Siemens SIPROTEC 7UT613 series Manual page 163

7UT613/63x Manual
C53000-G1176-C160-2
2.4 Time Overcurrent Protection for Phase and Residual Currents
Example:
Transformer used in the infeed of a bus supply with the following data:
Transformer
Current Transformer
The time overcurrent protection is assigned to the 110 kV side (= feeding side).
The maximum possible three-phase fault current on the 20 kV side, assuming an im-
pressed voltage source on the 110 kV side, is:
Assuming a safety margin of 20 %, the following primary setting value results:
Setting value I>> = 1.2 · 1224.7 A = 1470 A
For settings with secondary values the currents will be converted for the secondary
side of the current transformers.
Secondary setting value:
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:
With the same safety factor results:
Setting value I>> = 0.8 · II
Increased inrush currents, if their fundamental oscillation exceeds the setting value,
are rendered harmless by delay times (address 2013T 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 pro-
tection 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 over-
grades 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>>:
YNd5
35 MVA
110 kV/20 kV
u = 15 %
sc
200 A / 5 A on the 110 kV side
(rated current of the side).
NS
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