Design Options Advice

P

PV Noob

Member
Location
Midwest
Occupation
Designer
Hello all, I need some general advice on the best way to approach this design.

The situation is there is a large residence on a large property with an existing 800A 240/120V 1p,3w service. The homeowner would like to do a large ground-mount array in an empty field approximately 1500' from the house. The array will be somewhere between 130-150KW. It was discussed to upgrade the house to a 3 phase service but that would be $500K for the utility work so the homeowner doesnt want that.

My first thought was to do a large 1500V inverter, say 125KW, run the strings into a DC combiner and then run one large DC source circuit back to the inverter at the service point. My problem is is that obviously inverters that large are only 3 phase and the house service is single phase. I thought they had transformers that were also phase converters, and they do, but they are as not nearly as common as I thought, and are mostly made by questionable looking Chinese companies, which I'm not sure the owner would go for.

The next best solution I see is to have several smaller single phase 600V inverters, probably eight 15KW, stacked near the service. Still have DC combiners with one larger source circuit from each combiner to each inverter. The big problem I see with this is I am going to have much larger voltage drop issues, going from ~1400V Vmp to ~500V Vmp. I am afraid the conductors are going to be too costly or maybe even too large to land on the inverter.

I am not really sure what other/better options there may be. Any and all advice would be appreciated!
 
Regardless of what you do you'll have voltage drop, so it's a matter of crunching numbers to see if there’s a cost effectice way to lessen it.

Voltage drop on DC conductors is just an efficiency loss, it's less likely that voltage drop (or rise, as it's often called) on the AC conductors of inverter outputs. Basically it's fine as long as you don't drop out of the MPPT range of the inverters. So my bias would be towards running DC for the 1500'. But this has been debated and may depend on the pricing you get for wire or components.

If you want to run AC for the distance you need to consider the voltage rise very carefully to ensure it won't go out of the acceptable range at the inverter terminals. A step-up/step-down transformer setup would be an option but I doubt it will pencil out.

I don't think there's any real option to get around using single phase inverters.

You should do some actual calculations to compare options instead of just 'being afraid' of lower voltage.
 
I don't know what the utility rules are in your area but I would drop a dedicated service to the field and leave the house alone. A residential 800A service will not likely have a 150kVA utility transformer.
 
pipe_bender said:
I don't know what the utility rules are in your area but I would drop a dedicated service to the field and leave the house alone. A residential 800A service will not likely have a 150kVA utility transformer.
Click to expand...
That would likely be pointless if the intent is to backfeed the existing meter for net-metering.

I agree about the existing transformer. The interconnection application for such a system should be filed well in advance of building it, in case the utility is going to charge for upgrades or refuse permission to connect. That's a ridiculous sized PV system for residential (but so is an 800A service).
 
jaggedben said:
That would likely be pointless if the intent is to backfeed the existing meter for net-metering.
Click to expand...
I am sure it depends on the grid operator where you are, Here the rules are very different from 60kVA and up to 1MW. Its a lengthy process the customer has to pay a engineering study fee and you typically have a separate CT can and service. The standard 'expedited process' for residential net metering is 15kW or less single phase or 25kVA three phase.
 
Fronius inverters do 1000v strings on their Single phase inverters, you may look into that. Also I think some XHHW is rated 1000v so you could look into that instead of PV wire. If you did go with PV wire, keep in mind it's available in AL. Honestly without running Any numbers, I would guess that would be your most cost effective option. You could step up to MV, say 7200 and use a single #2 CN cable which is only about $3.75/ foot, but the cost of the transformers will probably kill it.
 
Just for some comparison with transformers, this is probably the most cost effective option you will find. These are nice new surplus units , I actually bought some smaller ones from this same place for a residential setup / step down recently. You can just set them on the ground and build a dog house over them. Well worth the cost when you compare to the cost and lead time of pad mounts.

www.ebay.com

Pole Mount Transformer, 167 KVA | eBay

Single Phase - 167 KVA Transformer Primary Voltage: 2400V Secondary Voltage 240V or 480V
www.ebay.com
 
jaggedben said:
That would likely be pointless if the intent is to backfeed the existing meter for net-metering.

I agree about the existing transformer. The interconnection application for such a system should be filed well in advance of building it, in case the utility is going to charge for upgrades or refuse permission to connect. That's a ridiculous sized PV system for residential (but so is an 800A service).
Click to expand...
It needs to be connected to the house because eventually they want to think about adding batteries and going off grid. The existing utility transformer is 110KW AC. We are oversizing the DC size of the array to 130-150KW.

