[RE-wrenches] Megger for array testing

[Bill Brooks]

Meggerers,

 

Although shorting the terminals to perform the megger tests may provide the
most uniform voltage profile, it is difficult to perform on many arrays and
adds unnecessary complexity to a test that needs to become routine for all
PV installations.

 

The appropriate way to perform the test at the combiner box level for
negatively grounded systems is as follows (voltage would be applied
backwards for a positively grounded array):

 

1.      Open all fuse holders and make sure no fuses are mounted in the
holders.

2.      Remove any connected surge arrestors from the circuits as these will
conduct during the test and may fail.

3.      Remove the grounded feeder conductor so there is no connection to
the inverter.

4.      Connect the negative of the Insulation Tester to the grounding
connection in the combiner box. 

5.      Connect the positive of the Insulation Tester to the negative
connection in the combiner box.

6.      Apply either 500V or 1000V to the negative connection and wait for
the insulation resistance to achieve at least 100kOhms.

 

Insulation resistance will vary based upon module construction, moisture on
the  modules and conduit systems, and cycle of the moon (oh wait, that's the
PMS procedure-sorry)-actually irradiance and temperature can make a
difference. By adding 500V or 1000V to the negative side of the strings, the
positive side is going to go to 1000V or 1500V. This does not violate the
listing of the product as this is a temporary test. These types of tests are
customary for 600Vac wiring systems and they should be mandatory in PV
systems of any size operating above 100V. If the module is damaged by this
test, it was a faulty module since all module internal wiring is required to
withstand 2200V to ground throughout the unit-diodes and all. By testing in
the normal voltage bias that the product sees during operation, it will more
thoroughly stress the higher voltage parts of the array which are more
critical for the long-term reliability of the PV system.

 

If a resistance of 100kOhms cannot be achieved, take all the grounded-side
string conductors loose from the combiner block and test each one
individually to see if there are variations in resistance. If no variations
exist, the modules might be a more leaky variety and a new, lower threshold
would need to be established with the help of the module manufacturer.

 

Happy testing and don't forget to use your gloves.

 

Bill.

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Bill Hoffer
Sent: Wednesday, November 17, 2010 4:36 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Megger for array testing

 

Joel 

You are correct about the UL testing requiring 2 x system voltage plus 1000
volts. The UL requirement is also to ramp up to the full voltage in 5 sec
and held for 1 min, which I believe most meggers are capable of.  The test
is considered a failure if greater than 50 uA current is measured.  The
difference is that the connection is between the shorted outputs of the
module and the ground.  In the case of the required factory tests one end of
the high pot testing device is connected to the frame (ground) and one point
is connected to both the negative and positive  module terminals shorted
together.  In the UL test it is done with the module submerged one terminal
of the high pot tester is connnected to a ground plane in the water and the
other terminal is connected to the negative and positive terminals shorted
together.  In both cases the voltage potential across the module terminals
(through the module)  is 0, but between the terminals and ground is the 2 x
system voltage plus 1000.  The module never sees 2200 volts potential across
the cells and diodes.  The test is normally run again reversing the
polarity, but the terminals still are shorted together.  I have performed
this test in the factory and observed it being done this way during UL
testing.  

UL1703 Section 21 Leakage Current Test specifies shorted module output
circuits System Voltage is the test level required

21.5 All accessible parts and surfaces are to be tested for leakage current.
The positive and negative
terminals of an unilluminated module are to be connected together and to one
terminal of a dc power
supply. Both polarities of the source connection are to be used, unless it
can be shown that one polarity
will represent both. Leakage currents are to be measured between the part or
surface and the other
terminal of the power supply.  System Voltage is the test level required and


UL1703 Section 26 Dielectric Voltage-Withstand Test does not specify shorted
module outputs, but does specify 2 x system voltage plus 1000V

UL1703 27 Wet Insulation Test does specify Shorted output terminals of the
module (and only 500vdc)
27.3 After two min of immersion in the solution, the insulation resistance
between the shorted output
terminals of the module or panel and the solution is to be measured, in both
polarities, with an instrument
having a voltage of 500 Vdc.

43 Factory Dielectric Voltage-Withstand Test  Also doe snot specify shorted
out puts

The IEC 61215 is more explicit in all test cases it specifies using the
shorted output terminals for all these tests.  IMHO the IEC is the better
document to follow (which is what the listing agencies all do when things
are not defined well in UL1703).  The industry will be much better off once
we standardize with the IEC!

In my experience all three tests for UL listing are performed in the same
way on shorted output terminals.  I would recommend that the megger test be
performed in the same way especially for field confirmation procedures for
commissioning a system.  This will still give you the intended results
(identifying a potential short to ground) without the risk of applying
unnecessary voltage across the module cells or diodes.  I do not want to be
explaining a failed module to the manufacturer or customer after subjecting
it to 2200 + volts across the cells or diodes!  You would never meg across
an inverter input either, even though they are also high pot tested for
listing.  Of course standard procedures for megging wire runs without the
inverter or modules in the circuit can be done at those higher levels
without a problem.

Bill

Bill Hoffer PE

Sunergy Engineering Services PLLC
2504 Columbia Ave NW
East Wenatchee WA 98802-3941
SunEngSer at gmail.com (509)470-7762
Cell(509)679-6165


On Wed, Nov 17, 2010 at 9:05 AM, Joel Davidson <joel.davidson at sbcglobal.net>
wrote:

Hello Drake,

 

I think you should talk to a Sanyo Solar engineer to get accurate
information. Every Sanyo and other brand UL listed modules must be
dielectric voltage-withstand factory tested to 2 times system voltage plus
1,000 volts. I hi-pot test modules in the factory at 3,000 volts so they
will meet UL and also IEC standards. I megger field test arrays at 1,000
volts to find module, connector, and wiring problems. Megging at 500 volts
is ok, but it may not reveal wire insulation problems.

 

Joel Davidson

 

----- Original Message ----- 

From: Drake <mailto:drake.chamberlin at redwoodalliance.org>  

To: RE-wrenches <mailto:re-wrenches at lists.re-wrenches.org>  

Sent: Wednesday, November 17, 2010 7:36 AM

Subject: [RE-wrenches] Megger for array testing

 

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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