessary to use a directional earth fault relay.
2.1.3.3
Solidly earthed neutral
The earth fault current is mainly inductive current, with magnitude being close to that
of the three-phase short-circuit fault currents. The contribution of capacitive residual
current from the cables is negligible.
Typical settings are :
•
2.1.4
in the case of a residual connection to three phase CTs:
−
[Io>>] is higher than 10% of the CT rated primary current, and
−
[tIo>>] = 100 ms
Unbalance [46]
Under normal motor running conditions only positive sequence current components
flow. The presence of a negative sequence component produces a field revolving in
an opposite direction to that of the rotor. It induces rotor winding currents at double
the supply network frequency. The skin effect in the rotor winding bars at this
frequency can cause a significant increase in the resistance of the rotor. The rotor will
overheat leading to deformation of the rotor bars and damage to them. This imposes
additional heating of the stator that is in excess of the manufacturers rating.
Even if the thermal protection provided by this relay takes into account negative
sequence component of the current, it will not account for the additional heating due
to high unbalance rate. In the event of the motor losing one phase of its supply,
considerable overheating would occur, hence protection for negative sequence is
employed separately
In order to provide this function, the P220 relay is equipped with two independent
negative sequence overcurrent elements. The first one, denoted by [Ii>], is an alarm
threshold associated with an adjustable constant time. The second, denoted [Ii>>], is
a threshold of tripping associated with a inverse time characteristic curve. The
features of this curve are described in chapter 5.3 of this technical guide.
The equation of this curve is :
for 0.2 < (I2/In) < 2 --> t = 1.2 / (I2/ In)
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This type of curve has the following advantages:
1.
For an external fault:
•
to desensitise the relay during a violent unbalance fault occurring upstream or on
external feeders, when the motor temporarily behaves like a negative current
generator ; selective tripping at the faulty feeder level is secured - the inverse time
characteristic curve allows co-ordination with the faulty feeder protection relay.
•
to avoid nuisance tripping which may occur due to high starting currents causing
the CTs to saturate.
2.
•
For a motor fault.
To ensure rapid fault interruption, but to retain co-ordination with protective fuses
when fused contactors are used.
It should be noted that the single-phase and two-phase faults also generate negative
currents. However, the value of the single-phase fault current is generally limited, and
in any case these faults are eliminated by relevant protection with a time shorter than
that afforded by the IDMT curve.
Typical settings are
2.1.5
•
alarm threshold : [Ii>] = 15% of the motor rated current, with a delay time of
about 8 to 10s,
•
tripping threshold : [Ii>>] = 20% of the motor rated current.
Excessive long start [48]
The start-up current is specific to each motor and depends on the start-up method
used (direct on-line, autotransformer, rotor resistance insertion, etc.). As for the startup time, it is dependent of the load connected to the motor.
During the start-up period, this current surge imposes a thermal strain on the rotor.
This is exaggerated as the rotor will have lost all of its ventilation because it does not
rotate at the full speed. Consequently, a long start-up causes a rapid heating of the
motor. For this reason, this protection is complementary to the thermal overload
protection, and makes it possible to check that the start-up sequence does not exceed
the parameters given by the manufacturer
The MiCOM P220 relay offers the choice of motor start-up detection as follows :
•
closure of the contactor/circuit breaker, or
•
closure of the contactor/circuit breaker and overshoot of the starting current
threshold [Istart].
The user can configure either option using the CONFIGURATION menu. Method 1 is
recommended. This detects the start sequence on the circuit breaker closure.
The function " Excessive long start " is initiated either by the detection of a start-up
sequence, or (under normal operation) by the detection of a phase of re-acceleration.
If at the end of delay time [tIstart] the current remains higher than the threshold [Istart],
then a trip takes place.
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Typical settings are :
•
[Istart] is equal to:
⇒ 1.5*[Iθ>] if the motor start-up current is lower than 4 times the rated
current;
⇒ 2*[Iθ>] if the motor start-up current is equal to or higher than 4 times the
rated current and lower than 8 times the rated current;
⇒ 3*[Iθ>] if the motor start-up current is equal to or higher than 8 times the
rated current.
•
2.1.6
[tIstart] = 120 % of the time of start-up and shorter than withstand time for the
motor.
