Volt Drop in pv installs

[hduz5]

Hi Guys,

Looking for a bit of advice re volt drop and PV installations as I'm getting confused.

For example, a domestic job where the property has an outbuilding with a south facing roof. The customer already has a 100m length of 6 mm cable feeding the outbuilding. They wish to install a 3.96kw system on the outbuilding, but don't want to pay to have a new supply cable installed. For FIT purposes they want the inverter and the generation meter installed in the outbuilding so they get paid for all the electricity they generate. Does locating the meter and inverter there contravene any regulations?

Secondly, the set up gets run through sma design software and it hypothetically says that this system will have losses of 5% on the cable (don't know the actual figures, just making these up for an example). Is this a problem, other than 5% of the electricity being produced being wasted? If you were to take this to the extreme, could the system operate with 99% losses on a cable run, and still successfully import the 1% into the grid? What are the consequences of this? Is it dangerous or just massively wasteful?

Apologies if this isn't the clearest post. If anybody can shed any light on any of these issues I'd be very grateful.

Cheers

 

[Graeme Harrold]

There are many problem posts on here at the moment steming from undersized cables. 1% VD is the guideline limit and by going much adrift from this, you are only going to end up causing problems, Adding 16A to an existing circuit (supply to buildings) is also going to upset the existing calcs on the supply. Work it out and see what you come up with as a VD.

e,g, SMA 4000TL inverter suggests 6mm cable run of up to 23m!, you still have another 77 to go.............

 

[hduz5]

Cheers for the response.

When you say it deviating from 1% will end up causing problems are you able to be any more specific about what those problems will be? Put another way if you had to explain to a potential client (with a little knowledge) why they should upsize the AC cable if they reject economic concerns about 5% loss, how would you convince them a larger AC is required? Is a smaller cable actually dangerous? Will it actually stop the system functioning at all?

Cheers

 

[dansk]

Think of it as flowing water, you want as much water at the end of the pipe as you put into the pipe in the first place, and the only way to acheive this is by haveing an oversized tube - make sense? Squezzing electric down a narrow tube (cable in this case) creates heat build up and loss of volts over a long distance, and hence loss of generation.

Oversize the cable and power will flow freely.

 

[hduz5]

Cheers Dansk.

Using your analogy what if the customer challenges your first sentence - and is happy to accept having less water at the end of the tube than they put in at the beginning because of the cost of enlarging the tube? Would this system operate albeit inefficiently, or once the loss hits a certain threshold will the system cease to function?

Or put another way putting aside economic and all other arguments, what are the electrical engineering consequences of 5% or greater loss?

Apologies if I'm not making sense, I'm just trying to get to the electrical theory behind the 1% vd guidelines.

 

[TedM]

Grid-tie inverters can only operate within a defined range of grid voltage - 230V +10% -6% so 216V to 257V. If the volt drop at the point where the inverter is located is too much then the inverter will simply not work. Same problem if the voltage is too high. The 1% guideline is designed to make sure that this is not an issue.

 

[hduz5]

Thanks very much!! That's exactly what I've been looking for.

 

[Zipp]

What would be the better option in the case of the system installed in an outbuilding - have the inverter and generation meter in the outbuilding with a short DC run from panels to inverter and a long AC run to the house, or run the DC to the house and have the inverter and meter there?

 

[JD6400]

You will have substantially larger losses on the dc side over the ac side .

 

[sambotc]

I remember seeing a post on here not so long back about a similar question, and it was mentioned that the inverter may pick up the VD as a fault and trip out, is this correct or no?

 

[JD6400]

If it enough to take it out of it's G83/G59 parameters then yes it will !

 

[BruceB]

What would be the better option in the case of the system installed in an outbuilding - have the inverter and generation meter in the outbuilding with a short DC run from panels to inverter and a long AC run to the house, or run the DC to the house and have the inverter and meter there?

The answer very much depends on the circumstances and it is worth putting the design into something like Sunny Design to see the losses on the ac and dc sides.

DC cables can be operating up to 600V and up to 1000V with some inverters. In that case the currents are lower than an equivalent 240V current carrying the same power in an AC cable so it is easier and cheaper to make the losses less with a long dc cable rather than ac. The downside is that you sometimes have more than one pair of dc cables. 4 core swa is not expensive though.

A long ac cable can result in a large voltage drop (or rise at the inverter). Under G83 that is often not a problem as the upper limit is 264V so there is some wriggle room. Under G59 the upper liimit is 253V, the same as the DNO's upper limit. So much less wriggle room depending on local supply characteristics.

Regards
Bruce