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.