Q & A: Series String OCPD Requirements for Grid-Direct Inverter Applications: Page 2 of 2
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Example 1: single series string.
In a one-string system, no fusing is required since there are no external sources of overcurrents. An unfused dc PV disconnect can be used on this type of system as long as the modules’ maximum series fuse rating is at least 1.56 Isc, and the connecting cables are rated at 1.56 Isc or higher.
Example 2: two strings in parallel.
In this example, each string of modules can generate a maximum of 1.25 Isc. If a fault occurs in one string, the electrical potential of the second, unfaulted string can result in a current of 1.25 Isc in the faulted string’s circuit. However, we know that the modules in the faulted string can withstand currents up to at least 1.56 Isc or higher (if their marked series fuse rating is higher), and the conductors have an ampacity of at least 1.56 Isc or greater. Therefore, with only two strings of modules, no currents exist in the PV array that can damage the modules or the wiring, and no OCPDs are required.
Example 3: three strings in parallel.
In this case, a fault in one string could be subjected to currents from the two unfaulted strings. Each of these unfaulted strings could deliver up to 1.25 Isc under worst-case conditions for a total of 2 × 1.25 Isc = 2.5 Isc. Suppose that the module manufac- turer had rated the value of the maximum series fuse at exactly 1.56 Isc, and the wiring was sized at exactly 1.56 Isc. The currents from the two unfaulted strings at 2.5 Isc would be greater than the series fuse rating of the module and ampacity of the conductors, and the modules could be damaged. OCPDs in all three strings at a minimum value of 1.56 Isc would be required.
However, modules rarely have a marked maximum fuse value of exactly 1.56 Isc. Typically modules will pass the UL reverse-current tests at a higher current such as 15 A. For example, assume the modules have the following characteristics: an Isc of 5 A, a marked maximum series fuse value of 15 A and interconnecting conductors rated at 15 A after the appropriate deratings for conditions of use have been applied. In a system with three series strings of this module, the two unfaulted strings could deliver 2 × 5 × 1.25 = 12.5 A. Since the cumulative current is less than the 15 A rating of the conductors and is also less than the module’s 15 A maximum series fuse requirement, OCPDs are not required. The actual conductor ampacity would not have to be 15 A, but it would have to be at least 12.5 amps after deratings for conditions of use.
In a final variation on Example 3, assume that the module interconnecting conductors are still rated at 15 A, as is the module’s series fuse rating, but the module has an Isc of 8 A. Two unfaulted strings could feed up to 2 × 8 × 1.25 = 20 A to the faulted string. The resulting 20 A exceeds both the conductor ampacity and the ability of the module to withstand reverse currents. To protect against these currents, OCPDs are required in each string of modules. The OCPD must be at least 1.56 Isc (1.56 × 8 = 12.48 A) and not greater than 15 A. In this case a 15 A OCPD would normally be used.
As can be seen from these examples, when more than two strings of modules are connected in parallel, a calculation is necessary to determine if an OCPD is required in each string. When three strings of modules are connected in parallel without fuses, the conductor ampacity may have to be greater than the normal 1.56 Isc. Most utility-interactive PV systems with only one or two strings of PV modules will not require OCPDs in direct current circuits between the PV modules and the inverter. Systems with three or more strings in parallel require a simple calculation to determine the OCPD requirements.
—John Wiles / Institute for Energy and the Environment / Las Cruces, NM / nmsu.edu/~tdi/