Long DC runs or long AC runs on ground mounted systems

[SolarCity]

This topic has sprung up a few times on the forum lately and I'd be interested to hear other opinions.

I believe that the inverter should be sited as close to the array as possible on ground mounted systems and that the AC cable should be sized to negate any possible power losses.

I've noticed other talking about running long runs in DC as the voltage is usually higher.

Pros and cons of both methods?

 

[BruceB]

I do not think there is a single correct answer. You have to balance a number of factors:

- power loss in ac cables vs power loss in dc cables
- respective costs of each cable
- convenience of locating inverters by array or in a building.
- Difficulty of keeping voltage drops low enough on long ac cables, particularly with g59 when protection is provided by or with iinverter rather than near origin of installation
- any safety issues with long dc cables

Going your way in all situations Biggs can result in a massive and expensive ac cable.

Regards
Bruce

 

[JulianC]

Personal preference is to keep DC short and inverter near array if possible. Dependent on mains voltage, it is often possible to size a reasonable AC run without volt drop being much of an issue, despite the much mentioned 1% being exceeded. Don't like the idea of long runs of DC at risk of being chopped through by Johnny farmer/gardener/groundskeeper. DC string voltage - the gift that keeps on giving...

edit: but obviously, as Bruce says, it's not always possible or feasible. However, in my limited experience to date it's not been a problem.

 

[jupestar]

I would love to hear more opinions on this. We are currently looking at one where the out building for the array is 150m from the incoming. AC is the way we have always done it, due to safety. but that is one hell of a big expensive cable to be run.

The Pro of running AC are safety.
The Con are cost.

Safety Vs Cost = Safety wins.

I'd be very happy to have someone convince me that DC is better or a safe way to run it.

 

[SolarCity]

I do not think there is a single correct answer. You have to balance a number of factors:

- power loss in ac cables vs power loss in dc cables
- respective costs of each cable
- convenience of locating inverters by array or in a building.
- Difficulty of keeping voltage drops low enough on long ac cables, particularly with g59 when protection is provided by or with iinverter rather than near origin of installation
- any safety issues with long dc cables

Going your way in all situations Biggs can result in a massive and expensive ac cable.

Regards
Bruce

Absolutely. I've just run 100m of 25mm 3-core cable in on a job this week. Although on this occasion it was the only option.

 

[Earthstore]

HVDC Cross-Channel - Wikipedia, the free encyclopedia
Try this link, 2000mw DC max transmission over 45 miles.
For the domestic install, my preference is still AC.
What would happen in 20 years time if house has had 4 owners, we are all dead from stress because of the FIT and some and some Muppet digs up a cable, if it's AC they would have protection (trips etc) if it's HVDC the only protection is the Armour.

 

[BruceB]

But in reality is there much danger from a buried dc cable dug up? The panels are double insulated. The digger bucket will hardly notice the current. An individual would have to be connected to both cables to get a shock.

Compare with live ac cables above our heads at 230 and 11,000 volts which regularly get pulled down by trees etc and breakers do not always trip when they touch trees or ground. Which is more dangerous?

 

[dansk]

But in reality is there much danger from a buried dc cable dug up? The panels are double insulated. The digger bucket will hardly notice the current. An individual would have to be connected to both cables to get a shock.

Compare with live ac cables above our heads at 230 and 11,000 volts which regularly get pulled down by trees etc and breakers do not always trip when they touch trees or ground. Which is more dangerous?

Good point, and they current limiting, not like AC side, but then thats protected.

 

[powervan]

Absolutely. I've just run 100m of 25mm 3-core cable in on a job this week. Although on this occasion it was the only option.Just getting my hacksaw out !! whats the address

 

[Maff]

100m of 25mm, better get a gaurd dog for when the local pikeys turn up

 

[JD6400]

But in reality is there much danger from a buried dc cable dug up? The panels are double insulated. The digger bucket will hardly notice the current. An individual would have to be connected to both cables to get a shock.

We have on more than several occasions been called to unearthed chopped cables and rather alarmingly regularly find that either the digger driver or ground worker has got down in the trench and moved the cable out of the way and just carried on !
Pound to a pinch of s""t when asked why !!!! They always say " well it must have already blown the fuse " :frown2: !

