Siemens siprotec 7SD5 User Manual

2.20 Circuit Breaker Failure Protection
359
7SD5 Manual
C53000-G1176-C169-1
Figure 2-144 Simplified function diagram of circuit breaker failure protection controlled by
circuit breaker auxiliary contact
Current Flow
Monitoring
Each of the phase currents and an additional plausibility current (see below) are fil-
tered by numerical filter algorithms so that only the fundamental component is used
for further evaluation.
Special measures are taken in order to detect a current interruption. In case of sinu-
soidal currents the current interruption is detected after approximately 10 ms. With
aperiodic DC current components in the fault current and/or in the current transformer
secondary circuit after interruption (e.g. current transformers with linearized core), or
saturation of the current transformers caused by the DC component in the fault cur-
rent, it can take one AC cycle before the interruption of the primary current is reliably
detected.
The currents are monitored and compared with the set threshold. Besides the three
phase currents, two further current detectors are provided in order to allow a
plausibility check (see Figure 2-145).
As plausibility current, the earth current (residual current I
E
(3·I
0
) is preferably used. If
the residual current from the starpoint of the current transformer set is connected to
the device it is used. If the residual current is not available, the device calculates it with
the formula:
3· I
0
= I
L1
+ I
L2
+ I
L3
Additionally, the value calculated by 7SD5 of three times the negative sequence
current 3·I
2
is used for plausibility check. This is calculated according to the equation:
3·I
2
= I
L1
+ a
2
·I
L2
+ a·I
L3
where
a
= e
j120°.
These plausibility currents do not have any direct influence on the basic functionality
of the breaker failure protection but they allow a plausibility check in that at least two
current thresholds must have been exceeded before any of the breaker failure delay
times can be started, thus providing high security against false operation.
www . ElectricalPartManuals . com

Siemens siprotec 7SD5 Specifications

General

Brand	Siemens
Model	siprotec 7SD5
Category	Relays
Language	English