Current Transformer Saturation Due to DC Offset

[jliu1]

Our project has GE's UR series T60 transformer protective relay. The percentage differential function has dual slope adjustability, with Break point 1 as the turning point. The selection for this point requires it to be below a current that could cause CT saturation due to DC component. The plant configuration is a typical generator with step up transformer connected to the switchyard. So what is the criterion for CT saturation due to DC offset? Seems that the relay manual T60 does not provide any guidance and requirement. Any input is appreciated.

 

[dpc]

The dc offset is primarily determined by the X/R ratio of the system looking back in from the point of the fault. The higher the X/R ratio, the higher the possible dc offset. Actual dc offset also depends on the voltage phase angle at the instant of the fault as well as the X/R ratio, so you have to consider the worst case which is when the fault occurs at voltage at 0.

As a worst case, you can assume about 1.6 to 1.7 times the symmetrical current.