Ungrounded PV Power Systems in the NEC: Page 6 of 12
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SolarEdge has developed a unique utility-interactive PV system that consists of module-level dc-to-dc power optimizers coupled with proprietary non-isolated inverters. According to John Berdner, the company’s general manager for North America: “Non-isolated inverters offer the best chances for future cost reductions since they do not include the large transformers found in low-frequency transformer-isolated inverter designs and have far fewer components than highfrequency transformer-isolated designs.”
Increased ground-fault sensitivity. When people refer to the safety benefits associated with ungrounded PV systems, they are almost certainly referring to the fact that non-isolated inverters are more sensitive to ground faults than isolated inverters. In a SolarPro magazine article (February/March 2011) identifying the limitations of GFDI systems used in listed isolated inverters, Brooks points out, “The only way to get ground-fault detection below 1 amp as part of the GFP scheme for large PV systems is to unground or resistively ground the array circuit, just as they do in Europe and Japan.” He continues: “Contemporary European inverters, for example, can detect changes in ground current as low as 300 mA, which is an order of magnitude lower than our solidly grounded systems.”
While the differential is less pronounced in residential applications, non-isolated string inverters are still three times as sensitive to ground faults as isolated string inverters. At present, transformer-isolated string inverters up to 15 kW in capacity typically use a 1 A GFDI fuse to provide ground-fault protection. UL 1741 allows higher-capacity inverters to use GFDI fuses with higher ratings. For example, inverters rated more than 250 kW in capacity are allowed to use a 5 A fuse. Since a fuse located between the grounded current-carrying conductor and the ground bond most commonly provides this protection, the time required to open this fuse is determined by the physical response time of the fuse itself, which varies depending on temperature and the amount of current flowing across it during the fault event.
The electronic GFP strategy employed by non-isolated inverters used on ungrounded PV arrays allows for much lower and more consistent current and trip-time settings. Non-isolated inverters 30 kW and below sold on the market today are tested to the current UL CRD requirements of 300 mA maximum fault current and 0.3 second maximum trip time. Additionally, there is a “sudden ground-fault current change and response time” requirement that causes the operation of this protection circuit at levels as low as 30 mA and as quickly as 0.04 seconds. Besides reducing the potential shock hazard in a PV array, this means that ground faults are identified and the fault current is stopped more quickly, before it turns into an arcing fault capable of starting a fire.
Furthermore, since non-isolated inverters test for ground-fault currents at the start of each day— before the inverter goes online—they can detect potential ground-fault conditions before a fault occurs. For example, compromised conductor insulation may first manifest as a high-resistance fault and only later as a low-resistance fault. The groundfault protection scheme used in transformer-isolated string inverters may respond to the low-resistance fault only, whereas the scheme used in non-isolated string inverters is more likely to identify the highresistance fault condition.
SMA America was the first manufacturer to certify non-isolated inverters to UL 1741, using UL as its Nationally Recognized Testing Laboratory (NRTL). Greg Smith, a technical training specialist with the SMA Solar Academy, notes, “Plan checkers and inspectors may mistakenly think that a non-isolated inverter is unsafe because it doesn’t have the isolation transformer in it.” The reality is just the opposite, Smith explains: “Because non-isolated inverters check for PV isolation resistance before connecting to the grid and producing power, current is never flowing in a potentially unsafe array with ground faults.”
Ground-Fault Protection in Non-Isolated Inverters
Since AHJs occasionally question the safety of ungrounded PV systems, it is helpful to understand how the ground-fault protection system works in a non-isolated inverter. UL developed the increased ground-fault protection requirements for non-isolated inverters in concert with the PV inverter industry. These requirements address the unique conditions that ground faults can present in an ungrounded PV system. The process is under way to formally add these requirements for the testing and listing of non-isolated inverters to the published UL 1741 standard.