Author Topic: Solar PV strings in parallel, blocking diodes or not.  (Read 2005 times)

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Offline bdunham7

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #25 on: August 22, 2019, 01:34:17 pm »

 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.
 

Offline SpeedyDave

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #26 on: October 10, 2019, 04:23:23 am »
**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 "  !!!
« Last Edit: October 10, 2019, 04:32:08 am by SpeedyDave »
 

Offline Technomaniac

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #27 on: November 05, 2019, 11:47:57 pm »
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.
 

Offline SpeedyDave

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #28 on: November 06, 2019, 12:56:11 am »
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.
 
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Offline bdunham7

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #29 on: November 07, 2019, 06:08:27 pm »
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.
« Last Edit: November 07, 2019, 06:10:21 pm by bdunham7 »
 

Offline SpeedyDave

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #30 on: November 08, 2019, 12:06:03 am »
***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 )

 
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Offline SpeedyDave

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #31 on: November 11, 2019, 12:49:31 am »
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.  :box:
 

Offline SpeedyDave

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #32 on: November 17, 2019, 05:20:27 am »
Actually fascinating to watch  :-DMM the MPPT adjust the load on the east/west string to regulate the total output of the inverter (5kW)

Very clever stuff  :)

 

Offline bdunham7

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #33 on: November 18, 2019, 07:33:40 pm »
What are the system voltages during that long flat spot?
 

Offline SpeedyDave

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Re: Solar PV strings in parallel, blocking diodes or not.
« Reply #34 on: November 19, 2019, 01:08:06 am »
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 !
 


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