Siemens 7SJ85 - Page 866

The following applies for the restraint current:
I
Rest,REF
= |I
gnd,A
| + |I
gnd,B
| + |I
gnd,C
| + |I
A
| + |I
B
| + |I
C
|
i
i
NOTE
If both 1-phase neutral point and 3-phase ground side are connected, only the 1-phase neutral point is
used by the restricted ground-fault protection. The 3-phase ground side is not considered.
Pickup
If the I
Diff,REF
calculated differential current exceeds the calculated current I
char.
(see Figure 6-264), pickup
occurs and the internal processing is enabled. The pickup is indicated.
Operate Curve
The operate curve represented in the following figure consists of 2 parts. In the right part of the characteristic
curve, you will find a case of an internal ground fault. Under ideal conditions, the angle between the two
residual currents (∠(
I
0
*, I
0
**)) equals 0. With current-transformer saturation, angle errors can result. The right
part of the characteristic curve is valid for angles ≤ 90°. The tripping current is the current flowing at the
neutral point (
I
0
*). It is compared to the set or increased threshold value.
[dwausken-170712-01.tif, 1, en_US]
Figure 6-268 Operate Curve Depending on the Phase Angle between l
0
* and I
0
** at |I
0
*| = |I
0
**| (180° =
External Fault)
In case of an external ground fault, the zero-sequence current calculated from the phase currents reverses by
180°. The phase angle between the residual currents thus (∠(I
0
*, I
0
**)) equals 180°. They are located in the
left part of the operate curve and recognize a clearly increased pickup value. Angle deviations of 180° are
caused by transformer saturation in conjunction with external short-circuits.
The current I
Angle,REF
for the Angle Decision is determined from the following subtraction and summation:
I
Angle,REF
= |
I
0
* - I
0
**| - |I
0
* + I
0
**|
The resulting current I
Angle,REF
results from the respective fault conditions, which are illustrated in the following
figure. With an internal fault (angle = 0°), a current results that has a negative sign. If angle errors occur, the
sign remains negative. The amount of the current decreases.
Protection and Automation Functions
6.43 Restricted Ground-Fault Protection
866 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017