Locked rotor [51LR/50S]
There are two possible conditions for the rotor becoming locked : at motor start-up or
during normal run. Whatever the case, a locked rotor produces an input current
equivalent to the direct on-line starting current.
The most frequent cause of a locked rotor is to a phase break (eg: melting of a fuse
protecting the motor, or one pole of a contactor remaining open.). A stationary motor
can not start and remains stationary with two phases feeding the stator. In the same
way, a locked rotor can take place after the loss of a phase after the motor has been
working normally. The appearance and the importance of a locked rotor depend on
the motor load at the time when the loss of phase occurs. In both cases the result is
likely to be a thermal overloading of the rotor windings.
Under healthy conditions, a revolving flux is induced in the rotor, which generates
balanced rotor current in the windings which produce symmetrical rotor heating. In
the event of the loss of one phase of the supply, a heterogeneous flux is induced in
the rotor as a result of the positive component and the negative components of the
current. This causes uneven heating of the rotor windings which depend on the
position of the rotor bars. This can lead to the damage of the rotor bars. For these
reasons, it is important to eliminate the fault as quickly as possible.
2.1.6.1 Locked rotor during the start-up stage [50S]
This function is enabled only during the motor start-up stage. In order to take
advantage of this function, the motor has to be equipped with a tachometric control,
which indicates if the motor turns. This information is carried to a digital input of the
relay so that the relay can detect whether the motor’s speed is or is not zero. A locked
rotor is detected if, after expiration of delay time [tIstall], the digital input indicates zero
speed (logic 0).
Motors for which the real start-up time is shorter than their locked rotor withstand
time can be protected against locked rotor condition at start-up without the help of a
tachymetric control device (speed switch). For such cases, the use of [tIstart] time setting
(refer to « [48] Excessive long start » function) shorter than the motor locked rotor
withstand time allows to provide efficient protection against both too long start-up
sequence and locked rotor at start-up conditions.
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2.1.6.2 Rotor stalled during normal run [51LR]
This function is valid only outside the re-acceleration and start-up stages. Tripping
takes place if the current remains higher than [Istall] for a time period equal to or
higher than delay time [stall].
Typical settings of the function [ 51LR/50S ] are :
•
[Istall] :
⇒ 1.5*[Iθ>] if the motor start-up current is lower than 4 times the rated
current;
⇒ 2*[Iθ>] if the motor start-up current is equal to or higher than 4 times the
rated current and lower than 8 times the rated current;
⇒ 3[Iθ>] if the motor start-up current is equal to or higher than 8 times the
rated current.
•
2.1.7
[tIstall] is 1 to 2 s for a pump and a fan, and 5 to 10 s for a crusher. In all the
cases, this setting must be lower than the withstand time for the motor with the
rotor stalled.
Loss of load [37]
This function makes it possible to detect the motor running without a load connected
on the output shaft. It is automatically disabled when the motor is off, and it is
reactivated after the inhibit time has expired [Tinhib]. This delay time [Tinhib] allows the
motor to perform an off-load start.
The use of this undercurrent protection function allows:
•
protection against the electrical pumps becoming unprimed.
•
protection against a drive belt or drive shaft breakdown.
Typical settings are:
2.1.8
•
[I<] = higher than the no-load running current of the motor and lower than the
normal running current of the motor in normal operation,
•
[Tinhib] = This setting depends on the load connected to the motor. If the motor is
started on load , this delay time is set to its minimal value, that is to say 0,05 s. If
the motor is idle-started, this delay time is set to be slightly longer than the load
increase time of the motor.
•
[tI<] = depends on the load driven by the motor (often set to a few seconds).
RTD probe [49/38] and Thermistor [49]
Prolonged overloads make the windings hot, which can cause a premature ageing of
the insulation. In the same way, the excessive heating of the bearings can lead to
irreversible damage.
As explained in the paragraph dealing with the thermal overload protection [ 49],
this function can be realised either indirectly, by the means of an overcurrent relay or
using the thermal replica, and/or by the means of direct temperature measurement.
Certain motors can be equipped with resistor probes, placed in the stator slots. They
are resistors made out of platinum, or sometimes out of nickel or copper, their
resistance varying with the temperature. There are generally six of them, distributed in
the stator winding.
