Optimizing long PV cable runs

[clivusmul]

Post for Half-Crazy Ryan...

Hello Ryan, I am BobJ (clivusmul) a new forum member and a former analog power IC designer... I really appreciate the work you put into the String Sizing Tool - thanks!  Your Classic is an impressive piece of engineering!

I'm building an Off-Grid solar power system for a mountain cabin.  Battery bank will be 8 ea. high Ah 6 volt AGMs wired either 24V or 48V.  I will use 8 ea. 250 watt solar panels (2KW total) with Voc ~ 37.5 V rm temp and, MPPV ~ 30.5V.

Problem is best South exposure is 250" distant.  Long cable runs and high cost wiring at either 2 runs @ 16A or 1 run @ 32A...   I have read all the forum discussions re: 2SX4P or 4SX2P, etc.  I understand the Voc Max tempco NTE constraint and the HyperVOC design scheme.  My question involves how the controller senses and adjusts for Vdelta between the panel and the controller... Max input controller voltage constraints is not my question...

I read that the desired V input delta between the array output and the MPPT controller input should be < 2% of the VMP stemming from the panels.  For me, that is 1.4V @ 2PV's in series X 4P.  Maintaining < 2% IR drop is an onerous task... the primary benefit to using MPPT vs. PWM is voltage transformation using the DC-DC isolation feature of the switching regulator scheme in the Classic.   If I were to use PWM, Max charge power would be in the 800 W range and if Vmp is 60 volts, I could easily tolerate 25 volts of cable IR drop... Now, a 30 % improvement using MPPT would increase this value to about 1100 watts +/-; a desirable goal.   Q - Is there any way I could actually shove 90 amps into a 25 volt B-Bank from a 2000 watt panel using a Classic 150?  I don't think so... If the user reports of performance delta over PWM are correct, then 1.2KW battery charge power is the best any optimally designed system can provide with 2KW of solar generation... That says with MPPT we are still wasting 800 watts of theoretical power. 

Having said that, why is it not OK to lose 500 watts in the PV cabling to the controller? 

If the above is true (might be totally wrong...), I was thinking that the problem a low IR drop cable feed is trying to resolve is not conserving power that can't be used; rather, perhaps the IR cable drop confuses the MPPT algorithm. 

Question:  Is there any way to utilize a remote Vsense (kelvin remote measurement) wire from the Classsic to the PV panel and sense PV Voltage around the cable IR drop?  Have you guys thought about remote Vpv sense?  and would this help? 

Bottom line - if IR drops need to be < 1.5 volts for MPPT operation, AND if the improvement using MPPT is around 40% avg, there are some valid reasons to consider using PWM at 70 volts and cost trade off 2 more panels ($700 vs. $K for #4/#6 URD cable) to get 8 more charging amps @ 70 V?? 

I get the idea that no matter how many panels I have over 2KW, including how they are wired in terms of Vout (as long as Voc is not violated), 1 Classic is good for about 1200 watts of charge power into a 24 volt Vbank...  Sorry for the long note but hopefully it is understandable and OK to post... Going to 36 volts is nice but no one makes a 36 V inverter... 48 volts gets into other issues.. Clivusmul (KB7GX)

[Halfcrazy]

Post for Half-Crazy Ryan...

Hello Ryan, I am BobJ (clivusmul) a new forum member and a former analog power IC designer... I really appreciate the work you put into the String Sizing Tool - thanks!  Your Classic is an impressive piece of engineering!

I'm building an Off-Grid solar power system for a mountain cabin.  Battery bank will be 8 ea. high Ah 6 volt AGMs wired either 24V or 48V.  I will use 8 ea. 250 watt solar panels (2KW total) with Voc ~ 37.5 V rm temp and, MPPV ~ 30.5V.

Problem is best South exposure is 250" distant.  Long cable runs and high cost wiring at either 2 runs @ 16A or 1 run @ 32A...   I have read all the forum discussions re: 2SX4P or 4SX2P, etc.  I understand the Voc Max tempco NTE constraint and the HyperVOC design scheme.  My question involves how the controller senses and adjusts for Vdelta between the panel and the controller... Max input controller voltage constraints is not my question...

Not sure I follow here why would there be 2 runs at 16 amps or 1 at 32 amps? Either way you need a single Classic so it should be a single wire run one + and one - if wired in 2 strings of 4 you would have around 16-18 amps if wired in 4 strings of 2 you would run about 32-36 amps

I read that the desired V input delta between the array output and the MPPT controller input should be < 2% of the VMP stemming from the panels.  For me, that is 1.4V @ 2PV's in series X 4P.  Maintaining < 2% IR drop is an onerous task... the primary benefit to using MPPT vs. PWM is voltage transformation using the DC-DC isolation feature of the switching regulator scheme in the Classic.   If I were to use PWM, Max charge power would be in the 800 W range and if Vmp is 60 volts, I could easily tolerate 25 volts of cable IR drop... Now, a 30 % improvement using MPPT would increase this value to about 1100 watts +/-; a desirable goal.   Q - Is there any way I could actually shove 90 amps into a 25 volt B-Bank from a 2000 watt panel using a Classic 150?  I don't think so... If the user reports of performance delta over PWM are correct, then 1.2KW battery charge power is the best any optimally designed system can provide with 2KW of solar generation... That says with MPPT we are still wasting 800 watts of theoretical power. 

