Siemens siprotec SJ62, Руководство пользователя

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2.10 Thermal Overload Protection 49
176
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
max
is described as a multiple
of the object nominal current I
Nom Obj.
:
I
max
= k · I
Nom Obj.
In addition to the k factor (parameter
49 K-FACTOR
), the
TIME CONSTANT τ
th
and
the alarm temperature
49 Θ ALARM
(in percent of the trip temperature
Θ
TRIP
) must
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.
Coolant Tempera-
ture (Ambient Tem-
perature) 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
u
= 104
°
F or 40
°
C is used for calculation with the ambient temperature detection
simply being ignored.
When detecting the coolant temperature, the maximum permissible current I
max
is in-
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.
Current Limiting 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
.
Extension of Time
Constants 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
th
by a programmable factor (k
τ
factor). The motor is con-
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
K τ -FACTOR
=
1
.
Blocking 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