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The setting of the tripping and alarm thresholds depends on the temperature class of
the motor, the ambient temperature and the altitude of the site where the motor is
installed.
When the correction of the thermal replica by the measurement of the motor outside
temperature is used, RTD1 probe should be placed near the cooling air inlet of the
motor.
2.2
AUTOMAT. CTRL Menu
2.2.1
Limitation of the number of start-ups during a given period of time [66]
The start-up of a motor is often carried out at the price of an increase in temperature
above the normal level – especially in the case of several successive start-ups having
relatively long start-up times. In this situation there exists the danger of premature
ageing of the motor insulation and, furthermore, the rotor undergoes high thermal
strains. For more precise details, see the paragraphs relating to the excessive start-up
times functions and locked rotor.
In order to limit the start-up repetition frequency for a motor, the P220 relay has a
counting and locking system based on four following parameters:
−
duration of the reference period;
−
the number of cold starts;
−
the number of hot starts;
−
the restart prohibition delay time [Tinterdiction].
The reference delay time is activated once a start-up is detected, and provided that it
was initially equal to zero for the reference period, a counter records the number of
hot starts, and another one records the number of cold starts. If one of them reaches
the upper limit threshold programmed by the user, the delay time [Tinterdiction] is
initiated but the start-up inhibition signal will be activated only at the moment when
the motor stops the next time. As long as this delay time has not expired, any start-up
is inhibited.
It should be noted that a start-up is considered as cold start if the motor’s thermal
status is lower than 50%, and that a start-up is described as hot start if the thermal
state is equal to or higher than 50%.
The recommended settings have to be compared to the motor characteristics
provided by the manufacturer. Nevertheless, the programming of these parameters
can also depend on the operation mode of the set “motor and/motor-driven unit” as
a whole.
If these data are not available, the default settings are as follows:
−
duration of the reference period = 60min;
−
the number of cold starts = 3;
−
the number of hot starts = 2;
−
the delay time of prohibition of restart-up [Tinterdiction] = 30min
It should be noted that this function does not make it possible to limit the repetition
frequency for any two successive sequences of start-ups as long as [Tinterdiction] has not
been initiated. This limitation is ensured by the complementary function " Time
between two successive start-ups " (see the corresponding paragraph).
P220/EN AP/B43
Technical Guide
Application Guide
MiCOM P220
Page 22/40
2.2.2
Time between two successive start-ups [66]
This function is complementary to that limiting the number of successive starts during
a given period of time. It makes it possible to prevent two consecutive motor starts .
This may be a limitation of the motor or of the motor starting equipment.
The setting of the delay time [Tbetween 2 start] should be based on the minimum time
which is required between 2 start-ups.
2.2.3
Re-acceleration
A fault in an installation, or a fault close to the sources of supply may cause high
voltage drops which will result in supply voltages lower than the minimal permissible
levels. For example, a three-phase fault in a point of the network produces voltage
dip in the equipment in the neighbourhood. This voltage dip could result in
difficulties, which do not necessarily disappear with elimination of the fault and the
ultimate return to a normal voltage.
As the voltage dip appears, the motor torque, which is roughly proportional to the
square of the voltage, undergoes a brutal reduction causing the deceleration of the
motor. This deceleration is a function of the amplitude and the duration of the
voltage dip. It is mainly governed by the moment of inertia of the rotating masses
and by the torque-speed characteristic of the motor-driven motor.
In the most unfavourable case, the motor can stall, the new torque that it develops
being lower than the braking torque of the motor-driven unit. This phenomenon is
illustrated on the following figure.
Torque
Cm1
Cm0 = Cr0
Cr1
Cm
Cm
Cm0
N1
Nn Ns
Speed of rotation
P0172ENa
The curves shown above represent, respectively:
•
motor torque Cm versus rotation speed N and corresponding to the rated voltage
Vn;
•
motor torque C'm versus rotation speed N and corresponding to a voltage V
lower than Vn;
•
braking torque Cr of the motor-driven unit, versus rotation speed.
When the voltage dip appears, the motor torque passes abruptly from the value
Cm0 = Cr0 to the value C'm0 < Cr0. Therefore, the motor-drive unit will slow down,
and when the voltage is restored, the motor torque abruptly increases to the value
Cm1, whereas the braking torque is of value Cr1. Hence the motor cannot accelerate
and would return to its normal speed only if Cm1 is higher than Cr1 (see the figure
above).
