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101 renewable - pv plant ground protections

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Last Updated
1st of October, 2018

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PV Grid Tie Grounding

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 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 above figure).

The second is 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

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 back feeding from other normal strings. This current is often called backed current (I back, or reverse current) back will flow into the fault point F and become a part of Ifg.

The another 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 backed 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

IV Curve Ground Fault

 

The IV curve shows the fact that the modules before F point of the short between active DC and the ground will work in the fourth quadrant of the IV curve and will behave as a load. Those panels will be back fed 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.

Ground-Fault Indicators:~

When a ground fault occurs, the inverter will shut down within seconds. Most inverters contain a GFDI fuse 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.

 

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