Solar PV strings in parallel, blocking diodes or not.

[bdunham7]

 If I were making my own, I would use 2  diodes rated at least 40A, 1000V, in series with a high-energy 10A 600V minimum fuse between them and robust 600V MOVs across each one. 

Thanks 

could you please explain exactly where you would connect the MOV's ??

Across the diodes.  But as Kleinstein says, the MOVs are probably overkill and are just my idea.  One diode and a fuse is also enough, with an MOV if you are as concerned about lightning as I am.  My main issue with these HV strings is that you don't always know when failures occur--you don't go up on the roof with a multimeter every month and test stuff--so critical parts, especially safety related, need to be very robust.

[SpeedyDave]

**Update**

Things are progressing and I have acquired a few panels, diodes, breaker etc.

Let the fun begin !!

Here is how I have decided to include blocking diodes for each string.
The way that the DC breaker (4 pole) is configured, I am going to use 2 diodes per string.

Diodes are 1000v 10A

Looking at the picture, the string positive enters the first breaker, passes thru the diode, then the next (second) breaker and onto the +ve input of the inverter.

The string negative enters into the fourth breaker, passes thru the diode, then the previous (third)  breaker and onto the -ve input of the inverter.

The strings on my system are configured exactly the same, but have buss-bars connecting the two breakers together.

I will also replace those buss-bars with diodes.

Easy to monitor and no fire hazard.

Should work a treat !


*note.  I was speaking to a solar installer the other day and he showed me some inline blocking diodes ( modular with MC4 connectors each end )  and he was like "  yeh, so what, use them all the time "  !!!

[Technomaniac]

Hi.  If you are talking about a diode in the conducting direction in series with each string of panels, the panel producing the higher voltage will reverse-bias the diode in series with the low voltage producing panel string. IT WONT CONDUCT AT ALL. So effectively you are switching off the weaker panel altogether. If you want all panels to contribute all day, you can't use series diodes. I have a 24v system and I have experienced this. I got around it by using separate regulators and battery pairs (12v car batteries) on each panel, or parallel groups of panels, and then bringing the groups together through series Schottky diodes.Even then, only the battery pair with the highest voltage is in use - same theory, reverse-biased diodes dont conduct. .But I have all panels horizontal, 8 feet off the ground, as I'm in a dusty area and have to clean them at least once a week. Incidently, if any readers of this are doing the 24v thing like me, put some reverse diodes across each 12 volt battery. It took me a long time to work this out, second hand batteries, stronger one is reverse charging the weaker one through the night (if the batteries are being discharged more than is healthy.)
We had a retirement home fire started in a panel on its roof a couple of weeks ago. I don't like so many panels in series, if one fails (its only a coat of semiconductive paint) then you have the 400 volts to jump across the gap and because its DC the arc keeps right on. I read once that when Edison had 110v DC mains around the houses, if a lamp hanging from the ceiling would blow, sometimes an arc was created and it would move up the wire to the ceiling and along the wire, burning down the house.

[SpeedyDave]

Nearly ready for final connections and testing.
I understand that reversed bias diddes dont' conduct, but my theory is that both strings will be contributing due to the fact that  PV panels are a high impedance current source. ie the voltage varies according to load.

In my existing system, OCV is roughly 440V, but that drops to around 330V when loaded by the inverter setting the MPPT load.

If a string is not under load, its voltage will rise to its OCV spec.
This will then be higher than the LOADED voltage (MPPT determined) of the second string, and as such, the diode will conduct and start suppling current.
This load will pull the voltage of the first string down to be the same as the second string, so that BOTH strings will contribute, although I recognise that they will not be contributing equally.

Once my setup is ready, I can easily add an extra panel to one of the strings ( 11 on one and 12 on the second ) and physically measure what is going on.
I am confident that my theory will be proven correct.

[bdunham7]

Hi.  If you are talking about a diode in the conducting direction in series with each string of panels, the panel producing the higher voltage will reverse-bias the diode in series with the low voltage producing panel string. IT WONT CONDUCT AT ALL. So effectively you are switching off the weaker panel altogether. If you want all panels to contribute all day, you can't use series diodes.

The blocking diode will only be reverse-biased and non-conducting if the MPPT (loaded) voltage of the first array is higher than the OCV of the second.  And in that case, you still would want the blocking diode because without it the second array would absorb power from the first.  In a small 24V system as you mentioned, with similar panels and a very light load, I suppose it is possible that the differences could be the same or even less than the forward voltage drop of the diode, but that would be best remedied by increasing the load and lowering the voltage a bit.  Any competent MPPT device should do this.

[SpeedyDave]

***update***

Wiring done.

some system tests etc over the next few days to get some base line numbers etc. , then a full system test planned for Sunday.

