Alarm and tripping logic
The modules generate alarm, restart inhibit and tripping signal.
When the thermal level exceeds the set value of the
Alarm thermal value
setting,
the ALARM output is activated. Sometimes a condition arises when it becomes
necessary to inhibit the restarting of a motor, for example in case of some extreme
starting condition like long starting time. If the thermal content exceeds the set
value of the
Restart thermal Val
setting, the BLK_RESTART output is activated.
The time for the next possible motor start-up is available through the monitored
data view from T_ENARESTART. The T_ENARESTART estimates the time for the
BLK_RESTART deactivation considering as if the motor is stopped.
When the emergency start signal START_EMERG is set high, the thermal level is set
to a value below the
Restart thermal Val
. This allows at least one motor start-up,
even though the thermal level has exceeded the
Restart thermal Val
.
When the thermal content reaches 100 percent, the OPERATE output is activated.
The OPERATE output is deactivated when the value of the measured current falls
below 12 percent of
Current reference
or the thermal content drops below 100
percent.
The activation of the BLOCK input blocks the ALARM, BLK_RESTART and OPERATE
outputs.
Thermal level calculator
The module calculates the thermal load based on the rotor resistances factors, true
RMS and negative sequence current. The rotor heating is determined by the squared
value of the load current.
However, in case of unbalanced phase currents, the negative-sequence current
also causes additional heating. By deploying a protection based on both current
components, abnormal heating of rotor is avoided.
The equations used for the thermal level calculations are:
=
+
∙ (
∙
)
2
+
−
∙ (
2
∙
)
2
∙ 1 −
−
∙ 100%
(
(
)
)
(Equation 45)
=
+
∙ (
∙
)
2
+
−
∙ (
2
∙
)
2
∙ 1 −
−
∙ % (
(
)
)
(Equation 46)
θA is the thermal level during overload,
θB is the thermal level during nominal conditions,
R
r+
is the rotor positive sequence resistance factor,
R
r-
is the rotor negative sequence resistance factor,
I
is the True RMS value of the measured max of phase currents,
I
IFLC
is the calculated internal FLC,
I
2
is the measured negative sequence current,
Table continues on the next page
1MRS759142 F Protection functions
REX640
Technical Manual
425
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