Different Panel Configurations on an MPPT Controller

Cariboocoot

Another of the questions that get asked a lot is "which is the best panel configuration for me?"
In this example we will examine the four possible ways of connecting the same six panels on one system. The panel specifications for this exercise are: Vmp 17.7, Imp 7.91, Voc 22.1, Isc 8.68 (140 Watt panels). The controller is an MPPT type with maximum input Voltage of 150 and 60 Amp output capacity. The nominal system Voltage is 12.

Possible configurations:

1). All six panels in parallel. Array specs: Vmp 17.7, Imp 47.46, Voc 22.1, Isc 52.08. Advantages: None really. Disadvantages: each panel needs circuit protection, high current and low array Voltage can be a Voltage drop problem for long wire runs.

2). Two parallel strings of three panels. Array specs: Vmp 53.1, Imp 15.82, Voc 66.3, Isc 17.36. Advantages: no need for circuit protection on strings, higher array Voltage with lower current good for reducing V-drop. Disadvantages: higher ratio of array to system Voltage reduces conversion efficiency (minimally).

3). Three parallel strings of two panels. Array specs: Vmp 35.4, Imp 23.73, Voc 44.2, Isc 26.04. Advantages: higher array Voltage with lower current good for reducing V-drop, better array to system Voltage ratio improves conversion efficiency marginally over #2. Disadvantages: needs circuit protection on each string.

4). All six panels in series. Array specs: Vmp 106.2, Imp 7.91, Voc 132.6, Isc 8.68. Advantages: high array Voltage can overcome significant wiring resistance for long runs between array and controller without the need for excessively large wires, no circuit protection needed on single string of panels. Disadvantages: high Voc could exceed controller's input max under cold temps, lowest conversion efficiency of the four possible configurations.

mryimmers

Re: Different Panel Configurations on an MPPT Controller

Can you clarify for me why you need circuit protection on 3 or more strings, but not 1 or 2 ? Thanks

BB.

Re: Different Panel Configurations on an MPPT Controller

Say you have Isc for a solar panel at 8 amps... And the series protection fuse requirement is 15 amps.

One string... There is no way for a panel to output more than (roughly) 8 amps Isc. No fuse needed.

Two strings in parallel... Short one panel/array wire. All the current from one side of the short 8 amps from first panel. And all the current from the second parallel string, another 8 amps. No way that you can get >15 amps.

Three strings in parallel. Short one panel/array wire. Current from one string, 8 amps. Current from other two parallel strings ~16 amps (8 amp + 8 amp). You now have >15 amp series protection fuse rating.

In reality, is not by much. And if you do worst case planning 16 amps * 1.25 NEC derating = 20 amp branch circuit/fuse rating (if was an actual sustained load of 16 amps).

That is the logic behind why one or two strings do not need series fusing per string, but 3 or more parallel strings do.

There are a few panels where you can get 3-4 or so panel strings in parallel, because they have Isc~2 amps and Iseries-fuse~15 amps. But I have only seen a few high voltage panels (such as DuPont 40-80 watt high voltage thin film panels)--But the are relatively rare in the systems we see around here (not sure these panels are even made anymore--mostly selling off old stock/used panels?).

-Bill

vtmaps

Re: Different Panel Configurations on an MPPT Controller

mryimmers wrote: »

Can you clarify for me why you need circuit protection on 3 or more strings, but not 1 or 2 ? Thanks

I agree with Bill (BB.), but I feel you should want to have circuit breakers on all strings, even if you have only one string. Notice that I wrote 'circuit breakers', not 'circuit protection'.

Circuit breakers make excellent switches for PV strings. At some point or another you will be glad to have switches on each string. You may want to switch your strings on, one at a time, to see if they are all performing equally. Flip all the breakers at the combiner and you have effectively de-energized your combiner-controller cable.

A combiner box (even for just a single string) makes a good spot to install a lightning arrester.

--vtMaps

niel

Re: Different Panel Configurations on an MPPT Controller

what one may want to do and what one has to do are 2 different things.

to add to what bill said, should you have 3 or more in parallel and accidentally wire one in backwards it could destroy the one in backwards. it had happened to one member here years ago.

mryimmers

Re: Different Panel Configurations on an MPPT Controller

ok, got it, so combiner box is good idea regardless. thanks a lot guys!!!

niel

Re: Different Panel Configurations on an MPPT Controller

mryimmers wrote: »

ok, got it, so combiner box is good idea regardless. thanks a lot guys!!!

with breakers i hope as that was the point for 3 or more in parallel.

