Wacky problem, 36kW 6 SMA Inverters GFCI Cleared

Hi people. I'm new to this forum and just looking for a place to gain some insightful knowledge as through my career I've surpassed most of my sources for knowledge and experience.

So I'm just over 1.5 MW installed and I've run into my first problem where I'm truly stumped. Ground Fault experts please read on!

I have six SMA Inverters (despite 1.5MW this is my first time using SMA) powering 152 Sharp 240watt modules on the roof of a 10 story high rise. 108 of those are on a large rack and power 4 of the inverters. Their are 4 combiner boxes on the top of the rack easily accessible by a pent house roof. My equipment ground runs across the tops of my rails and split bolts into each combiner box, leaving the equipment ground headed back to the inverters and the system ground in 4 places. the Two combiners on the left and right side feed two SMA 5000US inverters (2X11 strings) and the two middle combiner boxes feed two SMA 7000US inverters (3x10 strings).

So yesterday during commissioning, I booted up the two SMA 5000US inverters and one of the 7000's and they all operated normally. The 2nd SMA 7000US blew it's ground fault fuse almost immediately upon turning it on. Going up to the combiner boxes, at first I registered two strings that wouldn't show voltage potential from positive to negative. We fixed what we thought were bad MC-4 connector terminations and the strings came up reading from Positive to Negative. However, NONE of the six strings total in those two middle combiners will register any voltage to ground, period, and one of those combiners feeds an inverter that maintained properly operating conditions for the 90 minutes it was on. The ground is clearly not open in both combiners, as it is connected both to the system ground through the inverter and also to the system ground through the solid #8 running across the rails and into the other combiners.

I am stumped, and the only other ground fault issue I've had in the past I was able to fix by the standard test the voltage to ground and find the fault in the series string... difficult to do when they array is saying six strings have no potential to ground!

Any information anyone can provide would be a lifesaver. This system has a scheduled inspection of Monday the 8th and this inspector is 4 weeks out for inspection rescheduling.

Thanks in Advance,

Matt Murphy
NABCEP Certified PV Designer
Peck Solar - A Division of Peck Electric
South Burlington, VT

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,609 admin
    Re: Wacky problem, 36kW 6 SMA Inverters GFCI Cleared

    Sorry will be short as I am on a phone...

    You have 6 inverters and 4 combiners?

    I think you have already answered, but to be clear, each inverter has its own array wired directly to its own solar panel input. No shared +/-connections with any other inverter input. No ground wire connections of any type from array +/- to any other connections (ground, safety grounds, other equipment, etc.).

    If you have lightning arresters-are they dc rated and rated for the array open circuit voltage?

    Have you placed an ac/dc current clamp meter on the ground wires to look for circulating currents?

    My guesses.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Wacky problem, 36kW 6 SMA Inverters GFCI Cleared

    Sorry to confuse you.

    The remaining 2 combiners that feed the remaining 2 inverters are on top of the penthouse roof, which is about 20 feet above the roof the larger rack is on. The reason they are not involved in this problem is their equipment ground is not connected to the others, so they are completely different systems until they arrive back at the sub-panel that combines the inverter outputs inside the penthouse.

    The system doesn't share any of the same wiring at all, except for the shared equipment ground across the large rack, which is standard for systems that have a large racked system that runs off several inverters.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Wacky problem, 36kW 6 SMA Inverters GFCI Cleared

    Hard to diagnose remotely...sort of sounds to me like the ground is non-continuous somewhere.

    First question: If the ground runs from the combiner to the inverter, and the inverter tripped GFI, then did you reset the GFI back on?
  • System2System2 Posts: 6,290 admin
    Re: Wacky problem, 36kW 6 SMA Inverters GFCI Cleared

    GFCI in an SMA inverter is fused. I'm not going to replace the fuse until the system is operating correctly on my multimeter.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Wacky problem, 36kW 6 SMA Inverters GFCI Cleared

    Okay, well you have six strings that are not reading to ground - so have you verified that the ground is in fact grounded?

    Trying to picture it, they won't read positive to ground unless ground and negative are bonded - are they?

    From the backseat here, it still feels like a ground problem.
  • BB.BB. Super Moderators, Administrators Posts: 32,609 admin
    Re: Wacky problem, 36kW 6 SMA Inverters GFCI Cleared

    And there was no string to string voltage connection/voltage either?

    By the way, what setting(s) where you using on your multi-meter? AC or DC? Some mutli-meters will read Zero Volts when set to AC and reading a DC voltage--Others (typically True RMS type meters) will read AC, DC, or AC+DC when set to AC volts.

    In general, there has to be a complete circuit with the 1-5 amp fuse in the middle for enough current to flow and pop the fuse. That would mean + or - Lead on the solar panel tied to ground, or another solar panel +/-, or possibly a short to 120/240 VAC (not likely).

    The solar array with GFI fusing is not really "hard grounded"--Its only ground is through the ~1 to 5 amp fuse in the inverter. So--to blow the fuse, you have to have some AC or DC potential across the fuse.

    Have you tried to measure the voltage drop across the fuse holder (should be pretty close to zero volts AC and DC)?

    Again, if you have some lightning suppressors on the PV array side--they could be providing some sort of short if they have failed or have been wired wrong/internal problems).

    Also, what is the Voc reading on the arrays--I assume that it is within the SMA's input range.

    I know that you have done way more installs than I have (one on my own home)--Just trying to ask all of the questions that I can.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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