[foschwe3-170712-01.tif, 1, en_US]
So that the function is responsive, select a setting value of 0.08 I/Irated,S.
•
Recommended setting value (_:103) Threshold = 0.08 I/Irated,S
As gradient, the following results:
[fosteig1-170712-01.tif, 1, en_US]
•
Recommended setting value (_:105) Slope = 0.03
Because of the grounding resistance R
E
reduced protection range results. This can be estimated as follows: The
grounding resistance is much larger than the winding reactance, thus the following maximum ground current
results:
I
Gnd max
= 20 kV/(√3 · 12 Ω) = 962 A
You can determine the minimum tripping current from the setting value:
I
Gnd min
= side rated current * threshold value = 4042 A · 0.08 = 323 A
Thus, the following protection range results:
[foschwe4-170712-01.tif, 1, en_US]
The attainable protection range amounts to approx. 66.4 %. In order to capture ground faults near the neutral
point, it is also recommended to connect a sensitive ground current protection to the neutral point trans-
former. You must coordinate its operate time with the line protection.
NOTE
Ensure that the protection range further limits itself with larger neutral-point resistances, or the restricted
ground-fault protection can no longer be used.
Protection with Neutral Reactors on the Delta Side
To produce ground currents on the delta side of a transformer, neutral reactors (zigzag reactors) are used. A
resistance for ground current limitation can also be present. In this case, the resistance is much greater than
the reactor and delta reactance of the transformer. The following figure shows a typical implementation. The
restricted ground-fault protection works between output-sided phase current transformers and the current
transformer in the neutral point of the neutral reactor.
In the example, the fault current curve with the ground fault in a delta winding is also represented. The fault
location in which the lowest fault current flows can also be seen. This current influences the protection setting
set close.
Protection and Automation Functions
6.43 Restricted Ground-Fault Protection
876 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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