Siemens SIPROTEC 7SJ62 Instruction Manual page 242

Functions
k–Factor
6-102
The short time delays associated with fault protection elements do not allow sufficient
time for the orderly curtailment of load by operating personnel. In addition, fault pro-
tection elements set to trip for overload will not allow short-duration, non-damaging
overloads – a practice which is often required in real operating situations.
The 7SJ62 protective relay features an thermal overload protective function with a
thermal tripping characteristic curve which may be adapted to the overload tolerance
of the equipment being protected.
Thermal overload protection may be switched 21 or 2)) or $ODUP 2QO\ at address
 )&7  . If switched
The nominal device current is used as a basis for overload detection. The program-
mable .²)$&725 (set at address ) is calculated as the ratio of the thermally-per-
missible continuous current I
The thermally-permissible continuous current for the equipment being protected is
known from manufacturer's specifications. The thermal overload function is normally
not applicable to aerial lines since the actual current capability of aerial lines is depen-
dent on factors that are generally unknown (e.g. wind speed, ambient temperature,
etc.). For cables, the permissible continuous current is dependent on the cross-sec-
tion, insulating material, design, and the cable routing, among other things. It may be
taken from pertinent tables, or is specified by the cable manufacturer. If no specifica-
tions are available, a value of 1.1 times the nominal current rating is assumed.
When determining the nominal current rating of the protected equipment, it is impor-
tant to relate this current to the nominal current rating of the device. For example, the
maximum permissible continuous current for a motor and the nominal current rating of
the motor are given in primary amperes.
For the k factor to be set based on the nominal device current, the following equation
must be used:
Set the .²)$&725
where I Maximum permissible continuous motor current in primary
max prim
amperes
Nominal motor current in primary amperes
I
N Motor
I Current transformer primary nominal current
N CT prim
Example: Motor and current transformer with the following data:
Permissible Continuous Current:
Motor Nominal Current
Current Transformer Ratio
Set Value K-FACTOR
21
, tripping is also possible.
to the nominal device current I
max
I
max
k ---------- -
=
I
N
I
max prim
-------------------- -
k =
I
NMotor
I = 1.2 · I
max prim
I = 1100 A
N Motor
1200 A/1 A
1100 A
⋅
------------------ -
1.2 1.1
= =
1200 A
:
N
I
NMotor
⋅
----------------------- -
I
N CT prim
N
7SJ62 Manual
C53000-G1140-C121-1