[RE-wrenches] Megger for array testing

Drake drake.chamberlin at redwoodalliance.org
Wed Nov 17 09:24:27 PST 2010


Thinking more about the Megger test of an array, is seems impossible, 
despite that fact I've been told it could be done.  If I put the 
tester positive to the 500 V DC negative, that would put two 500 V 
sources in series for 1000 VDC.  If I put the positive to positive, 
the resulting voltage to ground would be zero.  What is the trick?



At 10:36 AM 11/17/2010, you wrote:
>Hello Wrenches,
>
>Looking through the archives on using a Megger  for testing modules, 
>it appears that it is best to keep voltages to 500VDC.  I read all 
>that I could find.
>
>Talking to a Sanyo representative, I was told that keeping test 
>voltage under 600 would not void the warranty or be an issue with 
>UL.  He said they have tested Sanyo modules up to 1000 VDC with no problems.
>
>As a final test on a system, I want to Megger test the array from 
>the terminations at the inverter.  The approach I'd plan to use is:
>    * Clamp the Fluke 1587 negative to the enclosuer ground.
>    * Put the positive test lead to the negative conductor from the 
> array and test
>    * Put the positive test lead to the positive conductor from the 
> array and test
>All tests would be done at 500 V.  The modules are illuminated, but 
>there is no significant reading from either positive or negative 
>terminal to ground.
>    * The conductors would be removed from their terminals at the 
> inverter and be in free air.
>    * The conductors would be continuous through the string of 10 
> Sanyo 210 modules.
>My questions are:
>    * Does anyone see a problem in this approach?
>    * Since the array often has 500 VDC open circuit readings and 
> shows no indication of a fault whatever, would this test accomplish 
> anything more than the standard voltage to ground tests I've already done?
>    * Is there any danger to diodes or other PV components?
>Thanks in advance.
>
>Drake
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