ABB RELION REX640 Technical Manual page 723

1MRS759142 C
REX640
Technical Manual
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-847B5476-5A87-47AD-B901-619870EE20C2 V1 EN
Figure 397: 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.
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-A3211D77-8748-429E-B7D5-1E30FF5D3CAF V1 EN
Figure 398: DC saturation
Figure
Figure 398 shows the secondary current of the CT in the fault
Section 4
Protection functions
397. A DC component in the current
717