I am not sure why the 2kw would only do 1200 watts. My experience is that you get about 80% of the nameplate rating do to the name plate being in perfect lab conditions. boB can correct me if I am wrong but if you took a Classic 200 and ran 2SX4P vs 4SX2P I would suspect less than a 1% hit for the greater MPPT Voltage.

Having said that, why is it not OK to lose 500 watts in the PV cabling to the controller? 

Because the cost to make the wire not lose hardly anything is cheaper than 500 watts of PV

If the above is true (might be totally wrong...), I was thinking that the problem a low IR drop cable feed is trying to resolve is not conserving power that can't be used; rather, perhaps the IR cable drop confuses the MPPT algorithm. 

Nope no confusion the MPPT Voltage will still be there may just be at a lower voltage at the controller vs the PV but the controller and the panels will not care as the Classic is simply looking for the most powerful spot in the voltage window

Question:  Is there any way to utilize a remote Vsense (kelvin remote measurement) wire from the Classsic to the PV panel and sense PV Voltage around the cable IR drop?  Have you guys thought about remote Vpv sense?  and would this help? 

Bottom line - if IR drops need to be < 1.5 volts for MPPT operation, AND if the improvement using MPPT is around 40% avg, there are some valid reasons to consider using PWM at 70 volts and cost trade off 2 more panels ($700 vs. $K for #4/#6 URD cable) to get 8 more charging amps @ 70 V?? 

Again I think this is all thinking about the PvV and Voltage drop. The MPPT tracking algorithm is simply looking for the most powerful spot.

I get the idea that no matter how many panels I have over 2KW, including how they are wired in terms of Vout (as long as Voc is not violated), 1 Classic is good for about 1200 watts of charge power into a 24 volt Vbank...  Sorry for the long note but hopefully it is understandable and OK to post... Going to 36 volts is nice but no one makes a 36 V inverter... 48 volts gets into other issues.. Clivusmul (KB7GX)

On a 24 volt battery the Classic 150 is good for 2300-2700 watts, The Classic 200 is good for 2000-2200 watts and the 250 is good for 1700 watts. With a 250ft wire run I would consider the 200 with 4 panels in series (Provided the cold temp does not drive you over the voltage limit). I think if you run the numbers the wire size does not have to be terrible. I came up with #4 being under 2% and #6 being under 3%. I have a similar array it is 15 panels for 3200 watts going 300ft and I used #2 Aluminum. I know some despise aluminum but my experience has been if it is in conduit and NoLoxed at each end when connected it is fine. And it is a boat load cheaper

Ryan

[niel]

i'll add to what ryan said.

i would opt for 2 paralleled series strings of 4 pvs to help overcome v drop losses and being the soc will be closest to the classic 150's max before hitting hypervoc then that makes the classic 200 best. to add even more efficiency to the mix go with a 48v battery bank. now any way you slice it you need to keep the v drop low to keep losses down. midnite recommends less than 2% drop, but that entails 500ft (250ft of + and 250ft of -) of #0 wire. a cheaper option may be running 500ft of #2 for a 2.72% loss. there isn't anything wrong in running separate runs for each string, but this only saves you going out 3 places on the gauge number and you would now need 1000ft of that wire. for example the #0 in a single run would need to be split into 2 runs of #3. (if you find some of that gauge or go to 2 runs of #2) likewise going from #2 would make it 2 runs of #5. you could compromise on the 2 runs and go with #4, but the loss % will be over 2% for 2.17%. using #4 would be 4 and 1/3% and all of these are for a common home run of 250ft.

the classic does not do remote voltage sensing, but that is primarily for the cc to battery anyway and not the pv to cc. note even if it did have v sense that has to be short extra large cables anyway being losses at lower voltages will be significantly higher. the v sense is only to insure the proper voltage is seen by the controller and using the proper wiring for low loss makes the need for the v sense nil.

now i based this on 2000w/48v=41.6a and i temperature compensate in my calculations to 90 degrees c which is the worst case as set by the nec. even relaxing requirements and compromising still results in a big investment into wire, as runs of #10 for example just won't cut it here, for the distance is too far.

