5 SETTINGS
The IEC255-8 cold curve is defined as follows:
The IEC255-8 hot curve is defined as follows:
In the above equations,
•
t
= time to operate.
op
•
= thermal protection trip time constant.
τ
op
•
I = measured overload RMS current.
•
I
= measured load RMS current before overload occurs.
p
•
k= IEC 255-8 k-factor applied to I
•
I
= protected element base (nominal) current.
B
The reset time of the thermal overload protection element is also time delayed using following formula:
In the above equation,
•
τ
= thermal protection trip time constant.
rst
•
T
is a minimum reset time setting
min
GE Multilin
2
I
------------------------- -
t
ln
=
op
op
2
I
kI
–
B
2
2
I
I
–
p
------------------------- -
t
ln
=
op
op
2
I
kI
–
B
, defining maximum permissible current above nominal current.
B
2
kI
B
---------------------------- -
t
ln
=
rst
rst
2
2
I
kI
–
B
D30 Line Distance Protection System
5.7 CONTROL ELEMENTS
2
2
T
+
min
(EQ 5.21)
(EQ 5.22)
(EQ 5.23)
5
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