NEWBIE QUESTION: Long run of wire

[realpinochet]

Correct on the panel arrangement. "s" just means series, "p" means parallel. Ideally you would want to combine as many as you can in series for a higher voltage and same current for more power and less power drop over the wiring. Having said that, depending on what your inverter can handle you may have to limit he number of panels in series and the number of strings in parallel that you can connect to an inverter. Also, you can't really connect strings of different length to the same inverter input as it would produce uneven voltage between the strings and one of them would operate at sub-optimal performance. You can compensate for this kind of discrepancy with optimizers which are essentially buck/boost converters that will change a panel's output voltage/current to meet the other panel's output voltage (strings) or current (panels within the same string).

As for the 40V panel voltage, that initially came from assumptions that were later confirmed by the original poster posting the spec sheet off the panel. But as to where the assumption came from: Solar Panels are made out of photovoltaic cells which are essentially reverse LEDs, rather than turning electricity into light, they turn light into electricity. Since they're semiconductors with PN transfer they behave like a diode and generate roughly 0.6V per cell.

Now, PV panel manufacturing has largely been standardized and large (roof mount) ones usually come in either 60 or 72 cells, or 120 and 144 half cell arrangements which means they typically have 60 x 0.6 V = 36V or 72 x 0.6 V = 43.2 V Voc. The 36V panels are your typical 350-380W panels (they call them residential) and the 43V panels are usually (called commercial due to larger size) are generally in the 400-450W range.

So when OP stated they have 400W panels it was an easy guess to assume that these panels would be your 72 cell designs which are ~43V Voc or thereabout.

Unlike many would like you to believe, solar panels are a commodity product that are largely one and the same. The difference between the most efficient "best" panels and the cheapest ones is marginal. Their specs are usually extremely close and you can interchange one with another in a design without having much-if-any ill effect on the rest of the design. The difference is what horse are you willing to bet on that they will be there for you 10-20 years down the line and provide warranty and which are fly-by-night companies who will leave you high and dry should a panel fail. I would not spend significant money on "premium" panels. Paying 20% premium for 10% increased production is a fool's bet. Unless you're severely strapped for space, your best bet will always be the cheapest panel per Watt.

Thanks greg for that update!

 

[740GLE]

AC and DC will suffer the same loss at the same voltage/current. The benefit is in the much cheaper and easier transformer solution to boost AC Voltage over boosting DC voltage.

One downside of AC over large distances is that the conductor will actually have a capacitance factor which due to fancy math may actually raise voltage at the remote terminal end under light loads. Where DC that capacitance isn’t an issue.

Not saying this will be an issue hear, but it an actual phenomenon/issue and one of the reasons DC is chosen for high voltage long distance power transmission.

 

[SolarTexan]

I am totally off-grid.

My power room is located in my shop building some 500+ feet from the house. I am using three Sol-Ark 12Ks in parallel, and I use a 37.5kVa step up transformer to get to 480v and then a 25kVa step down transformer at the house to bring it back down to 240v. I did this to reduce the wire size necessary to provide power to the house, and the smaller wire size was easier to install it ourselves. I have had this setup for almost a year with no problems.

I intend to build another house on the property, and the power room is centrally located between the two houses. The second house will need it's own step down transformer as well.

 

[GregTR]

Or call it an electric fence at 10kV, kill two (or three) birds with one stone?

But seriously, silicon is cheaper than copper (or aluminum) these days. The point is that 3% or so voltage drop only really matters for battery and AC wires. For PV wires, the goal is adequate performance at the best price. Much higher IR drop at peak current can be tolerated with a PV array.

You have to keep AC voltage drop to a few percent, or won't be able to start motors that draw 5x their rating.
PV DC, you could design a system with 50% voltage drop (at peak current) and make it work. That high loss would only occur during times of peak production, and you probably won't care. Other times at half the production you'd only lose 12.5%, so it wouldn't put a dent in winter production.

Two Sol-Ark, 30 x 400W = 12kW of panels 1000' away. 40 Vmp, 10A Imp.
Orient 3kW of panels SW, 3kW SE, connect in parallel. About 4200W peak, more hours of operation.
7s2p, use 10 awg copper (or slightly larger aluminum).
Second array 8s2p (if Voc limit not exceeded in cold) for second inverter.

