It's one of three symmetrical components used for unbalanced fault analysis, although often used as loosely equivalent with ground fault current, although that isn't quite correct.
Here's an on-line reference on symmetrical components:
Zero sequence current is one of the three symmetrical components used to study systems under unbalanced conditions. Its mathematical definition correspond to the third of the addition of the three current phasors, that is, the third of the neutral current value. So, zero sequence currents, an voltages, are directly asociated to ground faults.
Whith distorded waveforms, third harmonic and multiples behave as zero sequence currents and/or voltages. Julian
The reason why third harmonic currents (or voltages) are zero sequence is relatively straightforward.
Assume you have three identical non-sinusoidal waveforms periodic at 60hz and shifted from each other by 120 "degrees" based on 60hz (ie shifted from each other gby 1/180th of a second).
The third harmonics are also shifted by 1/180th of a second. But their frequency is 3x60hz=180hz. The time shift between waveforms of 1/180th of a second corresponds to 360 degrees based on 180hz. So the three 3RD harmonic currents are in-phase with each other, and by definition if they have same phase and same magnitude in each phase, they are zero sequence.
The same reasoning can be applied to determine the apparent phase sequence of any harmonic. BTW, saying "zero sequence" is just a lazy shorthand for "zero phase sequence", which is a bit more descriptive of what is being done mathematically.
The thing to remember is that it is all mathematical trickery to get a set of independent balanced phasors which are easy to deal with - they don't really exist in practice. And when you do things to threaten the independence of the sequence components (multiple simultaneous faults on different phase at different locations, for eg), you might as well revert to traditional per phase analysis.
Bung
Bung - you're right. Similar logic shows the following pattern
1st harmonic - positive sequence
2nd harmonic - negative sequence
3rd harmonic - zero sequence
4th harmonic - positive sequence
5th harmonic - negative sequence
6th harmonic - zero sequence
etc (repeating pattern).
The assumption required for the above is that we have three waveforms (currents or voltages) which are periodic and identical except that they are time-shifted from each other by 1/3 of their period (120 degrees based on fundamental frequency).
An interesting consequence of the above is negative sequence present in the voltage supply of a motor (due to imbalanced voltages or conceivably due to 2nd harmonics) will cause reverse rotating stator field which induces a reverse rotating current in rotor with two consequences:
#1 - Those two fields work together to produce torque in the wrong direction, decreasing motor output
#2 - Reverse rotating rotor current is traveling at almost twice speed with respect to the rotor, resulting in large skin heating of the rotor.
