Schottky Diodes

[tecnodave]

Can someone comment on the use of Schottky diodes on each string of solar panels. I have not used them in the past as I was using multiple controllers for the different types of panels but now I have most of the strings connected to a single classic. My concern is about the First Solar CdTe panels having a reverse flow from the other panels.  I have some late day shading mostly affecting the First Solar panels and have read on their website cautioning about damage from reverse flow.

Strings:

4 ea. Siemens M55  36 cell mono in series       X5       v.m.p.  69.6.     V.o.c.     85.6
2 ea. Suntech STP-280/24/Vd. 72 cell poly in series.   V.m.p.   70.4.    V.o.c.     89.6
10 ea First Solar FS-275. 154 cell  CdTe. In parallel.    V.m.p.   69.4.    V.o.c.     92.0

Tecno

[Robin]

You will not be getting reverse flow. All three strings should be combined in a combiner and separated by circuit breakers. That is for fire protection. All three strings will be at the exact same voltage when connected together.
By the way, the combiners job is to stop reverse current flow into a bad string. So if one of your panels shorted, the other two strings will gang up on it and reverse current would flow. Since there is a fuse installed in each string, the theory is that the two good strings will have enough current to blow the fuse and take the bad string off line, thus stopping reverse current flow.
Do not use diodes. They are terribly inefficient and not high enough voltage. As I remember, Schottky diodes are not high voltage diodes.

[tecnodave]

Thanks robin. That is what I was thinking....all 8 strings (5 strings of Siemens) hook to a MNPV-12 combiner with each series string having its own breaker but the CdTe panels are all in 2 groups of 5 panels in parallel at the panels with two feeds to the combiner box with two 6 amp breakers, slight economy on my part, the CdTe panels also have a fast blow AGC fuses right at the panels.

The good Schottky diodes are 50 piv but there are 600 v and 1000 v ones on Amazon but it is possible that they are not Schottky diodes, hard to tell by part numbers

I won't worry about it , I don't want to join the smoke um club

Tecno

[vtmaps]

By the way, the combiners job is to stop reverse current flow into a bad string. So if one of your panels shorted, the other two strings will gang up on it and reverse current would flow. Since there is a fuse installed in each string, the theory is that the two good strings will have enough current to blow the fuse and take the bad string off line, thus stopping reverse current flow.

There are two possible functions for the breakers in a Midnite combiner.  One function is to act as a switch and disconnect a string.  The other function is the one you mentioned: overcurrent protection from reverse current flow through a faulted string.

I notice that the polarized breakers in the Midnite combiner are connected with their (+) side wired to the (+) side of the PV string.  This is the orientation I would expect if the breakers were intended to function as switches.

If the breakers were to be used for overcurrent protection of a reverse current flow, their polarity should be the reverse of what the combiner box instructions call for.

--vtMaps

[RossW]

Do not use diodes. They are terribly inefficient and not high enough voltage. As I remember, Schottky diodes are not high voltage diodes.

Respectfully, I disagree.

I have 6 arrays, each of 6 series-connected panels. On trackers.

I have breakers on each array, and individual current monitoring and logging. And because I had something bizzare happen once and blew all the diodes in one array when it was initially connected to the rest of the arrays (I think DV/DT did it), I bought a bunch of DO-220 mounting, 200V, 10A schottky diodes. Each string has its own diode. Sure, I waste a very small amount of power, but they are not "horribly inefficent" (half a volt at 7 amps is 3.5 watts). I think my diodes cost all of about $1.50 each.

[boB]

Do not use diodes. They are terribly inefficient and not high enough voltage. As I remember, Schottky diodes are not high voltage diodes.

Respectfully, I disagree.

I have 6 arrays, each of 6 series-connected panels. On trackers.

I have breakers on each array, and individual current monitoring and logging. And because I had something bizzare happen once and blew all the diodes in one array when it was initially connected to the rest of the arrays (I think DV/DT did it), I bought a bunch of DO-220 mounting, 200V, 10A schottky diodes. Each string has its own diode. Sure, I waste a very small amount of power, but they are not "horribly inefficent" (half a volt at 7 amps is 3.5 watts). I think my diodes cost all of about $1.50 each.

I think the point here is that using diodes is more wasteful than not using diodes.  Diodes are not necessary in typical
PV combiner operation.   Of course, it also depends on your definition of "horribly ".   In my past years discussions, it seems that
most off gridders fight for every last watt they can get.

The higher voltage "Schottky" diodes do not have the nice and low forward voltage drop that the lower voltage Schottky's have.
They're almost as bad as a regular silicon diode.  However, their on voltage goes down as their temperature goes up, which is
what is going to happen to a certain extent when they dissipate  power which is in your favor.  Less semiconductors also mean
less to break, especially something that can be so close, electrically to lightning strikes.

Ross, I suspect the reason your diodes broke was because the modules were not designed correctly.
What kind were they ?  How old ?   There have been bad batches of certain modules.

Diodes in parallel with a PV module should not be subject to high reverse breakdown because
the forward conduction voltage (Voc) of the PV module will (should) clamp that way before the diodes'
reverse rating is reached.   That is another reason to suspect bad modules.

boB

[RossW]

Ross, I suspect the reason your diodes broke was because the modules were not designed correctly.
What kind were they ?  How old ?   There have been bad batches of certain modules.

