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Topic Title: Distribution Network Design
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Created On: 07 August 2012 10:23 AM
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 07 August 2012 10:23 AM
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Hi All,
I am currently a trainee design engineer for a DNO (HNC entrant not graduate) and would be most grateful for any advice and guidance on where I could find out info on how the network works and how to read network diagrams. I've only been in the position a few months and am struggling to comprehend how the distribution network operates and the plant involved. My HNC/HND studies only covered this topic in very general terms so any DNO specific advice or pointers would be fantastic.
Many thanks
 07 August 2012 10:52 AM
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I went into engineering through a similar rout to yourself. The lack of a degree should not hold you back at all, infact people with a more practical back ground often do better in a DNO than graduates.
I would be supprised if your company does not have some form of design manual which would pretty much descibe the design process. In addition many DNO's have a lot of material on the net, it's worth a trawl.
If you can find anyone who has done the power system protection course ask to borrow volume three it has chapers on rural and urban protection which explain a fair bit.
 07 August 2012 05:36 PM
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Thanks Arthur,
Your comments are appreciated, I,m having trouble with some of the DNO jargon like "backfeeding" a circuit and also what is a "link box" and what is "phase out" I can make an educated guess at what they are but I'm not entirely sure.
 07 August 2012 10:18 PM
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Hello Robert

A link box (also know as ground box) is LV apparatus which could be thought of as a junction box for 415V 3-ph cables typically 450mm deep in the pavement/footpath covered with a bell housing and then the lid which is visible on the footpath.

Consider a substation A and substation B and between the LV board in each sub runs an LV mains cable protected by fuses at each substation. The fuses will be out at one end otherwise you will be paralleling transformers. If there is a cable fault then all customers supplied, connected or 'fed' from the cable will go off.

If however the mains cable is split or sectionalised via a two way link box placed half way along the main with it's links out and fuses in at s/s A and s/s B. The link box basically brings up the three phases or each cable up onto stalks within the link box, so you will have three stalks on one side for R, Y, B for cable A and three stalks opposite them for R, Y, B of cable B with the neutral link of each cable always connected.

Links can be inserted to connect RD phase of cable A to RD phase of cable B, similarly with YW and BE phases.

Consider again the cable which runs from s/s A down to the 2-way link box. If that cable faults and the nature of the fault is a complete 3-ph open circuit fault. All customers btwn the location of the fault and the link box will be off supply. The quickest way to restore supplies to affected customers will be to insert links into the link box, meaning that the customers affected are now supplied from substation B via the link box, i.e. the customers have been 'BACK FED' . So a backfeed is just an alternative source of supply.

Now consider that substation A is a pole mounted transformer which is fed from an 11kV overhead line which is being isolated to allow tree trimming to be carried out. In order to keep supplies to customers fed from s/s A links cane be inserted into the link box and the fuses removed from s/stn A (to prevent the transformer back energising the 11kV line) so that s/stn B takes all the load. However before the links can be inserted the link box must first be PHASED OUT. That is, you check that RD phase stalk on one side of the link box is RD phase stalk on other with test lamps, similarly with YW and BE phases. If RD is RD on both sides the RD phase link can be inserted. If for some reason RD is not RD on both sides, i.e. it is YW or BE opposite RD, then the link cannot be inserted as you will be putting a phase to phase short on, which will cause a bang, sparks, fire, and an explosion right in front of your face as you will be on your knees, bent over the link box.

So phasing out is required to ensure that prior to making system parallels, you are gonig to connect RD phase to RD phase, YW to YW, BE to BE. Confirming network phasing can be achieved in various ways depending upon the voltage level, LV phase check at LV, HV or primary phase at 11kV or 33kV, cable colour checks............

Any Senior Authorised Person within you DNO worth their authorisation will be able to discuss all of your queries with you and indeed show you.

 16 August 2012 03:17 PM
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I've been away for a while Alan and just seen your reply thanks very much thats cleared that up for me.
Many thanks
 08 January 2013 10:58 PM
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could anyone enlighten me on backfeeding (like in a grid connected solar power system) and as to how it works. theoretically i do know.. but practically is it implemented in power system. how exactly does it work. is it helpful in smart microgrids?

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