Siemens siprotec 7SA6 User Manual page 203

Current
Transformer
Saturation
Circuit Breaker
Status
7SA6 Manual
C53000-G1176-C133-1
The current ratio may also be calculated from the desired reach of the parallel line
compensation and vice versa. The following applies (refer to Figure 6-12):
x l ⁄
I
E
-------- ----------------- -
=
or
2 x l ⁄
I –
EP
The 7SA6 contains a saturation detector which largely eliminates the measuring errors
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
detector. This setting can only be changed with DIGSI
Settings". If current transformer saturation is expected, the following equation may be
used as a thumb rule for this setting:
Setting value I-CTsat. Thres.
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 6.20).
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 changed 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
operation. The preset value is usually sufficient. This setting can only be changed with
®
DIGSI 4 under "Additional Settings".
The switch-on-to-fault activation (seal-in) time 6, 7LPH DOO &O (address $)
determines the activation period of the protection functions enabled during each
energization of the line (e.g. fast tripping high-current stage). This time is started by
the internal circuit breaker switching detection when it recognizes energization of the
line or by the circuit breaker auxiliary contacts, if these are connected to the device via
binary input to provide information that the circuit breaker has closed. The time should
therefore be set to be longer than the circuit breaker operating time during closing plus
the operating time of this protection function plus the circuit breaker operating time
x 2
-- - ------------------------ -
=
l 1
1 -------------- -
+
⁄
I I
E EP
®
4 under "Additional
n'
⋅
------------------- - I
=
1 ωτ
nom
+
N
Functions
®
4
6-21