24 pulse essentially means they have 4 separate 3 phase rectifiers feeding a common DC bus. They put transformers in front of each of them and they deliberately create phase shifts between each one by virtue of using Delta and Wye windings with careful phasing of the primaries and secondaries. So when each rectifier draws its non-linear current, the harmonics created on one are at a different phase angle than the others and end up canceling out some of the harmonics created on the neighboring transformers. 12 pulse is better than 6 pulse, 18 pulse is better than 12 pulse, 24 pulse is the best.

Yes. We generally use two transformers each with a delta and a wye secondary. The primaries are delta with slightly extended windings to give +7.5deg on one and -7.5deg on the other.

I suppose it might be worth pointing out that, with a balanced system, the lowest order harmonic should the the pulse number -1 and so, for 24-pulse, the lowest would be the 23rd. The series goes 24n ? 1. Or 23, 25, 47, 49 etc.

The significance of this is that the magnitude of the harmonic current reduces roughly as the inverse of the harmonic number.