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OK to connect parallel PV strings without fuses?


NigelL

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I am planning on having 9 x 330W panels installed as 3 parallel strings of 3 panels in series. I have calculated that this combined array will have a worst-case short-circuit current of 28.6A at max temperature.

The solar installer has said that they will use cascaded MC4 "Y" connectors to parallel the strings together under the panels - without any fuses between the strings. The combined array will connect to the DC-DB via two 6mm^2 cables (plus extra earth cable). The DC-DB will then just have a single fuse in each of these PV cables. The cable run is about 16m - 18m.

I thought that one must have individual fuses for each string - both for safety reasons and for obtaining a CoC. In this case it would make more sense to route 6 thinner cables (e.g. 4mm^2) to the DC-DB where one would do the combining/fusing and surge protection.

Am I just being over-cautious?

 

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Hi Nigel. 

1 hour ago, NigelL said:

I thought that one must have individual fuses for each string - both for safety reasons and for obtaining a CoC. In this case it would make more sense to route 6 thinner cables (e.g. 4mm^2) to the DC-DB where one would do the combining/fusing and surge protection.

This in my opinion remains your best and safest option. The fuses per string is needed to protect the strings "from each" other in the case of a fault condition.  

Please remember to put a fuse on the + as well as the - of each string. Because the wires on the bottom of the fuses will then combine into one wire, you can use one surge arrestor for all three strings. 

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I respectfully disagree.

Solar panels can be shorted without damaging them, secondly how are you going to rate the fuse,when you want panels to deliver to their maximum.

 Thirdly if a fuse blows (with age as sometimes happens) you may not know about it until your batteries are flat.

And fourthly you are going to have to go on to the roof to fix something that is not providing any value.

Stick an appropriate rated  MCB in your ground level DB that can act as functional isolator. 

Then put surge protection on the panel side of this MCB. ( for lightning)

For surge protection I recommend buying cheap ferrite cores of say 15mm ID and putting at turn or two of your dc cable through this.

This makes a choke that becomes high impedance to lightning, causing the voltage to rise at that point.

A ferrite core looks like this:

ferrite core

Do this on the +ve and the -ve cable, then directly on the panel side of this choke fit cheap axial surge arresters that breakdown at say 145V between each cable and ground.

Surge arrester

 Hopefully, the cheap surge arrester sacrifices itself before your batteries or MPPT depending on your setup. Lightning jumps air all the way from the sky,  a further 1 inch fuse is not going to stop it. 

The MCB will protect the cable to your panels from your batteries, in the event of lightning allowing the batteries to backfeed to your now blown surge protection.

The batteries  are the only DC component with enough oomph to blow a fuse anyway and fuses wont save you from lightning.

All you can do is stop the batteries continuing to feed the fault that lightning will create.

So I recommend no fuses between panels (especially on the roof) in any configuration.

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29 minutes ago, phil.g00 said:

I respectfully disagree.

Thanks For that opinion Phil, Here is a post where we looked at PV fuses and some of the regulations and reasons for installing them on parallel circuits. I am still searching for another post about regulations that wants a fuse on each  + and -. 

The cost of fuses is so insignificant that installing them will hurt no-one, but that remains your choice. 

Please look at this explanation from @Chris Hobson, it might shed some light on your concern. 

A.thumb.PNG.600cd6260e43ebbaeadd323c25aee4e9.PNG

Something else to look at. http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/technical-literature/bus-ele-an-10191-pv-app-guide.pdf

I also struggled to see the use of it till I asked and read about it. 

 

  

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38 minutes ago, phil.g00 said:

So I recommend no fuses between panels (especially on the roof) in any configuration.

My turn to disagree a little, and also agree a little.

Whether you need fuses between strings needs a bit more thought. When the strings become long enough and the possibility that a short condition, or shading condition can cause one string to make a much smaller voltage. Then you need to look at the breakdown voltage of individual cells or at least groups of cells. If there is significant risk of something like that, then you need fuses between the strings. But sometimes the string is just too small and such a thing cannot happen. But now the question is so complex that it becomes impossible to come up with a set of "best practices", what do you tell a newbie?

