CT saturation

CT saturation is one of the phenomena which is important for differential protections. During saturation, core of CT gets saturated due to higher level of flux density than designed value. After this no more change of flux occurs even the primary current is still changing. 

Due to no change in flux secondary current vanishes. We may get secondary current having distorted waveform during CT saturation.

For differential protections, this may lead to differential current for out of Zone faults and relay may give unwanted trip command. Most of the manufacturers compensate CT saturation with different techniques.

 Let us see two cases with and without CT stauration

For case-1 External fault without CT saturation:

Differential current = 1 + 1 + 2 – 4 = 0 kA
Bias current = 1 + 1 + 2 + 4 = 8 kA
Idiff / IBias = 0/8 = 0.0

For case-2 External fault with CT saturation, CT for fourth feeder is staurated and measuring only 1kA current instead of actual 4kA current: 

Differential current = 1 + 1 + 2 – 1 = 3 kA
Bias current = 1 + 1 + 2 + 1 = 5 kA
Idiff / IBias = 3/5 = 0.6

For case-2 we can avoid unwanted relay operation if we keep differential slope setting higher than 0.6

* For simplicity current is considered without any DC component.