Perhaps 'house' is the wrong word. Wouldn't call it a mansion but its definitely not an average house. For reference, we received their utility bills and the home uses 240,000 KWh per year. Don't ask me what all they are doing in there.
 
  • Wow
Reactions: Zee
I agree with everything said, but I would also include undergrounding the runs to avoid cable chew from wildlife. There is soy in some insulation types that attract rodents? At least I hear that is an issue with some of the outdoor communication insulation. There was a post here a while back that showed the damage. The covered cable tray wasn't enough to keep them out.

*Photos are from their post*

5 minutes later.jpg

rodents .jpg
 
PV Noob said:
It needs to be connected to the house because eventually they want to think about adding batteries and going off grid. The existing utility transformer is 110KW AC. We are oversizing the DC size of the array to 130-150KW.

Perhaps 'house' is the wrong word. Wouldn't call it a mansion but its definitely not an average house. For reference, we received their utility bills and the home uses 240,000 KWh per year. Don't ask me what all they are doing in there.
Click to expand...
240,000 kwh usage?
Probably resistively heating an infinity pool, on a hill, with winds blowing across that 30' long, thin sheet of water cascading over the lip of the pool and coming from the warmest top layer of the pool.
Probably also the uncovered hot tub kept at a nice 100° without a cover.
Also probably heating the whole house - maybe even the crawl space - with electric resistive heat.
Seen it🤣
 
Installing several 240VAC inverters doesn't seem that bad.
It makes sense that with such an unusual house you'd have an unusual design.
The usage is about 40 times an average home!
The 800 amp service you describe for the residence is about as far out of the normal as the number of inverters I would expect to install.

Google AI says the largest single phase inverter is 16 KW.... but it's a hybrid, so more $..... AND from an unknown brand.( SUNKYS or something?)

Fronius may win with its 15 kW single phase inverter, the Primo 15.0-1.
( primo means single phase and symo means three phase in fronius speak , -1 also indicates single phase).

SMA has been promising an 11.5 kW,.....but not sure if it has landed ... And that's a hybrid so you may pay a lot.

Eight or so inverters should do it.
8*15 kW=120 kW

Pros are:
You can decide to run either AC or DC the 1500 ft .
These inverters are easy to bolt onto the back side of the ground mount racking.
No special cranes/ pads/lifts needed ....all of this is "light" enough to manage by hand.

I can't speak to larger commercial inverters and transformers.

Good luck, sounds like an interesting project.
 
PV Noob said:
It needs to be connected to the house because eventually they want to think about adding batteries and going off grid. The existing utility transformer is 110KW AC. We are oversizing the DC size of the array to 130-150KW.

Perhaps 'house' is the wrong word. Wouldn't call it a mansion but its definitely not an average house. For reference, we received their utility bills and the home uses 240,000 KWh per year. Don't ask me what all they are doing in there.
Click to expand...
Taking that house with that much usage off grid would be a serious challenge. The storage needs would be rediculous.
 
PV Noob said:
The array will be somewhere between 130-150KW
Click to expand...
150kW of Micro inverters on 240v services @ peak capacity is ~652 x 0.95 = 619 Amps, with no house load.

If utility doesn’t want it, you need an auto shutdown, or unconventional energy storage, like a permanently connected Level-3 car charger with empty battery.
 
Thanks for the replies. It sounds like the best bet would be to parallel several of the Fronius 15K single phase inverters since they can do 1000V on the DC side. Mount the inverters near the service and do the long runs with DC.
 
PV Noob said:
Thanks for the replies. It sounds like the best bet would be to parallel several of the Fronius 15K single phase inverters since they can do 1000V on the DC side. Mount the inverters near the service and do the long runs with DC.
Click to expand...
So just super quick and dirty: if you can get the voltage all the way up to the top, assume that will be around 800 volts (without looking up the actual mppt range of that inverter and not knowing your cold temperature voltage). That would be two strings of 19, each with a current of 10 amps and about 17kw per inverter with 445W modules. #6 Al for the strings should be adequate. Say 8 inverters so 16 strings is 48,000 feet of conductor. At maybe .25/ foot is $12k, really not bad in the scheme of things. That's for XHHW which is 1kv rated these days.
 
You must log in or register to reply here.
Top