[clivusmul]

Thanks guys... I appreciate you both taking to reply.  I don't understand how the CC can "search" for the optimal Mpp operating point if the lead wire to the CC doing the Vpp computation is in error via the transmission line IR drop.  I still suspect the primary reason for keeping IR losses minimized to the controller's input is to allow a better determination of the Panel's optimal MaxPower operating point.  Respectfully, neither of you addressed the delta power lost between what the panels can output and what the controller can drive...  Ryan may have tried to respond here by informing that,
"My experience is that you get about 80% of the nameplate rating do to the name plate being in perfect lab conditions. boB can correct me if I am wrong but if you took a Classic 200 and ran 2SX4P vs 4SX2P I would suspect less than a 1% hit for the greater MPPT Voltage.

[Halfcrazy]

No simply put the Classic could care less what the voltage drop is period. In the simplest terms it loads the pv array down watches where it makes the most current and that is what it considers the MPPT V

[Halfcrazy]

Respectfully, neither of you addressed the delta power lost between what the panels can output and what the controller can drive...  Ryan may have tried to respond here by informing that,
"My experience is that you get about 80% of the nameplate rating do to the name plate being in perfect lab conditions. boB can correct me if I am wrong but if you took a Classic 200 and ran 2SX4P vs 4SX2P I would suspect less than a 1% hit for the greater MPPT Voltage.

Not sure I fully understand this The controller will pass through roughly 96-99% of what the panels can provide minus wire losses and 15-20 watts to run the controller while it is charging. I thought what you where actually asking was what would be the difference in output of a particular Classic if all things where equal and the PV array was doubled in voltage do to string reconfiguration. In this case it would be less than 1%

[clivusmul]

Sorry for the multiple posts; I fumble fingered the keyboard a couple of times... here is the full reply and Ryan, you did initially misunderstand my post... thanks for the clarification... here is the full last reply/post.


Thanks guys... I appreciate you both taking time to reply. 

I don't understand how the CC can "search" for the optimal Mpp operating point if the lead wire to the CC doing the Vpp computation is in error via any significant transmission line IR drop.  IF the Vpp was say, 65 volts and there was 10 volts of cabling IR drop, the controller would detect a 55 volt panel output unless it makes its meassurement when the controller is not loading the panels... This may be the way it works.. dunno...

All the YouTube videos I see concerning MPPT controllers show about a 25-30% increase in power relative to a PWM control scheme.  Ryan may have tried to respond here by informing that, "My experience is that you get about 80% of the nameplate rating do to the name plate being in perfect lab conditions. boB can correct me if I am wrong but if you took a Classic 200 and ran 2SX4P vs 4SX2P (I think these two terms should be reversed... clivusmul) would suspect less than a 1% hit for the greater MPPT Voltage." 

I have no problems buying a Classic 200, vs the Classic 150.  This may be the best solution for me. 

I am also looking at running individual smaller gauge wires for about 100' (this would be 8 pairs of smaller gauge wire) and then terminate them into an array combiner and from this conbiner, run a shorter piece of #2 or say, 2 ea. #4 Al wires to the PV controller.  I need to do some math-$ modelling...

I have one last question to test my understanding of MPPT battery charging.  Here it goes:

1) assume perfect conditions with 100% efficiency, no Voc limitations and all measurements done at room temp.
2) assume the model being tested is either of two array configurations:
3) configuration 1 is 4 ea. 250 watt panels for a total of 1000 watts connected 2 in series and these 2 strings paralleled; or 2x2.  Each string is capable of outputting 15 amps each.  This configuration is able to theoretically drive 30 amps at 70 volts Vmp.
4) configuration 2 is the same panels all connected in series.  Again 1000 watts but the all series connected amperage is now 15 amps; not 30 and the Vmp would be 140 volts.   This configuration is 4x1.
5) IS IT TRUE THAT BOTH CONFIGURATIONS IF USING MPPT CHARGING TECHNIQUES WILL PROVIDE IDENTICAL CHARGING WATTAGE TO THE BATTERY? 

If the answer is YES, then I am all in and have a fundamental grasp of the MPPT technique!  Again, a first order answer is what I am looking for; either mostly correct or they are not nearly  equal...

I get the idea that the primary power translation factor operating using MPPT is "energy" not just voltage or just current alone.  The method is akin to AC energy transfer in a transformer but in this case it is a type of DC energy transformation using switch mode technology. 

It's far easier for me to wire 4 panels in series and then either home-run each 4 panels or connect each of them in parallel to run just one + and one - wire to the controller.  I'd far rather use a 48 volt battery bank too as the inverters are more efficient operating at the higher input voltage.  Given the Classic 200 is on par cost wiser to the Classic 150, everything remaining gets  easier in terms of smaller wire, more flexible wire, lower inverter currents, etc. 

Please address the hypothetical model above... I appreciate your assistance again.... clivusmul - KB7GX.

