Let's as pure example assume each workstation takes 2A and there are to be 24 workstations.
It is always advisable to assume the PCs are non-PF corrected and have very sharp conduction angles. This means the Neutral can carry up to 3 x phase current.
So, in no transformer scenario:
24 PCs over 3 phases = 8 PCs per phase = 16A per phase and possible 48A on Neutral.
If we put in DY xfos and split the load between straight feed and DY feed, then:
9 PCs over 3 phases = 3 PCs per phase = 6A per phase and possible 18A on Neutral.
Next 15 PCs are on DY. Secondary currents are 10A per phase and possible 30A on local Neutral. However, primary side (Delta) has 10A per phase but no Neutral current therefore total current on each supplied phase is 16A per phase (back to our original).
The trick is the current curve presented to each phase is a combination of 2-pulse and 6-pulse resulting in a much reduced harmonic content, and the whole system is capable of running on a 3P+1N cable.
As for the Ground/Earth currents, I suspect circulating currents and would need to do tests to establish where they were coming from (because there is just no way on 7.5A per phase he can combine to be in excess of 22.5A - and as is 13A is already on the Neutral, that Ground current must be coming from another system).
M.
[This message has been edited by marcspages (edited 10-13-2005).]
