Siemens siprotec 7SA522 User Manual

2 Functions
300
7SA522 Manual
C53000-G1176-C155-3
Figure 2-121 Simplified function diagram of circuit breaker failure protection controlled by
circuit breaker auxiliary contact
Current Flow Moni-
toring
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 plausibil-
ity check (see Figure 2-122).
The earth current (residual current I
E
(3 · I
0
) is preferably used as plausibility current.
If the residual current from the star-point of the current transformer set is connected to
the device, then 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 7SA522 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.

Siemens siprotec 7SA522 Specifications

General

Brand	Siemens
Model	siprotec 7SA522
Category	Protection Device
Language	English