Technical Guide
Application Guide
MiCOM P220
P220/EN AP/B43
Page 23/40
After the fault has cleared, the motor has a value of internal impedance close to the
corresponding value when it is stopped. So, when the system voltage is reestablished, the motor draws a current close to its start-up current at the full voltage.
This current is higher if the motor slip becomes high.
This stage of re-acceleration does not always involve serious consequences, except if
a number of large motors are re-powered on simultaneously. In this case, these
motors can result in large voltage dip further restricting the re-acceleration of the
motors. So, it may be necessary to carry out load shedding of a certain number of
motors, in order to be able to ensure the re-acceleration of the priority motors.
The parameters of MiCOM relay can either be set so as to authorise a re-acceleration
of the motor after a voltage dip, or they can also be set to give a command to stop
the motor in the event of prolonged voltage dip.
2.2.3.1 Authorisation of re-acceleration
An external voltage relay connected next to busbars is used to report on any voltage
dip as well as to indicate any restoration of the voltage. This “voltage dip”
information is sent via a wiring link to a logic input of the MiCOM P220 relay
programmed at «VOLT. DIP»
Treacc delay time should be set to be equal to the maximum duration of voltage dip
of the network for which one wishes to authorise a re-acceleration of the motor. Thus,
for any voltage dip shorter than Treacc delay time, an authorisation of re-acceleration
will be activated. On the other hand, if the voltage dip lasts longer than Treacc delay
time, the relay does not modify its operation and any attempt of re-acceleration of the
motor could be seen by the relay as a “Rotor locked” condition (the amplitude of the
re-acceleration current being the same as that in “Rotor locked” condition) – and,
consequently, causing a possible tripping command.
2.2.3.2 Load shedding on voltage dip
The same voltage relay can be used to realise load shedding when the supply voltage
dips. Two cases are possible:
−
When it is desired to turn off the motor in the event of the voltage dip,
−
When it is desired to turn off the motor only if the voltage dip lasts longer than
the value it was assigned in the re-acceleration authorisation.
A programmed logic input on EXT. 1 (or EXT. 2) should to be connected to the
voltage relay detecting the presence of a voltage dip. Associated delay time tEXT1 (or
tEXT2) will be set as follows:
−
equal to the duration of voltage dip for which one wishes to carry out a load
shedding,
−
equal to Treacc.
The external tripping command EXT. 1 (or EXT. 2) will be programmed to send the
shutdown command (assignment on the output relay RL1).
Thus any voltage dip longer than the programmed duration will result in a shutdown
command.
P220/EN AP/B43
Technical Guide
Application Guide
MiCOM P220
Page 24/40
NOTE:
⇒ The determination of the maximum duration for which one wishes
to authorise re-acceleration of the motor is specific to each site and
it depends on the characteristics of the network (source impedance,
impedance of the other loads - in particular, presence of other
revolving machines) as well as of the characteristics of the given
motor and its load (magnitude of the direct-on-line start-up current,
inertia). The value of this duration is generally obtained as a result
of a study of the dynamic stability of the system.
⇒ The «voltage dip» information generated by the voltage relay must
exist as long as the conditions of voltage dip exist. With the return
of the voltage on the busbars, this «voltage dip» information must
disappear as soon as possible. The voltage relay used to generate
«voltage dip» information must have very short pick-up time and
drop off time, ideally less than one and half periods (times lower
than 30 ms for a 50 Hz system).
⇒ This assumes that the value of the voltage is the same on the
busbars as on the terminals of the motor. If this is not the case, it
will be necessary to estimate the voltage drop between the busbars
and the motor’s terminals and to take this into account when setting
the voltage relay thresholds corresponding to appearance and
disappearance of the of «voltage dip» conditions.
Technical Guide
Application Guide
MiCOM P220
P220/EN AP/B43
Page 25/40
3.
