Q & A: Series String OCPD Requirements for Grid-Direct Inverter Applications

When are overcurrent protective devices (fuses or circuit breakers) required in direct current circuits between the PV modules and grid-direct inverters?

Before answering this question directly, it is important to note that properly rated fuses and circuit breakers are equivalent in this application and are collectively referred to as overcurrent protective devices (OCPDs). This is true even though the required label on the back of certified and listed PV modules lists the “fuse” rating.

In most electrical systems, the NEC requires that every ungrounded circuit conductor be protected from overcurrents that might damage that conductor. OCPDs provide this function. All commercial scale systems with central inverters require an OCPD for each series string of modules, but some smaller grid-direct systems may not need OCPDs in the dc circuits between the inverter and PV modules.

In UL Standard 1703, Underwriters Laboratories has established that modules must have an external series OCPD if there are external sources of current that can damage the internal module conductors. In the case of an external or internal fault, the module can be damaged if reverse currents are present in excess of the value of the module’s maximum series fuse rating. However, if there are no sources of external currents that exceed this marked value, then no OCPD is needed to protect the internal module wiring.

PV modules are current-limited devices, and their worst-case, continuous outputs for code calculations are 1.25 times the rated short-circuit current (Isc). An exception to NEC Article 690.9(A) allows conductors, typically rated at 1.56 times Isc, to be used with no OCPD where there are no sources of external currents that might damage that conductor.

Backfeed considerations.

Many of the smaller grid-direct inverters (below approximately 10 kW) are designed so that they cannot backfeed currents from the utility into array faults. Established, abnormal operation tests for backfeed do not rule out backfeed during normal operation of the inverter. Currently there are no normal operation tests in UL Standard 1741 to validate the lack of backfeeding from the utility. As a result, the manufacturer should provide written certification that the inverter cannot backfeed from the utility into an array fault. Larger central, transformerless and bipolar inverters may require additional certification verifying that they cannot backfeed.

If the inverter can backfeed utility currents into the dc PV wiring, the NEC requires that an OCPD be installed in series with the output of all individual module strings to protect the cables and the modules from reverse currents from any backfed ac currents through an inverter. In large systems, fused combining boxes are typically mounted at the array. In these systems, assuming that the inverter is a potential source of overcurrents, an OCPD may also be needed at the inverter input. This OCPD will have a minimum rating based on the number of strings connected in parallel on that circuit and the short-circuit current of each string. It will be sized to allow maximum forward currents from the array (all strings of modules) to pass through without interruption while keeping the overcurrent device from operating at more than 80% of its rated value.

Guidelines for string inverters.

The following guidelines and examples relate only to grid-direct PV systems using inverters certified by the manufacturer to not allow backfed currents. The goal is to determine how many strings of modules can be connected in parallel and still meet NEC and UL requirements before an OCPD is needed on each string. Where required, one OCPD will protect all modules and conductors in a single series string of modules per Section 690.9(E).

Additionally, the NEC requires in Article 110.3(B) that the manufacturer’s instructions and labels be followed. These materials list the maximum value for the OCPD. Lesser values can be used, according to Article 690.8(A) and (B), as long as they meet the NEC requirement of 1.56 times the module Isc in order to protect the conductor associated with the module or string of modules. Note that in these examples we are not determining the rating of any required OCPD. We are merely making some calculations to indicate whether or not an OCPD is needed on each string of modules.

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.

Summary.

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/

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