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.