Cariboocoot

Re: Different Panel Configurations on an MPPT Controller

If we want to be precise it is the issue of how much current a panel can produce versus how much current it can handle as a conductor.

Battery back-feeding doesn't enter in to it if there is a charge controller, as that should prevent any occurring. But is a panel is shorted or in backwards others in parallel to it can feed up to their total Isc to the panel now acting as a conductor.

So to be picky the circuit protection per panel is needed any time the combined Isc of all panels (-1 panel's worth) exceeds the panels' series protection rating (providing all panels are alike - there are other exceptions for when they are not).

Badar

Re: Different Panel Configurations on an MPPT Controller

I am unable to understand the purpose of "protection" in panels/arrays. Although I have read such thing on panels and their manuals, so now I am curious. I have not seen any such thing being practised. As I have seen many installations, even by other installers.
Or such thing is important only for 150v MPPT charge Controllers?

Here What I have seen is, for example....

12 panels of 20v,150 watt each
48V, 40A Charge Controller

Connect 4 panels to get the formation for 48V. So three arrays for 48v(80V).
6 Wires from panels (3 Arrays) are joined in junction box(Parallel).
from there two wire are taken to charge controller......

Can you please explain, whats wrong with above circuit?

Cariboocoot

Re: Different Panel Configurations on an MPPT Controller

It is a problem of "if something goes wrong" the same as with any over-current protection.

With only two identical panels in parallel if one of them shorts (becomes a conductor) it can still handle the full Isc of the other. There will be no overloading of the panel as conductor.

With three or more panels in parallel if one of them shorts it will be subject to the combined Isc of the other panels: 2 or more times its current handling ability. Thus it will be overloaded, heat up, and possibly start a fire.

If a panel never shorts the problem never arises. The same can be said for over-current protection on any circuit, and grounding practices as well. None of this things are needed for operation; they are there to stop a small problem, if it arises, from becoming a large one.

BB.

Re: Different Panel Configurations on an MPPT Controller

There are several types of protection for solar arrays... The first is the "traditional" protection against short circuits.

Basically, solar panels are current mode sources... For the most part, they cannot supply more than rated (Imp/Isc--current maximum power/short circuit current). You have a panel with Imp~10 amps and Isc~12.5 amps, you cannot get more than ~12.5 amps in full sun if no matter what you do--Heavy loads or short circuit.

And this is why installing breakers on a single solar array is pretty much useless... The wiring is already designed/rated to carry Isc (x 1.25 derating for US electric code).

However, if you look at the solar panels, the modern specifications have a "series protection fuse" rating... Lets say your 10 amp/12.5 amps * 1.25 means Ifuse=16.625 or Ifuse=20 amps (next standard size fuse/breaker).

If you have two strings of panels in parallel--No matter how you "short circuit" any wiring in the panel, you cannot get more than 12.5 amps to flow in the array.

However, if you have three strings in parallel, then you can short one panel string and have 2x12.5-25 amps feeding the shorted string--And that 20 amp fuse/breaker will protect the wiring against excessive current flow (and our standard requirement of three or more parallel strings requiring a series protection fuse on each string).

The second standard protection scheme is against lightning strikes. That is placing surge suppressors on the array combiner box and/or on the entry box into the home/structure. If there are any near/direct lightning strikes, the excess energy is shorted to a ground rod instead of being brought into the building. Surge suppressors are generally only installed in regions where there is a good chance of lightning strikes. In areas where lightning is rare, surge suppressors are not usually used.

