Siemens SIPROTEC 4 7UT6 Series Manual page 148

Functions
2.4 Time Overcurrent Protection for Phase and Residual Currents
[ueb-sekeinstellwert-021026-rei, 1, en_GB]
i.e. for fault currents higher than 1470 A (primary) or 36.7 A (secondary) the fault is in all likelihood located in
the transformer zone. This fault may be cleared immediately by the overcurrent protection.
When setting in per-unit values, the rated current of the protected object (here equal to the rated current of
the side) is cancelled. Thus the formula gives:
[dreipolig-max-ins-030603-st, 1, en_GB]
With the same safety factor results:
Setting value I>> = 8 · Ι
Increased inrush currents, if their fundamental oscillation exceeds the setting value, are rendered harmless by
delay times (address 2013 T I>>). The inrush restraint does not apply to the stages I>>.
Using the principle of the "Reverse Interlocking" the multi-stage function of the time overcurrent protection
offers its advantages: Stage I>> is used as a fast busbar protection with a short safety delay T I>> (e.g. 50
ms). Stage I>> is blocked for faults at the outgoing feeders. Stages I> or Ip serve as backup protection. The
pickup values of both elements (I> or Ip and I>>) are set equal. Delay time T I> or T Ip (IEC characteristic)
or D Ip (ANSI characteristic) is set in such manner that it overgrades the delay for the outgoing feeders.
If fault protection for motors is applied, it has to be ensured that the setting value I>> is smaller than the
smallest (two-pole) fault current and higher than the highest startup current. Since the maximum appearing
startup current is usually below 1.6x the rated startup current (event in unfavourable conditions), the
following setting is adequate for fault current stages I>>:
1.6 · Ι < I>> < Ι
startup
The potential increase in starting current caused by overvoltage conditions is already accounted for by the 1.6
factor. The I>> stage can trip instantaneously (T I>> = 0.00 s), since there is no saturation of shunt reac-
tance for motors, other than for transformers.
The set time T I>> is an additional delay time and does not include the operating time (measuring time,
etc.). The delay can be set to ∞. If set to infinity, the pickup of this function will be indicated but the stage will
not trip after pickup. If a pickup threshold is set to , neither a pickup annunciation nor a trip is generated.
Definite Overcurrent Stages Ι>
The setting of the I> stage (address 2014 or 2015) is mainly determined by the maximum operating current.
A pickup caused by an overload must be excluded, as the device operates in this mode as fault protection with
correspondingly short tripping times and not as overload protection. For lines or busbars a rate of approx. 20
% above the maximum expected (over)load is set, for transformers and motors a rate of approx. 40 %.
The time delays to be set (address 2016 T I>) are derived from the coordination chart of the network.
The set times are purely additional time delays that do not include the operating time (measuring time, etc.).
The delay can be set to ∞. If set to infinity, the pickup of the corresponding function will be signaled but the
stage will not issue a trip command. If a pickup threshold is set to ∞, neither a pickup indication or a trip will
be triggered.
Overcurrent Stages Ι with IEC characteristics
p
The inverse time stages, depending on the configuration ("Functional Scope", address 120 (see Section
2.1.3.1 Setting
With the IEC characteristics (address 120 DMT/IDMT Phase = TOC IEC) the following options are available
at address 2026 ANSI CURVE:
•
Normal Inverse (inverse, type A according to IEC 60255-3),
•
Very Inverse (very inverse, type B according to IEC 60255-3),
148
(Seitennennstrom).
NS
sc 2pol
Notes), enables the user to select different characteristics.
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016