•
Internal short circuit, feed only from one side:
Then I
2
= 0
I
diff
= |I
1
+ I
2
| = |I
1
+ 0| = |I
1
|
I
rest
= Max (|
I
1
| ; |I
2
|) = |I
1
|
The tripping quantity (I
diff
) and the restraining quantity (I
rest
) are of the same size and correspond to the
short-circuit current.
Additional Stabilization with External Short Circuits
In case of an external short circuit, high short-circuit currents flowing through can lead to current-transformer
saturation. This saturation can be more or less strongly pronounced at the measuring points, and in this way
simulate a differential current. The differential current can lie in the tripping range of the characteristic curve
for a certain time and lead to unwanted tripping without any particular measures. In accordance with
Figure 7-46, there are 2 typical scenarios:
•
High-current, external short circuit
First, the current on both sides is transferred and leads to a large restraint current. After this, a trans-
former becomes saturated, resulting in a differential current I
diff
that can exceed the operate curve.
Simultaneously, the restraint current I
rest
drops. Once the transformer is no longer saturated, the differen-
tial current decreases and falls below the characteristic curve.
•
Low-current, external short circuit
Current transformers can also become saturated through the large direct-current time constant
(switching of transformers, motors), but small currents flowing through. This manifests itself as a phase-
angle rotation of the current. Exceeding the characteristic curve in a non-stabilized area is possible.
Intelligent saturation-detection methods capture this state and lead to a temporally limited blocking of the
Differential protection function.
[dwislauf-170712-01.tif, 1, en_US]
Figure 7-46 Principle Current Course in External Short Circuit and Temporary Current-Transformer Satura-
tion on One Side
Inrush-Current Detection
The inrush-current detection evaluates the calculated instantaneous values of the differential current. The
inrush current develops with Sympathetic Inrush – connection of a parallel transformer or Recovery Inrush –
inrush current subsequent to returning voltage after fault in the system considerably higher than in the side
currents. An assignment to the corresponding blocking phases is possible only in the differential current due
to the vector-group correction.
The inrush-current detection is activated when the pickup characteristic is exceeded. It is deactivated when it
is blocked by an external fault.
Capacitor Bank Protection
7.8 Differential Protection for Capacitor Banks
SIPROTEC 5, Overcurrent Protection, Manual 1045
C53000-G5040-C017-8, Edition 07.2017
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