bipolar off grid array

[cardamon]

Am I correct that there isnt really any off grid charge controllers that will work with a bipolar array?  I think two charge controllers will only work on a bipolar array if their input and outputs are isolated correct?  I dont think any off grid controllers use an isolated topology.

[birdhouse]

i assume you are referring to some panels on an east slope and some on a west slope of a roof? 

i would think one controller would do this just fine so long as a few parameters were met.  total amps off both arrays should be below amp rating of controller.  i would also think about having two parallel runs, one from each array with some hefty blocking diodes before they both join at the controller couldn't hurt. 

are you talking mppt or pwm? 

i'm guessing two mppt controllers could get you more wattage from both arrays in variable conditions, but depending on controller cost you may get more bang for you buck with two pwm controllers and more panels per array. 

i've always thought about adding a smallish west facing array to my system.  it would be really nice to add some juice to the bank as the sun is setting! 

adam

[DamonHD]

You should run (strings of) panels that point in different directions (or have different shading/exposure) on different controllers, especially MPPT, if the controller is to get maximum energy out of them.

I have east- and west- facing strings of panels on my roof, in fact 2 on each, each on separate MPPT controllers.

Rgds

Damon

[cardamon]

Thanks for the replies.  What I mean by a bipolar array is one with a center tap midway on a string. It's a way of effectivly getting to string up twice the normal voltage.  They are used a bit in grid tie systems but still relatively rare.  So you have three wires from the array a negative, center, and positive. My thinking is one cc would get a negative and center ( +), and the other would get center (-) and positive.  Only problem is if the cc's aren't isolated then this creates a short when the cc's   are connected to the batteries.  Just wondering if I was thinking about this correctly.

[Frank S]

If I understand your line of thinking you are trying to use the string similar to a center ZERO tap transformer .
 DC doesn't work the same as AC.
 I would think that basically what you would wind up with would be in effect 2 strings . sharing the center with a single conductor would not increase your total string output voltage but would require a much larger common conductor. then your charge controllers IE MPPT's would have to charge separate banks  otherwise if connected to the same bank I think you would create a reverse current short or or only one CC would have output due to blocking diodes.
 I never tried it but it just sounds to me as if it would be a bad idea

[Mary B]

Be a whole lot less work to series separate panels and use one CC

[cardamon]

Bipolar arrays can be very advantageous in certain situations.  What you are doing with a bipolar array is essentially doubling you string voltage which can dramatically reduce wire and installation costs.   I am brainstorming on this because I have a 1400 wire run.  Essentially you are putting two CC's (inverters in grid tie) in series so each one sees half the voltage.  The large grid tie units have this all in one package of course so you are only installing one inverter.  You could just connect two battery CC's in series if they were each loaded evenly the entire time, but that is where the problem lies: when one sweeps for MPPT the voltage would become unbalanced and out would come the magic smoke.....  IF you ran the "neutral" that would keep the voltage balanced however without isolation you still have a short through the cc's since a positive and a negative input are common.....Would appropriately placed blocking diodes be a way around this?  Just wondering if anyone has played around with this scheme....

[dnix71]

Sounds like a bad idea, period. If you have a problem with the length of a wire run, then micro inverters should be on each block of panels. The outputs from those is then sent to a central controller and that goes to the grid or your battery bank.

Enphase C250s support 3-phase 480v. Each panel or small cluster of panels has an MPPT/inverter that talks to a control box that is the combiner. If one goes micro inverter down, the rest stay up. Costs a bit more up front, but is fully code compliant and safe (plug and play).

http://enphase.com/microinverters/

If 480v is too high, the M250 supports 110v/208 reisdential/commercial voltages.

[cardamon]

That is an interesting proposal, I never considered micro inverters for off grid.  I think their anti islanding parameters would pose a challange.  I see a Schneider MPPT 600 as a simpler option.

[dnix71]

Enphase has an off-grid option that involves batteries. It is intended to supply power if the grid fails.

http://enphase.com/eblog/2014/new-enphase-energy-management-system-accelerates-mainstream-solar-adoption/

also pdf on the subject http://enphase.com/global/files/Enphase_Application-Note_AC-Coupled-Battery-Based-Systems.pdf

For this you need a separate inverter to create a micro grid and manage the battery charging. Normally you wouldn't want micro inverters in a setup like this, but if extremely long wire runs are unavoidable, then it will work.

To meet the NEC, you must have at least 2 fail-safes. There must be a dump load behind the battery charger in case the charger fails to cut off and the battery voltage rises uncontrolled. In real life I would want another dump load behind that or a full system auto kill should the dump load fail to keep the battery voltage within safe limits.