With two strings oriented about 90 degrees apart, peak wattage and current about 0.7x as much, 15A.
2000' of 10 awg is 2 ohms. I^2R = 15^2 x 2 = 450W loss, out of 14 x 400W = 8%. Not a big deal.

Is nobody going to comment on two Sol-Ark being a serious overkill for 30 panels?

 

[Hedges]

Is nobody going to comment on two Sol-Ark being a serious overkill for 30 panels?

Not if he wants 24kW of AC.

I use a 37.5kVa step up transformer to get to 480v and then a 25kVa step down transformer at the house to bring it back down to 240v.

Is that first transformer designed for step-up, or are you using a step-down transformer backwards?
I got a 480 delta step-down to 120/208Y and tried to use it backwards, but it doesn't seem happy.

3-Phase SMA Sunny Island (15kW) + Sunny TriPower (30kW)

Showing a work-in-progress assembling 3-phase SMA system. 3x SI-5048-US are configured for 3-phase and produce 120/208Y, rated 15 kW continuous. Sunny TriPower 30000TL-US-10 is 480V delta (or 277/480Y), rated 30 kW. TriPower combiner box will be used, but for now just one string being connected...

diysolarforum.com

 

[SolarTexan]

Not if he wants 24kW of AC.



Is that first transformer designed for step-up, or are you using a step-down transformer backwards?
I got a 480 delta step-down to 120/208Y and tried to use it backwards, but it doesn't seem happy.

3-Phase SMA Sunny Island (15kW) + Sunny TriPower (30kW)

Showing a work-in-progress assembling 3-phase SMA system. 3x SI-5048-US are configured for 3-phase and produce 120/208Y, rated 15 kW continuous. Sunny TriPower 30000TL-US-10 is 480V delta (or 277/480Y), rated 30 kW. TriPower combiner box will be used, but for now just one string being connected...

diysolarforum.com

I am using a step-down transformer backwards.

I consulted with the transformer company before I did this just to be sure it would work. They told me it would work, but to be aware of high inrush current, possibly several hundred amps, feeding into backwards transformer upon power up. I then contacted Ametherm, and pieced together an inrush current limiter/contactor solution to prevent tripping the load breakers on the Sol-Arks.

It works perfectly.

 

[niktak11]

Is nobody going to comment on two Sol-Ark being a serious overkill for 30 panels?

Not really. I only have about 6kW connected to my 15k at the moment.

 

[740GLE]

Not if he wants 24kW of AC.



Is that first transformer designed for step-up, or are you using a step-down transformer backwards?
I got a 480 delta step-down to 120/208Y and tried to use it backwards, but it doesn't seem happy.

3-Phase SMA Sunny Island (15kW) + Sunny TriPower (30kW)

Showing a work-in-progress assembling 3-phase SMA system. 3x SI-5048-US are configured for 3-phase and produce 120/208Y, rated 15 kW continuous. Sunny TriPower 30000TL-US-10 is 480V delta (or 277/480Y), rated 30 kW. TriPower combiner box will be used, but for now just one string being connected...

diysolarforum.com

Are you trying to use the delta transformer as split phase 120/240, or three phase 120/208? To source 120v you’re going to need a proper ground at the load point, or possible rewire the receiving transformer in a grounded wye configuration.

If you’re trying to use only two legs of three phase delta transformer that maybe the issue.

 

[burgerking]

can build your panel mounts nearer?

 

[Kornbread]

Din rail for 2/0 or 4awg?

Whichever. The link in post #38 is a Mouser search bar for terminal blocks. This link to din rail terminal block are a brand I used to take 2/0 al triplex to 4awg copper. Tidy solution to the problem of wire size/conductor type changes. Eventually, I'll extend the din rail and the bundle of wire on the left will get terminal blocks and redundant breakers. And no, I'm not a fan of polarized breakers, but did use them here.

 

[GregTR]

Not really. I only have about 6kW connected to my 15k at the moment.

That is a different scenario.

You have an oversized single inverter with option to expand later. That is just planning for the future and there are other benefits with going with a 15K over an 8K such as the 200A pass through.

