50kwp install, First G59 adivice, submains advice.

[GreggElectrical]

Hi, I have been approached to install a 50kwp Pv system for a very local industrial client.
I have been a little worried about + 4Kwp installs but as the client is only 100yds from my door I thought id get my feet wet.

First off i would appreciate any general advice when quoting, installing and commissioning a 50kwp installation.

Secondly I have an issue with a 35mm sub-mains cable my competitors all want to use to connect the system. The cable is 35mm 4 core swa with a total length of 160m approx. So far the two other companies have said this cable is fine but I cannot see how. By any calc I do it fails miserably on volt drop even at 3%. Ive read PV should be 1%! Am I doing something wrong or have my competitors not bothered to calculate this?

Hats of to mark c, you seem to know your stuff!

Thanks,

Austin Gregg

 

[edexlab]

It is recommended that you aim for 1% VD to allow for grid fluctuations ,loading conditions ie the supply voltage rising or falling outside the inverters parameters resulting in system shutdowns , obviously if you have a submain which is 3% and the pv circuit has a long run you could go to 2%
the DTI bipv says this

The a.c. cable connecting the inverter(s) to the consumer unit should
be oversized to minimise voltage drop. A 1% drop or less is recommended.
However in larger installations this may not be practicable or economic
due to the very large size of cable resulting. In this case the designer
should minimise voltage drop as far as possible and must remain within​

voltage drop limits as prescribed by BS 7671
I must admit I've had the same headache ,and spoke to several engineers and Nic techline they all agreed that you should ensure the submain cable can take the extra load ie the inverter/s max output and have minimal VD,without allowing diversity for the generation
so really they're basing this on consistent generation output which is not necessarily the case with pv but is for diesel generators or hydro

 

[Markc]

Agree the 1% is the recommend policy.

I am in the process of designing a 21kWp system. My concern over anything is bank rolling it. I don't see any option on credit lines on the product so have to consider if we as a company could even fund it. Before anything we will be cashing out and loosing cash flow. Even with a deposit (which commercial clients dislike and expect a minimum of 30days credit) we will still be looking at several thousand pounds outlay. Just don't think we can do it.

We will also be looking at string fuses, relays and G59 all new to us.

Think I will work a design and price just as an exercise but think it's out of our league.

 

[JulianC]

Speaking of the 1% volt drop limit, does anyone know for sure if this applicable/relevant to a G59 system? I think not but stand to be corrected. The 1% limit was originally laid down to prevent nuisance tripping in G83 inverters in the event of a high mains voltage + a high volt drop sending the output of the inverter above its G83 trip setting. However, in G59, it is the G59 relay that monitors the grid and shuts the system down in the event of voltage or frequency wandering out of spec. Since the relay is generally located near the origin of the installation, can we be less concerned with achieving 1% drop from relay to inverters? I suppose this will be to some extent dependent on the 'anti-islanding' settings in whatever inverter is used. Be interested in anyones comments.

Looks like we are finally going to be installing one of the many commercial systems I seemed to have been designing over the last few months. Should start onsite in around 6 weeks. 50kW system using SMA TriPowers - built in electronic string fuses and no need for type B RCD's despite the transformerless design should make it a relatively straightforward job - perhaps worth considering Mark C? Shall be speaking to a proper electrician about confirming the AC hook up design 'cos it's out of this ex-housebasher's league!

 

[SolarCity]

Interesting point, Julian. I wasn't aware that nusiance tripping was the reason that we aimed for 1% volt drop maximum.

However, in a G59 scenario you still use G83 inverters so even though there is the extra protection of the G59 relay, the G83 inverter would still be subject to the same conditions as it would be in a standard G83 install so the problem with nuisance tripping would still exist.

 

[edexlab]

Good point

 

[JulianC]

Yup - all the inverters I've looked at for various G59 systems are indeed G83 compliant, so yes - 1% it is.

 

[GreggElectrical]

Hmm, Very interesting points here.
I am thinking along mark's lines and deciding if the job is really worth it. Credit isn't really an issue as the arrangement is for the client to purchase the equipment up front straight from my supplier, (obviously there is still something in it for me). Still the G59 is confusing for someone like me. I keep thinking installing the panels and inverters is straight forward enough but I'm concerned there is more to the G59 than I'm seeing. Ive dealt with supplies up to 800A with my previous job so the big stuff doesnt really scare me, just the numbers the quote is coming to!

As far as the 1% volt drop it seems as though this is applicable which is going to mean installing at least a 50mm sub-main. Interesting that the client has been told by my competitors that this is not necessary....

 

[BruceB]

1% is advisable, but I suggest not set in stone.
How much does the larger cable cost compared to the cost of the power lost in a smaller cable?
Which end of the cable is the total generation meter? If at the inverter end then power lost will not reduce FIT payment. If at the DB end it will.
What is the actual mains voltage on site? Is it high enough that the voltage drop in the cable is going to be enough when added to mains to approach the inverter cut out figure of 264V? If not then you may be willing as the designer to go above 1%

Regards
Bruce

 

[edexlab]

Sorry BruceB I don't follow this ; can you explain what you mean a bit more clearly please
the way i'm reading it it sounds as though you are saying " if the supply voltage is high (eg 250v +) " and " the volt drop in the cable is going to be enough when added to the mains"
do you mean low enough , because this sounds like you are adding volt drop on to supply voltage? making a larger value
"to approach cutout figure of 264v" (this must be the higher parameter)
I'm thinking if there is a volt drop across the supply cable then this lowers the value of the supply voltage perhaps below the lower inverter voltage parameter thus causing the inverter shutdown
please bear in mind I'm not trying to say you're wrong , Its just this does'nt make sense to me ,and i've read it about 10 times

 

[BruceB]

Sorry, my english was a bit abbreviated as it was a fleeting visit to the computer during the afternoon. The voltage drop in the inverter circuit has to be added to the local mains voltage to find out what the voltage will be at the output terminals of the inverter. And it is that voltage that the inverter measures to decide whether to shut down.

So if you have a low mains voltage locally, say 230V, then the voltage drop in the inverter circuit is most unlikely to be enough to cause any problem in respect of upper voltage shut down. But if your mains voltage is high, say 250V, then a few volts of voltage drop creeps closer to the G83 figure of a 264V maximum and it takes only a small transient on the line to take it over the top and cause a shut down.

Also, it is possible depending on manufacturer that the inverter is set to switch off slightly below 264V, in order to ensure it always meets the G83 requirement.

Hope that is clearer

Regards
Bruce

 

[BruceB]

I was asked to explain more so here is a picture. Standing by to have it torn apart!
Regards
Bruce

 

[SolarCity]

Ah! That makes more sense.

I have to admit that I was one of the ones scratching my head. When we are talking about volt drop, we are concerned with what we have by the time our SSEG's power reaches the DNO. If the DNO voltage is high, then this makes it harder to hit.

 

[solgen]

I also have a 50kWp opportunity and am in need of some guidance.

Am I right in thinking the only way to connect back to the grid is via a local 11kV transformer.

What if the Tx is a long way from PV site?

Who connects to the transformer?

Advice appreciated.

Thanks

 

[Worcester]

Most DNO's are OK with up to 50kWp connected to local 3 phase, - it's only ~ 17kW / phase = 70 Amps / phase. (depends upon what's already being supplied)

11kW would be for large scale generation

Plus read the G59/2 Guide to Connections