The best thing to do is to always advise the same thing: Put in those fuses.

I do however agree about not putting the fuses on the roof. Put them where you can reach them, in a combiner box ideally.

You can however skip them on your 2 panel camping rig... :-)

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Nope, the OP described a string as panels in series, all that does is increase the voltage.

If 8 amps is the panel rating, then shorting out 10 panel in series will still only provide the same SC current of 8 Amps as shorting out 1 or shorting out a string of 100.

If they were in parallel then the current would increase multifold.

The question here is if a parallel string went short circuit would you want its fuse to blow so you could limp along probably unknowingly with 1 string less production or whether you'd prefer to know straight away as you'd have no production straight away. (Remember nothing will get further damaged as its quite permissible to SC PV panels.)

I submit I'd prefer I'd want to know about it straight away, however its never going to happen.

Why?

Now, I've just spent 5 min googling and I couldn't find an instance of a panel going short circuit.

The little cells themselves are like diodes.

Now when a diode blows it does go short circuit, but for series column of them with a  panels  to blow that would be winning the bad luck lottery.

For a column of series cells in every panel in a string to blow, borderline statistically impossible.

As to shading of cells that doesn't become lower impedance but a higher impedance. That lowers current and wont increase current which is what you need to blow a fuse.

 

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25 minutes ago, phil.g00 said:

I submit I'd prefer I'd want to know about it straight away, however its never going to happen.

As mentioned above.. 

59 minutes ago, Jaco de Jongh said:

but that remains your choice. 

There is recommendations from people and company's much bigger than myself, although it doesn't always makes sense to me, or even though I don't fully agree with it, I prefer to follow them. 

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1 hour ago, phil.g00 said:

As to shading of cells that doesn't become lower impedance but a higher impedance.

Yes, but the bypass diodes come into play. And even though a shaded cell does increase impedance, you can still force current through it if you have enough voltage, and it will get hot and can become damaged, which is exactly my point: large numbers of series cells which are in turn paralleled, that is precisely the scenario where a cell or group of cells might find itself in a breakdown condition. Rather put in the fuse. It's cheap peace of mind.

But as already conceded, there are places where you are absolutely right. A 2x2 series/parallel setup for example. Voltage too low to cause breakdown anywhere, a short in one string can draw no more than the other makes, which is already on spec... etc etc. Hence me saying that such shortcuts are okay on your camping rig.

The conditions are that you either have more than two strings, or two strings of many cells (3, maybe 4 panels or more), then the fuse becomes important. Or that's how I understand it in any case.

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8 hours ago, NigelL said:

I am planning on having 9 x 330W panels installed as 3 parallel strings of 3 panels in series. I have calculated that this combined array will have a worst-case short-circuit current of 28.6A at max temperature.

The solar installer has said that they will use cascaded MC4 "Y" connectors to parallel the strings together under the panels - without any fuses between the strings. The combined array will connect to the DC-DB via two 6mm^2 cables (plus extra earth cable). The DC-DB will then just have a single fuse in each of these PV cables. The cable run is about 16m - 18m.

I thought that one must have individual fuses for each string - both for safety reasons and for obtaining a CoC. In this case it would make more sense to route 6 thinner cables (e.g. 4mm^2) to the DC-DB where one would do the combining/fusing and surge protection.

Am I just being over-cautious?

 

No  you not being too cautious. One could have inline fuses if you wanted and still use cascaded MC4 Y connectors. I still prefer a combiner box which has good access (not that I have opened my combiner box in years).

Are your losses acceptable with 6mm2 over 16-18m?

 

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It's getting crowded in here. :)

@phil.g00, you seem to be missing the fact that in this case there are three strings in parallel. So if any group of cells gets shorted out, e.g. by a failed bypass diode, then the remaining cells in the same string, will have current forced through them in reverse, by the two healthy strings. Sure PVs can take the same current in reverse as they can deliver in the forward direction (into a short-circuit), but they can't usually take twice that current. So with only two strings in parallel you don't need string overcurrent protection, but with 3 or more strings in parallel you do if you want to avoid a possible fire.

