Low PV Isolation fault?

Hi, One of my grid tie inverters is showing low pv isolation. Right now this is really bad since it’s Monsoon here and we are getting periods of very heavy rain each day. I know this is caused by a fault in one of the wires or panels and none of the panels have any cracks that I can see. This system has 3 strings of 10 panels. With each string isolated from all the others by shutting off all breakers on strings not being tested, here’s what I have. String 1; 402v tested + to -, 20v tested + to ground and 20v tested - to ground. String 2; 402v tested + to -, 127v tested + to ground and 189v tested - to ground. String 3; 398v tested + to -, 95v tested + to ground and 253v tested - to ground. Each panel is a 300watts and has an open circuit test of about 36v. I’m assuming string 2 has problems after panel 3 and another before panel 6 and string 3 has problems after panel 2 and before panel 4. Am I correct? What would cause the readings on string 1? Could that be an issue at the breaker at the array or is that OK the way it is? the dog

It definitely sounds like an earth leakage issue on string 1. How old are the panels? What model?

Earth leakage is a common problem (especially with cheap panels) and is generally caused by PID (potential induced degradation), which is not a visible fault and is impossible to see.
A few options you could try:

  1. Make up a test cable and bypass each panel in string one and test the string voltage.
  2. Or disconnect all the panels on string 1 and do a Voc and Isc test on each panel to try to identify the faulty panel.
  3. Does the issue occur when the panels are dry? If the issue only occurs while the panels are wet then it can be very difficult to find the fault.

Hope this helps

Hi, The panels are about 4 years old. It has been happening for the last 3 years. They were originally installed by idiots that my wife hired. They knew nothing, based on the connections, panel positioning and wires used. After changing many of the connectors on the panels which were damaged by incorrect installation I have repurposed them into an installation of a 10kw grid tie system. They are 300 watt mono panels. They don’t have a brand name on them, they only say Germany. Which to me here, says they’re the cheapest thing available since that is the type of thing they do here to cover up cheep junk. Anyway, of course, since that’s the most difficult thing, they do give that problem every year during Monsoon Season. String #1 had never given problems before this season. Initially it was reading exactly the same as the other 2 strings, so I checked all the connections near the panels that added up to the numbers. After that it shows the 20v on the + and - to ground sometimes a bit lower to 15v on each. I don’t see any chaffed wires and I haven’t found any connections that are damp on anything. Now if the panels are leaking could I use something like a clear water seal around the edges? Or time to replace? Thanks the dog

Hi, Yes the jumper wires helped a lot. I was able to track one string down to 3 panels, each showing power to ground on both sides at a bout 5v each. I changed the connectors on the panels wires and now I have 0v at the panels but at the beginning and end of the string I have about 1-2v. There’s a small amount showing on each panels in the string between .01 and .20v. I’ve started on the 2nd string but that’s a bit harder to reach. Thanks for the suggestions. The dog