Volt drop calcs

[flyingsparks]

Ok, so have been back to the site today and had another meeting with the customer.

It turns out he wants to bring down the 3 phase grid supply to the house too, so increasing the volt drop to 6% for a private generator supply is out of the question.

Another idea which he actually had, is to use a couple transformers to increase the voltage at the digester end, and then drop it again at the house, to keep the cable size a bit smaller. Obviously depends on how much the Tx are but perhaps could be a possibly. I guess max could be 1000V AC as then you are out the remit of BS7671 and would mean getting high voltage engineers in?

 

[JohnW2]

It turns out he wants to bring down the 3 phase grid supply to the house too, so increasing the volt drop to 6% for a private generator supply is out of the question.

I have to say that I'm still a bit uncertain/confused as to where the 'origin of the installation' would be, either for the digester-powered or the grid-powered supply.

Another idea which he actually had, is to use a couple transformers to increase the voltage at the digester end, and then drop it again at the house, to keep the cable size a bit smaller. Obviously depends on how much the Tx are but perhaps could be a possibly. I guess max could be 1000V AC as then you are out the remit of BS7671 and would mean getting high voltage engineers in?

Indeed. As you may recall, my very first comment was:

I must admit that, although it's way beyond my experience, I might wonder whether LV was really ideal for distribution over a distance of 300m!

However, if you want to stay with LV (i.e. under 1000V), I would wonder whether the cost of the trannies would be justified (for an ~4-fold reduction in VD and required CCC). As you say, I guess the cost of the trannies would be a major deciding factor.

Kind Regards, John

 

[SimonH2]

as far as I'm aware they will get rid of the grid connection at the digester.

Generally, embedded generation is not capable of islanded operation - and before it can be connected to the grid the DNO will need to be satisfied that it will trip - not just if the site becomes disconnected, but if the local DNO network section becomes disconnected (eg the breaker in the feed to the village trips).
Key search term, "G.59" protection.

In addition, going off grid would preclude exporting excess to the grid (and hence missing out on FITs) or importing from the grid if generator output is insufficient or off.

It turns out he wants to bring down the 3 phase grid supply to the house too, so increasing the volt drop to 6% for a private generator supply is out of the question.

That makes more sense.

Another idea which he actually had, is to use a couple transformers to increase the voltage at the digester end, and then drop it again at the house, to keep the cable size a bit smaller. Obviously depends on how much the Tx are but perhaps could be a possibly. I guess max could be 1000V AC as then you are out the remit of BS7671 and would mean getting high voltage engineers in?

I can't help thinking that the cost of the transformers and medium voltage cable is likely to wipe out savings on the CSA of the copper.

 

[flyingsparks]

as far as I'm aware they will get rid of the grid connection at the digester.

Generally, embedded generation is not capable of islanded operation - and before it can be connected to the grid the DNO will need to be satisfied that it will trip - not just if the site becomes disconnected, but if the local DNO network section becomes disconnected (eg the breaker in the feed to the village trips).
Key search term, "G.59" protection.

In addition, going off grid would preclude exporting excess to the grid (and hence missing out on FITs) or importing from the grid if generator output is insufficient or off.

It turns out he wants to bring down the 3 phase grid supply to the house too, so increasing the volt drop to 6% for a private generator supply is out of the question.

That makes more sense.

Another idea which he actually had, is to use a couple transformers to increase the voltage at the digester end, and then drop it again at the house, to keep the cable size a bit smaller. Obviously depends on how much the Tx are but perhaps could be a possibly. I guess max could be 1000V AC as then you are out the remit of BS7671 and would mean getting high voltage engineers in?

I can't help thinking that the cost of the transformers and medium voltage cable is likely to wipe out savings on the CSA of the copper.

I hadn't thought of the cable voltage rating. Xlpe is rated to 600/1000V is it not? I guess the 100v rating is for AC, and hence suitable?

The generator is going to be kept connected to the grid to keep the FI tariff.

 

[flyingsparks]

It turns out he wants to bring down the 3 phase grid supply to the house too, so increasing the volt drop to 6% for a private generator supply is out of the question.

I have to say that I'm still a bit uncertain/confused as to where the 'origin of the installation' would be, either for the digester-powered or the grid-powered supply.

Another idea which he actually had, is to use a couple transformers to increase the voltage at the digester end, and then drop it again at the house, to keep the cable size a bit smaller. Obviously depends on how much the Tx are but perhaps could be a possibly. I guess max could be 1000V AC as then you are out the remit of BS7671 and would mean getting high voltage engineers in?

Indeed. As you may recall, my very first comment was:

I must admit that, although it's way beyond my experience, I might wonder whether LV was really ideal for distribution over a distance of 300m!

However, if you want to stay with LV (i.e. under 1000V), I would wonder whether the cost of the trannies would be justified (for an ~4-fold reduction in VD and required CCC). As you say, I guess the cost of the trannies would be a major deciding factor.

Kind Regards, John

Quite John, I'm starting to think this isn't going to go ahead, as the cost of a cable of suitable size is going to out way the benefits. Unless of course the tranny way is viable!

 

[JohnW2]

However, if you want to stay with LV (i.e. under 1000V), I would wonder whether the cost of the trannies would be justified (for an ~4-fold reduction in VD and required CCC). As you say, I guess the cost of the trannies would be a major deciding factor.

Quite John, I'm starting to think this isn't going to go ahead, as the cost of a cable of suitable size is going to out way the benefits. Unless of course the tranny way is viable!

Indeed. I should, of course, have said "reduction in VD or CCC (i.e CSA)" - you can't have it both ways!

Kind Regards, John

 

[Lectrician]

600/1000 is 600 to earth, 1000 between phases, as far as I am aware.

If you are bringing grid in too, your installation starts there, no questions, IMO.

Very rare to see generation that is able to generate in isolation from the grid. There is a site near me which does, but its controlled by National Grid, and essentially treated as a power station!

If the size of the generation meant an HV connection, with a private sub, the G39 would operate on the auto switching of the HV RMU. You could then treat your connection as a private sub.

 

[JohnW2]

If you are bringing grid in too, your installation starts there, no questions, IMO.

If the DNO supply is 'delivered to' (as in cutout and metering) the site of the generator, than that is clearly true, at least as far as the grid supply is concerned.

However, what about my original, more general, question which I posed when I didn't really realise that the grid was involved (except, perhaps, to sync the genny). If one has a private generation facility remote from (e.g. 300m away from!) the buildings it supplies, I would still have thought that (by analogy with DNO supplies), the 'origin of the installation' within a supplied building would be the point of entry of that supply into that building. Standard VD considerations would then apply within the building (VDs being measured relative to the 'local origin of the installation'). VD in the supply cable (from genny to building) would then, separately, be manifested as a requirement for the voltage delivered to the 'origin of the installation' to be within certain limits - like the standard 230V +10% -6%.

Is that thinking all wrong?

Kind Regards, John