Functions
2.9 Thermal Overload Protection 49
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
147
When the calculated overtemperature reaches the first settable threshold 49 Θ ALARM, an alarm annunciation
is issued, e.g. to allow time for the load reduction measures to take place. When the calculated overtempera-
ture reaches the second threshold, the protected equipment may be disconnected from the system. The
highest overtemperature 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
element. The current warning element may report an overload current prematurely, even if the calculated op-
erating temperature has not yet attained the warning or tripping levels.
Coolant Temperature (ambient temperature)
The device can take external temperatures into account. Depending on the type of application, this may be a
coolant or ambient temperature. The temperature can be measured via a temperature gauge or the I/O2 ex-
tension module. For this purpose, the required temperature sensor is connected to sensor input 1 of the first
RTD box (corresponds to RTD 1) or to the I/O2 extension module. Ambient temperature detection is, however,
disabled if there are communication problems between the sensor and the RTD box or between the RTD box
and the protection device or if incorrect temperature values are measured. In these cases, an error message
will be generated and the standard temperature of Θ
u
= 40 °C is used instead for the calculation.
When measuring the coolant temperature, the maximum permissible current I
max
is influenced by the temper-
ature difference of the coolant to the standard value of 104°F or 40°C. Because if the ambient or coolant tem-
perature is low, the protected object can sustain a higher current than when the temperature is high.
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 constant must be used for calculation. For a motor that is switched off, the
7SK80 increases the time constant τ
th
by a programmable factor (kτ factor). The motor is considered 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.
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 pro-
tection, the thermal replica is frozen (kept constant) as soon as the current exceeds the threshold value 1107
I MOTOR START.
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