Table of Contents
Advertisement
The restraining current counteracts the differential current. It is the total of the
maximum measured errors at the ends of the protected object and is calculated from
the current measured quantities and power system parameters that were set. There-
fore the highest possible error value 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. The total value, including the measured in-
ternal errors, is then transmitted to the other ends. This is the reason why the restrain-
ing current is always an image of the greatest possible measurement error of the dif-
ferential protection system.
The pickup characteristic of the differential protection (Figure 2-20) derives from the
restraining characteristic I
DIFF>. It complies with the formula
I
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
If not only an internal fault is to cause a TRIP command, but if a local current of a spe-
cific 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 cri-
terion does not become effective.
High-speed charge
The charge comparison protection function is a differential protection stage which is
comparison
superimposed on the current comparison (the actual differential protection). If a high-
current fault occurs, high-speed tripping decision is then possible.
7SD610 Manual
C53000-G1176-C145-4
= I-DIFF> + Σ (errors by CT´s and other measuring errors)
rest
Differential protection pickup characteristic, I
(45°-curve), that is cut below the setting value I-
= I
diff
rest
2.3 Differential Protection
> stage
diff
67
Advertisement
Table of Contents