How you size that protection is, first you read the panel data sheet and see what the max fuse size is that the panel manufacturer recommends. Failing that, you take the panel's short circuit current under standard test conditions (Isc) and multiply it by 1.5. You then use the next standard size above that, which must be less than 2 times Isc.

We used to use 1.25 * Isc in Australia, but there were still some nuisance trips, hence we now use 1.5 * Isc. That factor has little to do with temperature. PV short circuit current is hardly affected by temperature. It is however proportional to irradiance. During the sky condition known as "cloud halo" it is easy to get 1.25 suns worth of irradiance (i.e. 1250 W/m²) due to direct sunlight plus diffuse light from white cloud surrounding the sun.

Yes. Best not to have these protection devices on the roof. But if your installer refuses to do anything else, he could at least use inline weather-proof MC4 fuses tied up underneath the panels. When it is done on the ground, there is also the option to use a double-pole non-polarised DC circuit-breaker for each string (instead of fuses). This costs more but has the advantages that (a) you can see if one has tripped, and (b) you don't need a separate isolation device. I use NoArk brand.

Edited by weber
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A really big thank you to all the feedback! As an electronic engineer I understand the theory, but was not sure about the "best practice" and regulation aspects to this issue.

There is still some time before my panels will be installed, so I will request the installer to make some adjustments to their initial "plan".  I will give an update on the final implementation in a few weeks.

Edited by NigelL
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The bypass diode purpose is to allow other panels in the same series string to "bypass" a shaded panel, which when shaded has a high impedance.

No, a PV panel doesn't take as much current in reverse as forward, you will not pass any current in reverse through a pv panel. It is a series of diodes that will block current for many multiples of it output voltage applied in reverse. As will the bypass diodes.

The bypass diode is a pn junction in parallel with a string of series pn junctions within the panel. A pn junction is a diode.

A diode needs a 0.7V forward bias to conduct in forward direction, and will not conduct when reverse biased.

Panel strings in parallel will provide a voltage in the polarity that would attempt to reverse bypass all neighboring strings.

For a string to conduct in reverse (and then presumably blow a fuse) a series combination of diodes (pn junctions) would have to blow. (Highly unlikely).

A single blocking diode in a string would prevent this better than a fuse. 

But if you aren't going to believe that multiple diodes is series are already blocking any reverse current, why add an extra one?

 

 

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9 hours ago, Chris Hobson said:

Are your losses acceptable with 6mm2 over 16-18m?

 

I was also a bit sceptical about this.  My preferred alternative, of routing each string via a pair of 4mm^2 cables, will give far lower losses.

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2 minutes ago, NigelL said:
9 hours ago, Chris Hobson said:

Are your losses acceptable with 6mm2 over 16-18m?

 

I was also a bit sceptical about this.  My preferred alternative, of routing each string via a pair of 4mm^2 cables, will give far lower losses.

If you run the stings all the way to a combiner close to the inverter, separate 4mm will do. If you take them to a combiner box close to the pv array and them move the combined power over 18 meters you will be above the 3% recommended volt drop. Using a 10mm in this case will bring the volt drop to below 3%. Although Sans regulation requires a volt drop below 5 percent, in the Solar world the recommendation is below 3%. in your setup, every volt you loose is equals to 111 watt loss.    

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1 minute ago, phil.g00 said:

No, a PV panel doesn't take as much current in reverse as forward, you will not pass any current in reverse through a pv panel. It is a series of diodes that will block current for many multiples of it output voltage applied in reverse. As will the bypass diodes.

Some kinds of diodes have lower reverse breakdown voltages. The most well known type is called a Zener diode, and they are designed to break down at a specific voltage. Silicon diodes usually have such a high reverse breakdown voltage that you rarely even have to consider it, but other types like shottkys have a lower breakdown and it has to be considered. A PV cell is another such diode that has a breakdown that has to be considered. The number seems to be somewhere between 30 and 100 times its nominal voltage, which is about an order less than a cheap silicon diode can do.

