Differential vs. zero-sequence currents

[w8wer]

I have an application for a CT based protection system in a 115/200 VAC 400 Hz 3 phase electrical system in an aircraft. The spec requires both differential protection and zero-sequence protection. Differential protection in an aircraft uses 2 sets of CTs, one at each end of the feeder to make sure that what goes into the feeder comes out the other end. I think I understand that the zero-sequence current is the same as ground fault current. If there is a differential fault, where else would the current go but to ground, so how do these 2 requirements differ?

I see from my research that ground faults are often measured using a single CT with all 3 phases passing through it. My application is not a new one, only new to me and the existing design has only the 3 phase CTs.

 

[dpc]

The differential relay checks current in versus current out. Any type of fault that involves current leaving a phase conductor will be seen by the differential circuit. So if there was a three-phase fault in the differential zone, there is no zero sequence current, but plenty of differential current.

The flux summation or zero sequence CT just measures the resultant flux of the three phase conductors. If the three phase currents are equal, the net current and the net flux is zero. When there is a ground fault, some current in one of phase conductors is going into the earth or ground system, so the three phase currents will not sum to zero. If you have three phase CTs, you can also sum the three CT secondaries and run this through another relay to provide ground fault detection. This is called a residual connection. The two methods each have pluses and minuses.

Cheers,

Dave

 

[w8wer]

Thanks dpc. Shows the power of writing something down - as I was writing the original post, I started thinking about phase-to-phase faults, just hadn't finished the thought. Now that you remind me, I have seen the residual connection before. I just needed a nudge in the right direction. Thanks

Bill

 

[stevenal]

Another difference is that differential current will only be produced for faults located between the two CT sets. Zero sequence current can be produced by ground faults external to this area, and also by unbalanced phase to neutral connected loads.