Harmonics of different orders have different phase sequences. What is phase sequence any way? In simple terms, a phase sequence is the order of rotation of phase vectors relative to each other. Positive sequence has a phase order of "abc", a Negative sequence has a phase order of "acb" and a Zero sequence system, the phase vectors has the same phase angles.
Harmonics of different order form the following sequence set:
Positive sequence: 1, 4, 7, 10, 13, ...
Negative sequence: 2, 5, 8, 11, 14, ...
Zero sequence: 3, 6, 9, 12, 15, ...
Zero sequence is also called triplen as it is in the multiples of 3 and this is the current flowing into the neutral of a three phase system.
From Effects of harmonics on Electrical Equipment, a 6-pulse VSD has the following harmonic orders:
5th Harmonic (250 Hz): 20.0% x I1
7th Harmonic (350 Hz): 14.3% x I1
11th Harmonic (550 Hz): 9.1% x I1
13th Harmonic (650 Hz): 7.7% x I1
17th Harmonic (850 Hz): 5.9% x I1
19th Harmonic (950 Hz): 5.3% x I1
23rd Harmonic (1150 Hz): 4.3% x I1
25th Harmonic (1250 Hz): 4.0% x I1
29th Harmonic (1450 Hz): 3.4% x I1
31th Harmonic (1550 Hz): 3.2% x I1, etc
The above table indicates that a 6-pulse VSD theoretically does not produce zero sequence currents.