Siemens siprotec SJ62, Руководство пользователя

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2.10 Thermal Overload Protection 49
180
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
with
I
max prim
Permissible thermal primary current of the motor
I
Nom Obj.
Nominal current of the protected object
I
Nom CT prim
Nominal primary CT current
Example: Motor and transformer with the following data:
Time Constant
τ
The overload protection tracks overtemperature progression, employing a thermal dif-
ferential equation whose steady state solution is an exponential function. The
TIME
CONSTANT τ
th
(set at address
4203
) is used in the calculation to determine the thresh-
old of overtemperature and thus, the tripping temperature.
For cable protection, the heat-gain time constant
τ
is determined by cable specifica-
tions and by the cable environment. If no time-constant specification is available, it
may be determined from the short-term load capability of the cable. The 1-sec current,
i.e. the maximum current permissible for a one-second period of time, is often known
or available from tables. Then, the time constant may be calculated with the formula:
If the short-term load capability is given for an interval other than one sec, the corre-
sponding short-term current is used in the above formula instead of the 1-sec current,
and the result is multiplied by the given duration. For example, if the 0.5-second
current rating is known:
It is important to note, however, that the longer the effective duration, the less accurate
the result.
Permissible Continuous Current I
max prim
= 1.2 · I
Nom Obj.
Nominal Motor Current
I
Nom Obj.
= 1100 A
Current Transformer 1200 A / 1 A