Siemens 7SJ85 - Page 1044

calculation of the difference is determined through the current-direction definition. The current direction is
defined as positive to the protected object. The current difference results from the vector addition of the
currents.
[dwprin2s-170712-01.tif, 1, en_US]
Figure 7-45 Basic Principle of Differential Protection Using the Example of 2 Sides
For safe operation, additional functions are required. The implementation of these additional functions is
described in chapter I-DIFF Differential Protection Function, Page 1049.
Current Stabilization
If very large currents flow through the protection range in the case of external faults, with different transmis-
sion behavior of the transformers T1 and T2 (Figure 7-45), a corresponding differential current I
diff
occurs.
When this is larger than the corresponding pickup threshold, the device can issue a trip command, even
though no fault is present in the protection range. Such a malfunction of the protection is avoided by the
restraint current (I
rest
) and is implemented in the form of a characteristic curve I
diff
= f(I
rest
).
The largest current of 2 or more measuring points of the protected object is used for stabilization.
The definition for 2 measuring points is:
A trip or differential current is
I
diff
= (
I
1
+ I
2
)
as well as a restraint current of
I
rest
= Max (|I
1
| ; |I
2
|)
For more than 2 measuring points, this definition is expanded. This restraint method allows a reliable trip deci-
sion for more than 2 measuring points and different feeding behavior for internal short circuits.
I
rest
= Max (|
I
1
| ; |I
2
| ; ... |I
n
|)
To clarify the effect, 3 important operating states are observed with ideal and adapted measurands.
•
Through fault current in healthy operation or externally caused error:
I
1
flows into the protection range, I
2
flows out of the protection range, that is, negative compared with
the prefix definition, thus I
2
= -I
1
;
in addition |I
2
| = |I
1
|
I
diff
= |
I
1
+ I
2
| = |I
1
- I
1
| = 0
I
rest
= Max (|I
1
| ; |I
2
|) = |I
1
| = |I
2
|
No tripping quantity (I
diff
= 0); the stabilization (I
rest
) corresponds to the flowing current.
•
Internal short circuit, feed from both sides with equal amount of currents, for example:
Then I
2
= I
1
; In addition |I
2
| = |I
1
|;
I
diff
= |I
1
+ I
2
| = |I
1
+ I
1
| = 2 · |I
1
|
I
rest
= Max (|
I
1
| ; |I
2
|) = |I
1
| = |I
2
|
The tripping quantity (I
diff
) is twice as large as the short-circuit current and the restraining quantity (I
rest
).
Capacitor Bank Protection
7.8 Differential Protection for Capacitor Banks
1044 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017