Siemens SIPROTEC 7UT613 series Manual page 221

Standstill Time
Constant in Ma-
chines
7UT613/63x Manual
C53000-G1176-C160-2
The protection function thus represents a thermal profile of the equipment being pro-
tected (overload protection with memory capability). Both the previous history of an
overload and the heat loss to the environment are taken into account.
In steady-state operation the solution of this equation is in an e-function whose asymp-
tote represents the final temperature Θ
first settable temperature threshold Θ
warning alarm is given in order to allow a preventive load reduction. When the second
temperature threshold, i.e. the final temperature rise or tripping temperature, is
reached, the protected object is disconnected from the network. The overload protec-
tion can, however, also be set to Alarm Only. In this case only an indication is issued
when the final temperature is reached. For setting block. Relay allows to operate the
protection but the trip output relay is blocked.
The temperature rises are calculated separately for each phase in a thermal replica
from the square of the respective phase current. This guarantees a true RMS value
measurement and also includes the effect of harmonic content. The maximum calcu-
lated temperature rise of the three phases is decisive for evaluation of the thresholds.
The maximum permissible continuous thermal overload current I
multiple of the nominal current I
I = k · I
max N Obj
I
is the rated current of the assigned side of the protected object:
N Obj
• For power transformers, the rated power of the assigned winding is decisive. The
device calculates this rated current from the rated apparent power of the transform-
er and the rated voltage of the assigned winding. For transformers with tap changer,
the non-regulated side must be used.
• For generators, motors, or reactors, the rated object current is calculated by the
device from the set rated apparent power and the rated voltage.
• For short lines, branchpoints or busbars, the rated current of the protected object is
directly set.
In addition to the k-factor, the thermal time constant τ
ture Θ
must be entered as settings of the protection.
warn
Apart from the thermal alarm stage, the overload protection also includes a current
overload alarm stage I
warn
is imminent, even when the temperature rise has not yet reached the alarm or trip tem-
perature rise values.
The overload protection can be blocked via a binary input. In doing so, the thermal
images are also reset to zero.
The differential equation mentioned above assumes a constant cooling represented
by the thermal time constant τ
However, the thermal time constant of a self-ventilated machine during standstill
differs substantially from that during operation due to the missing ventilation.
Thus, in this case, two time constants exist. This must be considered in the thermal
replica. The time constant for cooling is derived from the thermal time constant multi-
plied by the a factor (usually > 1).
Stand-still of the machine is assumed when the current drops below the threshold
PoleOpenCurr.S1, PoleOpenCurr.S2 to PoleOpenCurr.S5 (the minimum
current of the feeding side below which the protected object is assumed to be switched
off, refer also to Subsection 2.1.5).
. When the overtemperature reaches the
End
, which is below the overtemperature, a
alarm
:
N Obj
, which may give an early warning that an overload current
(thermal resistance × thermal capacity).
= R · C
th th th
2.9 Thermal Overload Protection
is described as a
max
as well as the alarm tempera-
th
221