2/3 and 5/6 winding pitch in AC Generators, what are the advantages and disadvantages?

It is produced with an AC generator with 2/3 or 5/6 winding pitch. The winding spacing of AC alternators has an impact on the shape of the waveform and the base voltage level. Both winding pitches have some pros and cons. Application choices will have an impact on cost.

The winding pitch of a generator is an important design parameter to reduce cost. Low winding pitch ratios require the use of more copper electrical steel.

In terms of harmonics:

2/3 pitch windings; suppresses third harmonics. However, it can amplify the 5th and 7th harmonics.

5/6 pitch windings suppress 5th and 7th harmonics. However, it can amplify third harmonics.

When comparing generator specifications, the value listed for output voltage total harmonic distortion (%THD) may be similar for a 5/6 or 2/3 pitch winding configuration. A well designed generator can probably achieve very similar voltage waveform THD% with both steps.

In low voltage generators, a winding pitch of 2/3 is generally recommended. Because neutral is used in these systems. For both 50 and 60Hz low voltage, a 2/3 pitch winding configuration will eliminate triple harmonics. An additional benefit of 2/3 windings is that they prevent circulation currents from the star points of our generators when connected in parallel. In addition, the 2/3rd step is better in terms of motor starting capability (lower source impedance)

It is understandable to use 5/6 winding pitch in medium voltage and high voltage generators. Because neutrals are rarely used to directly serve loads, and only low levels of triple harmonics as such are present. In addition, 5/6 step winding increases the efficiency and reduces the cost of the generator.

Neutral point grounding and equipotential bonding are often confused. The winding pitch affects only the neutral point bonding requirements. It detects the touching potential of conductive parts of the installation, etc. To avoid electrically coupled equipotential bonding, it is necessary regardless of the choice of winding pitch. Additionally, when the generator star points are grounded, it is important that neutral current is not allowed to circulate in the ground path; because this can cause problems in telecommunications systems as well as cause constant voltages in the ground. The method of connection of the star point to ground depends on the type of application, voltage level and generator winding spacing.

For parallel operation; Can 2/3 and 5/6 step generators be connected together?

If a 2/3 pitch is parallel to a 5/6 pitch winding there will be differences in the harmonic 'structure' of the voltage waveform. These differences, and they are electrical 'potential differences', cause currents to flow between the two generators. However, if the star points of generators with different winding pitches are not interconnected, then there is no circulating current, although the generators may not fully share the reactive load proportionally.

CONCLUSION:

The 2/3 winding pitch is mainly used in three-phase four-wire low voltage generators because the load is directly connected to the generator without a transformer and single-phase and non-linear loads are most likely present.

Machines with 5/6 winding pitch are more suitable for three-phase three-wire MV and HV systems as the load will not be directly connected and therefore triple harmonics will not be present as the sum of third harmonics will be zero in line-to-line voltages.

Usually MV and HV generators are connected to delta/star transformers and this is a significant advantage of 5/6 winding pitch for MV and HV machines as the transformer will act as a barrier for triple harmonics.

If the star points of all operating generators must be connected together, parallel connection of generators with different winding spacing will require careful consideration. The 3rd harmonic current circulating in the system neutral cables must be taken into account at the beginning of the project and necessary steps must be taken to control it.

References : https://www.stamford-avk.com/