Blocking diode vs 2nd charge controller

[Tinman]

Assumptions for an off grid configuration:

1. If there are two mismatched arrays (series to parallel) the conventional wisdom is to use a 2nd charge controller so that each array has it's own controller.

2. When using a charge controller, blocking diodes aren't needed since the charge controller performs that function.

3. SPDs are always installed from at each array input,leading to common bus bar, from the combiner (out) to the Classic and then from the batteries to the Classic

____________________

Then does it follow that you could put blocking diodes between the arrays and the combiner and thus use mismatched arrays connected in parallel at the combiner?

The reason I ask is the following scenario:

With dual axis panel trackers, the possibility exists that one or more motion controllers can fail. Since they are run by electric motors that rely on sensors to control them things can and eventually will happen, just the way it is. If one or more trackers experienced a failure on a single axis motor, it is possible that at the highest harvest time during the day, one or more arrays would see the sun much less than the others. The effect would be the same as severe shading and both voltage and amperage are affected.

Since this is similar to an array mismatch, would blocking diodes be recommended to reduce those effects? If true, would blocking diodes allow the use of different series arrays on the same controller if the blocking diodes are placed in line with each array before the combiner?

Trying to accommodate Murphy.

[boB]

If I am understanding the amount and type of mis-match of the modules/strings correctly, then I would say no, you do not need diodes.

Unless if the Voc, series fuse ratings and Imp of the strings are so mis-matched that you need diodes, then the Max Power Point voltage would be so far off that you would be significantly reducing the output power without multiple controllers.  And that would be mainly because of the mis-matched MPP voltage alone which also means highly mis-matched Voc.

As long as the Voc of the different strings are semi-close, no diodes should be necessary.  This is because one string cannot drive another string backwards unless that one string exceeds the other strings Voc.  Voc is temperature related and not related to how much sun is hitting the modules so much.

I think that's what you were thinking about ?

boB

[Tinman]

Right, all panels and arrays are sized exactly the same.  Same everything including manufacturer.

My concern is that if one array had an axis motor failure then it could end up pointed in a direction other than full on to sun and therein rests the concern. The motor failure would be detected easily enough but it could be hours and not immediate.

Thanks

[Robin]

Given two identical arrays on trackers..... Let's say each array has a maximum power voltage of 100 volts and puts out 20 amps. That would be a 2000 watt array on each tracker. Now let's say one tracker get's stupid (like mine). One array is still capable of 100V and 20 amps because it is pointed at the sun. The other is now only capable of 80 volts at 15 amps. The controller has to make a decision of what to track. It sees two humps of max power voltage. It will average the two, so it will park the voltage at around 90 volts. You will get about 17.5 amps from each array.  So instead of getting 4000 watts from the total arrays, you now will be getting 3150 watts out of both arrays. That is a 22% loss of power.

[Tinman]

Perfect. Thanks all. At least there won't be a smoke test.

[TomW]

Yes the error will be detected easily and in order to correct that error it will take time :

This is pure SPAM and has zero to do with this thread.

Just sayin.

Tom

[boB]

Yes the error will be detected easily and in order to correct that error it will take time :

This is pure SPAM and has zero to do with this thread.

Just sayin.

Tom

Did somebody already delete that spam message ??

boB

[Halfcrazy]

Yes I got it. I was hungry around lunch.

Ryan

[Westbranch]

Monty Python comes to mind....spam, spam, spam , spam.....

[TomW]

Yes I got it. I was hungry around lunch.

Ryan

Ryan;

Figures. Next time nuke my whine post, too, OK? 

I have a genetic flaw that makes me lodge a knee jerk complaint on SPAM.

For lunch, its nearly as good fried as bologna! Especially if some Velveeta is involved!

Sorry for the diversion.

Tom

[Halfcrazy]

Yes I got it. I was hungry around lunch.

Ryan

Ryan;

Figures. Next time nuke my whine post, too, OK? 

I have a genetic flaw that makes me lodge a knee jerk complaint on SPAM.

For lunch, its nearly as good fried as bologna! Especially if some Velveeta is involved!

Sorry for the diversion.

Tom

No worries. Plenty of spam to go around.

Ryan

[boB]

For lunch, its nearly as good fried as bologna! Especially if some Velveeta is involved!

Sorry for the diversion.

Tom

Mmmmmm....   Velveeta !!!   Great stuff !

And sorry for the shunt regulation !

boB

[Allen15]

Make sure you're not confusing blocking diodes with bypass diodes...

Many commercial panels will have bypass diodes to help with partial shading problems, and if your panels have those, then it probably becomes a moot point about trying to plan for it, because I suspect from your description, that you really are wanting the function of a bypass diode, NOT a blocking diode set?

For more info, here's a link to a page describing it probably better than I can:
http://www.pveducation.org/pvcdrom/modules/mismatch-effects-in-arrays

Hope this helps.

Allen