On one of last one's only about 3 months ago , they had somehow got through a 100A incoming concentric with a big flash so just pinned it out of the way with two metal stakes and finished off the trench then laid the bloody soil pipe as well because the nearest one of our lads were over 3 hours away and they had described it as private swa , or we would have told them to get the DNO in , and they needed to get on .
I ended going out to it in the end , I take one look and say we need the DNO , just out of interest I get down to have look and check it is safe ?
And no it was not , how they managed to chop it in half ( taking a fair bit of bucket with it ) and not blow anything ? I can still picture the old boys face that put those metal stakes in , as they say priceless !!!!!

 

[JD6400]

100m of 25mm, better get a gaurd dog for when the local pikeys turn up

Just done a 480 m 95mm 4 core !
Forgot to leave the offcuts on site !

 

[powervan]

Just done a 480 m 95mm 4 core !
Forgot to leave the offcuts on site !

500 mts of 95mm i could retire on that after ive run the pen knife down it (checking for DEAD ) first lol

 

[JD6400]

500 mts of 95mm i could retire on that after Ive run the pen knife down it (checking for DEAD ) first lol

One of the larger ones that we have had for a while !We did two about four years ago in Linc's , on neighbouring property's not more than a month or so apart owned by two different people ! 2 x 580m 120mm 4 core , normally we do not take payment until completion of the job but on these occasions !!!!

We did pull a 3.6km and 2x 1km run of fibre on a locall estate last year , now that was a absolute nightmare , especially after we realised it was the wrong speck after 2.6km was already in !:furious3:

 

[SolarCity]

100m of 25mm, better get a gaurd dog for when the local pikeys turn up

They're welcome to try.

It took 3 men most of the day to pull it through a 90m duct. Absolute mission.

 

[Gary K]

They're welcome to try.

It took 3 men most of the day to pull it through a 90m duct. Absolute mission.

But it will only take about 30 seconds to pull it out with a truck if you are not worried about the damage!!!

 

[powervan]

i do recall pulling in a 35mm (or sim) with a toyota celica st (choc brown) at reading university !! the engineer i was a scream !! wot fun my apprenticeship was)) good old days!!!!

 

[synergy power]

Great thread guys. I was actually pricing up 2 4kw ground mounted systems today and i was wandering whether to mount the inverter by the array and have long AC run or vice versa...

I already decided to keep the DC run short, but this thread has now confirmed my decision.

Thanks

 

[JD6400]

We actually do use a Yamaha 350cc big bear quad thats kitted out with a winch , to move the moiling equipment trailer around when on site which is also used to pull in the larger cables when it is safe to do so .
I can not remember what the name is for the type of transmission it uses but it has not got a clutch as such and is sort of rev and go which with experience you can feel the tension that you are pulling at through the bike and means you do not stress the cable as you may do using other methods that i have seen ! ( 4x4's , tractors , teleporters , diggers even cherry pickers ! )
It has saved a fair few trips to the chiropractor !

 

[moggy1968]

it'll be a PTO (power take off) I would imagine.
I've got a 350m metre runof 16mm to price up. I am expecting to have to use 50m sections of cable and use resin joints to connect them together, does that sound feasable. We don't generally get involved in this type of work so don't have the kit to manage massive rolls of SWA.

A South African mate of mine was doing this sort of stuff for a couple of years. He used to load the off cuts into his landcruiser at the end of the week and take them down the scrappie. On the proceeds He built his own yaught (i could never spell that!) and spent the next 5 years sailing round the world!!

 

[Earthstore]

It is spelt boat (with a sail)

Hope this helps

 

[JD6400]

Hi moggy , The 16mm should be quite easy i would have thought you will just need some extra help ( be it mechanical or several pairs of hands ? ) to get it on to the cable jacks .
The jacks or rollers are essential and if you do not have them ask at your wholesalers as they usually keep a set to hire / lend if needed !
As for the joints , i to be honest would deem it to be bad practise to introduce unnecessary joints in to run .
Good look with it and just make sure you have allowed for enough man power !!!!!

 

[powervan]

it is spelt boat (with a sail)

hope this helps

posh boat

 

[powervan]

Hi moggy , The 16mm should be quite easy i would have thought you will just need some extra help ( be it mechanical or several pairs of hands ? ) to get it on to the cable jacks .
The jacks or rollers are essential and if you do not have them ask at your wholesalers as they usually keep a set to hire / lend if needed !
As for the joints , i to be honest would deem it to be bad practise to introduce unnecessary joints in to run .
Good look with it and just make sure you have allowed for enough man power !!!!!

