112kW Grid Tied PV System

cdcengineer

Lot's of great info on this site. I've been trolling for info for months as I've been working through a number of smaller residential and small commercial grid tied PV systems.

I am now involved with a team designing a 112kW grid tied PV system and have started my research on what the requirements are. I have contacted the local utility company, and this would be their largest project as this is a rural area and a smal rural utility cooperative.

Any advice on things to watch out for and what the major stumbling blocks might be for a larger system such as this? This is a rack mounted system which is connecting to a 480V, 3-Phase, 4-wire distribution system. We have (40) strings of (12), 235w panels which connect through a number of combiners before tying into a 100kW PVP Powered inverter.

My concerns are grounding and lightning protection.

Input is greatly appreciated.

niel

Re: 112kW Grid Tied PV System

welcome to the forum. we don't usually address large systems like that as most of us lack that kind of familiarity with something that huge. now is this one centralized system or is it spread out at various points? in reality, you may have access to more info than we do being you actually have this going up.
as what to watch for is an inspector always running up costs because he wants it his way, but that's normal and i think there would be little you could do about that.
do up a grand favor and elaborate on your proposed system here with some details if you would.

mike95490

Re: 112kW Grid Tied PV System

My concerns are grounding and lightning protection.

Consider what oil companies do - a row of towers around the site, with galvi steel cable strung between them, to intercept any lightning. each tower grounded , but not tied to your electrical system ground.

Of course, then you have shadow issues from the towers and wires, but even putting up flag poles with Franklin rods on top, will cause shadows.

Will you have any penalties imposed if you fail to produce power in fair weather ?

BB.

Re: 112kW Grid Tied PV System

From this Working Thread for Solar Beginner Post/FAQ for all new posters... Lots of different solar projects and information:

BB. wrote: »

A couple threads about Lightning:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's consolidated FAQ page:

www.windsun.com
Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section. [comment was regarding off-grid systems--but probably applies to grid tied too]

For a larger system--you probably will have to have it professionally designed--So there is less wiggle room for your specific requirements.

One suggestion about rural areas--Their line voltage can vary quite a bit, and also you can have surges/dirty line issues from heavy power users in the area (processing plants, etc.).

The inverters monitor the line voltage and frequency--And will drop out for 5 minutes (at least the "residential sized units") before reconnecting.

Many times, rural line may run near maximum voltage to allow for summer loads (pumping/AC/etc.) to drop the line voltage some.

With a large solar PV GT system, you will actually have voltage increase (instead of voltage drop) because you are sending lots of current out to the utility grid.

So, monitoring your line voltage/quality and getting an understanding of what voltage they will guarantee at the meter for your installation--Will the adjust the voltage/installation if it ends up being too high (with solar PV power adding to line voltage) and who will pay for any issues--May be important for you (you don't want high or low line voltage stranding your expensive Solar PV System because the inverters keep shutting down for safety).

As far as I can tell--You are pretty much at the mercy of the Utility (and local government/building inspectors) for what and where they will for your installation.

Also, monitoring... As discussed above, you need to be able to watch for grid interface issues--But you will also need to know how well your installation is performing for warranty/maintenance issues.

If this is a central inverter type system--You will have 10's to 100's of solar panels per large inverter. Ask them what happens if one panel (or a few random panels) fail or fail to meet specifications (80% minimum power).

Are you going to have somebody go out once a month with a DC clamp meter checking every solar panel string, send somebody out when output "looks low", or have current/voltage monitoring on every string.

Solar is highly variable and it is difficult to "see" 10% loss of power (dirty panels, failed panels, failed electrical connections, etc.) -- But that 10% drop in output may be your "profit" for your system.

My two cents worth (I am certainly no expert in this corner of the field).

-Bill

cdcengineer

Re: 112kW Grid Tied PV System

Thanks for the responses.

I am not the installer. I am contracted by the installing contractor to put together the electrical one-line diagram for the utility company. I haven't been involved with a system this large until now. I've been in discussions regarding a 100kW system, but the largest design I had done to date was an 11kW GT PV system.

The contractor sold this town-owned, wastewater plant a huge system. It was design-build, but the utility (surprisingly not the electrical inspector) wants an electrical engineer to put provide an electrical one-line diagram.

I'm just trying to get a feel for pitfalls and stumbling blocks to watch out for. As far as I can tell, it's just a larger version of the smaller residential systems which many of us deal with regularly. NEC 690 and local utility requirements are the major guidelines.. I would post photos and single-line diagrams if I could, but I don't think I have those types of privileges on this forum.

Thx

BB.

Re: 112kW Grid Tied PV System

You should be able to upload pictures and small documents. PDF files are the largest allowed. On the top line of the edit tool box is a paperclip where you can upload files:

[FONT=Fixedsys]
Filetype/Max Filesize/Max Width/Max Height
bmp     117.2 KB        620     280
doc     19.5 KB          -     -
gif     117.2 KB        620     280
jpe     19.5 KB         620     280
jpeg    117.2 KB        620     280
jpg     253.9 KB         -     -
pdf     781.3 KB         -     -
png     117.2 KB        620     280
psd     19.5 KB          -     -
txt     19.5 KB          -     -
[/FONT]

-Bill

cdcengineer

Re: 112kW Grid Tied PV System

Attachment not found.

Testing attachment

mike95490

Re: 112kW Grid Tied PV System

One line and photo - way cool. Re one line, condouit and wire gage looks small, is my only comment, and I'd install breakers instead of fuses, I worry the fuses will let loose randomly, but they are cheaper than breakers. Breakers, you can switch off if needed.

cdcengineer

Re: 112kW Grid Tied PV System

Good catch on the feeders. The #4's are now #2's (#3's would've worked if not for temp rating of the terminals). I'll try and post my one-line once complete.

cdcengineer

Re: 112kW Grid Tied PV System

My one-line file size is too large to upload. Perhaps I can shrink it down...

wdegrace

Re: 112kW Grid Tied PV System

You may have already caught this but most utilities are going to want to see required disconnects (definitelt an AC and most times a DC) Most are starting to require the proper signage, fuse sizing, GEC sizes etc. Also whether a net metering system or sell - all. Go to Bill Brooks' website - he has a Photovoltaic one-line that is interactive and has just about all the information that you would need to provide. Hope that helps.

BB.

Re: 112kW Grid Tied PV System

Is this the website you are typing about?

Brooksolar.com

-Bill

cdcengineer

Re: 112kW Grid Tied PV System

Thanks. We got this one dealt with. It should be up and running this week. Cool interactive website Brooks has.

arcturusk1

Re: 112kW Grid Tied PV System

I just wanted to say thanks so much for the Brooks link. That interactive PDF is great. Also, I've been using Brooks' drawings as a base and then making my own modifications for my drawings and everyone that I've submitted drawings to loves the layout! No need to reinvent the wheel, right?

Getting back on topic, though, this sounds like an exciting job. I echoed the same concerns on conduit and wire sizing, although it seems like you've tackled them. I'm jealous: You guys have 480V service so your currents coming out of such a large inverter isn't as bad. I'm dealing with slightly smaller ground-mount systems here (70-105kW) and we're going to feed services of 208V, meaning way higher currents.

I would love to see your combiner box and wiring layout as-is. That is, where did you locate the combiner boxes and are they just basic combiner boxes or do they have integrated DC disconnects? How long were your wire runs? EMT I assume? What was your conduit fill? Would you be able to or allowed to provide any of this information? Any tips or even just pictures would be much appreciated.