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 element. The current warning element may report an overload current prematurely, even if the calcu-
lated operating temperature has not yet attained the warning or tripping levels.
Extension of the Time Constants
When using the device to protect motors, the varying thermal response associated with stationary or rotating
machine can be correctly evaluated. When running down or stationary, a motor without external cooling
looses heat more slowly, and a longer thermal time constant must be used for calculation. For a motor that is
switched off, the 7SC80 increases the time constant τ
th
by a programmable factor (kτ factor). The motor is
considered switched off when the motor currents drop below a configurable minimum current setting
BkrClosed I MIN (refer to "Current Flow Monitoring" in Section 2.1.3 Power System Data 1). For cables or
transformers, the Kτ-FACTOR is 1.
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 tripping times thereby perhaps affecting time grading of the
short circuit protection, the thermal replica is frozen (kept constant) as soon as the current exceeds the
threshold value 1107 I MOTOR START.
Blocking
The thermal memory can be reset via a binary input (
>RES 49 Image
) and the current-related overtempera-
ture value is thus 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 element.
When motors must be started for emergency reasons, temperatures above the maximum permissible over-
temperatures can be allowed by blocking the trip signal via a binary input (
>EmergencyStart
). Since the
calculated operating temperature may be higher than the maximum allowable operating temperature after
drop out of the binary input, the thermal overload protection function features a programmable run-on time
interval (T EMERGENCY) which is started when the binary input drops out. Tripping by the thermal overload
protection will be defeated until this time interval elapses. The binary input affects only the trip command.
There is no effect on the trip log nor does the thermal replica reset.
Functions
2.11 Thermal Overload Protection 49
220 SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
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