Functions
2.3 Differential Protection
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
69
The restraining current counteracts the differential current. It is the total of the maximum measuring errors at
the ends of the protected object and is calculated adaptively from the current measured quantities and power
system parameters that were set. For this purpose, the maximum error of the current transformers within the
nominal range and/or the short-circuit current range is multiplied with the current flowing through each end of
the protected object. Consequently, the restraint current always reflects the maximum possible measuring error
of the differential protection system.
The pickup characteristic of the differential protection (Figure 2-20) derives from the restraining characteristic
I
diff
= I
rest
(45°-curve), that is cut below the setting value I-DIFF>. It complies with the formula
I
rest
= I-DIFF> + Σ (errors by CT´s and other measuring errors)
If the calculated differential current exceeds the pickup limit and the greatest possible measurement error, the
fault must be internal (shaded area in Figure 2-20).
Figure 2-20 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 7SD610 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.
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