1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
333
Technical Manual
Saturation of current transformers
There are basically two types of saturation phenomena that have to be detected; AC
saturation and DC saturation. The AC-saturation is caused by high fault current, where the
CT magnetic flux exceeds it’s maximum value. As a result the secondary current will be
distorted as shown in Figure 158. A DC component in the current will also cause the flux
to increase until eventually the CT saturates. This is known as DC saturation.
Figure 158:
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 159 shows the secondary current of the CT in the fault
situation. Because of the DC component , the flux reaches its maximum value at 0.058sec
causing saturation. As the DC component decays away, the CT recovers gradually from
the saturation.
Figure 159:
DC- Saturation
0.016 0.033 0.05 0.066 0.083
0.033 0.066 0.1 0.133 0.166
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