But why bother buying two 15Ks when one could easily handle the current number of panels?

Buy one now, hook up the panels and buy the second inverter when you're in need of expansion. Putting $8k+ into something up front that is not needed just does not seem like a financially smart idea.

I guess the only reason would be if they really need 24kW power occasionally without grid, two 15Ks with batteries would allow for that while a single one wouldn't.

 

[OzSolar]

Din rail terminal blocks are a cheap and reliable way to make the splice Din Rail Terminal Block

Interesting idea, I'll have keep that in mind for future projects. I do think it would be more labor to install DIN rails vs. Polaris lugs. At shop rate we have to be pretty careful when adding labor to save money on material.

Regardless the point was that in my case it didn't pencil out to mess around with stepping up to much larger wire. "your mileage my vary"

 

[Hedges]

They told me it would work, but to be aware of high inrush current, possibly several hundred amps, feeding into backwards transformer upon power up. I then contacted Ametherm, and pieced together an inrush current limiter/contactor solution to prevent tripping the load breakers on the Sol-Arks.

So far, it wasn't a problem for me. Feeding 120V legs I captured a peak of 80A for one cycle, within inverter's surge capability. Feeding in the forward direction (I have another transformer that does what I need), I saw 70A at 277V, similar current almost 2.5x the power during surge.

Sounds like you use a delayed relay to short out the Antherm thermistor, which should be an improvement over always having it in the circuit.

How much steady-state current does the the transformer draw? When I drove mine in the forward direction, from a 277/480Y transformer into its 480 delta, I got 1 Arms, something around 800 VA no-load. Haven't determined how much of that is "real" loss vs. reactive, and haven't checked inverter power consumption feed it.

Are you trying to use the delta transformer as split phase 120/240, or three phase 120/208? To source 120v you’re going to need a proper ground at the load point, or possible rewire the receiving transformer in a grounded wye configuration.

If you’re trying to use only two legs of three phase delta transformer that maybe the issue.

I initially fed a single 120V phase from an extension cord which gave the worst result. It has 3-leg "E" core, so the three phase windings interact.
I then drove 120/208Y from 3x Sunny Island inverters configured 3-phase into the secondary. I left midpoint of transformer disconnected (wired with 3 + ground romex), but that shouldn't matter no load or symmetric load since neutral current could be zero.

My purpose in using the transformer is to step up 120/208Y to 480 delta, for a 3-phase solar inverter Sunny TriPower. That will work without neutral, but I want to protect inverter against seeing 480Vrms from ground (in the event a line shorts, becomes corner-grounded delta). I think I could do that with two single phase transformers wired in a "T" to find the midpoint, but I bought a "zigzag" transformer rated 600V 15A, which is the accepted way to do it. That would provide a neutral for 277/408Y.

But my problem with this particular transformer is it seems to be driven too far into saturation operating steady-state with 120/208Y input. Something about how it was designed, I suppose. It is rated 5.99% impedance, which does not seem to be uncommon. Some other brands are rated 2%. That may relate to how much leakage inductance on primary (and how much resistance.) This apparently means if 5.99% of rated voltage is applied to 480V secondary, and primary is shorted, it will carry full-load amps. At first I thought that also related to prospective fault current, but maybe not because core not saturated during such a test.

For OP, this topic is a heads-up that if stepping AC up and back down with transformers, using a transformer designed as "step-up" is the only guaranteed satisfactory solution. Transformers are not as reversible as I thought, and using a pair of identical transformers for step-up and step-down may or may not work; we're seeing differences between models.

 

[740GLE]

Single phase step up and step down transformers are the same thing just depends how it’s wired.

When dealing with single phase units it’s much easier than three phase all in one units as you discovered.

 

[SolarTexan]

So far, it wasn't a problem for me. Feeding 120V legs I captured a peak of 80A for one cycle, within inverter's surge capability. Feeding in the forward direction (I have another transformer that does what I need), I saw 70A at 277V, similar current almost 2.5x the power during surge.

Sounds like you use a delayed relay to short out the Antherm thermistor, which should be an improvement over always having it in the circuit.

