Siemens 7SR242 Duobias, Техническое руководство

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Chapter 1) 7SR242 Duobias Description Of Operation
©2015 Siemens Protection Devices Limited Chapter 1 Page 38 of 60
3.9 Current Protection: Thermal Overload (49)
Optionally a phase segregated thermal overload element is provided, this can be selected to either winding 1 or
winding 2. The thermal state is calculated using the measured True RMS current.
Should the current rise above the 49 Overload Setting for a defined time an output signal will be initiated.
Operate Time (t):-
2
B
2
2
P
2
I k I
I I
t
ln
Where
T = Time in minutes
= 49 Time Constant setting (minutes)
In = Log Natural
I = measured current
I
P
= Previous steady state current level
k = Constant
I
B
= Basic current, typically the same as In
k.I
B
= 49 Overload Setting (I )
Additionally, an alarm can be given if the thermal state of the system exceeds a specified percentage of the
protected equipment’s thermal capacity
49 Capacity Alarm setting.
For the heating curve:
100% ) e (1
I
I
t
2
2
Where: = thermal state at time t
I = measured thermal current
I = 49 Overload setting (or k.I
B
)
The final steady state thermal condition can be predicted for any steady state value of input current where t > ,
100%
I
I
2
2
F
Where:
F
= final thermal state before disconnection of device
49 Overload Setting I is expressed as a multiple of the relay nominal current and is equivalent to the factor k.I
B
as defined in the IEC255-8 thermal operating characteristics. It is the value of current above which 100% of
thermal capacity will be reached after a period of time and it is therefore normally set slightly above the full load
current of the protected device.
The thermal state may be reset from the fascia, a user logic reset or externally via a binary input.
Thermal overload protection can be inhibited from:
Inhibit 49 A binary or virtual input.
User Inhibit Reylogic (graphical logic).