Siemens SIPROTEC 7UT613 series Manual page 186

2 Functions
Overcurrent Stage
I
with ANSI Char-
Ep
acteristics
186
If the inverse time trip characteristic is selected, it must be noted that a safety factor
of about 1.1 has already been included between the pickup value and the setting
value. This means that a pickup will only occur if a current of about 1.1 times of the
setting value is present.
Using the overcurrent stageIEp (address 2421) earth faults can also be detected with
weak fault currents. Since the starpoint current originates from one single current
transformer, it is not affected by summation effects evoked by different current trans-
former errors like, for example, the zero sequence current derived from phase cur-
rents. Therefore, this address can be set to very sensitive. Consider that the inrush
restraint function cannot operate below 10 % nominal current (lower limit of harmonic
filtering). An adequate time delay could be reasonable for very sensitive setting if
inrush restraint is used.
Since this stage also picks up with earth faults in the network, the time multiplier (ad-
dress 2422 T IEp) has to be coordinated with the grading coordination chart of the
network for earth faults. In most cases, shorter tripping times than for phase currents
may be set since a galvanic separation of the zero sequence systems of the connect-
ed power system sections is ensured by a transformer with separate windings.
The time multiplication factor may also be set to ∞. If set to infinity, the pickup of this
function will be indicated but the stage will not trip after pickup. If the IEp–stage is not
required, select address 124 DMT/IDMT Earth = Definite Time when configuring
the protection functions.
If under address 2424 TOC DROP-OUT the Disk Emulation are set, dropout is pro-
duced in accordance with the dropout characteristic, as set out in the functional de-
scription of the inverse time overcurrent protection under margin heading „Dropout Be-
haviour".
The inverse time stage, depending on the configuration, enables the user to select dif-
ferent characteristics. With the ANSI characteristics (address 124 DMT/IDMT Earth
= TOC ANSI) the following is made available in address 2426 ANSI CURVE:
• Definite Inv.,
• Extremely Inv.,
• Inverse,
• Long Inverse,
• Moderately Inv.,
• Short Inverse, and
• Very Inverse.
The characteristics and equations they are based on are listed in the „Technical Data".
If the inverse time trip characteristic is selected, it must be noted that a safety factor
of about 1.1 has already been included between the pickup value and the setting
value. This means that a pickup will only occur if a current of about 1.1 times the
setting value is present.
Using the time overcurrent stage IEp (address 2421) earth faults can also be detect-
ed with weak fault currents. Since the starpoint current originates from one single
current transformer, it is not affected by summation effects evoked by different current
transformer errors like, for example, the zero sequence current derived from phase
currents. Therefore, this address can be set to very sensitive. Consider that the inrush
restraint function cannot operate below 10 % nominal current (lower limit of harmonic
filtering). An adequate time delay could be reasonable for very sensitive setting if
inrush restraint is used.
7UT613/63x Manual
C53000-G1176-C160-2