ABB RELION 620 Series Technical Manual page 424

Section 4
Protection functions
418
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
increase until the CT saturates. This is known as DC saturation.
GUID-0A569D8E-6A69-43F1-848B-6AA364E130B7 V1 EN
Figure 190: 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 191
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.
GUID-505C1447-5FA4-43B7-8AD7-9E2449CC10C3 V1 EN
Figure 191: DC saturation
190. A DC component in the current also causes the flux to
shows the secondary current of the CT in the fault
1MAC504801-IB E
620 series ANSI
Technical Manual