Functions
2.7 Thermal Overload Protection 49
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
114
Example:
Belted cable 10 kV 150 mm
2
Permissible continuous current
I
max
= 322 A
Current transformers 400 A / 5 A
Setting value
49 K-FACTOR
=
0.80
Time Constant
The thermal time constant
τ
th
is set at address
4203
TIME CONSTANT
. This is also provided by the manufac-
turer. Please note that the time constant is set in minutes. Quite often other values for determining the time
constant are stated which can be converted into the time constant as follows:
1-s current
permissible current for application time other than 1°s, e.g. for 0.5°s
t
6
–time; this is the time in seconds for which a current of 6 times the rated current of the protected object may
flow
Example:
Cable as above with
Permissible 1-s current 13.5 kA
Setting value
TIME CONSTANT
=
29.4 min
Warning Temperature Level
By setting a thermal alarm stage
49
Θ
ALARM
(address
4204
) an alarm can be provided before the tripping
temperature is reached, so that a trip can be avoided by preventive load reduction or by switching over. The
percentage is referred to the tripping temperature rise.
The current overload alarm setpoint
I ALARM
(address
4205
) is stated as a factor of the nominal device current
and should be set equal to or slightly below the permissible continuous current k ·
I
N
. It can also be used instead
of the thermal alarm stage. In this case the thermal alarm stage is set to 100 % and thus practically ineffective.
