I´m going to tell my experience.

Five years ago i had an install wiht 9 solar panels, 200 w 60 cells, in three series of three, 3+3+3. I discoverd that one diode was short so i thought the next:

We have two series of three with 180 cells each one and a one whit 60+60+40 cells. Then, Vo of the two complete series in paralel is 108 V, against 36+36+24=96 V of the other series. Mixing three series we could think that the equilibrium would be with the three series on 96 V, and passing current from the higher, but that wasn´t what i saw, because the current was only the leackage current of the bypss diodes. Could yoy explain this?

Edited October 24, 20187 yr by Javi Martínez

Which diode blew a bypass diode or an internal cell diode?

Edited October 24, 20187 yr by phil.g00

1 minute ago, phil.g00 said:

Which diode blew a bypass diode or an internal cell diode?

Bypass

It sounds like one of the bypass diodes failed short circuit, turning the module from a 60-cell module into a 40-cell module, and in turn changing the 180-cell string into a 160-cell string.

So he ended up with two dissimilar strings.

I'm don't fully understand how much current this was making (language barrier?), but I too would expect the voltage to pull down to the voltage of the shorter string and some of the current from the longer one to pass through the shorter one.

@Javi Martínez, is that what you saw?

Then you would have a short circuit of that entire panel.

(Assuming 1 bypass diode per panel)

It would be a 3||3||2 panel situation.

Still wouldn't blow a fuse.

 

Edited October 24, 20187 yr by phil.g00

I can test now. I have a small solar pannel, V0=21V, Isc 300 mA, and a charger CC-CV charger 0-30 V 0-10 A. I´ll see what happens

6 minutes ago, plonkster said:

It sounds like one of the bypass diodes failed short circuit, turning the module from a 60-cell module into a 40-cell module, and in turn changing the 180-cell string into a 160-cell string.

So he ended up with two dissimilar strings.

I'm don't fully understand how much current this was making (language barrier?), but I too would expect the voltage to pull down to the voltage of the shorter string and some of the current from the longer one to pass through the shorter one.

@Javi Martínez, is that what you saw?

No, the Vo of the mix was the 60+60+60 Vo

I have done de test yet.

Firstly Vo of the solar panel was 19.4 V. If i put the charger it starts to flow current whit 20.1 V against solar panel.

@phil.g00 Once again I failed to understand the exact nature of your misunderstanding. Sure, if the diode was in series with the current source, it would necessarily be forward biased in normal operation. But if you had just googled the equivalent circuit, you would have seen that the diode is in parallel (or rather anti-parallel) with the current source, not in series.

If I cover the solar pannel, is starts to flow current on 14.0 V.

17 minutes ago, phil.g00 said:

Assuming 1 bypass diode per panel

Modern panels usually have three bypass diodes built in, bypassing part of the panel, in this case 20 cells.

21 minutes ago, phil.g00 said:

Then you would have a short circuit of that entire panel.

(Assuming 1 bypass diode per panel)

It would be a 3||3||2 panel situation.

Still wouldn't blow a fuse.

 

No, three bypass per panel

7 minutes ago, weber said:

the diode is in parallel (or rather anti-parallel) with the current source, not in series.

If I read that correctly, it means that applying a higher voltage (from another string) to the output terminals will bias that diode forward?

My goodness, it's only when you really start thinking about this that you realise you still have much to learn. Thanks @weber.

So, what is better, an additional diode per string or a fuse. Fuses don´t avoid reverse current.

Just now, Javi Martínez said:

So, what is better, an additional diode per string or a fuse. Fuses don´t avoid reverse current.

Por que no los dos? :-P

9 minutes ago, plonkster said:

Por que no los dos? 😛

Yes, i think it would be right

Edited October 24, 20187 yr by Javi Martínez

I think we might be forgetting what fuses are for. A fuse is there to protect the cable, in this case also some of the busbars inside the module (google for "pv panel hot spots" to see what I mean). If I have 3 strings in parallel and each of them makes 10A, then a fault in the middle one could potentially draw 20A down that path. If that path isn't designed to take 20A, put in a fuse.

