Rockwell Automation 865 Differential Protection Relay - Manual (Page 68)

3-24 Protection
Functions
865-UM001A-EN-P – July 2009
k
Q
I
MAX70
I
MAX40
Q
AMB
(°C)
AmbientTemperatureCompensation
0.6
10 20 30 40 50 60 70 80
0.8
1.0
1.2
Figure 3.14 – Ambient Temperature Correction of the Overload Stage T>
Example of a Behavior of the Thermal Model
Figure 3.15 shows an example of the thermal model behavior. In
this example
τ = 30 minutes, k = 1.06 and kτ = 1 and the current has
been zero for a long time and thus the initial temperature rise is 0 %.
At time = 50 minutes the current changes to 0.85xI
N and the tempera-
ture rise starts to approach value (0.85/1.06)
2
= 64 % according the
time constant. At time=300 min, the temperature is about stable, and
the current increases to 5 % over the maximum defined by the rated
current and the service factor k. The temperature rise starts to
approach value 110 %. At about 340 minutes the temperature rise is
100 % and a trip follows.
Initial Temperature Rise After Restart
When the relay is switched on, an initial temperature rise of
70 % is used. Depending of the actual current, the calculated
temperature rise then starts to approach the final value.
Alarm Function
The thermal overload stage is provided with a separately settable
alarm function. When the alarm limit is reached the stage activates
its start signal.