Retrofitting Non-Isolated Inverters in Legacy Arrays

The aging fleet of fuse-grounded string inverters presents a potential challenge for service providers since the industry has largely transitioned to non-isolated inverters. Prior to 2012, the vast majority of interactive inverters fielded in North America utilized a traditional fuse-grounded isolation transformer–based topology. Since then, non-isolated string inverters have become the de facto industry standard in residential and commercial applications. This evolution is because transformerless inverters offer improved performance—in terms of both cost and efficiency—and improved safety relative to older transformer-based models.

As fuse-grounded inverters reach the end of their warranty term, which is typically between 5 and 10 years, end-of-life failures occur with increasing frequency. The challenge for service personnel is that direct replacement models are not available for any inverter that is more than 5 years old, unless you uncover new old stock via a secondary market such as eBay. As a result, service personnel generally need to install a non-isolated inverter in place of a failed transformer-isolated model. This new inverter is much more attractive than old stock, since it carries a valid manufacturer’s warranty and offers enhanced safety features, such as dc arc-fault detection and superior ground-fault protection. However, some AHJs could interpret the National Electrical Code in ways that effectively disallow this inverter upgrade.

In this article, I examine relevant Code requirements, including revisions introduced in NEC 2017, and provide recommendations about how solar companies and AHJs can move forward on this issue. Replacing legacy fuse-grounded PV inverters with currently available non-isolated inverters not only is consistent with the most recent Code revisions, but also is allowed under previous editions.

Brief History of PV System Grounding

The basic concern about retrofitting non-isolated inverters in legacy PV arrays arises from the conceptual issues surrounding PV system grounding in accordance with the NEC. Prior to approximately 2012, most PV systems in the US included transformers to isolate the grounded ac grid from the grounded dc conductors. These isolation transformers are expensive, inefficient and heavy. To eliminate the isolation transformer and still connect a PV system to the grounded utility grid, inverter manufacturers must remove the system grounding bond on the dc side, as shown in Figure 1.

Technically, a PV system that has had the isolation transformer removed is classified as non-isolated. However, many PV practitioners refer to these systems as ungrounded since the ground bond to the dc conductors is intentionally removed. This terminology use is unfortunate, as it is inaccurate to describe non-isolated PV systems interconnected to grounded ac services as ungrounded. Because most utility services in the US are grounded, and most non-isolated PV inverters require installation on a grounded ac service, typical non-isolated PV systems in the US are grounded via the ac service when operational; these systems are ungrounded only when nonoperational.

This ungrounded-when-non-operational state is no different from what occurs in a fuse-grounded PV system when the ground-fault fuse blows. According to NEC Section 690.5(B)(2) and interactive inverter certification standards, a PV system with a blown ground-fault fuse must “cease to supply power to output circuits.” After a ground fault is detected and interrupted in this manner, fuse-grounded PV systems are ungrounded and nonoperational. By comparison, PV systems with non-isolated inverters enter this ungrounded-when-nonoperational state every night after disconnecting from the utility grid. Most importantly, the non-isolated system does not present a fault or safety hazard, as its ground-fault detector is 50 times more sensitive than that of the fuse-grounded system.

Legacy vs. New Single-Phase Inverters

As legacy transformer-isolated inverters fail, O&M providers have two options for getting the PV system back on line. The first option is to find a transformer-isolated inverter that works with the array. The second option is to retrofit a non-isolated inverter, which is the preferred approach.

Transformer-isolated option. Inverter manufacturers have generally phased out the production of transformer-isolated inverters in favor of safer and cheaper non-isolated models. However, you can occasionally find new old stock or lightly used replacement inverters on eBay or in the dusty corner of someone’s warehouse. Unfortunately, any inverter that is more that 5 years old will not have dc arc-fault detection, which is a standard feature on new non-isolated string inverters since undetected dc arc faults present a potential fire hazard.

In addition, the older inverter has a very simple ground-fault protection system that uses a 1 A fuse, located in the grounded conductor-to-ground bond, to detect and interrupt dc ground faults. A ground fault with 1,500 mA of current will clear (open) this 1 A fuse in about one minute. By comparison, the ground-fault detector in a non-isolated string inverter will trip at 30 mA of ground current in less than one second. Based on these detection levels and clearing times, the fault energy required to clear a ground fault is 3,000 times greater in a legacy residential inverter system than in a system with a non-isolated string inverter.

Non-isolated inverter option. Newer non-isolated inverters clearly provide a far safer PV system. However, differing opinions about system grounding classifications and requirements may complicate this option. Since I regularly work with AHJs, I have posed the following question to them many times: “If an existing PV system experiences an inverter failure and a contractor pulls a permit that includes replacing the failed inverter, would you accept the installation of a safer inverter even though some engineers consider the repaired system ungrounded rather than solidly grounded?”

I get a fairly consistent response to this question. If the new PV system is safer, most AHJs will approve the proposed installation based on the following language in NEC Section 90.4: “By special permission, the authority having jurisdiction may waive specific requirements in this Code or permit alternative methods where it is assured that equivalent objectives can be achieved by establishing and maintaining effective safety.”

Via this allowance, even AHJs who believe that non-isolated inverters are subject to ungrounded PV system requirements can approve the use of non-isolated inverters for retrofit purposes. It is also possible to show that ungrounded requirements simply do not apply in this scenario.

Non-Isolated ≠ Ungrounded

In 2005, the Code-Making Panel (CMP) responsible for Article 690 introduced Section 690.35, “Ungrounded PV Power Systems.” A close reading of the language makes it clear that these ungrounded PV system requirements do not apply to systems deployed with non-isolated inverters: “Photovoltaic power systems shall be allowed to operate with ungrounded PV source and output circuits where the system complies with 690.35(A) through (G)” [emphasis added].

