Siemens siprotec 7SD5 User Manual page 96

2 Functions
If the calculated differential current exceeds the pickup limit and the greatest possible
measurement error, the fault must be internal (shaded area in Figure 2-22).
Figure 2-22
High-Speed Charge The charge comparison protection function is a differential stage which is superim-
Comparison posed on the current comparison (the actual differential protection). If a high-current
fault occurs, high-speed tripping decision is then possible.
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 ac-
cording to the following formula:
It includes the integration interval of t
the period
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.
Right after a fault occurrence within the protected zone a charge difference emerges.
For high fault currents which can lead to saturation of current transformers, a decision
is taken 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 possi-
ble differences resulting from the saturation are excluded. Generally it is assumed that
96
Differential protection pickup characteristic, I
1
/ .
4
> stage
diff
to t , which is selected in the 7SD5 device to
1 2
7SD5 Manual
C53000-G1176-C169-1