The detector has 2 essential tasks:
•
Blocking in the case of a definite high-current external fault
The differential current and the restraint current are monitored on a phase basis in the Diff/Restr level
immediately after fault inception. To ensure the stability of the saturation detection for low saturation-
free times of the primary current transformer, estimated values based on the difference quotients of the
instantaneous values are used.
If the estimated restraint current exceeds the Threshold add-on stabiliz. parameter and the
expected value of the differential current of all phases is in the additional stabilization range (parameter
Slope 1) (Figure 7-51) at the same time, the detector is active and the indication
Blocked by ext.
fault phsx
appears. The tripping of the Differential protection function is blocked.
If an external fault arises and is cleared by the protection responsible for it, the additional stabilization
drops out. If the dropout ratio (0.7) of the RMS value of the restraint current does not reach the
Threshold add-on stabiliz. parameter (Figure 7-51), the additional stabilization drops out. The
indication
Blocked by ext. fault phsx
is reset.
If the differential current exceeds Slope 1 as a result of transformer saturation, that is, there is a pickup
phsx, the blocking is limited in time (parameter: Time of add-on stabiliz.).
Starting with the indication
Blocked by ext. fault phsx
, the other phases can be blocked with it
via the Crossblk. time add-on st. parameter. If differential currents due to transformer satura-
tion spread to the other phases through the vector-group correction or residual current elimination, this
blocking is necessary.
•
Removal of the blocking with a change in fault location from external to internal
An internal fault can arise as a consequence of an external fault. This fault change must be recognized
and the blocking must be removed. The critical case is a short-circuit with current-transformer saturation.
The saturation detection examines here the instantaneous values of the differential current. With trans-
former saturation, the instantaneous values fall below an internal threshold in the saturation-free time. If
this threshold is permanently exceeded and a pickup is present in the corresponding phase, an internal
error is assumed. The other phases are also examined during the parameterization with crossblock.
The blocking of the tripping is removed and the indication
Blocked by ext. fault phsx
is reset.
DC-Component Detection
Low-current external faults with large DC components can also lead to exceeding of the operate curve. The
additional stabilization range is not attained, however.
To avoid an overfunction, the following steps are taken:
•
The restraint current I
rest.;phsx
is monitored for a jump.
•
If the jump occurs, the DC component is calculated, with a temporal delay (1 period), from the instanta-
neous value of the restraint current (current at the measuring point with the largest current) i
rest.,phsx
=
i
phsx,Mm
. The result is I
rest.DC,phsx
. This current is compared to the restraint current I
rest., phsx
and is checked for
exceeding of the internal threshold.
•
The differential current I
diff,phsx
must lie below the dropout characteristic curve (characteristic curve · 0.7)
in all 3 phases.
•
If the first 3 points lead to positive results, then the operate curve is raised by the parameter Factor
increasing char. with a default setting of 2.3. Here, only the threshold value and the slope 1 are
raised (see Figure 7-52). The indication
Increase of char. (DC)
is output. If the product of param-
eter Factor increasing char. and Slope 1 exceeds 0.85, the maximum increase of the raised
Slope 1 is limited to 0.85.
•
As additional criterion, the pickup indication
Pickup
(internal fault is present) or the indication of the
additional stabilization
Blocked by ext. fault phsx
(high-current external fault) is interrogated. If
one of the 2 conditions is present, the characteristic-curve increase is stopped. A retroactive pickup no
longer resets the characteristic-curve increase. A pickup with DC-component detection occurs only if the
raised operate curve is exceeded.
Capacitor Bank Protection
7.8 Differential Protection for Capacitor Banks
SIPROTEC 5, Overcurrent Protection, Manual 1051
C53000-G5040-C017-8, Edition 07.2017
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