Third harmonics, and it multipales current, will get path through ground, if ground is connected and will increased the harmonic loading., which will not be there in delta and star without ground.connection.

Triple harmonic currents  may find a closed  path  as already stated above. It depends on the transformer connections. You may find attached pdf of use where this is demonstrated for a particular case in Pspice.

Cheers

On further thought it occurred to me ..you might have something more than

what has been discussed. perhaps the following:(just my fancies..)

1. If we ground a circuit to a metallic conductor there might be

nonlinear coupling effects  so the correct modelling should include details of

the earth return path..

2. Is it possible the actual earth for say for longer distribution  lines (single  wire

supplies (to save on copper) could potentially couple harmonic voltages

3. some effects such as acoustic noise may also be possible

if we consider saturation effects of transformers .. not to forget

ferro resonance possibilities.

4. unbalanced loads is another aspect that might affect grounding

5. lightning /solar flares might cause resonance effects too

Cheers

Yes this is very important.

You must Know the Grounding is very critical.

I suggest you refer to grounds for grounding a circuit-to-system handbook.

This is very good and useful for question.

Best regard Majid  

I suggest you refer to grounds for grounding a circuit-to-system handbook.