Siemens SIPROTEC 4 7UT6 Series Manual page 201

In steady-state operation the solution of this equation is in an e-function whose asymptote represents the final
temperature Θ
End
below the overtemperature, a 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 protection 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 calculated temperature rise of the three phases is decisive for evaluation of
the thresholds.
The maximum permissible continuous thermal overload current Ι
current Ι :
NObj
= k · Ι
Ι
max NObj
is the rated current of the assigned side of the protected object:
Ι
NObj
•
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 transformer 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
entered as settings of the protection.
Apart from the thermal alarm stage, the overload protection also includes a current overload alarm stage Ι
which may give an early warning that an overload current is imminent, even when the temperature rise has
not yet reached the alarm or trip temperature rise values.
The overload protection can be blocked via a binary input. In doing so, the thermal images are also reset to
zero.
Standstill Time Constant in Machines
The differential equation mentioned above assumes a constant cooling represented by the thermal time
constant τ = R
th
ventilated machine time constant differs substantially from that during operation due to the missing ventila-
tion.
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 multiplied 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 Section
Motor Startup
On startup of electrical machines the overtemperature calculated by the thermal replica may exceed the alarm
overtemperature or even the trip overtemperature. In order to avoid an alarm or trip, the starting current is
acquired and the resulting increase of temperature rise is suppressed. This means that the calculated tempera-
ture rise is kept constant as long as the starting current is detected.
Emergency Starting of Machines
When machines must be started for emergency reasons, operating temperature above the maximum permis-
sible operating temperature can be allowed by blocking the tripping signal via a binary input (
O/L ). After startup and dropout of the binary input, the thermal replica may still be greater than the trip
temperature rise. Therefore the thermal replica features a settable run-on time (T EMERGENCY) which is
started when the binary input drops out. It also suppresses the trip command. Tripping by the overload protec-
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
. When the overtemperature reaches the first settable temperature threshold Θ
· C (thermal resistance × thermal capacity). However, the thermal time constant of a self-
th th
is described as a multiple of the nominal
max
as well as the alarm temperature Θ
th
2.1.5 Setting
Functions
2.9 Thermal Overload Protection
, which is
alarm
must be
alarm
alarm
Groups).
>Emer.Start
201
,