For a transformer, for example, the (primary) value is:
[fo_Idiff-error, 1, en_US]
The I-DIFF unrestrained stage also acts if, for example, due to current-transformer saturation because of a
DC component in the short-circuit current, a 2nd harmonic occurs. The inrush-current detection could inter-
pret this as the closed-circuit current.
The I-DIFF unrestrained protection stage operates both with the fundamental component of the differential
protection and with the instantaneous value.
Metrologically, 2 methods are combined:
•
Instantaneous-value method
•
Filter-based method
Instantaneous-Value Method
With the instantaneous-values method, an immediate evaluation of the sampled values of the differential
current i
Diff
(t) occurs. The absolute-value and vector-group adapted currents (i*) are also the basis.
Fundamental-Component Method
Parallel to the instantaneous-value method, the fundamental component works with a temporal delay. Here,
the same measured values are processed as with the I-DIFF method. The fundamental component of the
differential current I
diff
is ordered in the operate curve Figure 7-61 and the trip decision is made.
[dwidffunres-100117-01, 1, en_US]
Figure 7-61 I-DIFF Unrestrained Characteristic Curve
Figure 7-62 shows the overall logic.
Capacitor Bank Protection
7.8 Differential Protection for Capacitor Banks
1058 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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