As with any job its all about having the KIT!! once ur backs gone its gone for LIFE!!

 

[TedM]

My wind turbine is connected with 450m of 3 core 16mm SWA (the turbine output is 3-phase wild AC). Putting it in was pretty easy even from a 500kg drum held just off the ground on a scaffold frame. I'd also suggest avoiding joints.

 

[moggy1968]

thanks for the tips guys. I was thinking hiring a cable roller trailer to attach on the back of my landrover might be the way ahead. just drive along slowly and spool the cable off the back.
do you know any good suppliers of large SWA like this. can't see CEF doing it too cheaply!!

 

[JD6400]

You could use CEF , but newys tend to be better on price !
You can deal direct with the likes of Batt cables or Cleavland if you can be arsed to open up account .

 

[yellowvanman]

thanks for the tips guys. I was thinking hiring a cable roller trailer to attach on the back of my landrover might be the way ahead. just drive along slowly and spool the cable off the back.
do you know any good suppliers of large SWA like this. can't see CEF doing it too cheaply!!

Any wholesaler can supply that sort of cable, they would just get it delivered direct from the supplier. I've ordered 560m of 10mm SWA before. I generally use 2 core if I can, saves a bit of money.

Ask all the suppliers in Blandford for a price!

 

[moggy1968]

beleiev it or not, no accounts with suppliers in Blandford!
Most of my business is actually in Poole so have accounts there, might try Kew electrical tomorrow.

 

[danesol]

All cables should be as short as possible as good practice IMO, but remember that the most efficient way of transferring electricity is High Voltage Direct Current, so with this in mind, I would say more notice should be taken over AC connections ( ie. size of cable used and the length used - always best to oversize perhaps seeing the cost of cable is really insignificant especially SWA) , however the absolute length is really out of your control when the inverter is close to the PV array in the loft and the cable is therefore routed by the most direct route to the CU

In the case of ground mounted systems - most I've seen have the Inverters immediately under the panels, but I would think you would be better off having the Inverter "safe" in an actual building and running long DC cables with SWA given that say DC is the most efficient way of transport ??

Summary:- In essence I think this depends on the actual size of the system - the bigger the system ( G59 ), the greater important of using longer DC runs possibly - check the sums !
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[Mark42]

… do you know any good suppliers of large SWA like this. can't see CEF doing it too cheaply!!

Have a look at:

Industrial SWA Cables

I bought some 10mm 4-core recently and they were a pound a meter less than anyone else. (And it didn't have mud all over it and the end hacksawed off )

16mm 2-core £3.40/m Not sure how it compares. TLC is £4.05.

Evel cables also used to be good, but their web site has been ‘closed’ for ages.

CSE, last time I got a quote for SWA, was expensive.

 

[powervan]

My wind turbine is connected with 450m of 3 core 16mm SWA (the turbine output is 3-phase wild AC). Putting it in was pretty easy even from a 500kg drum held just off the ground on a scaffold frame. I'd also suggest avoiding joints.

Get U MY WIND TURBINE !!! lol 450mtrs wots volt drop on that?? oh yeh is it that far cause u have a big garden or u nicked a supply from the local wind farm ps NO JOINTS always trouble (in the end)

 

[yellowvanman]

16mm 2-core £3.40/m Not sure how it compares. TLC is £4.05.

Got mine @ £3.30/m last week from my local wholesaler, sorry!:smug2:

 

[moggy1968]

who was that off matey?

 

[SolarCity]

£3.30/m for 16mm 2core? That's almost '90s prices!

 

[yellowvanman]

who was that off matey?

Cribbs

 

[Mark42]

Got mine @ £3.30/m last week from my local wholesaler, sorry!:smug2:

Oh don't apologise It's not ME who's flogging cable - that was a link to an internet reseller I use. I don't know if I misinterpret it, but they do appear very cheap.

 

[TedM]

Get U MY WIND TURBINE !!! lol 450mtrs wots volt drop on that?? oh yeh is it that far cause u have a big garden or u nicked a supply from the local wind farm ps NO JOINTS always trouble (in the end)

My 'garden' is 18.5 acres. :43:

 

[JD6400]

Just got a price out of interest from our rep at CEF and we get it at £3.18/m , with our early settlement discount it would be £3.10/m .
Reckon you could batter your suppliers down even more !:6:

 

[moggy1968]

I'm looking at 4 core, cos I'm running two strings. (the run is for the DC)
the internet guys above came out at about £2700 for 400m