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#141133 07/05/04 02:34 AM
Joined: Sep 2002
Posts: 1,498
Likes: 1
C
C-H Offline
Member
I can think of two reasons:

- The pylon will remain there even after a lightning strike.

- If a conductor touches the pylon, the current to earth/ground (the soil) is sufficient for the ground fault protection to intervene.

#141134 07/05/04 03:02 PM
Joined: Aug 2001
Posts: 7,520
P
pauluk Offline OP
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I think C-H has it. On British systems, it's the big all-metal suspension towers used for 132kV and above which have the ground wire strung between them. Lower voltage distribution lines such as 11kV and 33kV which are generally run on wooden poles don't have it.

It's possible to run a 3-phase network with no neutral and delta-connected transformers at the load ends but still have the originating end wired with a wye secondary.

In that case the system can still be referenced to earth by having the star/neutral point of that secondary grounded (either solidly or via an impedance). So if you ran an 11kV 3-wire line that way, each of the distributed phases would be at about 6.6kV to earth.

Even if the transformer feeding the line is delta wired, the phases can still be balanced to earth by the use of auxiliary windings with a center-point earthed.

Finally, even if the transformer is delta and no attempt is made to ground the system at all, you'll still get variable voltage readings from phases to earth due to capacitance along the route.



[This message has been edited by pauluk (edited 07-05-2004).]

#141135 07/06/04 10:26 AM
Joined: Jun 2004
Posts: 54
I
Member
It used to be the case that transformers of different vector groups were used on distribution systems.

The majority of our network was constructed with DY1 transformers, but there are numerous areas which still have DY11. When replacing a DY11 transformer where we cannot re-align the phasing as there are 3 phase customers being fed off the transformer, we use a DY1 transformer but swap the yellow and blue phases on both the HV and LV sides.

There are areas of the UK distribution network where there are a variety of different vector group, and this might explain the different configuration of the OHL.

Ian

Edited to correct spelling etc

[This message has been edited by ianh (edited 07-06-2004).]

#141136 07/07/04 05:38 PM
Joined: Aug 2001
Posts: 7,520
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pauluk Offline OP
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Ah.... That sounds interesting.

Do you have any more info on when these different trnasformers were installed?

#141137 07/08/04 08:54 AM
Joined: Jun 2004
Posts: 54
I
Member
Not to hand, but I believe that it has been down to the distribution company. Some companies might use DY1 whereas others may use DY11. Some might still use a combination of the two. As far as I know both types are still available.

The choice of vector group makes no difference to the operation of the system, until you end up trying to parallel the two!

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