Additional Stabilization In The Case Of Current Transformer Saturation; Local Current Threshold; In-Rush Stabilization With Second Harmonic - Siemens SIPROTEC 7SD600 System Manual

4
Method of operation
Numerical line differential protection SIPROTEC 7SD600 V3.0
4.3.5 Additional stabilization in the case of
current transformer saturation
The saturation of the current transformer in the case
of high fault currents and/or long system time con
stants is of practically no significance for an internal
short-circuit (on the line to be protected), as the error
in the measured value affects the differential current
and the stabilizing current in equal measure. The fault
characteristic in Figure 4.6 in general, also applies
here. Naturally, the secondary current must at least
exceed the pick-up threshold of the 10I-DIF> para
meter (segment "a" in Figure 4.6), as well as the cur
rent threshold (in accordance with section 4.3.6).
In the event of an external fault causing a high pas
sing fault current, current transformer saturation, par
ticularly when of different levels at the two line ends,
can simulate a significant differential current which, if
the operating point I /I
diff stab
the stabilization characteristic (Figure 4.6), would lead
to false tripping unless special measures are taken.
7SD600 is equipped with a saturation detector and
initializes corresponding stabilizing measures. The
saturation detector functions dynamically in the area
designated "Additional stabilization" in Figure 4.6.
Current transformer saturation during external faults,
is recognized by the fact that initially a large stabilizing
current is present, which is located in the area desi
gnated as "Additional stabilization" in Figure 4.6. On
the other hand, the operating point immediately fol
lows the fault characteristic during an internal fault, as
the stabilizing current hardly exceeds the differential
current. The saturation detector reaches its decision
within a quarter period. The indication "B.DIFS" (Fno.
3024) is then generated which indicates high-current
external faults.
If an external fault is recognized, the differential pro
tection is blocked for a maximum of 1 second when
the differential current subsequently moves into the
tripping area. However, this blocking is canceled as
soon as the operating point of the currents is in a
steady-state position, i.e. for more than two periods,
close to the fault characteristic. This enables fast re
cognition of sequential faults on the line to be protec
ted.
4 - 8
is in the tripping area of
System manual Order no. E50417-G1176-C069-A1
4.3.6

Local current threshold

The line differential protection 7SD600 has a local cur
rent threshold. This represents an additional criterion
for tripping and combines the differential protection
trip command with an overcurrent criterion.
The current threshold is independent of the threshold
for the differential current (segment "a" in Figure 4.6)
and can also be set independently (parameter
"10I1>Frg"). If only the differential protection criteria
must be used, the threshold must be set to "0".
The local current threshold may cause single-ended
tripping at the infeed end in the event of internal
short-circuits, when there is not infeed from both
ends. However, the 7SD600 also enables both-sided
disconnection via the transfer trip function (see sec
tion 4.4), provided that the device is equipped with
this function (see section 2.3 order data).
4.3.7 In-rush stabilization with second
harmonic
In cases where a transformer is directly connected to
a line, it is possible to select an in-rush stabilization
with the second harmonic (parameter "10 2HARM").
The amount of second harmonic required to activate
the stabilization can be set under parameter "10
Harm2%". The ratio between the second harmonic
and the fundamental component of the differential
current is compared with the set parameter value.
If the limit is exceeded, the differential protection is
blocked (the indication "B.DIF.H" (Fno. 3026) is gene
rated).
This blocking is canceled as soon as the differential
current exceeds a threshold value (parameter "10
Harm2I"). The stage"10 Harm2I"only works in this
context and is not a tripping stage.
If the in-rush stabilization function is activated, the
differential protection is delayed by approx. 10 to 15
ms to enable evaluation of the stabilization with the
2nd harmonic and to generate the blocking indication.
Siemens AG January 1999