Very long run of LV Cabling

If you are supposed to consider all the options, another to think on is a pair of private transformers - step up to 690 single phase, or even 999V split phase,  (anything not quite HV )  and then step down again at the load end. Allows a smaller, cheaper cable size that may pay for the transformers, and quieter than the genset.

 There is a magnetising current in the transformers but that is mostly inductive and can be tuned out if need be with shunt capacitors.

Done in large  factories, and at least one poultry farm in Essex.

Mike. 

PS  late edit.

A single 20kVA unit in a box will weigh in at about 150-180kg, be a cube of about 50-60cm per side, and have thermal losses of about 5% at full load.

A pair of transformers is certainly an option, but remember that the customer will be paying forever for the iron losses, and also paying for the copper losses when loaded.

If going down this route, select the transformer taps to give about 250 volts at the load end, NOT 230 volts. This will give more leeway for voltage dropping under load. If a voltage regulator is installed at the load end then this will help with proper equipment operation but it adds another layer of cost, complexity, and another single point of failure. Remember that a voltage regulator compensates for voltage drop, but that the losses still exist and are being  paid for.

The simplest option is almost certainly either three phase 400 volt or split phase 460 volts if available at the supply end.

The cheapest option is 2km of DNO overhead line. Going up in voltage is possible, but the change to under a kV is probably not worthwhile and does have the snag of continuous losses, probably around 500-1000 VA for 2 20 kVA transformers. This would cost around £2000 a year at today's prices, similar to 4000 litres of red diesel which would operate a 20 kVA generator on little load for at least 2000 hrs. The next thought is the load profile, how much, how long and when is important. If it is power 24/7 the DNO line is certainly best, if little load for a few hours a day the generator might be cheaper overall, if occasional full load but usually nothing, again the generator would work.

The next point is that if you use regulated LED lighting you can allow much more volt drop, it will not cause flicker with load changes. For most other appliances it makes little difference and electronics doesn't care at all. So we could say that 10% regulation is ok, saving a large amount of money, but with the snag that we will lose some power in the resistive loss in the cable. The cost of this loss can again be calculated and informed choices made.

The cheapest option is 2km of DNO overhead line.

But check options with your DNO - my local one seems to have a policy of all new HV lines being underground - and only allow overheads if it's not physically possible to trench (e.g. crossing waterways). Oddly new pole mounted transformers are still done, fed from underground cables. Underground is likely to be significantly more expensive than overhead would have been.

   - Andy. 

Thanks all for the contributions, to give more background. The site is due expansion in the next couple of years where a HV network will be integrated more readily, within 300 metres or so.

 I also have seen what Andy has mentioned on DNOs being overhead line averse especially with a recreational airfield and way leaves involved.

This is the reason the Genset is my preffered option, at 50% load PRP rating the set only uses 2.5 l/h, I’ve proposed a 2000L above ground tank and based on 6 hours a day, and a business week where the user switches the generator on/off, you’ll be looking at under 20K over a 3 year period. (even with the red diesel ban in April) and maybe 10K in maintenance  over three years including a service for every 450 hours of work done, and maybe 30k for the installed equipment, this would then also facilitate a sensible LV DNO connection in the future.

I still think the 850m of LV is viable, but defiantly on the extreme side from a cost perspectiv, with a potential risk of theft also

thanks again all

Consider also the risk of theft of the diesel as well as the cable if it is an unsupervised site but with vehicle access.

The fact that it will all be un-needed after 2 years is an interesting factor - certainly there is a reason very short life events use hired gensets - it may be worth a quick call to the usual suspects asking about very long term rental prices.

Mike.

If a generator is needed for two years, outright purchase is probably cheaper. After the two years it might be worth keeping it for standby purposes.

If a generator is to be regularly used for two years, it might be worth utilising the heat from the engine cooling system for central heating. Free heating, apart from the cost of the plumbing works.

Dont forget a battery bank for lighting, refrigeration, and small important power loads. Either with an inverter or specify ELV DC for lighting and refrigeration. It is exceedingly inconvenient to manage without electric lighting on demand these days.

davezawadi (David Stone): 
What red diesel ban? 

From April 2022 the range of permitted usages of rebated (red) diesel is being reduced - it will continue to be allowed for its core usage of things like agriculture and domestic heating, but will be removed from things like commercial heating, construction etc.

As announced in the 2020 budget, the red diesel reduction of scope has been coming for a while, although the reduced tax rate will remain, for now, for forestry and farming and non-commercial heating. 

Building sites, commercial heating, generators at pop festivals and so forth will need to switch to DERV and pay duty at that rate. As you say that alters the economics quite sharply.

Why do you think industrial estate roofs are being plastered with solar panels etc for the last 2 years and folk like aggreko and pytch are practising towing pre-charged batteries onto festival pitches so the thing can run on inverter for half the time instead of burning half a litre of diesel per head per weekend  ? 

Economic stimulus works, - round here  there can't be many square feet of sawtooth roofs left to do, and the tow on battery pack idea  is certainly taking off.

Mike