Blocking (Isolation) Diode question

lazza · October 2011

HI Forum

I'm about to add 6 120W Kyocera panels with their own Xantrex C40 to an existing installation, to increase energy production.

The 6 panels will be wired in 3 series of 2 panels, to give a 24V system.

I plan put a 10A fuse (Isc of panels is 7.49A) in each of the 3 strings... and then this is the big question-

should I put in blocking/isolation diodes in each string to prevent reverse current flow from other strings? Or is a 24V system too low for this to make sense

Thanks for any assistance

Larry

BB. · October 2011

Re: Blocking (Isolation) Diode question

Not usually--You only have "reverse current flow" if the panel is in the dark.

If the panels are all out the sun (possibly with variable shading), then you will not have any problems.

Also, the reverse current block is to prevent the battery from self discharging back through panels at night. Normally, the charge controller will prevent reverse current flow at night.

If you have a battery bank and no charge controller, you don't really need blocking diodes for 12 volts, should have it for 48 volts, and is optional for 24 volt battery banks.

-Bill

RCinFLA · October 2011

Re: Blocking (Isolation) Diode question

Your controller blocks any reverse flow from the battery.

As to one series array dragging down the other two, that depends on the quality of the panels. Worse case you might get a 1 amp drag from a partially shaded panel on the other two series panels. Normally the leakage is not this high. By quality, it depends on the individual cell leakage tolerance acceptance range by the manufacturer. Monocrystaline will likely have least leakage, followed by poly-crystaline, and thin film being the worse. Again, not terribly concerning unless the individual string leakage constitues a significant amount of other strings generation.

For a 24 vdc system I would not worry about blocking diodes as you will lose some in normal operation due to the approximately 1 volt drop of blocking diode that will likely waste more then the abnormal situation where one panel might get shaded on one of the series strings.

lazza · October 2011

Re: Blocking (Isolation) Diode question

Great- Thanks for your replies

So, if I understand correctly. Blocking diodes bewteen strings are not worth it for low voltage, small installations to prevent leakage between strings, and will probably consume more energy due to the diodes' voltage drop.

Thanks again
Larry

BB. · October 2011

Re: Blocking (Isolation) Diode question

Yep, that is correct.... And solar panels pretty much approach Vmp/Voc with weak sun or sky backscatter during the day (unless you are in deep shade under a tree/building--probably would not have put them there in the first place).

You should not see any appreciable losses from "back driving" a shaded array during daylight hours.

The blocking diodes where originally there to prevent batteries from back feeding at night.

Bypass diodes, another kettle of fish--They are there to prevent damage to shaded cells in a large array (shaded cells produce little current and will actually reverse voltage--like a single dead battery in a flashlight). If the back driven voltage is over ~12 volts, the cells, which are diodes, will exceed their working voltage and fail.

-Bill

niel · October 2011

Re: Blocking (Isolation) Diode question

you could investigate what happens in your circumstance by measuring the current in the shaded string as opposed to what a good string outputs. odds are the shaded string will still have some + current output due to diffuse sky radiation making protecting the output unnecessary.

if in the long run you decide to put in blocking diodes there are some things to consider here.

1> a diode (preferably low forward resistance type) will need to be large enough to handle the current that needs to pass through it.

2> the diode will present a v drop of between .5v and .75v and multiply that by the current that passes from the string equals the constant loss seen by the string.

3> you need to properly mount the diodes and many of today's pvs don't provide an area to mount the extra parts from the weather or it is cramped for room. some with junction boxes of the old school pv types may have the ability to accommodate them, but this varies nowadays as some may accommodate the diode.

4> in the winter the snow can shade the pv and if some power gets dumped into the shaded pv for even a little that bit of power that it could heat the pv enough to melt off the snow faster.

as i said, you can make measurements with your system to see what occurs in it and weigh it for the losses a blocking diode would pose. only good thing is that you'd only need 1 blocking diode per string. in any case when using more than 2 strings each string needs to be fused.

boB · October 2011

Re: Blocking (Isolation) Diode question

lazza wrote: »

HI Forum

should I put in blocking/isolation diodes in each string to prevent reverse current flow from other strings? Or is a 24V system too low for this to make sense

Larry

Yeah, diodes are another way around PV combiners using fuses or breakers. But as was said, you have to heat sink them (somewhat) because they will dissipate somewhere around 5 to 10 watts each and will waste some valuable energy.

If you are already going to put fuses in series with the strings, then you shouldn't need the diodes. The C40 has back to back FETs to eliminate night time losses, if any. (modules are better these days for that anyway)

What is the fuse rating of the modules ?? If it's somewhere near 15 amps, then your Isc X 2 would be around that value. 10 amps for the fuse sounds pretty good.

boB

Blocking (Isolation) Diode question

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