[Gllug] OT: Merging UPS outputs

[Ken Smith]

>    That sounds as though they actually generate and distribute a 3-phase
> supply, then use a 3 to 6 phase transformer to give a local 6-phase supply
> as you cannot earth more than one point at a time.
>    Standard practise here is to generate and distribute 3-phase, with the
> centre tap tied to earth (via earth spikes, etc) at every transformer and
> every main switch. This earthed centre tap is provided as the "neutral"
> for
> single phase operation, with multiple earthing for additional safety.
> 

They generate and distribute in three phase just as we do and the local
substation voltage is 4.1KV, as far as I recall, fed by a traditional 3 wire
3 phase system - 3 lives no neutral - delta wired. The final transformers
are mostly up street poles, as the volt loss in long 115V distribution lines
is a problem. The transformers have what we would recognise as a normal 3
phase primary and are delta connected

The 3 230V secondaries are centre tapped and the centre taps are joined and
grounded yielding 6 x 115V supplies or 3 off 115 - 0 - 115 supplies. They
are then sent to customers homes as either 115 single phase or 115 - 0 - 115
for a 230V/115V service. The neutral is only grounded at the transformer
substation, like here, and cannot be assumed to be at earth potential
anywhere else or any use whatever as a safety ground.

What you say about neutral burnouts is quite right - it is all too easy to
assume that the load on all three phases of a star connected three phase
system (as we often use here) are equal and therefore there is no neutral
current - not so - there can be a substantial neutral current - surprisingly
sometimes at harmonics of the supply frequency - because not all loads are
clean nor is the supply really pure. If you do the maths, the harmonics are
not at the vital 120degree phasing that the 3 phase system relies on and so
lead to current flowing in the neutral. 

Bringing us back closer to GLLUG - does anyone know if Linux is used in any
electricity supply monitoring/management systems?

Ken

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