Struggling with spec'ing panels to max out 2x Victron MPPT RS 450/200

[l00semarble]

I am working on a design for a system of about 25kw to power my off-grid house with consumption of about 30kwh/day.

My PV arrays will be on a ground mount some distance from the house (maybe 300m). In order to transmit power most efficiently over this distance I was going to run the PV cabling off the arrays at high DC voltage to the house. I was thinking that with the Victron MPPT RS 450/200 I could transmit power at over 400v which would require smaller cables etc.

For those that don't know the Victron nomenclature for their MPPTs is XXX/YYY which is (Max PV input VOC)/(Max output current) so a 450/200 can take up to 450v pV input and can make up to 200 amps output to batteries. The RS 450/200 can do 11,520w and I was going to overpanel them a bit.

But now looking more closely at specifics on this arrangement I see that the RS 450/200 has 4 separate MPPT trackers and each needs to be wired separately with its own array so I will need 4 separate arrays into each Controller. Each array can be 450VOC and up to 18a (before allowed overpaneling.) 450v*18a = 8,100w but the total output can't be over 11,520w so no reason to put 8,100w on each of the 4 trackers.

So I was playing with some numbers to max out two of these controllers with large format panels for example the Trina Vertex bifacial 500w which is:
51.5VOC, 43.4MPP V, 12.13A short circ I, 11.53A MPP I.

So to "fill up" a RS 450/200 I can only do 6 panels per array which gives me each array with:
309 VOC, 260.4v MPP, 3,000w, 11.53A MPP I and 4 strings like this is 12,000w which is a nice little 4% overpaneled. All good, right? I can "Max Out" 2x MPPT RS 450/200 and get about 23,000w output....

but...

the working voltage of the panels to the MPPT in this config is only 260v. I really was hoping to do the long distance transmission at over 400v. 260v is not much better than just putting the inverters at the ground rack and transmitting 240v AC. And....I was imagining using two high voltage DC conductors for each MPPT but in reality I will need 8 conductors per MPPT or 16 conductors total.

So I am realizing that the 'huge' 450v/200a MPPT controller is really 4 separate 450v/50a controllers and it isn't easy to make a string to max the voltage.

I need PV panels with higher VOC and lower current but they don't seem to go much above 50VOC. Sorry....post got long. Probably hard to read and follow but if anybody cares to it is the folks here. Thoughts?

 

[FilterGuy]

I was thinking that with the Victron MPPT RS 450/200 I could transmit power at over 400v which would require smaller cables etc.

Depending on your climate and the temperature coefficient of the panels, 400V might be too close to cover cold temperature voltage rise. The good news is that a lot of these new large format panels tend to have a low temperature coefficient.

Adjusting Solar Panel Voc for temperature

When designing you solar panel system, it is important to adjust you solar panel Voc for temperature in order to ensure you do not over-voltage the PV inputs of your solar charge controller. This paper shows how to calculate the Temperature...

diysolarforum.com

 

[l00semarble]

It just occured to me that maybe the solution is to not use all 4 trackers. I have to research if this is permissible but if so I could use 3 trackers with 8x500w panels and push the VOC to 412v with MPP voltage of 347v. That would still be the same 12,000w through the Controller.

 

[l00semarble]

Depending on your climate and the temperature coefficient of the panels, 400V might be too close to cover cold temperature voltage rise.

I knew this comment would come and I should have addressed it in the post. This is the tropics(Costa Rica). Annual low of maybe 60F? Normal nighttime lows around low 70s.

 

[CoolWill]

Enersol makes high voltage solar panels. I don't know about availability. I bought 54 used Sunpower 435 watt panels on eBay that have a VOC of 85.6 V. I guess you just have to dig.

 

[rhino]

So I was playing with some numbers to max out two of these controllers with large format panels for example the Trina Vertex bifacial 500w which is:
51.5VOC, 43.4MPP V, 12.13A short circ I, 11.53A MPP I.

So to "fill up" a RS 450/200 I can only do 6 panels per array which gives me each array with:
309 VOC, 260.4v MPP, 3,000w, 11.53A MPP I and 4 strings like this is 12,000w which is a nice little 4% overpaneled. All good, right? I can "Max Out" 2x MPPT RS 450/200 and get about 23,000w output....

Since you are in the tropics you can do 8 panels per string. You have to stay at or below 8 * float voltage of your battery bank for each tracker on that charge controller. Which if you are using LiFePO4 battery would be about 53.6 * 8 = 428.8 which is below the 8 * 51.5Voc = 412 (and only a bit higher for your low temp of 60F). You would be able to use all 4 trackers with 8 panels per tracker.

You should even technically be able to do 2 strings in parallel per tracker since the short circuit is 12.13A * 2 = 24.26A which is still under the 30A absolute max for each MPPT. This would be very over paneled though since each tracker can only output 4000W max.

 

[niktak11]

Victron charge controllers aren't great for large DC coupled arrays with long wire runs. This is one of the reasons why I went with a sol-ark system over a victron system. I can put 8kW+ through a single MPPT input.

You could have 18 (maybe even 20 in your climate) of the panels you mentioned on a single input with a sol-ark 15k.

 

[l00semarble]

I was planning on using what I can get in large format panels. My supplier (In Costa Rica) can get me right now:

Jinko JKM535M-72HL4-TV 535w panels

49.1 VOC

40.63 MPP V

13.85a Short Circuit I

13.17a MPP I



So if I want to use these on the MPPT RS 450/200 I could put 6 in series to each tracker (24 total panels) and put 3,210w into each tracker. This would over-panel the MPPT by about 11%. It seems perfect to max out the controller but the problem is the voltage of each array would only be 295 VOC/244 MPP V so the working voltage I would transmit over the long distance would be only 244v when I was hoping to do around 400v.

I am wondering if I should consider only using 3 trackers on the device. I 99% sure this is permissible while still being able to get the maximum rated output of the MPPT. Can someone confirm? . In that case I could put 9 panels in series into three trackers and get 442 VOC/366 MPP V and same array MPP I of 13.17a. This would put 4,815w into each of three inputs thus a total of 14,445w so 25% over-paneled. This will be wasted capacity in the dry season but during rainy season I would appreciate the over-paneling.

442 VOC puts me close enough to worry about the temp coefficient even though this is the tropics. In recent years the lowest temp recorded in my neighborhood is about 69F/20.5C. I was going to use 20c as design temp so 5 deg offset from STC at 0.28%/DegC for these panels.

.28%/degC * 5degC = 1.4%

49.1VOC * (1+1.4%) = 49.818VOC

49.818VOC * 9S panels = 448.4VOC String

So I'm just squeaking in at the limit. Technically even 19C would give me 449.6VOC. 18C and I blow up my controllers! This temp is unheard of here and even if somehow it was that cold it certainly would not be sunny at the same time. Our low temps come with rain/fog.

I think using 3 trackers per controller accomplishes my goal. This would be 28,890w of total PV which is a 25% overpaneling of the 2x MPPT RS 450/200 so I would get pretty much all I can out of them and I would do much better in the rainy season. My working PV voltage will be around 366v so better for the long transmission and by using 3 trackers instead of 4 I have one less pair of cable to pull too.


Thoughts?

 

[MrSteve]

Hi, just reading your post.. Did you managed to connect the panels like you wanted to the MPPT RS?