So again I'm in that space where there are just too many variables. On the off chance that I missed one, I'm fusing things :-)

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12 hours ago, phil.g00 said:

And fourthly you are going to have to go on to the roof to fix something that is not providing any value.

Stick an appropriate rated  MCB in your ground level DB that can act as functional isolator. 

Hi Phil

Thanks - your suggestion to use a couple of MCBs makes a lot of sense, but does come at a bit of a cost premium. Note that I was not planning on installing any fuses under the panels - only in the DC-DB.

12 hours ago, phil.g00 said:

For surge protection I recommend buying cheap ferrite cores of say 15mm ID and putting at turn or two of your dc cable through this.

This makes a choke that becomes high impedance to lightning, causing the voltage to rise at that point.

This approach is surprisingly effective, even for low-level ESD protection of low-power electronics (my area of expertise).

12 hours ago, phil.g00 said:

 Hopefully, the cheap surge arrester sacrifices itself before your batteries or MPPT depending on your setup. Lightning jumps air all the way from the sky,  a further 1 inch fuse is not going to stop it. 

The MCB will protect the cable to your panels from your batteries, in the event of lightning allowing the batteries to backfeed to your now blown surge protection.

The batteries  are the only DC component with enough oomph to blow a fuse anyway and fuses wont save you from lightning.

Lighting is somewhat of a rare event in Cape Town, but the induced currents from a strike over 1km away can still damage exposed/unprotected equipment. 

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25 minutes ago, Jaco de Jongh said:

If you run the stings all the way to a combiner close to the inverter, separate 4mm will do. If you take them to a combiner box close to the pv array and them move the combined power over 18 meters you will be above the 3% recommended volt drop. Using a 10mm in this case will bring the volt drop to below 3%. Although Sans regulation requires a volt drop below 5 percent, in the Solar world the recommendation is below 3%. in your setup, every volt you loose is equals to 111 watt loss.    

Out of interest, what cable power-loss calulator are you using? I have come across the site http://www.solar-wind.co.uk/cable-sizing-DC-cables.html, but is only confirmes whether a cable run will be within a given loss percentage.

Edited by NigelL
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15 minutes ago, NigelL said:

Out of interest, what cable power-loss calulator are you using?

I use a App on my phone called Electrical Calculations (Pro version) , but using the one you refer to gives the same answer. Vmp of a 330watt panel is in the region of 37 volts so under load I expect your strings to run at 111 Volt , lets work on 28.5 amps as stated above. 

cc.JPG.351f8a794b162d5e6b4898fa5f996b58.JPG

I am just looking for the online one I sometimes use. 

Edit: Found it:  https://photovoltaic-software.com/DC_AC_drop_voltage_energy_losses_calculator.php

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Hi @NigelL Phil didn't suggest "a couple of MCBs". He only suggested one MCB, to protect the cable to your panels from your batteries. That protection is necessary too. But Phil would have you connect your 3 strings in parallel with no over-current protection between them. It was I who suggested three MCBs (one per string), as a (convenient but expensive) alternative to a pair of fuses per string. And if you have your individual strings protected in your DC-DB on the ground, then these 3 fuse-pairs or breakers also provide protection of your cables against current sourced from your battery, e.g. via a faulty SCC.

There is a first line of defense against nearby lightning that should be done on all jobs and costs nothing. That is to minimise the loop area enclosed by your PV cables, so they don't act as an antenna for the electromagnetic pulses. Think of twisted-pair cables, although you don't need to actually twist them.

Hi @phil.g00 You're still missing something. But this time you're in good company because @plonkster is missing it too. :o ;) And that is: Although PV cells are indeed silicon diodes, they point in the opposite direction to what you're claiming. They are forward biased in normal operation. Don't take my word for it. Google "solar cell equivalent circuit" WoTQ. Or better still, do the experiment. Connect a string of two solar panels (or a current-limited power supply of a similar voltage) in parallel with a single panel, all out in the sun. Then measure the current.

