Saturation of current transformers
There are basically two types of saturation phenomena that have to be detected:
the AC saturation and the DC saturation. The AC saturation is caused by a high
fault current where the CT magnetic flux exceeds its maximum value. As a result,
the secondary current is distorted as shown in
Figure 264
. A DC component in the
current also causes the flux to increase until the CT saturates. This is known as DC
saturation.
Figure 264: AC saturation
When having a short circuit in a power line, the short circuit current contains a
DC component. The magnitude of the DC component depends on the phase angle
when the short circuit occurs.
Figure 265
shows the secondary current of the CT
in the fault situation. Because of the DC component, the flux reaches its maximum
value at 0.07 seconds, causing saturation. As the DC component decays, the CT
recovers gradually from the saturation.
Figure 265: DC saturation
4.3.1.6 Signals
Table 476: MPDIF Input signals
Name
Type Default Description
I_A1 Signal 0 Phase A primary cur-
rent
I_B1 Signal 0 Phase B primary cur-
rent
Table continues on the next page
1MRS757644 H Protection functions
620 series
Technical Manual
515
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