I am only familiar with the terms zero sequence and positive sequence in another context. When analyzing the effects of harmonics on three phase motors, the harmonics are variously classed as positive, negative, and zero sequence harmonics. You are looking at something else, the impedance of a supply system to various sequence currents, but I think that the basic meaning is the same, and that the below will help.
In a _balanced_ three phase system, the current flowing in each of the legs is 120 degrees out of phase, but with the same magnitude and frequency. In a motor, this current would set up a rotating field. The harmonic currents in each leg have relative phase angles that are _different_ from the base (or fundamental) phase angle. For example, in a balanced three phase system, the third harmonic has a phase angle difference of 360 degrees, which is another way of saying that the third harmonic in all three legs is in phase with the third harmonic on the other legs. For fifth harmonic, the phase angle is 600 degrees (5*120), which happens to be the same as 240 degrees or -120 degrees.
For the fundamental, the current that flows produces the rotating field. For third harmonic, all three third harmonic voltage components are _in phases_, and there is no voltage difference between leg terminals. The net result is that third harmonic currents cannot flow unless you have a neutral available. For fifth harmonic, current can flow, but the rotating field produced rotates in the reverse direction; in other words the phase sequence is reversed. These various states are the positive, zero, and negative sequence harmonics. It is just a very crude way of describing the relative phase relations between the three legs of the system.
For your application, I believe that the situation is that any three phase _unbalanced_ current flow can be described as the sum of three different _balanced_ current flows. Each one of these mathematically derived three phase sets has the same current flowing in each 'phase', and when you vector add the phase currents from each of these balanced sets, your sums are the real unbalanced current flow. No harmonics are involved; the terms positive, zero, and negative sequence are used to describe the relative phase angles of the current flow for the (mathematically derived) three phase sets. The reason for this mathematical nicety appears that you can analyze the system in terms of its _balanced_ response to these various current flow conditions, and since _any_ unbalanced condition can be described in terms of these balanced conditions, you can then use this to figure out how the system will respond to unbalanced situations. An analogy would be to describe the position of a point in space by its X,Y,Z coordinates, then describe how a system would respond to an object at X=1, how the system would respond to an object at Y=1, and finally how the system would respond to an object at Z=1, and then be able to use this information to figure out how the system would respond to an object at any location. If I understand them correctly, then the zero sequence impedance is the impedance of the system when all the current in the conductors is in phase, as could occur in a single phase to ground fault.
I found the following reference, which should prove useful. Like I said, my only which involved sequence current was harmonics in motors, so I am only guessing that the below is applicable. (These are all links into the same page.)
http://xnet.rrc.mb.ca/janaj/1_2_1_symmetrical_components.htm http://xnet.rrc.mb.ca/janaj/1_0_faults_on_power_system.htm http://xnet.rrc.mb.ca/janaj/1_2_2_single_line_to_neutral_fault.htm Regards,
Jonathan Edelson