Because they are forward biased in normal operation, they will be even more forward biased in a string fault scenario. Hence the need for the fuses when there are 3 or more strings in parallel.

You claim that blocking diodes would be better protection than fuses. But you also mention that diodes can fail short circuit. I hope you can see that those two things can't both be true. An overcurrent protection device that can fail short circuit is no overcurrent protection device at all. Blocking diodes can be used, but they still need to be backed up by fuses if you have 3 or more strings in parallel.

Edited by weber
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1 hour ago, weber said:

It was I who suggested three MCBs (one per string), as a (convenient but expensive) alternative to a pair of fuses per string. And if you have your individual strings protected in your DC-DB on the ground, then these 3 fuse-pairs or breakers also provide protection of your cables against current sourced from your battery, e.g. via a faulty SCC.

Hi @weber, thanks - I skimmed through the responses and mis-ascribed the recommendation 😉

7 hours ago, weber said:

We used to use 1.25 * Isc in Australia, but there were still some nuisance trips, hence we now use 1.5 * Isc. That factor has little to do with temperature. PV short circuit current is hardly affected by temperature. It is however proportional to irradiance. During the sky condition known as "cloud halo" it is easy to get 1.25 suns worth of irradiance (i.e. 1250 W/m²) due to direct sunlight plus diffuse light from white cloud surrounding the sun.

I had heard reports of the "cloud halo" effect from people with solar installations, but never seen it properly described or quantified. This is very useful info!

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1 hour ago, weber said:

They are forward biased in normal operation.

This I actually knew, but I have never fully worked out what that means in a parallel situation... except to note that whatever is happening likely isn't on your side and is waiting for the first opportunity to bite you :-)

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I know they are forward biased in normal operation otherwise they wouldn't work.

Think of a diode as a non-return valve.

So you connect the +ve of all the parallel strings together and all the -ve  legs.

All of the internal non-return valves point in the same direction.

The convention is current flow from +ve to -ve outside of the source terminals. ( think of a battery).

Conversely current flows from negative to positive inside the source. 

If one string was to drive current through another string it would try a drive in the opposite direction to normal operation to make its circuit. 

In other words it would try and cause current against the non-return valves.

It would try to drive from positive terminals to its negative terminal inside the source thereby reverse biasing a whole bunch of diodes, so current wont flow.

The above description of a Zener diode is correct, but Zener diodes are used in electronic circuits for special voltage clamping purposes and not just used willy-nilly.

You wont find them on a PV panel, never, ever, not a variable.

Diodes do blow short circuit, but we have to consider their statistical failure rate.

Lets just consider rectifiers, for example they do fail every now and again, but they are forward and reversed biased 50 times a second, 60 times in the states.

Probably more times in the time its takes me to write this, than they'll be expected to do on your roof in 25 years. Now factor in that you need multiple series failures to create a short circuit through a series on panels. Lightning aside, this is borderline statistically impossible in normal operation. 

 

 

 

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9 minutes ago, phil.g00 said:

The above description of a Zener diode is correct, but Zener diodes are used in electronic circuits for special voltage clamping purposes and not just used willy-nilly. 

You wont find them on a PV panel, never, ever, not a variable. 

And it was never my intention to suggest that they are used on PV panels, just that not all diodes are alike. Given that I was probably giving information that is old news to you, it was probably over-explaining it and using too many examples to bolster my case.

Fwiw, I do have two parallel strings of 3 x 36-cell panels in use that doesn't have fuses between the strings, precisely because I reasoned in the same manner that it serves no purpose. If I had three strings, I would fuse them. If I had 4 panels in series, I'd probably do the same. In fact, I've already done the work to split it into two separate strings (cabling is installed, just not connected yet) and put the fuses on the ground, mostly because I intend extending that array soon.

Edited by plonkster
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