[RE-wrenches] Megger for array testing

[Joel Davidson]

Perhaps someone has an electronic copy of "Field Wet Resistance Test" revised 7/18/94 that was developed by the PVUSA Project, PG&E, and Bechtel that they could share with the group.

Joel Davidson


----- Original Message ----- 
  From: Matt Lafferty 
  To: 'RE-wrenches' 
  Sent: Wednesday, November 17, 2010 6:29 PM
  Subject: Re: [RE-wrenches] Megger for array testing


  Hi Drake,

  Disclaimer #1: i do not recommend megger-testing an array (or module or string) unless you have an approved procedure from the module manufacturer for the installed configuration. 
  Disclaimer #2: i am not the foremost authority on the list for this topic... Mr. Brooks? (Note: Bill's post just came in, but i want to share this anyway)

  i have, however, conducted these tests, as part of the commissioning requirements, on about a dozen systems which were installed under R&D grants. Back in the day, pretty much all systems bigger than 4 or 5 kilowatts were required to be megger tested, if they were "on the radar" in any way. This was a function of the fact that public dollars were being spent in the HOPE that PV could some day become a mainstream technology. And the fact that PV modules were so expensive. The combination of these factors meant that, in any meaningful sense, only projects that received subsidies were built, and grant requirements were in play. One of the conditions was that these tests be performed. This is a logical requirement in many ways. 

  Each system, that i've done this on, was different from the others. In each case, the procedure and analysis methodology was different. Sufficiently different, that i learned early on to NOT MAKE UP MY OWN PROCEDURE. Manufacturer procedures varied in lead placement, array sections to be tested, surface wetting techniques, test voltages, temperature compensation, and Pass/Fail values. i've made up lots of procedures over the years, but i'm not touching this one. I don't understand it well enough.

  The basic theme and theory was the same, pretty much across the board. But the actual methods... The steps... They were different. The very limited information i was able to extract from manufacturers' engineers varied. Some warned about reverse-biasing diodes and others didn't think it was a concern. Some insisted that test-lead arrangement was super-critical and others didn't. Some wanted to test at voltages lower than the system voltage and others said 1kV was fine on a 500V circuit. Some wanted to connect one pole to ground. Most didn't.

  Applying test voltages to, and measuring the resistance of, an installed, illuminated source circuit or array, is very different than applying test voltages to, and measuring resistance of, a single, unilluminated and shorted module. Myriad additonal factors and considerations must be accounted for. 

  My observation is that at least half of the manufacturers' engineers were only guessing, and crossing their fingers that everything would be fine after performing megger testing on a source circuit or array. Another observation is that at least half of these engineers had never contemplated megger-testing an array as a completed system... Only as a single module within the hi-pot test regime for manufacturing and listing purposes. As a group, they all were resistant to provide a documented procedure and acceptable test values. None, as in Z-E-R-O, were able to predict actual test results with any reasonable degree of accuracy. As in, off by megohms in many cases. Even though the subjects under test were designed, and largely manufactured, by their companies. Even though these guys were responsible for the actual system design, in most cases. Even though my experience leads me to believe these guys are at least half full of crap, i do believe this test is valid and has merit. i believe that all arrays should be tested for their dielectric resistance during the commissioning process, in fact.

  If we are going to megger our field-installed conductors... And we absolutely should be... It's a simple step. In order to do this, as an industry, we just need a better understanding of the characteristics and test procedures. My requests that manufacturers publish a procedure for each module they make, with acceptable test results in common circuit configurations, have resulted in squat. Going forward, i hope somebody forces them to do it, 'cause they ain't likely to bother otherwise.

  Under controlled conditions, such as those found during module manufacturing, predicting a test result within a reasonable degree of accuracy has a much higher chance for success than a prediction under uncontrolled conditions in a multi-subject combination. I get that. Nevertheless, we are deploying these products into an installed configuration. There is shipping to the distributor. Shipping to the installer. Shipping to the jobsite. Unpacking and handling. Getting it to the roof. Bolting it down. Wiring it up. All before it actually gets to start its productive life.