Once base line numbers are done, the big theory test will be on Monday, where I will test 2 strings (east and west) with two differnt outputs ( 11 panels vs 12 panels ) thru blocking diodes, feeding into one MPPT at the inverter.
I can then directly measure voltages at the breaker, either side of the diodes, of both paralleled strings.

If my theory is correct, the loaded voltage out of each string will be the same ( current will be different of course )

[SpeedyDave]

The numbers are in !

Data valid as of 8:30am, 22 degrees C, clear skys.  ( both strings are at 20 degrees inclination)

Due East facing string of 11 panels,  OCV 455v
Due West facing string of 12 panels, OCV  495v

Parallelled, thru blocking diodes into a single MPPT on grid tied inverter.

Both panels loaded voltage now reads 358v each ( obviously, as they are parallelled ! )


Volt drop over the blocking diodes.  about 0.8v on the east string, about 0.7v on the west string. Yep, sounds right as the current flow is differnt.


current draw,
East string = +4.2 amps

West string = +2.3 amps


So there we have it, my theory is correct 
Two enirely different strings, facing different directions can positively contribute to a solar PV grid tied installation. 

[SpeedyDave]

Actually fascinating to watch  the MPPT adjust the load on the east/west string to regulate the total output of the inverter (5kW)

Very clever stuff 

[bdunham7]

What are the system voltages during that long flat spot?

[SpeedyDave]

Mppt raises voltages (reduced current draw) to about 410v

Under full load (earlier) the voltages are pulled down to about 340v. (higher current though)

So this is how the inverter manages its max output, even when being overdriven.

Clever stuff !

[sibianul]

Hello everyone,
Dave, thanks for you posts, I'm in almost the same situation as you, in the future I want to connect 2 different strings (EST String has 7 x 290w mono panels, SOUTH String has 7 x 295w mono panels).
Those strings are now connected each to a different ONGRID inverter.

This year I decided to try an OFFGrid system, I purchased an MPP Solar 8048 MAX inverter (8kw) and a Hyundai Ioniq Lithium Battery (28kwh), I also mounted a third String facing WEST, 10 x 370w Mono TrinaSolar Panels. The inverter has 2 MPPT inputs, each 4000w and max 18A. TrinaSolar string will be connected on MPPT1 and if that string won't be enough to recharge the big Lithium battery, I will extend the SOUTH string wires (~45meters) and connect it to the second MPPT on the offgrid inverter.

But I was wondering if it will be an even better idea, at least in the winter, to connect the EST and SOUTH strings in parallel, and than connect both to the second MPPT input of the offgrid inverter.

Regarding your picture, I don't quite understand how you connected two strings in parallel with that breaker, but I want to ask you regarding the heat, do those diodes get hot ?

I wanted to buy MC4 connectors with embeded diodes, and also an Y MC4 cable to join the 2 strings, as I want to join the strings on the roof, under the pannels, and directly from that point to extend the cable, using a thicker one: 10mm² compared to the existing  one which is 6mm² . Would this work too ?

The strings have each maximum 9Amp, should I get a 10Amp Diode MC4 connector, or a 15Amp diode connector, to be on the safe side with that diode?



Bellow you can see a drawing with my strings placement, and how I would extend  the cables to reach the OFFGRID INVERTER, that is placed inside my garage. The ONGRID inverters (green and yellow) are IP65 and are placed outside on my terrace. I'm not able to use a shorter cable from East adn SOuth string to the offgrid inverter, as there are windows all around the house, I can only go under the roof tiles, and on the back of my house, where I already cut the termoisolation of the house to insert a big tube inside wich I have the DC cables.



Any advices, from anyone, are welcome.

Thank you

UPDATE. Is this the way you connected your string in paralel ?

[SpeedyDave]

Lots to go thru, but sounds like you are on the right track.

In my system, the diodes get warmish.  Maybe 50 degrees C ?

The end point of the parallel strings is at the MPPT at the inverter ( inverter has 2 sets of inputs for each tracker )

on my inverter (sunny boy) ALL of the negative inputs are commoned within the inverter.

The 4 pole breaker is a specific solar breaker ( magnetic arc quenching) and it is polarity conscious, meaning that it has +ve and -ve terminals.

i'll try to explain; 

the +ve from the PV string enters the (first of 4 ) breaker at the top left. current flows thru the (closed) breaker down to one diode, thru that diode and then back up thru the next  ( second of 4) breaker to the +ve input on the inverter.

The -ve from the PV string enters the (fourth of 4) breaker at the top right. current flows thru the (closed) breaker down to the second diode, thru that diode and then back up thru the next  ( third of 4) breaker to the -ve input on the inverter.

I have 4 strings of PV

I have 4 x 4pole breakers ( 2 diodes per breaker, one on the positive lead and one on the negative lead ) 

Yes, I could have just used one diode per PV string, but it was easy to use two as the bridges between the positive pair and the negative pair of breakers.