Diodes in parallel with a PV module should not be subject to high reverse breakdown because
the forward conduction voltage (Voc) of the PV module will (should) clamp that way before the diodes'
reverse rating is reached.   That is another reason to suspect bad modules.

Bob, I got 30 panels in one lot. All identical.
Being the typical, impatient, offgridder, I would build a tracker, get the panels on it, and connect it ASAP.
Usually this meant I connected them near the end of the day with little sun.
The final array however I got finished just on lunch time. So I had already 5 arrays connected, in full sun, making full power. I connected the last array and went around plugging in the panels together (I know, wrong!). As I put the connectors together for the last connection, there was this popping and tinkling sound above me.




The PV cells all survived. The supplier was going to replace the modules, but was happy I was more than happy to just (try) replacing the diodes, which he sent me by international express next-day-delivery. Replaced them all (over 3 1/2 years ago now).

My gut feeling is that the 150V "instantly" applied was enough to punch through the diodes.
After adding the diodes, I've never had any concerns about anything, and I also see zero reverse leakage overnight.

[boB]

Ouch !

I had this happen about 2 years ago at a small energy fair in Oregon....  We had a small 50 watt panel
and one small-ish 12V car battery.  Our table was outside and I hooked up the module directly to
the battery so it would charge.  Normally this is just fine BUT the manufacturer (Chinese) had
marked the polarity wrong !  Bang went the diodes just like yours.  I quickly discovered it was
marked backwards, removed the two broken diodes in the J-box and connected it up the other
way and voila...  It worked.

Right on the leakage especially if you are using a charge controller which should be the case.

Again... Ouch !  But luckily all was still good.

boB

[dgd]

Ross,
So this was a complete panel string of bypass diodes that blew apart? Was one of the panels faulty? Did you suspect that the diodes, which look like rectifier types, were somehow below spec or wrong types?
Did you then replace all of the bypass diodes in all of the strings since they are identical panels?

I think tecnodave is talking about blocking diodes but why fast schottky types. I always thought these would be unnecessary when each string is combined via breakers.

dgd

[RossW]

Ross,
So this was a complete panel string of bypass diodes that blew apart? Was one of the panels faulty? Did you suspect that the diodes, which look like rectifier types, were somehow below spec or wrong types?
Did you then replace all of the bypass diodes in all of the strings since they are identical panels?

I think tecnodave is talking about blocking diodes but why fast schottky types. I always thought these would be unnecessary when each string is combined via breakers.

I have 6 modules in series per array.
Something roughly like this one which was the first.


This was the final array to come online. 6 modules, 2 diodes per module, only 2 diodes didn't explode. I replaced them all.
They were indeed schottky, and they were there for (I presume) "shaded cell" bypass. None of the diodes in any of the other panels has given problem, and none of the replaced diodes have given problems.

I too am talking about adding series "blocking" diodes at the "combiner".
Yes, I have breakers. One for each array.
But breakers don't stop one array being potentially "reverse biased" if it is in shaddow. If the rest of my arrays are in sun, I could have close to 150V "on the bus". An array in partial shade (or full shaddow, as a cloud passes) might be making as little as 30-40V, but in a hardwired scenario with breakers, it will have 150V across it, while making only 30-40. Leakage current is a given.

It's changed since this picture was taken, but the "metering" box (wide open), above the DC breakers. Schottky diodes are the half dozen DO220 devices bolted to the copper busbar in middle of frame. The very thin red wires from connectors to diodes are "fusible links" in the event the worst happened.

Is it standard practice? No. Is it best practice? I don't know. Would I do it again? In a heartbeat.

[dgd]

Ross,
Thanks for posting those pics. That looks like the backs of moving coil meters, ammeters I assume?, for each string and on the right side another? Is that an A to D device on back of meter, you are using the meter as a shunt? ( i think I remember you said this some time ago).
I have been using an MN-EDC quad box with 10A breakers on +ve only inputs from my 4 pv arrays of 5by 140w, 91v @ 7.85A
but want  to change to dual pole breakers for the arrays.
I divert two strings to water heating via SSR. I found that one bypass diode  in one of the strings was burnt out and I suspect one of the strings was badly shaded and the other was full power when this happened.  Possible because these strings are 50 metres apart. The PVs kept working and nothing seemed damaged so I just replaced the diode.
Looks like placing some blockers may be a good idea, those to220 packaged types look a good idea and I like the fusible wire idea too.

dgd

[tecnodave]

dgd.   

