Question re: central array with multiple remote connections/loads

DG98

Greetings, all.  I have perused this forum many times before as a lurker, and routinely see it referenced as the go-to knowledgebase on solar matters.  2018 has arrived, and I decided it is time for me to register and post a noob question for your experienced input -- perhaps it is simple, but it seems complex to me.  Please humor me a post which I suspect will become lengthy.  I see that many of the questions here are about very small scale, or even mobile, installations.  I am on the other end of that spectrum, with questions about what I’m guessing is a large residential installation, say 100 panels or so. 

My family and I live in East Texas (zip 75707) on a small ranch property.  Well, if you’re a major midwestern farmer, then you think we live on a postage stamp.  If you call NYC or LA home, then you think we have half a county to ourselves.  Despite being only 500 yards from the city limits, our location is electric-only as far as utilities go – no natural gas lines are available.  There is a rural water supply, but we have a well on-site and are planning for an additional deeper well to serve a new home (also in planning stages).  Propane is an option, but not one I favor for more than my cooktop. 

I’d like input about building out a grid-tie system, with a twist or two from the “normal” installation.  Our ranch has multiple structures which I would like to connect.  I have plenty of open / unshaded pasture area for a ground mount array.  The problem I perceive is the distance between the pasture and all the structures I would like to connect.  Approximate distances are something like this, measured from the proposed array site:

   - Existing Barn – 250’

   - Planned house – 700’

   - Existing house – 1200’

   - Shop – 1000’ 

(Note - existing house and shop are near one another, say 200' apart.  Planned house and existing barn are probably 500-600’ from one another.)

We’d be looking at what I’d call a “large residential” array – probably in the 30kw range.  The question is, can we build a single large array, and not have to do a smaller grid-tie system at each location?  From what little I know about electrical stuff, there’s a serious voltage-drop problem over some of those distances, even if you’re using the right wire and running 240v.  So finally I get to my question:  what about installing a step-up transformer at the central array site, and step-down transformers at each structure? 

I have no idea what the cost looks like, if the idea is even technically feasible.  Cost is not the only factor, though; avoiding multiple inverter systems and having panels installed everywhere would sure be preferable.  Additionally, the cost of bringing in utility company primary lines and transformers to two or three locations is obviously far higher than bringing in a single line to the central array.  The last time I checked, bringing in primary was $8-10 per foot, and none of my structures is closer than 600’ from the lines at the road.  In my mind, as long as it’s to code and safe, there really is no reason for the power company to have any idea what I am doing beyond the meter they set.  (Also, there is no permitting or review required in my area, being outside of city limits.) 

Now…just to add one more variable, I also have a diesel-driven 24kw generator, and what amounts to an unlimited fuel supply.  I realize that grid-tying a system with a generator presents unique issues.  However, power here can be intermittent at times, and the ability to disconnect from the grid and run off of solar and generator power would be very desirable.  I know most grid-tie installations do not have solar power available when the grid goes down, but I think that can be worked around.  It’s easy not to backfeed the grid using a transfer switch, but I believe that only certain inverters are set up not to backfeed a genset.  Thoughts on this would be appreciated.   

How’s that for a ridiculous first post?  I do appreciate you reading it, and the time anyone might take to chime in with input or responses to these issues.  Thanks very much.

Cheers, John

Estragon

IMHO, the distance issue would be best addressed by using (eg) 600vdc system available from Schneider and others in a central location, maybe a power shed located close to the load adjusted middle of the structures and grid service. There will be some drop running 240vac to loads in structures, and to/from grid, but shouldn't be huge at the distances involved.

I'm pretty sure there are ways to safely include generator power in the system.

You may not need permits from the town, but you may at least want to run the idea past the local utility. Larger (eg > 10kw) grid tied systems may need special utility engineering review, and may turn out to be impractical or impossible depending on their local infrastrucure.

mike95490

You could get separate meters for buildings, and limit arrays to 10kw.  The electric rates in Tx are pretty low, and net metering is not always possible with some of the small co-ops.  Over 10kw, and you may need special insurance as a power generation plant.

