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  • Ground Protections 




    Warning Grounding Equipment

    Ground fault in photovoltaic (PV) arrays is an accidental electrical short circuit involving ground and one or more normally designated current-carrying conductors. Ground-faults in PV array draws safety concerns because it generates DC arcs at the fault point on the ground fault path. If the fault is not cleared properly, the DC arcs could sustain and cause a fire hazard.

     Conform to the NEC Article 690.41, there are two types of groundings in PV arrays:
     The first one is system grounding: the PV system with system voltage over 50 volts should be solidly system-grounded. To achieve that, the negative conductor usually is grounded via the GFPD in the PV inverter at point G (see figure below);
     The other one is the equipment grounding: the exposed non-current-carrying metal parts of PV module frames, electrical equipment, and conductor enclosures should be grounded;

    PV Grid Tie Ground Fault

     [Tech Support]  Knowledge Base Information for:  Ground Protections. 
    Read from Knowledge Base and download a pdf file @  Article: 101 RENEWABLE - PV PLANT GROUND PROTECTIONS
    Fold | Unfold [ click ]  Read more...  

    Ground fault is the most common fault in PV and may be caused by the following reasons:-

     Insulation failure of cables, i.e. a rodent animal chewing through cable insulation and causing a ground fault;
     Incidental short circuit between normal conductor and ground, i.e. a cable in a PV junction box contacting a grounded conductor incidentally;
     Ground-faults within PV modules, i.e. a solar cell short circuiting to grounded module frames due to deteriorating encapsulation, impact damage, or water corrosion in the PV module;

    The fault changes the configuration of the PV array and causes subsequent fault currents. After the fault, String 1 only has two modules left operating, since the rest of modules (Module 3 ~ Module n) are short circuited by two ground points F and G. As a result, String 1 is significantly mismatched with other normal strings. Meanwhile, the operating voltage of the PV array might be even larger than the open-circuit voltage of faulted String 1. Therefore, instead of supplying power, String 1 may be forced to work as a load in the 4th quadrant of its I-V curve (see Fig. 3). Now String 1 has a negative current backfeeding from other normal strings. This current is often called backfed current (I back, or reverse current). Iback will flow into the fault point F and become a part of Ifg.

    The other part of Ifg is I1-, which is the current coming from other (n-2) modules in String 1. Since Module 3 ~ Module n in String 1 are short-circuited by ground points, I1- will be equal to the short-circuit current of each PV module (Isc) under standard test conditions. Finally, the backfed current (I back) and the current from other modules (I1-) will merge as the ground-fault current (Ifg) at fault point F.

     At the positive busbar: Ipv+ = -Iback+I2++…+In+
     At the negative busbar: Ipv- = I1-+I2-+…+In-
     At the ground-fault point F: Ifg=Iback + I1-, where I1- = Isc
     At the positive busbar: Ipv+ = -Iback + (n-1) Isc = 0
     At the negative busbar: Ipv- = n Isc
     At the ground-fault point F: Ifg = n Isc

    The IV curve shows the fact that the modules before F point of the short between active DC and ground will work in the forth quadrant of the IV curve and will behave as a load. Those panels will be back feed by the ground fault current and they are a hazard for the whole installation and become a source of fire for the installation. DC current needs one quarter of the time to generate the same amount of heat as AC the current.

    IV Curve Ground Fault

    Ground-Fault Indicators:-

    When a ground fault occurs, the inverter will shut down within seconds. Most inverters contain a GFDI fuse (GFDI = Ground-Fault Detector Interrupter) that trips when any ground-fault current exceeds the ampere rating of the fuse. For residential grid-direct string inverters, this is a 1 A midget style fuse, typically a type KLKD or equivalent. However, due to the higher levels of leakage currents found in large arrays, central inverters usually have GFDI fuses with higher trip ratings: 2 A, 3 A, 4 A, 5 A and even higher values are typical. In some cases, for ground-fault protection central inverters utilize a resettable circuit breaker or a current transformer inside the inverter with an adjustable trip value. Several central inverters also have early warning circuitry. This circuitry monitors any current on the ground bus or grounding jumper and may shut down the inverter before the actual GFDI fuse has blown.

    At times inverters can register a false ground fault, shutting down when there is no actual fault. In addition, electrical storms and lightning strikes can cause conditions that resemble ground faults. In some cases, a simple inverter glitch might cause a false ground-fault indication. A defective inverter may repeatedly shut down, displaying a ground fault error, when no fault is present. In that case, it is probably time to start the return merchandise authorization process and get an RMA number from the inverter manufacturer. A blown GFDI fuse or a tripped GFDI breaker, however, is a reliable indicator that a legitimate ground fault exists, and hazardous conditions may be present in the system. This indication simplifies the first step in troubleshooting ground faults: determining if a ground fault has actually occurred. It should also raise a red flag for electricians, installers and service personnel.

    The information posted herein has been compiled by Clean Energy Brands from OEM product data and reputable publications. All rights reserved!



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