ABB RELION 620 Series Technical Manual page 510

Section 4
Protection functions
504
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
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 254: 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 255
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 255: DC saturation
Figure 254. A DC component in the current
shows the secondary current of the CT in the fault
1MRS757644 F
620 series
Technical Manual