Powerlines

From: Daniel T. Burrows <danburrows_at_mindspring.com>
Date: Mon Apr 26 16:33:36 1999

>2. Phase.
>
>As I understand it the usual practice in the US is take _either_ a phase
and
>earth (neutral) _or_ two phases of the HV supply and feed a single phase
>transformer from this. The LV side of the transformer is (say) 240V with a
>centre tap. The centre tap is earthed and provides the neutral connection,
the
>outer two taps are then both hot at 120V, 180 degrees apart. This is true
_even
>if_ the primary (HV winding) is connected between two phases of the HV
3-phase
>system.

Close but not quite right. For single phase installations the power company
supplies 2 hots and neutral. This is a center tapped secondary and the
neutral is earthed at the service entrance. The primary of this transformer
is fed from only 1 high voltage phase.



>In the UK the usual practice is to connect all 3 phases of the HV (almost
>invariably 11kV phase to phase) to a delta-star 3-phase transformer. The
>secondary, the star-connected winding provides 3 phases and neutral.
Neutral is
>earthed at the transformer. Protective earth can be provided by _either_ a
>separate wire from the main earth at the transformer _or_ an earthing
electrode
>at each customer's site _or_ the neutral is earthed at a large number of
points
>on the system and the customer's protective earth is bonded to neutral at
the
>point of entry of the supply. In all cases the customer sees 240V line to
>neutral; if he gets more than one line (hot) connection, they will be 120
>degrees apart.


The delta-star is the same but with 120 phase to neutral and 208 phase to
phase. In the UK it is the same but 240 phase to neutral and 416 phase to
phase.
You can also commonly find 480 3 phase delta- star 277 phase to neutral.
This is then used for lighting (277 florescent is common) and large loads
like air conditioning. In these installations the customer has to provide
their own transformer to 208/120.


>In remote locations, only 2 phases of the 11kV system are taken to the
site.
>Small loads get a single phase 11kV/250V transformer with one end of the LV
>earthed; larger loads get a single phase 11kV/500V transformer with a
centre tap
>earthed. Some farm equipment (I am told - I have never seen this) is rated
for
>480V single phase.

I don't think this is avail. in the US.
>
>It is very unusual for a domestic installation to get more than one phase
and
>neutral. But 3-phase can be done - the supply company don't mind putting
it in
>if they think you'll buy a lot of electricity!

They will here also and I checked into getting it here but I am a few miles
from the closest point that 3 phase is available for distribution. I would
either have to guarentee a large $ per month usage or pay them to add the
additional lines. Over $10,000 was mentioned.

Dan
Received on Mon Apr 26 1999 - 16:33:36 BST