How much steady-state current does the the transformer draw? When I drove mine in the forward direction, from a 277/480Y transformer into its 480 delta, I got 1 Arms, something around 800 VA no-load. Haven't determined how much of that is "real" loss vs. reactive, and haven't checked inverter power consumption feed it.

I haven't measured the current being drawn by just the transformers. I made sure to get DOE2016 rated transformers for the efficiency, and the specs state "no load loss" of the 37.5kVa and the 25kVa is 85+54=139 watts, and the "full load" losses are 1470+1090=2560 watts.

At night, I see the load on my three Sol-Ark 12Ks drop down to 375-400 watts at their lowest, but keep in mind that I have 8 LED flood lights on, the air pump for the septic system, refrigerator, and other parasitic draws.

 

[Hedges]

Single phase step up and step down transformers are the same thing just depends how it’s wired.

Difference is also how they are wound.

Secondary is wound against core, primary is would on top of secondary (at least some designs.)
My transformer (pictures in thread I linked) also has air gap between each layer of primary winding. That may be for cooling, but also would mean even more "leakage inductance", inductance to air rather than coupled through core to secondary. This reduces transformer inrush.

Basic transformer model is two inductors with 1.0 coupling factor. Turns-count ratio equals voltage ratio, and inductance goes as turns squared.
By measuring inductance with other winding open/short, we separate out leakage inductance. Coupling factor k = 0.999, or 99.9% of inductance is assigned to 100% coupled inductors, 0.1% assigned to separate inductor. Resistance of winding is also seen by measurement.

Further refinement determines saturation, remanence magnetization, coercive current required to return magnetization to zero. Besides saturation (related to inrush when first powered) and maximum operating voltage, this also models core loss. Various "S" shape curves of cores are used for various purposes (power transformers, common-mode chokes, amplifiers, ...)

I've gotten as far as measuring various transformers with impedance analyzer, with variac, current probe, scope, simulated with "Chan" model in LTSpice.

I was going to make a model of this transformer, but it behaved baldly in the reverse direction I tried. It was only well behaved up to 1/2 of rated voltage. I suspect high leakage inductance of widely spaced 480V windings help keep it out of saturation.

This one was 240 lbs for 30kVA, had copper windings. Maybe one weighting 350 lbs and not wound with large air gap would be better for reversing.

 

[Hedges]

I haven't measured the current being drawn by just the transformers. I made sure to get DOE2016 rated transformers for the efficiency, and the specs state "no load loss" of the 37.5kVa and the 25kVa is 85+54=139 watts, and the "full load" losses are 1470+1090=2560 watts.

At night, I see the load on my three Sol-Ark 12Ks drop down to 375-400 watts at their lowest, but keep in mind that I have 8 LED flood lights on, the air pump for the septic system, refrigerator, and other parasitic draws.

My 4x SI split-phase system has 4 x 25W = 100W no-load loss, and the 3-phase 75W no-load loss.
Splitting the difference between your two transformer specs, I could expect 70W loss from a suitable transformer.
My plan is to connect it through a relay, disconnect at night.

The relay could also be used for over-temperature shutdown, disconnect in the event of a phase becoming grounded (depending on whether my zig-zag can trip a breaker), or for ground-fault. I might do an outdoor 480V outlet for yard tools. Some of those features could alternately be provided by coupled breakers. I'm considering linking a GFCI breaker to trip the breaker feeding transformer, either based on current through the zig-zag neutral/ground bond, or using a ferrite to couple L1/L2/L3/N to GFCI.

1.1kW loss out of 25kW, 4% at full load. Seems reasonable. Some, I see loss at partial load labeled, looked to me like more than 4% at full load if I assume it goes as current squared.

 

[HPaulPayne]

I'm not sure if it was clear from the OP's post that the shop is 50 ft from the panels or 50 ft from the house.

Either way, if he runs AC from the shop to the house, and we assume the shop is 50 ft from the panels but 950 ft from the house, the problem remains largely the same: he still needs to do a ~1,000 run with 240V AC instead of making the same run with ~380V DC.

Current on that AC would actually be 60% higher and he'd be needing even thicker wire to make a 240V AC run that he'd need to make a 380V DC run.