Distance is solved with proper wire size, and aluminum wire is cheaper than copper for long heavy runs, unspliced lengths in conduit don't have the issues aluminum had in houses in the 70's

BB.

You specifically said grid tied... Are you planning on one meter for the property or multiple utility meters?

More or less, for Grid tied... Assuming is it legal and cost effective (and approved by the utility engineer and the size of array you pare proposing), you would put your GT system right next the the power line drop/meter. You do not have to distribute GT solar inverters around your property--There is no reason to do that (at least from what I know about your system at this time).

Now, you are just looking at how to cost effectively distribute power around the property. You need to know the peak loads at each point (either in general, a 125-200 amp 240 VAC service typical), or the exact loads (residence, heating, cooling, hot water, cooking, well pump, etc.). If you are a "typical" high energy usage (i.e., ~1,000 to 3,000 kWH per month home--who knows how much for shop/etc.), you have to have a pretty beefy setup (lots of power). May make sense to bring in "higher voltage" AC and distribute to pole/ground mounted transformers (or even possibly have the utility bring power to each point and meter).

If you are looking to be a very energy efficient home (no AC, no electric hot water, cooking, drier, etc.), then you would have to look closer at setting up your own grid. It will always be a spread sheet type cost analysis. heavy cable and 120/240 VAC split phase (no transformers, but heavy cabling to buildings) or some version of higher voltage and step up/step down transformers (and their limitations/losses/costs).

Regarding the genset... If you have a main drop/meter/GT solar at the "curb", place your genset there with a transfer switch and power "everything". If you distribute high voltage power (600 or 12,000 VAC), still possible, but not nearly as straight forward (high voltage switching, high cost gear for something like an office building setup, etc.). Otherwise, it may be cheaper/easier to simply put your backup gensets at each site (say your houses, and one shop/barn) and not try to run a "mini-grid" backup system.

The system you are describing seems to be something that you would need a professionally licensed power engineer to design and oversee the installation of.

"We" can help you understand the building blocks and how some of this may connect together... There are a lot of safety (and cost) issues here.

-Bill

DG98

Estragon said:

IMHO, the distance issue would be best addressed by using (eg) 600vdc system available from Schneider and others in a central location, maybe a power shed located close to the load adjusted middle of the structures and grid service. There will be some drop running 240vac to loads in structures, and to/from grid, but shouldn't be huge at the distances involved.

You may not need permits from the town, but you may at least want to run the idea past the local utility.

I'll have to look into this -- higher voltage DC systems are new to me.  Of course, I thought DC was even harder to "push" over distance than AC.  Without revisiting the whole Edison vs. Westinghouse thing, I thought distribution was the main reason that AC is our standard.  The power shed idea is a good one, but I envisioned it being located with the array.  I'll have to look into locating it elsewhere. 

As for the utility, the local planner and project manager is a friend and has helped me with a couple of prior construction projects -- won't have any issue working with them.  If I could depend on (say) a 200A 240V circuit at those distances, then my question would be entirely moot.  Even if that required expensive cable, it would almost surely be cheaper than my transformer idea.  I didn't know that 10kw was a magic number, but lots of installs around here exceed that size.  The primary lines running past my property are 7200v, I believe. 

mike95490 said:

You could get separate meters for buildings, and limit arrays to 10kw.  The electric rates in Tx are pretty low, and net metering is not always possible with some of the small co-ops.  Over 10kw, and you may need special insurance as a power generation plant.

Distance is solved with proper wire size, and aluminum wire is cheaper than copper for long heavy runs, unspliced lengths in conduit don't have the issues aluminum had in houses in the 70's

On average, bringing primary to each of the four buildings, setting transformers, and installing a meter will cost me $10,000 -- so I'm $40k into getting power, all for the privilege of paying four bills with minimum charges each month, etc., etc.  This is precisely what I am trying to avoid with a central array -- bring in power ONCE.  I'm in a dual-service area, so I can throw over the crappy co-op for the "proper grid" with deregulated pricing, net metering, and so forth. 

