ABB RELION REX640 Technical Manual page 801

1MRS759142 F
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
current also causes the flux to increase until the CT saturates. This is known as DC
saturation.
Figure 456: 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.
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 457: DC saturation
4.3.7.7 Signals
REX640
Technical Manual
Figure 457
shows the secondary current of the CT in the
Protection functions
Figure 456
. A DC component in the
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