  I was talking about blocking diodes.  They look the same as the ones in RossW's pic of his blown diodes.  These were field installed in my Siemens panels when I bought them used.  These panels have 2 junction boxes having a total of 2 shaded cell bypass diodes factory installed and 1 field installed blocking diode in each panel. (Blocking diodes sloppily crimped and soldered) They were used when I bought them from a ham radio operator, he said that they were from a 2 meter mountain top repeater in Southern California. I originally used them with my Chinese tracer controllers, removing these blocking diodes as they were in 2 strings of 4 panels (440 watts each 390 watt controller)  I use Multi-9 din rail breakers to switch in 5 first solar CdTe panels (375 watts) in parallel with the Siemens strings to boost power in foggy conditions. The first solar panels each set of 5 panels in parallel each feed 2 breakers one goes to the controller the other to a set of switch mode T-8 fluorescent  lamps to keep the panels loaded at all times. Only one breaker for each CdTe string is on at a time. The amount of power in full sun would be too much for the tracers and the fluorescent  lamps are to dummy load the CdTe panels in full sun as per the manufacturers recommendation not to let the panels go to v.o.c.  I have never heard of anybody else saying any such thing but it is covered in a white paper on their website. The original buss bars in the MNPV-12 Combiners are not used as the breakers feed separate controllers and the fluorescent lamp bank dummy loads.

I recently reinstalled the blocking diodes and now have drastically reduced power output. I will need to remove them again to restore power as I am dropping to 50% s.o.c. sometimes

The tracers now do other duty as I recently bought a classic 150 to replace the 3 tracers.


RossW.       I like your metering setup.....I'm doing sort of the same thing with brainy deal digital volt ammeters 0-200 volts/0-200 amps  with shunts on each string......cheap on Amazon....but I had to futz with them and power the driver electronics with a 7809 regulator chip to get reasonable accuracy.


I'm still looking for a way to post some pic's but my iPad has too high a resolution for the allowed file size.

EDIT:  the fluorescent lamp ballasts are Sylvania Quicktronic nominally rated 120-277 volts a.c. 50-400hz. and in very small print 65-165 volts d.c.   They work fine with the 69 volt CdTe panels directly connected.

tecno

[tecnodave]

dgd

Those look like copper buss bars to me......two rows of 3 meters each.....nice busses

Tecno

[RossW]

Ross,
Thanks for posting those pics. That looks like the backs of moving coil meters, ammeters I assume?, for each string and on the right side another? Is that an A to D device on back of meter, you are using the meter as a shunt? ( i think I remember you said this some time ago).

They are indeed inexpensive but "adequate" 10A DC moving coil meters, and yes, I'm using them as shunts for my logging. I like the meters as they're instantly meaningful. (I have made prettier labels since this pic!)



tecnodave has it right, there are copper busbars to join the meters. When I took this picture I had not finished building my logger.


(The stuff on the right hand side was leftover rectifiers from my turbine, but that's all in another box now)

[dgd]

...  These were field installed in my Siemens panels when I bought them used.  These panels have 2 junction boxes having a total of 2 shaded cell bypass diodes factory installed and 1 field installed blocking diode in each panel. (Blocking diodes sloppily crimped and soldered) They were used when I bought them from a ham radio operator, he said that they were from a 2 meter mountain top repeater in Southern California. I originally used them with my Chinese tracer controllers, removing these blocking diodes as they were in 2 strings of 4 panels (440 watts each 390 watt controller)

..sorry a bit off topic but..
1997/8  I bought new about fifty of the Siemens M55 panels, they came boxes of 4 about $2500 per box. I thought these were excellent mono pvs, wired in pairs to an Heliotrope CC120 24v, they would work near max power. The junction boxes at each end for +ve and -ve were ok, the diode in each was crimped in.
It was only a couple of years later I found a pair was at zero, on close inspection the tape/traces at one end going to JB were blackened and one was dissolved.. dead panel.  After general panic mode set in I got the ladder out and went inspecting the rest and many were ok, looked normal, bright silver traces BUT at least six were various shades of brown. I also noticed on some good ones lots of little holes in the solder that appeared to cover the traces.
Other issues I had was water getting under the junction box as the white glue dried and powdered away. The flat tinned copper  trace to the junction box then corroded away. Fixing was not always possible without stripping away the tedlar backing to find some buried tinned trace.
WHat an absolute ballsup they were, warranty worth nothing as dealer vanished
Took me 3 months to dispose of the lot and get BP panels, 170watt types at over $2k each. ANother disaster but another story 
I hope your's are ok and reliable

I use Multi-9 din rail breakers to switch in 5 first solar CdTe panels (375 watts) in parallel with the Siemens strings to boost power in foggy conditions. The first solar panels each set of 5 panels in parallel each feed 2 breakers one goes to the controller the other to a set of switch mode T-8 fluorescent  lamps to keep the panels loaded at all times. Only one breaker for each CdTe string is on at a time.

Do you get fog very often? Seems like a nuisance having to manually switch in/out pvs like this 

The amount of power in full sun would be too much for the tracers and the fluorescent  lamps are to dummy load the CdTe panels in full sun as per the manufacturers recommendation not to let the panels go to v.o.c.  I have never heard of anybody else saying any such thing but it is covered in a white paper on their website. The original buss bars in the MNPV-12 Combiners are not used as the breakers feed separate controllers and the fluorescent lamp bank dummy loads.

This would put these panels in the too complicated, avoid at all costs basket for me.  I have never heard of this requirement for PVs before. I see they are thin film type PVs, are they so delicate that going OC in bright sunlight will damage them? they must be somehow optimised for poor light conditions.

dgd