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 295. A DC component in the current also causes the flux to
increase until the CT saturates. This is known as DC saturation.
GUID-0A569D8E-6A69-43F1-848B-6AA364E130B7 V1 EN
Figure 295: 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 296 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.
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Figure 296: DC saturation
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 583
Technical Manual
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