I understand the distance-vs-wire-size thing, but most of my sources are telling me that 500-600' is about the end of practicality when you're talking about a 240v circuit.  I'm open to learning otherwise, but this is coming from multiple electricians that have done work for me before and who would be involved in this installation,

BB. said:

You specifically said grid tied... Are you planning on one meter for the property or multiple utility meters?

More or less, for Grid tied... Assuming is it legal and cost effective (and approved by the utility engineer and the size of array you pare proposing), you would put your GT system right next the the power line drop/meter. You do not have to distribute GT solar inverters around your property--There is no reason to do that (at least from what I know about your system at this time).

Now, you are just looking at how to cost effectively distribute power around the property. You need to know the peak loads at each point (either in general, a 125-200 amp 240 VAC service typical), or the exact loads (residence, heating, cooling, hot water, cooking, well pump, etc.). If you are a "typical" high energy usage (i.e., ~1,000 to 3,000 kWH per month home--who knows how much for shop/etc.), you have to have a pretty beefy setup (lots of power). May make sense to bring in "higher voltage" AC and distribute to pole/ground mounted transformers (or even possibly have the utility bring power to each point and meter).

If you are looking to be a very energy efficient home (no AC, no electric hot water, cooking, drier, etc.), then you would have to look closer at setting up your own grid. It will always be a spread sheet type cost analysis. heavy cable and 120/240 VAC split phase (no transformers, but heavy cabling to buildings) or some version of higher voltage and step up/step down transformers (and their limitations/losses/costs).

Regarding the genset... If you have a main drop/meter/GT solar at the "curb", place your genset there with a transfer switch and power "everything". If you distribute high voltage power (600 or 12,000 VAC), still possible, but not nearly as straight forward (high voltage switching, high cost gear for something like an office building setup, etc.). Otherwise, it may be cheaper/easier to simply put your backup gensets at each site (say your houses, and one shop/barn) and not try to run a "mini-grid" backup system.

The system you are describing seems to be something that you would need a professionally licensed power engineer to design and oversee the installation of.

"We" can help you understand the building blocks and how some of this may connect together... There are a lot of safety (and cost) issues here.

-Bill

Yes, grid-tied, single meter located at a central array, remote buildings connected back to that site.  I guess I wasn't clear or said too much, but you hit the nail on the head.  Run po-co primary to the central array site, set their transformer, set their meter, and set my grid-tie system.  On the other side, I'd just want to run a circuit to each of the structures mentioned above, at the approximate distances noted.  How to overcome those distances is the real question of the thread. 

Loads are only estimated at this point.  Shop and barn are both low consumers but with medium peak loads (air compressor starting, occasional use of MIG/TIG welders).  The existing all-electric house runs 2000-3000kwh/month depending on season, and the new house will probably be comparable (much larger, but much more efficient).  The solar aim is to offset what amounts to a luxury house -- my wife and kids aren't going to be without air conditioning in Texas in August.  I'm not wasteful about energy, but we work hard so that we don't have to live like survivalists. 

Safety issues are easily overcome by local professionals, and cost issues are easily overcome by working a little extra and saving a little harder.  Right now this question remains very much at the feasibility stage, and I'm still not sure I've got an answer on that.

If these extra details help, I appreciate the comments already offered and welcome any others.

Thanks!
John

DG98

Bump...any other thoughts following extra detail?  Thanks.

Estragon

AFAIK, it's not really a matter of being able to "push" DC over distances being harder vs AC. HVDC is used for utility scale transmission, as it has advantages over AC (eg skin effect), but is generally used only for quite long distances (100s-1000s of KM)because of the cost of going from AC generation -> DC transmission -> AC distribution. In your case, generation is DC to begin with (so no rectification loss), and losses to AC will be whatever, wherever you do it. My thought was to bring 600VDC from pv to a point close to the load adjusted center, and put DC/AC inverting and grid tie there, and that distances from that point would be workable for 240v split phase.

As Bill said, the wire size vs transformer thing will be a spreadsheet exercise. Knowing loads and distances, you can find wire size needed for acceptable voltage drop, and compare versus transforming from 3ph or whatever. Bulk aluminum wire may or may not make more sense, depending on local prices.