Siemens siprotec 7SA522 User Manual page 171

Current
Transformer
Saturation
Circuit Breaker
Status
7SA522 Manual
C53000-G1176-C119-2
I
E
--------
=
I
EP
The 7SA522 contains a saturation detector which largely eliminates the measuring er-
rors resulting from the saturation of the current transformers. The threshold above
which it picks up can be set in address  ,&7VDW 7KUHV. This is the current
level above which saturation may be present. The setting ∞ disables the saturation de-
tector. This setting can only be modified with DIGSI
If current transformer saturation is expected, the following equation may be used as a
thumb rule for this setting:
where
P P
+
N i
n'= n · = actual over-current factor (accuracy limit factor)
------------------ -
P' P
+
i
P = rated burden of the current transformer [VA]
N
P = internal burden of the current transformer [VA]
i
P' = actual connected burden (protection device + connection cable)
ω = 2πf = system frequency
τ
= system time constant
N
In order to function optimally, several protection and supplementary functions require
information regarding the state of the circuit breaker. The device contains a circuit
breaker state recognition function which processes the status of the circuit breaker
auxiliary contacts as well as recognising switching operations, close and open, by
processing of measured values (refer also to Section 6.17).
In address  the remaining current 3ROH2SHQ&XUUHQW, which will definitely not
be exceeded when the circuit breaker pole is open, is set. If parasitic currents (e.g.
through induction) can be excluded when the circuit breaker is open, this setting may
be very sensitive. Otherwise this setting must be increased correspondingly. In most
cases the preset value is sufficient. This setting can only be modified with DIGSI
under "Additional Settings".
The remaining voltage 3ROH2SHQ9ROWDJH which will definitely not be exceeded
when the circuit breaker pole is open, is set in address . Voltage transformers
are presumed to be on the line side. The setting should not be too sensitive because
of possible parasitic voltages (e.g. due to capacitive coupling). It must in any event be
set below the smallest phase-earth voltage which may be expected during normal op-
x l ⁄
----------------- -
or
2 x l ⁄
–
Setting value I-CTsat. Thres.
x 2
-- - ------------------------ -
=
l 1
1 -------------- -
+
⁄
I I
E EP
®
4 under "Additional Settings".
n'
⋅
------------------- - I
=
1 ωτ
nom
+
N
Functions
®
4
6-23