A third possible failure is "arc fault"... This is where you have a "soft short circuit" (hot to ground, hot to return) or a poor series connection (crimp or connector fails, small animal chews through wiring, etc.). In some conditions you can get an arc started--And because solar array breakers do not trip in many short circuit situations and never in series arc fault conditions, there is a real fire issue of having a fire started by sustained arcs. And DC current/power is really good at sustaining arcs vs AC power... At this point, there are few good solutions for protecting against arc faults/short in solar array DC wiring. There is a DC Ground Fault type detection using ground current to trip (many safety issues). And there is (at least) one mfg (Midnite Solar) that is working on/has some types of "arc fault interrupters" (arc fault circuit breakers). This monitors the radio frequency energy on the wiring and open if there is the signature for an "arc". Still an open issue for solar power systems regarding the best/correct solution for fire protection (this discussion is really beyond your basic question--but has been an ongoing question).

An application engineer just sent me a link to a new(?) product of theirs that provides DC ground fault detection circuit breakers for DC... Very interesting and a huge improvement over the DC GFI scheme suggested by the NEC.

http://www.morningstarcorp.com/en/support/library/GFPD-ds-Eng.pdf

-Bill

Badar

Re: Different Panel Configurations on an MPPT Controller

Ok. Now I got the point. yet two more questions.

1: why a panel shorts? I mean why it become a conductor (Problem discussed in this thread)
2: How fuses are to be installed to avoid possible problem? Positive? Negative?

Attachment not found.

Cariboocoot

Re: Different Panel Configurations on an MPPT Controller

Wiring convention is to put over-current protection (fuse/breaker) on the positive or 'non ground' leads (negative in a positive grounded system). However since it is a circuit they work the same on 'either side'. With the new DC ground-fault rules neither side is grounded, and in some circumstances both positive and negative may be treated as 'hot' as with 240 VAC: utilize a dual breaker which trips and disconnects both sides in the event of an over-current problem.

A PV is basically a series of diodes. Under normal circumstances they conduct in one direction only. But it is possible for failures to occur, causing them to short and conduct in both directions making them function as an ordinary conductor. (Okay, it's slightly more complicated than that as diodes have reverse breakdown Voltage and in the case of PV's generate current due to photons bombarding thee PN junction).

Some of what is done for safety reasons is the topic of much debate and argument among engineers, as solving one problem may create another under the circumstances. You can't protect against every eventuality (particularly not lightning) so you go with what will provide the best protection under the most likely to occur circumstances.

BB.

Re: Different Panel Configurations on an MPPT Controller

In my humble opinion, the 4 fuses (one per string) is correct, the 5th 60 amp fuse is not really neededl. Your wiring should be big enough to manage the maximum current of the array (1.25 * Isc-array minimum).

Technically, for a floating power system, you would need fuses in both + and - leads from the array.

In practice, most systems have a negative grounded DC Battery bus (tied to the green wire safety ground everywhere). So you only need fuses/breakers in the positive wires from the array.

Also, the battery bus side of the drawing is playing pretty fast and loose with the code/safety/wiring of a large battery bank.

Start with how to "balance the DC wiring" for the battery bank.

Fuses/breakers are near the source of the highest current levels in the system (typically the battery bank). You need to cable / bus bars very carefully (and properly made). If something goes wrong, lead acid batteries are "scary" in their ability to output 1,000s of amperes of current into a dead short.

-Bill

Cariboocoot

Re: Different Panel Configurations on an MPPT Controller

I was ignoring that drawing because it is not well done as Bill said.
The full array fuse is not actually necessary as he said, there's no indication of the negative wiring on the input side of the controller, and the batteries are not wired correctly for even current sharing.

And that's a 'professional' diagram, eh? :roll:

vtmaps

Re: Different Panel Configurations on an MPPT Controller

BB. wrote: »

there is (at least) one mfg (Midnite Solar) that is working on/has some types of "arc fault interrupters" (arc fault circuit breakers). This monitors the radio frequency energy on the wiring and open if there is the signature for an "arc".

Midnite has arc fault protection built into their Classic charge controller. If there is a series arc fault in the PV array it will unload the array to extinguish the arc. It does nothing to help with a parallel arc fault (between PV hot and ground or neutral). The best safety solution (and another Midnite product) is an arc fault combiner box. It is very expensive.

Thanks for that morningstar GFPD link. Looks good, but I don't think its weatherproof. If its in the power room it can't de-energize the cable from the PV... it has the same problem as the arc fault in the controller.

It is better to put the protection as close to the power source as possible. Disconnecting the array at the combiner is better than disconnecting it at the controller.

