how to test for defective panel

balee123

I have a large grid-tie system from 2001.  This system is made up of 12V panels.  Several sets of four panels are set up in series and then  paralleled into an old STXR2500 grid tie inverter.  One of the sets of 4 panels is not producing any current as measured by a clamp on meter.  What is the easiest way to determine which panel (assuming it's 1 of the 4) is bad? It appears all have correct voltage across the output leads, but I'm assuming one is defective and thus all 4 in series are not producing.

Any experience would be much appreciated.

Thanks,
Andy

mcgivor

One method is to use a clamp on ammeter across the lead, short circuit the panel and read the current  Isc

BB.

Short out each panel individually. Measure the Isc with your clamp meter (in full sun). Current is proportional to amount of sunlight (i.e., a light cloud cover, you may get only 50% of rated Isc). If all panels read about the same Isc in the same sun--They passed that test.

Next test is Voc--Just measure the open circuit voltage of each panel in sunlight (amount of clouds do not matter much--As long as the sun is up and there are not dark storm clouds, the Voc should be the same across all panels). Voc does drop by 10-20% if the panels are hot (full noon time sun on a hot day). If Voc is ~rated Voc or 10% below--The panel is probably fine.

The above two tests will usually find a bad panel. However, you really need to pull energy from the panel (current at rated voltage) to be sure. Connecting to your charge controller and measuring the current flow is the best test.

In your case, you will need to disconnect the "bad 4 panel string" and measure Voc and Isc to see if you can find a bad panel.

Also, check your electrical connections. A bit of corrosion in the Junction Box can cause a connection to overheat and fail.

There are also "bypass" diodes in the solar panels--Your probably have 2 (or possibly 3) diodes. The diodes can fail shorted or open. A shorted diode will kill the output voltage of the panel (by 1/2 if two diodes). The diodes can be replaced if bad (if you can get into the J-Box).

An open diode will not affect the panel output--but an open bypass diode can cause a shaded panel (or a few cells) to go bad (a "dark" cell goes high resistance and will get too much reverse voltage from the rest of the series panels--and ruin the cell/panel).

-Bill

balee123

Hi Bill,

Thanks for  advice.  While I wasn't able to see if there was any corrosion in the "Junction Box", it appears that's where the problem was. The STXR2500 has integrated "?buses?" (if that's the term) for both the positive and negative wires from each string of 4 panels.  I believe there were either 6 or 8 individual connectors where the positive wires could be connected.  Each of these has a fuse associated with it; the fuse for the bad string of 4 panels was fine.  I moved the positive wire for this string to a different connector and everything works fine now.

Thanks again,
Andy