His only advance would be that he could use a transformer to boost the 240V then run a higher voltage from the shed to the house and transform it back down to 240V at the house. This has been proposed to them in earlier comments. Doing such thing right is going to cost money. Doing it with microwave transformers might be cheaper and as long as they're out in the country they can do whatever they feel comfortable with but within city limits I doubt they can do anything that is not by the book.

IF I run 240v AC from the shop to the house (950' - 1,000') what would be the best and most cost-effective way to do that?

BTW, the reason I am looking at two Sol-Ark 15k inverters is that I have a very high demand, especially when running the shop and house at the same time.

 

[Kornbread]

Interesting idea, I'll have keep that in mind for future projects. I do think it would be more labor to install DIN rails vs. Polaris lugs. At shop rate we have to be pretty careful when adding labor to save money on material.

Regardless the point was that in my case it didn't pencil out to mess around with stepping up to much larger wire. "your mileage my vary"

The Polaris connectors are nice. Bought these to reroute the main entrance cables in the shop, they were a little cheaper than the equivalent Polaris connectors. This is their website but you can find them considerably cheaper elsewhere Morris Insulated Connectors

 

[GregTR]

IF I run 240v AC from the shop to the house (950' - 1,000') what would be the best and most cost-effective way to do that?

BTW, the reason I am looking at two Sol-Ark 15k inverters is that I have a very high demand, especially when running the shop and house at the same time.

I did a quick google and the answer seems pretty hashed out: https://diy.stackexchange.com/questions/157675/what-size-wire-gauge-for-1000ft-service-run

No reason to regurgitate all that here.

As for two Sol-Ark 15K inverters, that is all fine and dandy, but what are you going to use as power source for those two inverters? 30 panels are not going to cut it. If you want to be able to do 24 kW safely all the time then you'll also need a battery bank that can handle 500A or two battery banks with 250A each to be more specific.

If you're looking at the 100Ah battery packs, you'd need 10 of them to run them at 0.5 C so make sure you account for 2x25 kWh battery storage otherwise you will never get the power you want.

 

[Hedges]

IF I run 240v AC from the shop to the house (950' - 1,000') what would be the best and most cost-effective way to do that?

BTW, the reason I am looking at two Sol-Ark 15k inverters is that I have a very high demand, especially when running the shop and house at the same time.

Start by determining what loads to be run at the far end. Continuous load and motor starting surge.
That will determine required wire gauge.
Also, how many kWh/day at each location?

You may also be able to do a system with an inverter at each location. Whether parallel, series, or independent. That would give full power of one at each location, possibly with other picking up some of the load.

 

[niktak11]

That is a different scenario.

You have an oversized single inverter with option to expand later. That is just planning for the future and there are other benefits with going with a 15K over an 8K such as the 200A pass through.

But why bother buying two 15Ks when one could easily handle the current number of panels?

Buy one now, hook up the panels and buy the second inverter when you're in need of expansion. Putting $8k+ into something up front that is not needed just does not seem like a financially smart idea.

I guess the only reason would be if they really need 24kW power occasionally without grid, two 15Ks with batteries would allow for that while a single one wouldn't.

He probably needs more power than a single 15k can provide

 

[GregTR]

He probably needs more power than a single 15k can provide

Again, see my earlier question: where is he going to get it from? 30x400W panels?

 

[niktak11]

Again, see my earlier question: where is he going to get it from? 30x400W panels?

Batteries

 

[GregTR]

Batteries

OP has 24x12Vx120Ah AGM in Montana at 45 degrees latitude with "not very good" winter sun hours.

I doubt that the bottleneck in his setup is going to be the single Sol-Ark 15K....

Building a 48v system for my off-grid workshop

I have a battery bank consisting of 24 12v 120ah AGM batteries series-parallel wired for 48v and 34,560wh total (17,280wh at 50% DOD // 8,640wh at 25% DOD). Will soon have the Aims PICOGLF10KW48V240VS 10,000w 48v split-phase inverter-charger. Using Outback Flexmax 80 charge controllers...

diysolarforum.com

Seems like they're putting lipstick on a pig... But hey, you do you...