[Halfcrazy]

well first the controller will not care if there is voltage drop as it simply loads the array down and lowers its voltage. It watches this and looks for the most current into the battery and where the 2 lines cross is where it sees the MPPT V

Now as far as the specs and array config "If we assume 100% efficient" then yes both array configs will give equal result. In reality any MPPT controller will have a slight bit more loss do to heat with the higher input voltage.

[clivusmul]

Thanks Ryan... appreciate the great response.  Midnite is an impressive company...

[Kent0]

I'll try to add a little to Ryan and Niel's excellent information.

1) Go with the 48-volt battery. The charge controller will be more efficient, the inverter will be more efficient too.

2) 4 panels in series is going to require a Classic 250 Classic 200, and that is probably the best way to go. Higher PV voltage is your best friend for long circuits.

3) There is no 1% or 2% maximum voltage loss requirement. The charge controller doesn't care. One should look at the cost of using larger wire to see if it is justified. Just keep in mind that X% voltage loss in the wires also means X% power loss. With four panels in series and 2 sets in parallel the voltage is nominally 122 volts and the current is 16 amps. A 250-ft circuit of #6 AWG copper conductors will have losses of 2.8%. You can reduce that to 1.8% loss by using #4 AWG copper conductors. Is 1% more power, about 20 watts, worth an extra $150? No, that is $7.50 per watt and way more than the value of the PV modules.

You can run four circuits with two in series only if the battery voltage is 24 volts. Then the PV array operating voltage is 61 volts and the current is 32 amps. You'll have to use #3 AWG copper wires just to keep the losses down to 5.7% and it would take 1/0 wire to match the 2.8% loss obtained with four modules in series.

You might consider a compromise and buy nine modules to be wired three in series and three in parallel. This arrangement allows you to use the Classic 150. The wire losses would be 2.8% with #3 AWG copper wires that cost twice as much as #6 AWG.

[Vic]

Assuming that the OP WAS from a Ham,  then,  Robert may well be near Tempe,  AZ.  He did mention a Mt cabin,  but would be surprised if a CL 200 would have Voc problems with strings of four of the PVs mentioned.

What is the approx lowest temps known for the area of the installation?   Record low for Tempe  is abut 12 F.   More data is always helpful.   Thanks,  GL,  Vic

[niel]

i don't understand why you guys feel a cl 200 won't work when the voc of each panel is 37.5v? 37.5v x 4 = 150v voc. i have difficulty believing the cold temps would push it to 200v.

[Kent0]

You guys are right about the Classic 200. The 250 is a typo, it should be 200. I'll edit that post.

[Westbranch]

I'm building an Off-Grid solar power system for a mountain cabin. 
I will use 8 ea. 250 watt solar panels (2KW total) with Voc ~ 37.5 V rm temp and, MPPV ~ 30.5V.
Long cable runs and high cost wiring at either 2 runs @ 16A or 1 run @ 32A...  Clivusmul (KB7GX)

I believe there is an error in the calcs, based on 250W @ 30.5V  should =~ 7.625 A per panel. 
Shouldn't it  be ~= 7.6A (2 runs from 4S) and 15.2A (1 combined run from 2 @ 4S runs)  I think Amps were added instead of  Voltage in 2S and 4S arrays.

This being correct, there would be a cascading error if 16 / 32 Amps was used for line loss calcs, as stated above. 

[clivusmul]

Thanks to Niel and the rest for the good inputs and suggestions. 

Yes, the installation locale is south of Heber, Az at an elevation of about 6100 ft.  The lowest recorded temp was 40 Degs F below zero in 1971 at Hawley Lake.  I think Maverick, Az may also share the cold temp record.  The normal lows in the area of my installation are +0 to +20 degs F.  Most winter days warm to about 35 degs F. Early AM hours can hit -5 to +5 degsF.   Importantly, the site is "winterized" for the months between Thanksgiving and about the 15th or March so I am not concerned about violating Voc worst case conditions...  It is winterized due to the difficulty keeping water pipes from freezing... cheaper to stay in Tempe and stay warm!   

I'm very comfortable with 2 strings of 4 panels in series.  My evals show this is close to optimum.  Another aspect I omitted was the need for two polar/mast top mounts given very large Ponderosa pine shadowing avoidance - why the arrays are 250' distant from the battery bank.  I'll put 4 panels on one pole and 4 on the other.  A small clearing area allows me to focus one array a tad to the East of South and the other a tad West of South.  I can get full sun from about 10:30 AM until about 3:00 PM in the summer.  Winter elevation changes are OK as well in this one spot.  The only concern about having 4 panels on each array is the challenge of adding 2 additional panels to each array to expand the capacity and to keep the array's balanced.  Existing panels are about 1.5 m^2 each.  Going from 4 to 6 panels/mount creates some incremental mechanical design challenges to overcome (weight, snow load, etc.).. clivusmul.