Ungrounded PV Power Systems in the NEC: Page 7 of 12
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The ground-fault protection system used in non-isolated inverters includes a regular test of PV array insulation resistance. This test is performed by an isolation monitor interrupter (IMI), which UL defines as “a device that monitors the insulation resistance of a PV array circuit to ground and prevents energization of the inverter ac output circuit or disconnects an energized output circuit when the PV array input resistance drops below a predetermined level.” The IMI performs the PV array insulation resistance test in the early morning hours, when the PV source-circuit voltage is high but there is not enough current for the inverter to begin operating.
The IMI measures any current leakage between all the conductors in the PV circuit to ground and identifies levels of leakage current above set values. This technique is very similar to the insulation tests that electricians perform on unenergized electrical conductors using portable megohm meters. If the tested source-circuit conductor insulation resistance is below a minimum level, the inverter will not interconnect with the utility. If the source-circuit conductor passes the test, the inverter will initiate its normal startup procedures.
It is common for ground-fault protection systems in nonisolated inverters to use what UL refers to as a functional ground, which is an intentional high-impedance connection between the ungrounded circuits that are being monitored and the equipment-grounding system. This connection exists for the sole purpose of fault detection. Since this intentional high-impedance path only exists when the inverter is operating, the PV system is not solidly grounded, according to the definition in Article 100 of the NEC. UL allows this strategy since it recognizes the role that these detection circuits play in reducing the potential for property damage due to stray ground-fault currents. An NRTL evaluates a non-isolated inverter’s ground-fault protection system, including any functional ground, as part of the product testing and listing.
As described previously, once the inverter is online, if it measures ground-fault current above the maximum level allowed or if it measures a sudden increase in fault current, even at very low levels, it will cease operating and indicate the presence of a fault.
Residual-current detector. Rather than using GFDI fuses to identify and interrupt ground-fault current as is typically done in isolated inverters, non-isolated inverters include a residual-current detector to continuously monitor the PV array. This detector circuit is similar to the ac ground-fault circuit interrupter (GFCI) devices with which most electrical professionals are familiar. Like a GFCI, the residual-current detector in a non-isolated PV inverter is an electronic monitoring circuit that identifies ground-fault current before it reaches destructive levels. Unlike a common household GFCI, this device functions to identify fault currents that could cause damage to property and is not specifically set to levels to protect people from electrical shock.
Residual-current detectors constantly monitor the current in an operating PV power circuit, and associated software looks for any imbalance. The outgoing and returning current in the dc circuit should be offsetting—equal in intensity but opposite in direction. In an ideal circuit, the sum of these currents would equal zero. If the residual-current detector indicates that the currents are imbalanced, then the control logic interprets this as a fault. The most likely fault path is to ground through the equipment-grounding system, but stray current in any other parallel circuit path would also be detected. If an imbalance is indicated, then the inverter ceases to operate and indicates that a ground fault has occurred.
In practice, all PV arrays have some small amount of residual leakage current due to a capacitance effect that is dependent on the specific module, the mounting system and the environmental conditions. This means that a residual-current detector system used in non-isolated inverters cannot actually be set at zero, as this would result in nuisance tripping, especially on very large arrays. However, since the residual-current detector is an electronic protection device, its trip points are much lower than the conventional GFDI fuse ratings commonly found in isolated PV inverters. The UL 1741 Standards Technical Panel, which includes manufacturer representatives, determined that the 300 mA ground-fault trip limit for nonisolated inverters up to 30 kW was adequate to prevent groundfault arcs that could ignite fires.