Retrofitting Non-Isolated Inverters in Legacy Arrays
Inside this Article
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.