For two strings, a fault can only cause 10A from the other string to be drawn, and this won't blow a fuse even if it was there, nor would it be more than the string can handle already. If I recall, this is what the regulations on most countries say in any case, there is a special n+1 rule that we discussed before on another thread.

The diode, on the other hand, is there to prevent power from the battery feeding back into the panels. This in itself isn't as big a problem as you might think. The voltage needs to be high enough to forward-bias that little internal parallel diode, an for the typical 36-cell panel connected to a 12V battery, the voltage isn't high enough to bias them all forward, so in general you will only have a bit of leakage current flowing. Also: solar charge controllers tend to take care of this for you, so no additional diode is needed.

See attached sketch showing what I think is happening in @Javi Martínez paulty panel.  A significant portion of the current generated by the good string is being shunted by the string with the shorted bypass diode.

5 minutes ago, NigelL said:

See attached sketch showing what I think is happening in @Javi Martínez paulty panel.  A significant portion of the current generated by the good string is being shunted by the string with the shorted bypass diode.

Yes, that is the reason why i think it's better to install a diode per string.

11 minutes ago, Javi Martínez said:

Yes, that is the reason why i think it's better to install a diode per string.

This will need to be quite a large high-power (>10W) diode, with suitable reverse-voltage rating. Heat dissipation becomes an issue and will probably need a heatsink.

Alternatively one can make up an "ideal diode" with almost 0V forward voltage-drop using an active circuit and some large Mosfets. No heatsinking needed, but more costly.

Edited October 24, 20187 yr by NigelL

36 minutes ago, Javi Martínez said:

Yes, that is the reason why i think it's better to install a diode per string.

You can install a diode per string. But you also need a fuse per string in case the diode goes short. But only if you have 3 or more strings in parallel.

Considering Weber's quite correct diagram as to the diode orientation.

i'll have to think about things, still not convinced about fuses.

 

 

 

 

 

 

 

1 hour ago, phil.g00 said:

Considering Weber's quite correct diagram as to the diode orientation.

i'll have to think about things, still not convinced about fuses.

That's good @phil.g00  To help you in your deliberations, you may be interested to know that in most jurisdictions, string overcurrent protection for (what usually works out to be) 3 or more strings in parallel, is not merely considered a good idea, but is a legal requirement. The best I could find online for free is this old draft version of AS5033, the Australian standard for PV arrays. Because of course, it's all about safety, so you have to charge people AU$254 to read the real thing!

Start with section 3.3 on PDF page 21 and read through to at least the end of section 3.3.5.1 (a) on PDF page 22. You may also want to see the definition of I_mod_max_ocpr on PDF page 9. It's what the panel manufacturers usually call "maximum series fuse rating". And you can see a circuit diagram on PDF page 16. I expect the corresponding South African standard has almost-identical requirements.

I noticed one difference from the present release version of the Australian standard. In 3.3.5.1 (b) the first occurrence of "(n_g - 1)" has been corrected to just "n_g".

Edited October 25, 20187 yr by weber

I just noticed another difference from the present release version of the Australian standard. Option 3.3.5.1 (b) is only allowed if I_mod_max_ocpr > 5 x I_sc_mod, i.e. only if the manufacturer's max series fuse size is more than 5 times the panel's short circuit current. This never happens, so you can ignore the (b) option.

Also see section 4.3.10 regarding blocking diodes on PDF page 33.

Edited October 25, 20187 yr by weber

Oops! I failed to give the link to the old draft standard. Here it is:

http://southerngreenenergy.com.au/wp-content/uploads/2013/01/DR_AS_NZS_5033_Installation_and_safety_requirements_for_photovoltaic_PV_arrays.pdf

This forum allows way too short a time for edits. On the AEVA forum we don't have a time limit on edits at all. This has never been a problem.