The current sharing "magic" happens when the inverters MPPT loads up the PV circuit thus dropping the voltage. When the loaded voltage goes below the open circuit voltage of the SMALLEST PV string, it will be able to forward bias its diodes and thus contribute power to the system.  So obvioulsy the two strings that you want to parallel need to be fairly close in OCV.

On my system, I have noticed MPPT voltages between about 280 - 380 volts, so as long as the OCV of each string is higher that that, they will both contribute.

Now I can almost hear the naysayers keyboards clattering !


YES !!!   I do know that this is not the most efficient way of harvesting solar power.
YES !!! individual MPPT's or optimisers or even inverters for each string WILL harvest more power !!!!!


but that was not the intention of this experiment   ( i got a bunch of second hand panels and had a spare input on one MPPT on my inverter. this was an experiment to see HOW to parallel two different PV strings, that are on different orientations into one MPPT on my inverter)

On my system, I have gone from a daily max of about 40kWh per day to about 47kWh per day.
The added PV string was 2.2kWh and daily irradiation of 6hrs.
So the potential of 12kWh ended up being 7kWh, these arew west facing panels anyway so not a bad outcome.
Here in Australia, we are limited to a 5Kw inverter anyway, so now, instead of having a 5Kw peaking bell curve at midday, I have a nice big 5Kw flat-top curve from about 9:30 to 3:00 PM

great outcome for a couple hundred bucks in second hand panels.

[bdunham7]

I have a nice big 5Kw flat-top curve from about 9:30 to 3:00 PM...
great outcome for a couple hundred bucks in second hand panels.

Excellent!  I think more and more systems will go to this 'clipped' model as the panels get cheaper and more people install them.  It's not just your inverter or your panel that limits the practicality of peak power, it is the grid itself.  Google "CAISO duck curve" and see what I mean.

[sibianul]

In my system, the diodes get warmish.  Maybe 50 degrees C ?

Ok, not to hot at 50deg C

The end point of the parallel strings is at the MPPT at the inverter ( inverter has 2 sets of inputs for each tracker )

on my inverter (sunny boy) ALL of the negative inputs are commoned within the inverter.

I think its normal to have the negative input common inside the inverter, all GND are common, no matter the POSITIVE voltage as I know. In my MPP Solar 8048 MAX inverter the GND are common too, and I think I opened an Growatt inverter and it also had common GND

I have 4 x 4pole breakers ( 2 diodes per breaker, one on the positive lead and one on the negative lead ) 

This made everything clear to me Now I understand your setup, you have diodes on negative wire also, I think I will use an diode on positive wire only, would you think this might be a problem ?

Regarding the diodes, anyone knows what are the disadvantage of using a bigger diode than the current generated by my string? My panels have a maximum 9A current, will it be more safe to get a 15A or 20A diode, instead of the 10A ? Is there any disadvantage going with bigger diodes ? Is the loss trough the 20A diode much higher than the loss trough the 10A diode ?

Thank you

[SpeedyDave]

you have diodes on negative wire also, I think I will use an diode on positive wire only, would you think this might be a problem ?

Regarding the diodes, anyone knows what are the disadvantage of using a bigger diode than the current generated by my string? My panels have a maximum 9A current, will it be more safe to get a 15A or 20A diode, instead of the 10A ? Is there any disadvantage going with bigger diodes ? Is the loss trough the 20A diode much higher than the loss trough the 10A diode ?

Thank you

One diode on the positive would be fine.
If your strings are 9 amps max, then a 10 amp diode is a bit close to max, so a bigger ( 20 amp ? ) would offer a margin of safety.  ( no real difference in "losses" in a bigger diode , just cost )

My strings max out at just over 5 amps, hence the 10 amp rated diodes ( 1000 volt blocking rated )  were plenty ( and cheap )

And yes, I only used 2 diodes ( one in +ve and one in -ve ) per string because             a; I had spares        and       b;  it was an easy and neat way of bridging the two beakers together.



The circuit breaker I used is this one  https://www.ebay.com.au/itm/124216950191?hash=item1cebe83daf:g:hKgAAOSwm3BazuIn

The diodes were these https://uk.rs-online.com/web/p/switching-diodes/7512709/   but they have been discontinued.

This would be fine, but you can choose what suits you !   https://americas.rsdelivers.com/product/on-semiconductor/ud1006fr-h/on-semi-600v-18a-diode-2-pin-to-220f-ud1006fr-h/1867562

Diodes must be rated to withstand the full (reverse)  voltage of your string, as well as to safely pass the full max current of the string.
So just figure out what those two values are and add say a 50% safety margin (or a bit more )  and then select a diode that can handle those values.