Zero Sequence Reactance

For the three phase unbalance fault current analysis, the symmetrical components (positive sequence, negative sequence and zero sequence) are used. See the earlier thread http://cr4.globalspec.com/thread/64674/What-is-Positive-Sequency for more detail of the symmetrical components. In this analysis, three types of networks are prepared – (1) positive sequence network, (2) negative sequence network and (3) zero sequence network. In each types of network, the corresponding components of current, voltage and impedance are used. The impedances used in zero sequence network is the zero sequence impedance. It is the impedance of the circuit when only zero sequence currents are flowing. For the rotating machines, the positive sequence impedance, the negative sequence impedance and the zero sequence impedance differs, they are generally provided by the manufacturer. For the transformers, the positive sequence impedance, the negative sequence impedance and the zero sequence are generally considered same. For the transmission line, the positive sequence impedance and the negative sequence impedance are same but the zero sequence is 2 to 2.5 times higher.

In the case of the ground fault, the ground fault current depends on the grounding system, (three time of) ground impedance and the zero sequence impedances of the network.

This links also help:

http://cindulkar.tripod.com/lab44.pdf

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/power-system/ui/Course_home-7.htm (chapter 7)

http://www.mty.itesm.mx/etie/deptos/ie/profesores/jabaez/clases/E00888/Fallas_asim%E9tricas/fallasasimetricas.pdf

http://www.basler.com/downloads/ZeroSequenceCircuit.pdf

- MS