Siemens 7SJ85 - Page 864

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Figure 6-265 Basic Principle of the Function
In accordance with the logic diagram, Figure 6-264 the protection function consists of 3 parts:
Effect of Pickup Value
The differential current and the restraint current are calculated from the residual currents. The reference
arrows are defined as positive when pointing to the protected object (see Figure 6-265). Consider that the
ground current in the fault record and in the DIGSI 5 Test Suite are displayed as positive if the current flows to
ground from the protected object.
i
i
NOTE
The following calculation applies to the configurations with a current transformer for the side. In a special
case of several current transformers per side, the calculation of the restraint current must be performed in
greater detail.
I
0
* = k
m
· 3I
0
'
I
0
** = k
m
· 3I
0
'' → 3I
0
'' = I
A
+ I
B
+ I
C
with k
m
= I
N,transformer
/I
N,side
I
Diff,REF
= |
I
0
* + I
0
**|
I
Rest,REF
= |I
0
*| + |I
A
| + |I
B
| + |I
C
|
where:
3
I
0
' Measured zero-sequence current at neutral point
3I
0
'' Zero-sequence current calculated from the phase currents
k
m
Factor for magnitude adaptation
I
rated,transformer
Primary transformer rated current
I
rated,side
Primary rated current of the transformer side
I
Diff,REF
Differential current
I
Rest,REF
Restraint current
Using the calculated restraint current, a current I
char. curve
which represents the pickup value for the tripping is
determined from the characteristic curve (Figure 6-266). In this way, the protection function is stabilized in
the event of external, multiphase ground faults, for example, a 2-pole ground fault. This means that the
protection function becomes less sensitive.
If the Slope = 0 is set here, the set Threshold of the operate curve is delivered independent of the restraint
current.
Protection and Automation Functions
6.43 Restricted Ground-Fault Protection
864 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017