Siemens siprotec SJ62 User Manual page 178

Coolant Tempera-
ture (Ambient Tem-
perature)
Current Limiting
Extension of Time
Constants
Blocking
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
old, the protected equipment may be disconnected from the system. The highest over-
temperature calculated from the three phase currents is used as the criterion.
The maximum thermally-permissible continuous current I
of the object nominal current I
I = k · I
max Nom Obj.
In addition to the k factor (parameter 49 K-FACTOR), the TIME CONSTANT τ
the alarm temperature 49 Θ ALARM (in percent of the trip temperature Θ
be specified.
Overload protection also features a current warning element (I ALARM) in addition to
the temperature warning stage. The current warning element may report an overload
current prematurely, even if the calculated operating temperature has not yet attained
the warning or tripping levels.
The device can account for external temperatures. Depending on the type of applica-
tion, this may be a coolant or ambient temperature. The temperature can be measured
via a temperature detection unit (RTD-box). For this purpose, the required tempera-
ture detector is connected to detector input 1 of the first RTD-box (corresponds to RTD
1). If incorrect temperature values are measured or there are disturbances between
the RTD-box and the device, an alarm will be issued and the standard temperature of
Θ
= 104° F or 40° C is used for calculation with the ambient temperature detection
u
simply being ignored.
When detecting the coolant temperature, the maximum permissible current I
fluenced by the temperature difference of the coolant (in comparison with the standard
value = 104° F or 40° C). If the ambient or coolant temperature is low, the protected
object can support a higher current than it does when the temperature is high.
In order to ensure that overload protection, on occurrence of high fault currents (and
with small time constants), does not result in extremely short trip times thereby
perhaps affecting time grading of the short circuit protection, the thermal model is
frozen (kept constant) as soon as the current exceeds the threshold value
1107
I MOTOR START.
When using the device to protect motors, the varying thermal response at standstill or
during rotation may be correctly evaluated. When running down or at standstill, a
motor without external cooling looses heat more slowly, and a longer thermal time con-
stant must be used for calculation. For a motor that is switched off, the 7SJ62/63/64
increases the time constant τ
sidered to be off when the motor currents drop below a programmable minimum
current setting BkrClosed I MIN (refer to "Current Flow Monitoring" in Section
2.1.3). For externally-cooled motors, cables and transformers, the Kτ-FACTOR = 1.
The thermal memory may be reset via a binary input („>RES 49 Image"). The
current-related overtemperature value is reset to zero. The same is accomplished via
the binary input („>BLOCK 49 O/L"); in this case the entire overload protection is
blocked completely, including the current warning stage.
When motors must be started for emergency reasons, temperatures above the
maximum permissible overtemperature can be allowed by blocking the tripping signal
via a binary input („>EmergencyStart"). Since the thermal profile may have ex-
ceeded the tripping temperature after initiation and drop out of the binary input has
taken place, the protection function features a programmable run-on time interval (T
2.10 Thermal Overload Protection 49
:
Nom Obj.
by a programmable factor (kτ factor). The motor is con-
th
is described as a multiple
max
th
) must
TRIP
max
and
is in-
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