Utility Interactive String Level Blocking Diodes

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JasonC

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I was hoping to get some assistance about blocking diodes at the string level (source circuit.)

I have an RFP design requirements that the engineer has a requirement for "All series connected strings of modules (source circuits) must also include blocking diodes to prevent reverse current flow. These diodes should have a low voltage drop, and voltage and current ratings (at temp) of at least twice the Voc and Isc of the source circuit."

Now I know the reasoning when you have an energy storage device (which there is not) so is this more of a case were it was in a book that they read and now it is being spec'd. I have done plenty of systems and have NEVER installed them. I know the panels have them but that is for shading purposes at the cell level.

My max Voc would be 421 and my Isc is 8.9. So with these requirements I would need the diode to handle a forward voltage of at least 842 V and 16.8 A. If anyone has any suggestions as to a product or possibly how I may be able to get persuade the omission of this from the RFP, I would appreciate it.

Thanks.
 
JasonC said:
My max Voc would be 421 and my Isc is 8.9. So with these requirements I would need the diode to handle a forward voltage of at least 842 V and 16.8 A. If anyone has any suggestions as to a product or possibly how I may be able to get persuade the omission of this from the RFP, I would appreciate it.

Thanks.
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1. Actually, you mean a reverse voltage of 842 volts. :)
2. This comes up a lot in PV discussions, and the general agreement is that it is not necessary. The reason being that a good string will not accept enough reverse current to either cause it any harm or cause a detectable power loss. (Any string that is exposed to any light at all, even moonlight levels or below, will have roughly the same Voc as more brightly lit strings, so there will be no reason for reverse current of anything more than the cell internal leakage resistance in total darkness.)
3. I have not seen this stated by an "authority" anywhere, but the recognized string configuration guides do not include a blocking diode even when they include everything else.
4. If panels in the string are damaged in a way that would allow reverse current, the panels will not be harmed by it unless the current exceeds the maximum series OCPD value specified by the manufacturer. In that case, the protective function is supplied by the string OCPD, with no need for a blocking diode.
 
And when you install the system, the AHJ may want to see the UL listing on the diode assembly. Good luck on that.

If you want to fabricate your own assembly, there are suitable diodes, search STTH3010 on Mouser, Digikey, etc. It will need a heatsink, enclosure, isolation, etc. Could be assembled and documented by a UL-508 shop.
 
String level blocking diodes should not be necessary. It sounds like someone dug up the text for an old product or design standard and cut and paste it into a modern RFP package. Unfortunately, goo like this is all too common in the PV section of an RFP.
 
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