Functions
2.3 Differential Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
99
Figure 2-30
Differential protection pickup characteristic,
I
diff
> stage
If not only an internal fault is to cause a TRIP command, but if a local current of a specific quantity is to exist
additionally, the value of this current can be set at address
1219
I> RELEASE DIFF
. Zero is preset for this
parameter so that this additional criterion does not become effective.
High-speed charge comparison
The charge comparison is a differential protection stage which is superimposed on the current comparison (the
actual differential protection). It produces high-speed tripping decisions in the event of high-current faults.
The charge comparison protection function does not sum up the complex current phasors at the ends of the
protected object, but the integral of currents calculated according to the following formula:
It includes the integration interval of t
1
to t
2
, which is selected in the 7SD5 device to period
1
/
4
.
The calculated charge Q is a scalar value which is faster to determine and to transmit than a complex phasor.
The charges of all ends of the protected object are added in the same way as done with the current phasors of
the differential protection. Thus the total of the charges is available at all ends of the protected zone.
Immediately after a fault has occurred in the protected zone, a charge difference emerges. For high fault cur-
rents which can lead to saturation of current transformers, a decision is thus reached before the saturation
begins
The charge difference of external faults is theoretically equal to zero at the beginning. The charge comparison
protection function immediately detects the external fault and blocks its own function. If saturation begins in one
or more current transformers which limit the protected zone, the before-mentioned function remains blocked.
Thus possible differences resulting from the saturation are excluded. Generally it is assumed that an initial sat-
uration of current transformers only takes place after the expiration of at least one integration interval (
1
/
4
cycle)
that commenced with the occurrence of a fault.