Looking to the future... there is a debate going on in NEC circles about the need to de-energize roof top arrays for the safety of firefighters. There may be a requirement for a method to de-energize the arrays from an accessible ground level spot. Once you have all the relays in place to make that possible, there will be other ways to trigger the de-energization of arrays in the event of a fault.

--vtMaps

Badar

Re: Different Panel Configurations on an MPPT Controller

Well, sorry for that drawing. I was also unable to understand anything from that drawing and even from its description.
You can remove it, if inappropriate. I don't mind

Cariboocoot

Re: Different Panel Configurations on an MPPT Controller

Badar wrote: »

Well, sorry for that drawing. I was also unable to understand anything from that drawing and even from its description.
You can remove it, if inappropriate. I don't mind

No, that's rather the point in fact: you couldn't make sense of it because it isn't well done. The fault lies not within ourselves but in the drawing.

BB.

Re: Different Panel Configurations on an MPPT Controller

Cariboocoot wrote: »

No, that's rather the point in fact: you couldn't make sense of it because it isn't well done. The fault lies not within ourselves but in the drawing.

Oh Marc--Showing off that classical education that is long dead in the US:

http://www.goodreads.com/quotes/64051-the-fault-dear-brutus-is-not-in-our-stars-but

“The fault, dear Brutus, is not in our stars, but in ourselves.”
― William Shakespeare, Julius Caesar


For Badar--In case he has not browsed through Shakespeare during his free time in Pakistan.

-Bill

Badar

Re: Different Panel Configurations on an MPPT Controller

BB. wrote: »

Oh Marc--Showing off that classical education that is long dead in the US:

http://www.goodreads.com/quotes/64051-the-fault-dear-brutus-is-not-in-our-stars-but

“The fault, dear Brutus, is not in our stars, but in ourselves.”
― William Shakespeare, Julius Caesar


For Badar--In case he has not browsed through Shakespeare during his free time in Pakistan.

-Bill

Too much of technical English

I have always used "browse" button in MS-Windows, but never tried to find its meanings. So, unable to understand, whose free time in Pakistan, shakespeare's?
(I think my stay here would surely improve my English language, unsure about my technical knowledge. )

westbranch

Re: Different Panel Configurations on an MPPT Controller

BROWSE means to casually look at a lot of stuff, Google in effect 'browses' the internet when you do a search, ie it looks at a lot of stuff that 'might' fit the description of what you are looking for eg, "Shakespeare Plays" vs "Shakespeare" will give different results.

BB.

Re: Different Panel Configurations on an MPPT Controller

Badar wrote: »

Too much of technical English

I have always used "browse" button in MS-Windows, but never tried to find its meanings. So, unable to understand, whose free time in Pakistan, shakespeare's?
(I think my stay here would surely improve my English language, unsure about my technical knowledge. )

Oh boy Badar--There you go showing my poor English skills and total lack of cultural awareness (even when making a post on cultural awareness). And English is my only language.

I intended to say your "free time" reading Skaespeare's plays. Browse also means to graze--Deer browsing on (eating) grasses and brush.

William Shakespeare

William Shakespeare (/ˈʃeɪkspɪər/;^[1] 26 April 1564 (baptised) – 23 April 1616)^{[nb 1]} was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist.^[2] He is often called England's national poet and the "Bard of Avon".^{[3][nb 2]} His extant works, including some collaborations, consist of about 38 plays,^{[nb 3]}154 sonnets, two long narrative poems, and a few other verses, the authorship of some of which is uncertain. His plays have been translated into every major living language and are performed more often than those of any other playwright.^[4]
Shakespeare was born and brought up in Stratford-upon-Avon. At the age of 18, he married Anne Hathaway, with whom he had three children: Susanna, and twins Hamnet and Judith. Between 1585 and 1592, he began a successful career in London as an actor, writer, and part-owner of a playing company called the Lord Chamberlain's Men, later known as the King's Men. He appears to have retired to Stratford around 1613 at age 49, where he died three years later. Few records of Shakespeare's private life survive, and there has been considerable speculation about such matters as his physical appearance, sexuality, religious beliefs, and whether the works attributed to him were written by others.^[5]
Shakespeare produced most of his known work between 1589 and 1613.^{[6][nb 4]} His early plays were mainly comedies and histories, genres he raised to the peak of sophistication and artistry by the end of the 16th century. He then wrote mainly tragedies until about 1608, including Hamlet, King Lear, Othello, and Macbeth, considered some of the finest works in the English language. In his last phase, he wrote tragicomedies, also known as romances, and collaborated with other playwrights.

