Siemens siprotec 7SA6, Manual

Page: 457 / 876

Functions
6-267
7SA6 Manual
C53000-G1176-C156-2
Example:
Belted cable 10 kV 150 mm
2
Permissible continuous current
I
max
= 322 A
Current transformer 400 A/5 A
Setting value
K-FACTOR
=
0.80
Time Constant
τ
The thermal time constant
τ
th
is set under the address
4203 TIME CONSTANT
. This
is also provided by the manufacturer. Please note that the time constant must be 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
Alarm Stages By setting a thermal alarm stage
Θ
ALARM (address
4204
) an alarm can be output
before the tripping temperature is reached, so that a trip can be avoided by early load
reduction or by switching over. The percentage is referred to the tripping temperature.
The current overload alarm setpoint
I ALARM
(address
4205
) is stated as a factor of
the rated 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.
Calculating the
Overtemperature The thermal replica is calculated individually for each phase. Address
4406 CALC.
METHOD
decides whether the highest of the three calculated temperatures (
Θ max
) or
their arithmetic average (
Average Θ
) or the temperature calculated from the phase
with maximum current (
Θ @ Imax
) should be decisive for the thermal alarm and trip-
ping stage.
k
322 A
400 A
----------------
0.805 = =
τ
th
min
----------
1
60
------
perm. 1-s current
perm. contin. current
-------------------------------------------------------------
è ø
æ ö
2
⋅
=
τ
th
min
----------
0.5
60
--------
perm. 0.5–s current
perm. contin. current
-------------------------------------------------------------
è ø
æ ö
2
⋅
=
τ
th
min
----------
0.6 t
6
⋅
=
τ
th
min
----------
1
60
------
13 500 A
322 A
-----------------------
è ø
æ ö
2
1
60
------
42
2
29 4
,
=
⋅
=
⋅
=