When you connect a non-isolated inverter to a grounded ac service, the system is grounded whenever the inverter is operating. Therefore, the proper application of the NEC does not require implementing 690.35 (A) through (G) for non-isolated PV systems connected to grounded ac services. Until recently, most engineers did not recognize this ac service–ground connection as a PV system ground.

As a result, PV systems deployed with non-isolated inverters are widely misidentified as ungrounded PV systems. This is a misnomer. Ungrounded systems operate without a connection to earth; non-isolated inverter systems are connected to earth when operating, but floating in reference to earth when not operating. Unfortunately, this misnomer is also ubiquitous. For several years, most solar professionals and AHJs have diligently, if mistakenly, applied 690.35(A) through (G) to non-isolated inverter systems.

Ungrounded system requirements. The practical requirements for ungrounded PV systems are well known to solar practitioners. Subsection 690.35(A) requires disconnecting means in both poles of the array for ungrounded PV source and output circuits; 690.35(B) likewise requires overcurrent protection in both poles of the array. Meanwhile, 690.35(D) mandates the use of PV Wire for exposed single conductors, which effectively rules out the use of USE-2 conductors in these systems.

This latter requirement had the effect of slowing the adoption of non-isolated inverters in the US. While nearly all PV modules sold today have PV Wire cable whips, this was not always the case. Prior to 2013, very few PV modules were manufactured with PV Wire cables. Since older PV modules are unlikely to have PV Wire cables, some jurisdictions have questioned whether they should allow retrofit installations of non-isolated inverters in legacy PV arrays.

Alternative means of compliance. The NEC has long addressed alternative methods of system grounding for PV power sources. Since the mid-1990s, Section 690.41 has allowed for the use of solidly grounded systems as well as systems that “use other methods that accomplish equivalent protection in accordance with 250.4(A).” Since non-isolated inverter systems fit this description, they are technically not subject to the ungrounded PV system requirements in 690.35. Therefore, it is fully acceptable to retrofit non-isolated inverters in legacy PV arrays, even those deployed using standard wiring methods for grounded PV systems. Since this was not clear to many AHJs and solar practitioners, Code-Making Panel 4 (CMP 4) addressed this as part of the 2017 cycle of revisions.

System grounding in NEC 2017. The most recent edition of the NEC resolves the confusion regarding grounded versus ungrounded system grounding designations. CMP 4 introduced a term used in Europe—functional grounded PV system—which NEC  690.2 defines as having “an electrical reference to ground that is not solidly grounded.” An informational note clarifies that both PV systems with fuse-grounded inverters and those with non-isolated inverters meet the definition of a functional grounded PV system. In addition to adding this new system grounding definition, CMP 4 eliminated Section 690.35, “Ungrounded PV Power Systems,” in its entirety.

These changes mean that all PV systems are subject to the same installation requirements under NEC 2017, regardless of inverter topology. As detailed in Figure 2, these unified installation standards are as follows: overcurrent protection is required in one leg of a PV circuit only [690.9(C)]; disconnecting means are required in both legs of a PV circuit [690.15]; and both USE-2 and PV Wire are allowed as single-conductor cable in a PV array [690.31(C)].

Recommended Practices

Based on this understanding of existing Code requirements and factoring in the relevant changes introduced in NEC 2017, I recommend the following practices when retrofitting non-isolated inverters in place of legacy transformer-isolated inverters.

1. There is no need to replace existing USE-2/RWH-2 cables with PV Wire. PV systems installed more than 3 years ago are unlikely to have PV Wire cable whips or source-circuit conductors. It is not possible to retrofit these modules with PV Wire, nor is it necessary. The USE-2/RHW-2 cable installed within these arrays is perfectly good and is safe for the operating life of the PV system. NEC 2017, the most recently adopted Code edition, supports this practice.

2. If the existing array has white wires for the previously grounded conductors, simply re-identify these as ungrounded conductors. Whereas one pole of legacy transformer-isolated PV arrays is connected to ground via a fuse, both poles of non-isolated PV arrays are balanced on either side of the ac ground reference. This means that a PV array operating at 300 Vdc has a voltage to ground of 150 Vdc for both the positive and the negative poles. In other words, neither conductor is at ground potential even though the circuit is referenced to ground through the grounded ac service transformer. Since neither pole of the array is intentionally grounded, service personnel should re-identify any dc conductors with a white marking or insulation, since these wires will no longer be at ground potential.

Prior to the introduction of non-isolated inverters, installers commonly used white markings to identify intentionally grounded conductors in a PV array. This practice was intended to meet NEC Section 200.6, which includes a special allowance for re-identifying grounded single conductors in PV systems with a white marking [200.6(A)(6)]. Where existing USE-2/RHW-2 conductors are identified in this manner, service personnel can simply remove or cover the white marking. In the event that existing conductors have white insulation, I recommend re-identifying these white conductors by some suitable means rather than removing and reinstalling new conductors, based on the precedence that 200.6(A)(6) sets for the re-identification of small PV system conductors.

3. Installing a dc disconnect that opens both positive and negative poles of the PV array will bring the existing system into full compliance with NEC 2017. While not required in existing installations, the safest and best approach is to replace the dc disconnect on each inverter with one that opens both the positive and negative poles of the PV array. You can easily rewire even the standard Square D HU361 disconnect used on many thousands of systems to open both positive and negative conductors. This practice makes the inverter much safer to service in the event of a ground fault. Fortunately, most replacement inverters on the market today have an integral dc disconnect that opens both poles. It is very easy and straightforward, therefore, to upgrade an existing PV array for full compliance with the newest edition of the NEC.

Bill Brooks / Brooks Engineering / Vacaville, CA / brooksolar.com

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