EXAMPLE OF NUMERICAL APPLICATION
3.1
Network data
•
MiCOM P220 current input ranges:
− Phase current input : In = 5 A ;
− Earth current input : Ion = 1 A
•
Breaking device type : circuit breaker
•
Maximum value of the three-phase short-circuit current on the busbars 6.2 kV:
Icc3φ = 9 kA
•
Neutral point connection mode for the network 6.2 kV : by a resistance limiting
the maximum value of the earth fault current to 30 A
•
Length of the feeder cable connecting the motor to the busbars 6.2k V : 100 m
•
Measuring transformers :
•
3.2
−
CT ratio: 300 / 5A
−
Core balance CT connection: ratio 25
"Speed switch" device available
Motor data:
Induction motor:
–
rated power
==> Pn = 2200kW - cosϕ = 0.8
–
rated voltage
==>
Vn = 6.2kV – 50 Hz
–
rated current
==>
In = 256 A
–
open-circuit (no-load) current
==>
Ino-load = 134A
–
start-up type (direct-on -line, soft)
==>
Direct
–
start-up current
==>
---
–
direct start-up current (if soft start used)
==>
Id = 5.4*In i.e. 1382A
–
start-up time
==>
td = 4 s
–
maximum repetition frequency of starts
---->
Hot = …x 2, cold = …x 3
–
withstand time for locked rotor (for hot & cold start)
---->
2s
–
heating curve
==>
---
–
time-constants of: heating , start-up, cooling- down ==>
–
transient characteristic curve at unbalance
---->
---
–
permanent allowable unbalance
---->
---
–
motor service use (driven equipment: compressor,
crasher, mill, pump, fan...)
==>
pump
–
start-up : no-load/ load
---->
no-load
(30 s loading after the end of
start-up)
14min, 10min, 28min
P220/EN AP/B43
Technical Guide
Application Guide
MiCOM P220
Page 26/40
3.3
List of settings:
OP PARAMETERS Menu
Password
AAAA
Reference
ALST
Frequency
50 Hz
CONFIGURATION Menu
CONFIG.SELECT Submenu
Change Group Input
EDGE
Setting Group
1
Default display
% I LOAD
Start detection criterion
52A (closing of the breaking
device)
Analogue output type (optional)
4 - 20 mA
Value transmitted by the analogue output (optional)
% I LOAD
RTD type (optional)
PT100
CT RATIO Submenu
Primary rating of the phase CT
300
Secondary rating of the phase CT
5
Primary rating of the earth CT
25
Secondary rating of the earth CT
1
LED 5, LED 6, LED 7 and LED 8 Submenus
LED 5
LED 6
LED 7
LED 8
Assignment: thermal tripping (overload)
No
No
Yes
No
Assignment: thermal alarm θALARM
No
No
No
Yes
Assignment: tI>>
Yes
No
No
No
Assignment: tIo>
Yes
No
No
No
Assignment: tIo>>
Yes
No
No
No
Assignment: tIi>
Yes
No
No
No
Assignment: tIi>>
Yes
No
No
No
Assignment: tI<
Yes
No
No
No
Assignment: tIstart (excessively long start)
No
Yes
No
No
Assignment: tIstall (stalled rotor when running)
No
Yes
No
No
Assignment: locked rotor at start
No
Yes
No
No
Assignment: emergency restart
No
No
No
No
Assignment: forbidden start
No
No
No
Yes
Assignment: tRTD1 ALARM, tRTD2 ALARM, tRTD3 ALARM
(optional)
No
No
No
Yes
Assignment: tRTD1TRIP, tRTD2 TRIP, tRTD3 TRIP (optional)
No
No
Yes
No
Technical Guide
Application Guide
MiCOM P220
P220/EN AP/B43
Page 27/40
LED 5, LED 6, LED 7 and LED 8 Submenus
LED 5
LED 6
LED 7
LED 8
Assignment: tRTD4 ALARM, tRTD5 ALARM, tRTD6 ALARM
(optional)
No
No
No
Yes
Assignment: tRTD4 TRIP, tRTD5 TRIP, tRTD6 TRIP (optional)
No
No
Yes
No
Assignment: Thermist 1 and Thermist 2 (optional)
No
No
No
No
Assignment: tEXT1
No
No
No
No
Assignment: tEXT2
No
No
No
No
Assignment: motor stopped
No
No
No
No
Assignment: motor running
No
No
No
No
Assignment: successful start
No
No
No
No
Configuration Inputs submenu
Inputs : 54321
11111
COMMUNICATION MODBUS Menu
Communication enabled ?
