Andy Cazzato
Member
- Location
- Aptos, CA, Santa Cruz Co.
Thanks for the article on finding ground faults. I read it online after SMA service line could not help me with a peculiar problem. I am hoping that you can.
An insulation resistance error appeared on a customer’s TL series. Zero watts. SMA told me this is indicates a ground fault. System was built by others about ten years ago and the first fault noted about three weeks ago by the customer.
Module string of 12 with pv wire in open air read 444V from pos to neg, polarity normal. This is the expected Voc.
Pos to Gnd read below ten volts and descending. Neg to Gnd also below 10V and descending toward zero.
I reconnected the string to the inverter dc disconnect switch. In free air at the disco I read ‑446 from pos to neg BUT THE POLARITY WAS REVERSED, meaning my meter positive probe on the positive RED conductor and my meter negative probe on the negative WHITE conductor, with the minus sign before the 446 in my display. I double checked to be sure my leads were not crossed and they were not. Pos to Grd read -423 (polarity reversed also, meaning my pos probe on pos and my negative probe on Gnd and a minus sign in display). Neg to Gnd read less than 1 volt (I didn’t note the polarity from N to G).
With the string still connected at the array and the conductors still in free air at the disco, I returned to the array and read 450V Pos to Neg POLARITY NORMAL. Neg to Gnd read 24 V with the positive probe lead on the Gnd and the negative probe on the negative WHITE conductor. Air had gotten cooler, probably accounted for slightly higher Voc.
I cannot grasp how the polarity can reverse, but suspect there is faulty insulation inside the buried conduit between the array and the Dc disco. WHERE WOULD YOU SUSPECT THE PROBLEM TO BE FOUND? To clarify, the array is about 250 feet from the inverter- mounted DC disco. Replacing conductors without a clear idea of which one(s) are at fault would be a risky and costly task. I do not have a megger. I suspect that either the positive conductor or the negative conductor or maybe even both of them have abraded insulation somewhere in the conduit. I strongly believe that no one has reversed the red and white conductors. I am sure that rain water has entered the buried PVC conduit. It may be helpful to note that the conduit serves two arrays and two inverters but only one of the inverters has the fault and the strange polarity. In the conduit are 5 conductors, two white negatives, two red positives, and one green ground common to both arrays and both inverters. One inverter is operating normally.
I am about to run a temporary test conductor about 250 ft. over the surface from array to dc disco to substitute for one of the buried conductors. If that is inconclusive, I will substitute for the other. If still inconclusive, substitute for both with two temporary test conductors over the surface, hoping that it will not be necessary to pull more than one or two conductors out for replacement. I hesitate to do this without agreement from a trusted source because I am so confused by the voltage readings.
Your thoughts, please?
Andy Cazzato
[FONT="]SANTA CRUZ SOLAR[/FONT]
831-685-2786 office
831 251-2786 mobile
An insulation resistance error appeared on a customer’s TL series. Zero watts. SMA told me this is indicates a ground fault. System was built by others about ten years ago and the first fault noted about three weeks ago by the customer.
Module string of 12 with pv wire in open air read 444V from pos to neg, polarity normal. This is the expected Voc.
Pos to Gnd read below ten volts and descending. Neg to Gnd also below 10V and descending toward zero.
I reconnected the string to the inverter dc disconnect switch. In free air at the disco I read ‑446 from pos to neg BUT THE POLARITY WAS REVERSED, meaning my meter positive probe on the positive RED conductor and my meter negative probe on the negative WHITE conductor, with the minus sign before the 446 in my display. I double checked to be sure my leads were not crossed and they were not. Pos to Grd read -423 (polarity reversed also, meaning my pos probe on pos and my negative probe on Gnd and a minus sign in display). Neg to Gnd read less than 1 volt (I didn’t note the polarity from N to G).
With the string still connected at the array and the conductors still in free air at the disco, I returned to the array and read 450V Pos to Neg POLARITY NORMAL. Neg to Gnd read 24 V with the positive probe lead on the Gnd and the negative probe on the negative WHITE conductor. Air had gotten cooler, probably accounted for slightly higher Voc.
I cannot grasp how the polarity can reverse, but suspect there is faulty insulation inside the buried conduit between the array and the Dc disco. WHERE WOULD YOU SUSPECT THE PROBLEM TO BE FOUND? To clarify, the array is about 250 feet from the inverter- mounted DC disco. Replacing conductors without a clear idea of which one(s) are at fault would be a risky and costly task. I do not have a megger. I suspect that either the positive conductor or the negative conductor or maybe even both of them have abraded insulation somewhere in the conduit. I strongly believe that no one has reversed the red and white conductors. I am sure that rain water has entered the buried PVC conduit. It may be helpful to note that the conduit serves two arrays and two inverters but only one of the inverters has the fault and the strange polarity. In the conduit are 5 conductors, two white negatives, two red positives, and one green ground common to both arrays and both inverters. One inverter is operating normally.
I am about to run a temporary test conductor about 250 ft. over the surface from array to dc disco to substitute for one of the buried conductors. If that is inconclusive, I will substitute for the other. If still inconclusive, substitute for both with two temporary test conductors over the surface, hoping that it will not be necessary to pull more than one or two conductors out for replacement. I hesitate to do this without agreement from a trusted source because I am so confused by the voltage readings.
Your thoughts, please?
Andy Cazzato
[FONT="]SANTA CRUZ SOLAR[/FONT]
831-685-2786 office
831 251-2786 mobile