Shakespeare is, arguably, the person with the single most influence/importance regarding the English language.

The Complete Works of William Shakespeare

Interestingly, even though he is widely known and even famous during his life--Almost nothing is known about Shakespeare personal/private life. There have been arguments that his is not even a single person but several people of the time.

I am certainly not an English Major (more of an English moron)--And Shakespeare's play are a difficult read. Many/most believe that Shakespeare should be read aloud to be appreciated.

-Bill

Cariboocoot

Re: Different Panel Configurations on an MPPT Controller

Such reading was required in my day, and I shall not apologize for how it has influenced both my way of thinking and my manner of expression. Nor shall I regret editing my own posts to correct such minor typographical errors as may occur.

(These days the method used for correcting English is to edit the dictionary to make any common mistakes "accepted use".)

If Bill had not elaborated on my paraphrasing of Shakespeare poor Badar would have seen only a curious turn of phrase and not had to suffer such slings and arrows of outrageous explanations.

westbranch

Re: Different Panel Configurations on an MPPT Controller

Keep this up Marc and we may start accusing you of writing like Joke K Lark spoke when he was PM....
Remember the quote when the poor chicken farmer was asked about "the totality of his estate?"

Badar

Re: Different Panel Configurations on an MPPT Controller

Cariboocoot wrote: »

If Bill had not elaborated on my paraphrasing of Shakespeare poor Badar would have seen only a curious turn of phrase and not had to suffer such slings and arrows of outrageous explanations.

Believe me Marc (I hope it would be your good name ), it took me at-least 15 to 20 minutes to understand what was going on from post 19 to 24. And I succeeded when I started to read sentences like a child, breaking into small parts, translating them into Urdu and then combining them to understand their actual meanings

So after this I can say that
"The fault, dear Bill, is not in my English, but in me.”

Badar

Re: Different Panel Configurations on an MPPT Controller

BB. wrote: »

Oh boy Badar--There you go showing my poor English skills and total lack of cultural awareness (even when making a post on cultural awareness). And English is my only language.

I intended to say your "free time" reading Skaespeare's plays. Browse also means to graze--Deer browsing on (eating) grasses and brush.

William Shakespeare



Shakespeare is, arguably, the person with the single most influence/importance regarding the English language.

The Complete Works of William Shakespeare

Interestingly, even though he is widely known and even famous during his life--Almost nothing is known about Shakespeare personal/private life. There have been arguments that his is not even a single person but several people of the time.

I am certainly not an English Major (more of an English moron)--And Shakespeare's play are a difficult read. Many/most believe that Shakespeare should be read aloud to be appreciated.

-Bill

I learned English just by reading business and economics text books from United States and then on this forum. So I was not having any idea what was going behind the posts. However my younger sister completed her masters degree in English Language, and I was pretty much afraid by just looking her books. However I am also well aware of the name of Shakespeare.

Actual problem with us is that we need to speak Urdu, Saraiki, Punjabi, Sindhi(Local Languages) and off course English, so we are unable to learn any language

Simple is that " “The fault, dear Bill, is not in your explanations, but in my English Language skills.”

BB.

Re: Different Panel Configurations on an MPPT Controller

Badar,

Frankly, your English reads like you are a native speaker. It is incredibly clear and concise. I have not seen one hint of awkward wording or misunderstanding of straight forward writing.

The only place where you got into (minor) trouble was with cultural references.

Be Like A Duck. Stay Calm On The Surface But Paddle Like Hell Underneath

Technical writing is probably much easier to understand/translate (ignoring the technical content) than poetry/literature/slang/etc. that uses the knowledge of the secondary meaning of words, a wider vocabulary, local culture, etc.