Yes
Data transmission rate
19 200 Bauds
Parity
No
Number of data bits
8
Number of stop bits
1
Relay address
1
Date Format
PRIVATE
Programming for Group No.1 of PROTECTION Menu:
THERM.OVERLOAD[49]
Submenu
Primary
setting
Secondary
Comments
setting
Thermal overload function
enabled ?
Yes
Thermal inhibition on start
enabled ?
No
Threshold Iθ>
270 A
0.9In (CT)
5.5% of overload authorised
= 1.055 x In(motor)
Ke
3
Te1
14 min
See motor characteristics
Te2
10 min
See motor characteristics
Tr
28 min
See motor characteristics
Influence RTD (optional)
No
θALARM enabled?
Yes
Thermal alarm threshold
θALARM
92%
θ FORBID START enabled ?
Yes
FORBID START
78%
0,92 > (256 / 270)2
0,78 < (1382 / 270)2 * (1-exp(4
/ 10*60)) +exp(5 / 10*60))
P220/EN AP/B43
Technical Guide
Application Guide
MiCOM P220
Page 28/40
Submenu [50/51]
SHORT-CIRCUIT
Primary
setting
Short-circuit function enabled ?
Threshold I>>
Yes
1800A
tI>>
Submenu [50/51]
EARTH FAULT
Secondary
Comments
setting
6In
0.1s
Primary
setting
Secondary
Comments
setting
Earth fault function enabled ?:
threshold Io>
No
Thresholds Io>
0.002Ion
tIo>
0s
Earth fault function enabled ?:
threshold Io>>
Yes
Threshold Io>>
2A
tIo>>
Submenu [46]
UNBALANCE
0.08Ion
Only one earth fault current
threshold can be programmed
Setting to 6,7 % of maximum
earth fault current
0.1s
Primary
setting
Function Unbalance enabled ?:
threshold Ii>
Threshold Ii>
130% of motor start-up current
25.6 A
Secondary
Comments
setting
Yes
Alarm threshold enabled
0.085 In
(CT)
Setting to 10% of Inmotor
tli>
10s
Function Unbalance enabled ?:
threshold Ii>>
Yes
Tripping threshold enabled
Setting to 20% of Inmotor
Threshold Ii>>
51.2A
0.171 In
(CT)
Submenu [48]
EXCESS LONG START
Primary
setting
Secondary
Comments
setting
Excess long start function
enabled ?
Threshold Istart
tIstart
Yes
540A
2Iθ
Id = 5.4*Inmotor ! Istart = 2*Iθ
5s
1.2 * td = 4.8s
Technical Guide
Application Guide
MiCOM P220
Submenu [51LR/50S]
BLOCK ROTOR
P220/EN AP/B43
Page 29/40
Primary
setting
Secondary
Comments
setting
Block rotor function enabled ?
Yes
tIstall
1.8s
Stalled-in-run rotor function
enabled ?
Yes
Threshold Istall
540A
Blocked-at-start rotor function
enabled ?
Submenu [37]
LOSS OF LOAD
Primary
setting
Loss of load function enabled ?
See motor characteristics
2Iθ
Id= 5.4*Inmotor ! Istall = 2*Iθ
Yes
Presence of zero speed detector is
necessary (speed switch)
Secondary
Comments
setting
Yes
Motor-driven pump
0.55In
Higher than no-load current
tI<
3s
Depends on process
Tinhib
40s
Tinhib > (5 + 30)
Threshold I<
Submenu [66]
START NUMBER
165A
Primary
setting
Secondary
Comments
setting
Start number limitation function
enabled ?
Yes
Treference
60 min
Hot starts number
2
Cold starts number
3
Tinterdiction
30 min
Submenu MIN TIME
BETW 2 START
Primary
setting
Secondary
Comments
setting
Time between starts function
enabled ?
Yes
Tbetween 2 start
10min
Submenu RE-ACCEL
AUTHORIZ
Primary
setting
Parameters depend on trade-off
of motor characteristics against
process requirements
Parameter depends on trade-off
of motor characteristics against
process requirements
Secondary
Comments
setting
Re-acceleration authorisation
function enabled ?
Yes
Treacc
0.2s
Parameter depends on trade-off
of motor characteristics against
process requirements