[Mary B]

I fail to see the advantages, I run 4 12 volt panels in series for ~86 volts that is fed to the house and the MPPT controller via 125 feet of #6. Some MPPT controllers can now handle 600 volts so for a super long run I would just series enough panels to feed it and go.

[dnix71]

Mary B He said he has a 1400 foot wire run. I don't see any good options. High voltage ac or dc requires poles a certain height and warning signs in case there is a brush fire (for the responders to know) or if anyone might trespass or hunt on the land.

That far you would want 480vac 3-phase, but that's heavy industrial voltage. Touch it and die voltage.

http://forums.outdoorsdirectory.com/showthread.php/56970-Need-quot-Used-amp-Cheap-quot-Electric-Line-%28BIG-1-4-mile-run%29

Here is a thread about a guy who wants to run 1300 feet. I have 4/0 in my attic running less than 50 feet from the meter to my apt panel. A 4/0 aluminum bundle is the diameter of my arm. The person in the forum is asking about 600+ volts, which is very high voltage and beyond the rated limits of most common insulation.

1400 feet isn't going to work unless some serious losses are allowed and the transmitted power is kept to a minimum.

It might work if there was a very large battery bank at the end use and that was used to power the dwellling. The solar array would have an off-grid inverter next to it sending 208 3-phase to a battery charger on the other end. Charge all day, but run off the batteries at all times and you don't have to deal with high/surge currents on the long run of wire.

[cardamon]

This distance is  not a problem at all.  There are two 600 volt battery based MPPT charge controllers (Schneider and morningstar) on the market that will work fine at 1400 feet using #6 aluminum wire which is .15 a foot.   I have a mini trencher - its like a lawnmower with the blade tilted up and down -  that makes a 1 inch wide 6 inch deep groove that 3/4 pvc conduit fits right it - im not as fan of direct burying.  I will be stringing up 3250 watts of PV, which is actually quite as bit over the rating of the CC (80 amps at 24V) but 'i want more output on less than ideal days.  If I remember correctly from the last time I ran the numbers, at full output, there is only 150 watts lost in the wire - cheaper to buy that PV than bigger wire.  So if this all works fine what is the problem?  Well I was just future tripping about adding another 3KW of PV some day and wondering if I could pull that off without running another wire run by being creative....

[dnix71]

600v is nasty lethal. If your area is that remote then maybe it doesn't matter. I live in south Florida and twice in the recent past people have been killed at bus stops by 480v because a lazy contractor didn't properly ground the transformer that stepped it down to 120v. That 480 was up on a pole.

Up on a roof for a short distance inside conduit to a grid-tie inverter is okay. No one is going to get close by accident. 1/4 mile at 6 inches deep, even inside non-metallic conduit is a different matter. A hog rooting around could rip up your feed.

[cardamon]

600v is nasty lethal. If your area is that remote then maybe it doesn't matter. I live in south Florida and twice in the recent past people have been killed at bus stops by 480v because a lazy contractor didn't properly ground the transformer that stepped it down to 120v. That 480 was up on a pole.

Up on a roof for a short distance inside conduit to a grid-tie inverter is okay. No one is going to get close by accident. 1/4 mile at 6 inches deep, even inside non-metallic conduit is a different matter. A hog rooting around could rip up your feed.

I think your concern is a little excessive.  Utility companies typically direct bury cables with up to 15,000 volts.  The NEC has no different rules or protections for a 120 volt cable vs a 600 volt cable.  Yes I am cheating the burial depth a bit - its all in the woods on a pretty steep grade, 120 acres, pretty much impossible to do with anything other than this mini trencher.  Interestingly, the NEC requires the same burial depth for direct buried cables as it does for conductors in PVC conduit, and further more I could just lay this on the ground and be code compliant.   Sure, I dont think anyone should work on an electrical system that they are not comfortable with, but I am an electrical contractor and have no problem utilizing the highest voltages codes and equipment will allow.  Almost any grid tie system will have string voltages approaching 600, and 1000 volt will be common soon.

[XeonPony]

One guy made a 2Kv step up system for his hydro, so if running an inverter in a power shack with your panels use a step up/down system, by my calcs even at 2Kv for that 6 guage alu you're at 6 to 7% drop @ 60A load

[cardamon]

2kv at 60 amps is 120 KW.....is there a typo?

[XeonPony]

No typo, but I gues you won't be generating a 120Kw! (Just forgot to do the conversion steps is all)

6Kw / 2Kv = 3 a

2Kv @ 3A @ 1400f run, you can run 14 awg Alu with a 2.7% Vdrop