Siemens SIPROTEC 4 7UT6 Series Manual page 93

[diff-grundprinzip4enden-020926-rei, 1, en_GB]
Figure 2-18
[diff-grundprinzip-3-wickltrans-1ph-020926-st, 1, en_GB]
Figure 2-19
Current Restraint
When an external fault causes a heavy current to flow through the protected zone, differences in the magnetic
characteristics of the current transformers CT1 and CT2
cause a significant current flow through the measuring element M. If it is greater than the respective pickup
threshold, the device can trip even though no fault occurred in the protected zone. Current restraint (stabilisa-
tion) prevents such erroneous operation.
In differential protection systems for protected objects with two terminals, a restraining quantity is normally
derived from the current difference |Ι
the relevant ranges of the stabilisation characteristics. For protected objects with more than two ends, such as
multi-winding transformers, busbars etc, only the arithmetical sum method is possible. The latter method is
used in 7UT6x for all protected objects. The following definitions apply for 2 measuring points:
a tripping or differential current
= |Ι + Ι |
Ι
diff 1 2
and the stabilisation or restraining current
= |Ι | + |Ι |
Ι
stab 1 2
The current sum definition is extended for more than 2 measurement locations, e.g. for 4 measuring locations
(Figure 2-18 or
= |Ι + Ι + Ι
Ι
diff 1 2 3
= |Ι | + |Ι | + |Ι
Ι
stab 1 2
is derived from the fundamental frequency current and produces the tripping effect quantity, Ι
Ι
diff
acts this effect.
To clarify the situation, three important operating conditions with ideal and matched measurement quantities
are considered.
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
Basic principle of differential protection for four ends (single-phase illustration)
Basic principle of differential protection for 4 measuring locations — example of a three-
winding power transformer with 4 measuring locations (single-phase illustration)
– Ι | or from the arithmetical sum |Ι
1 2
Figure 2-19), therefore:
+ Ι |
4
| + |Ι |
3 4
(Figure 2-17) under conditions of saturation may
| + |Ι |. Both methods are equal in
1 2
Functions
2.2 Differential Protection
counter-
stab
93