-Bill "just some idiot" B.

timh

Cariboocoot said:

Another of the questions that get asked a lot is "which is the best panel configuration for me?"
In this example we will examine the four possible ways of connecting the same six panels on one system. The panel specifications for this exercise are: Vmp 17.7, Imp 7.91, Voc 22.1, Isc 8.68 (140 Watt panels). The controller is an MPPT type with maximum input Voltage of 150 and 60 Amp output capacity. The nominal system Voltage is 12.

Possible configurations:

1). All six panels in parallel. Array specs: Vmp 17.7, Imp 47.46, Voc 22.1, Isc 52.08. Advantages: None really. Disadvantages: each panel needs circuit protection, high current and low array Voltage can be a Voltage drop problem for long wire runs.

2). Two parallel strings of three panels. Array specs: Vmp 53.1, Imp 15.82, Voc 66.3, Isc 17.36. Advantages: no need for circuit protection on strings, higher array Voltage with lower current good for reducing V-drop. Disadvantages: higher ratio of array to system Voltage reduces conversion efficiency (minimally).

3). Three parallel strings of two panels. Array specs: Vmp 35.4, Imp 23.73, Voc 44.2, Isc 26.04. Advantages: higher array Voltage with lower current good for reducing V-drop, better array to system Voltage ratio improves conversion efficiency marginally over #2. Disadvantages: needs circuit protection on each string.

4). All six panels in series. Array specs: Vmp 106.2, Imp 7.91, Voc 132.6, Isc 8.68. Advantages: high array Voltage can overcome significant wiring resistance for long runs between array and controller without the need for excessively large wires, no circuit protection needed on single string of panels. Disadvantages: high Voc could exceed controller's input max under cold temps, lowest conversion efficiency of the four possible configurations.

I have read through this thread and believe I understand the requirements of wiring 3 panels in parallel but I got a little lost in some of the jargon. I have 3 panels that I am wiring in parallel and would like to avoid using a c-box due to limited space on my van roof. The panels have Isc 6.65A, vpmac 20v, voc 22.8v, in my understanding if one panel short circuited then I would still be within the wire 10awg rating for one panel which would be rated for 30amps and wouldn't need individual fuses for each panel. Is that a correct understanding?

I have the panels feeding into a breaker before it gets to the controller but no breakers or fuses on individual strings before they are combined. The total wire run length is about 10ft. I originally wanted to connect the cables on the roof to a 6awg cable to help with voltage drop but couldn't find a combiner box that wasn't A) too big for my limited space and with a breaker that would connect 10awg to 6awg cable, hence why I am looking at if I can safely run them in parallel without a c-box on the roof. If there was some type of weatherproof in-line fuse I could put on each string that would be an option that wouldn't take up a lot of space but I'm not sure if those exist. 

Thanks for your help!

Estragon

I think the issue is the wiring IN the panel, not just the wiring TO the panel. The panels should have a spec for max string breaker size (often 15a). If you had two panels feeding a shorted third one, current could be roughly Isc x 2 (about 13-14a), which is close to or maybe above the max.

littleharbor2

There are inline fuses for MC-4 connectors.

BB.

Yes, the current limitation is the panel itself (i.e., 15 amps maximum). Just to be clear, DC current is actually quite a bit "more dangerous" regarding arcing vs AC... Here is an example of a load with DC vs AC feed and the difference in opening a knife switch:

https://www.youtube.com/watch?v=Zez2r1RPpWY

So, you want to make sure that you follow code and good electrical design. Arc faults are not the same as over current faults. I.e., a nice 8 amp @ 24-400 VDC will not trip a standard 15 amp breaker (15 amp DC arc will continue to "burn" very nicely). That is why the new codes are starting to require arc fault detection (cut loads/current flow if there is an arc detected--But this has its own issues with false trips and, in my mind, some questionable ground fault detection schemes).

Yes, requiring a fuse/breaker per string for 3 string of solar panels is typically a very close call (it depends on the panel ratings). 4 or more parallel strings is much more obvious for the per string fuse requirement.

And, solar panels (and their wiring) do catch fire if something goes wrong, or if gray market product, or inappropriately rated devices (i.e., AC rated breaker in a DC circuit, AC rated switch in DC circuit), etc.

https://www.youtube.com/watch?v=Cup5fMGaE2g

http://forum.solar-electric.com/discussion/3375/panel-fire-question/p1

-Bill