  It is reasonable to be able to expect and require that these tests be performed during the commissioning process. After all, when would you, as an installer carrying the warranty, rather find out if something has a higher chance for failure during its lifetime? At the time of installation when you can still get your hands around the neck of somebody or five years later when the sales-slug you know works somewhere else?

  The fundamental purpose of megger testing an array is to measure the dielectric resistance between conductive circuit elements (positive and negative) and conductive non-circuit elements (module frames & racking). The higher the resistance (megohm) value, the greater the dielectric resistance. Busbars and cables and switches and receptacles... They all need dielectric resistance in order to work. The entire theory of useable electricity depends on dielectric resistance. Without dielectric resistance, you have a short circuit.

  Each model of module is a little different in its construction. As a consequence, the absolute values for each are different. This may or may not mean there is a different minimum acceptable value for a given class of module. Resistance is resistance, after all.

  One key function of any test is to compare the results of the test to expected results... More accurately, to compare the test results to an acceptable threshold value. In our case, absent some other authority, that value has to come from the manufacturer. Furthermore, this threshold value, which i'll refer to as the minimum acceptable resistance, changes downward as the surface area of the array being tested increases. This phonomena is consistent with wires, too.... The larger and longer the conductor, the lower the theoretical minimum acceptable insulation resistance is.

  Test lead placement is a fundamental piece of the puzzle. But it's only part of the puzzle. You need to know several things that are specific to your application... For example, the procedure for testing a single module may differ greatly from testing a string of 10 in series. Testing two paralell strings of 10 may have a different procedure... In any case, the expected values for each setup will be different. Based on my experience, the key items you need to know, from the manufacturer and for your specific application, are:
    1.. Any pre-megger tests to be performed
    2.. Test lead placement (May be different for pos to ground than neg to ground)
    3.. Circuit conductor configuration
    4.. Test voltage
    5.. Time on test
    6.. Acceptable irradiance during test
    7.. Number of times to repeat the test
    8.. Wet or Dry 
    9.. If Wet, what solution to use
    10.. Temperature compensation
    11.. Irradiance compensation
    12.. Minimum acceptable resistance for each test (May be different for pos to ground than neg to ground)
  If you are testing at the inverter, you will need to account for the resistance of the insulation of your wiring. You will also need to be sure that each module frame has a good connection to the grounding conductor you are using as a reference.

  In my experience, i have seen differences in which test lead connects to which circuit conductor. Some had different Pass/Fail values for positive and negative and some did not. Some included temperature compensation and others didn't. Some were done wet and others done dry. Test times varied. Some said a single test was fine and others said three passing tests were required before the system passed. And they all had different minimum acceptable resistance values. Most systems passed the tests. Some didn't on the first try and required troubleshooting and module replacements. One never did pass and had to be removed. One thing i have NEVER SEEN is two identical test results on the same circuit.

  As you push for a procedure, expect them to request that you sign an NDA before they give it to you. You can decide how you choose to respond to that. For what it's worth, i'm somewhat confident that they will work with you on this. In large part, this is because it's Sanyo. They are a grown-up company who really believes in quality products and customer relationships. Can't say that about many others. If you run into resistance, get them on the phone, cough, then say "Solar Ark".

  i certainly wish you the best of experiences with this and hope you find the problem(s) one way or the other.

  Pray for Public Array Megger Procedures!

  Solar Janitor


------------------------------------------------------------------------------
  From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Drake
  Sent: Wednesday, November 17, 2010 9:24 AM
  To: RE-wrenches
  Subject: Re: [RE-wrenches] Megger for array testing


  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: 
      a.. Clamp the Fluke 1587 negative to the enclosuer ground. 
      b.. Put the positive test lead to the negative conductor from the array and test 
      c.. 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. 
      a.. The conductors would be removed from their terminals at the inverter and be in free air. 
      b.. The conductors would be continuous through the string of 10 Sanyo 210 modules. 
    My questions are: 
      a.. Does anyone see a problem in this approach? 
      b.. 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? 
      c.. Is there any danger to diodes or other PV components? 
    Thanks in advance.

    Drake  



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