Lv distribution cables Ring or radial

[calum63]

Hi all, I have the above question. Lets say there is a estate with 300 houses all with gas boilers fed by a 11kv substation.Lets say there is lv 3 phase and neutral cable coming out the substation supplying 100 houses, I know the main cable runs past the houses then each house gets jointed directly from it, each house on alternate phases RYB,RYB ect, but what happens when the cable reaches the last house or the end of its run?,does it just finish there? or does it go to a linkbox?, or does it double back to the substation forming a ring?. how would it work out? also what rating would the transformer likely be?.

everyday is learning day for me. thanks in advance

 

[JohnW2]

...I know the main cable runs past the houses then each house gets tapped directly from it, each house on alternate phases RYB,RYB ect, but what happens when the cable reaches the last house or the end of its run?,does it just finish there? or does it go to a linkbox?, or does it double back to the substation forming a ring?.

I'm no authority, so am open to correction, but I think that the main (and any branches thereof) usually simply 'finishes' at the end of the run(s).

... also what rating would the transformer likely be?.

That will be determined by the DNO's assessment of the average after-diversity demand, together with the number of properties supplied In estimating that, they use a surprisingly small average figure - IIRC, around 2.5kW for an average house. Voltage drop, and the permitted limits of supply voltage variation, will usually dictate the number of services (properties) that can be supplied by one main.

Kind Regards, John

 

[Lectrician]

It depends, there is so many different aged parts of the install, and nothing in consistent across the country.

The cable would of been laid from the substation, and pot ended with a joint at the end. This pot end joint *may* also feed a few houses.

You also see LV feeders leaving one substation, passing the buildings it feeds, and then terminating in a second substation. The fuses would only be in place in one the substations at any one time, but the cables would be phased correctly to allow it to be fed from both ends for a short duration until the other fuses were removed. This allows work to be carried out and power to rerouted during power cuts etc.

The HV system is done in a ring, with RMU (ring main units) in each substation. Essentially three switches on a busbar, two in/out, one to the TX. As it's a ring, you can isolate each section of cable (and earth it down) between substations to allow work. LV doesn't require the ability to switch out sections of the network without disruption to customers, as most works is done live. Occasionally you see the "link boxes" half way along a run of LV cable between the two substations.

 

[Screech20]

Lv networks is simply a 'radial'

The underground network could either go to a pot end, or a link box where it can be backfed from another sub.

On the overhead network, the end of line could go to a terminal pole, or an open point where jumpers can be made round, or a set of PC400's for backfeed.

Transformer size depends on what it's feeding.
Could be 50kva, 100kva, 300kva....

Simply really.

 

[calum63]

thanks for the replies, so it is run in a radial finishing at a either pot end or at a linkbox ready to be backfed by another substation. Just one thing though I am going a bit off topic here but I was wondering, I have read on the news few years back about a lv substation getting damaged either by a car running into it. The reporter says that over 1000 or so homes have lost power but from one substation getting damaged? I know dno's use mad diversity but most substations I see only have 2 or 3 lv cables coming out switchgear, I highly doubt that those 3 lv cables will supply 1000 customers. This substation only had 2 lv cables coming out of it.

however I did think about the two 11kv ring main cables supplying the sub in question. could these 11kv cables being damaged affect other lv substations around the area?

what do you think?.

sorry if it doesn't make much sense

 

[Lectrician]

The substation being damaged could affect the HV supplying it, killing other subs, until rerouting or repair work was carried out. HV has fuses, and damage to a sub could blow those.

 

[PBC_1966]

Problems on the HV network can be a problem, and if a fault (due to faulty equipment, a tree coming down across 11kV lines in a storm etc.) can't be cleared immediately, what is sometimes done is to isolate that part of the network which can be isolated in order to restore power to other areas as quickly as possible.

Traditionally that required going out to sectional switches or other points at which a feeder could be disconnected, but increasingly the DNO's are using remote-control sectionalizers to enable such operations to be carried out much more quickly and restore power to those customers to whom it can be restored while they get the crews rolling to attend to the fault.

A few years ago just before I left England my local DNO was installing such remote-control sectionalizers all around the rural area in which I lived.