Substring Optimizers Explained

The past several years of residential PV installation have been characterized by a marked shift in the dominant technology used for optimizing solar arrays. String inverters ruled the roost for years, offering simplicity and fast installations but providing limited performance control. Micro-inverters, initially a niche offering, became a ubiquitous approach for residential projects and ushered in a new era of evangelizing the importance of isolating individual modules.

Along the way, DC optimizers emerged and promised the same tight control and monitoring as micro-inverters, but without some of the warranty implications of placing inverter electronics on a vulnerable rooftop or potential inefficiencies of locating the DC/AC inversion so far from the building’s distribution panel.

National Electrical Code (NEC) 2014’s inclusion of Article 690.12, colloquially known as “Rapid Shutdown,” introduced for the first time a code mandate to create a PV disconnect located at the array. This shift handed module-level power electronics (MLPEs) such as micro-inverters and DC optimizers an advantage over traditional string inverters. Since 2014, Rapid Shutdown increasingly has been adopted at the state level. As of December 2016, 35 states had moved to NEC 2014, according to the Electrical Code Coalition.

With the major move to Rapid Shutdown, PV installers have been getting much more experience working with MLPEs. In some cases, the transition has been smooth. In some others, MLPEs have been shown to have limitations in quality. In most cases, MLPEs add more equipment and installation time to residential jobs than traditional string inverters do.

So, improvements have been made to solar optimizer technology, but MLPEs might not have nailed the right mix of performance, cost, simplicity and reliability.

Enter substring optimizers. Offering performance control, maximum powerpoint tracking and Rapid Shutdown compliance, substring optimizers have many of the benefits of MLPEs without some of the increases in part count and installation time.



What is a substring optimizer?

Substring, or sub-array, solar optimizers are DC/DC power converters typically connected to an inverter using a high-performance DC bus. These inverters have moved into center stage as an attractive new blend of performance, smart operational features and, in some cases, advanced functions like transformerless islanding and battery integration.

As part of the DC-bus-based inverter class, substring optimizers offer a technical set of benefits that include high transmission efficiency, powerline carrier communication and built-in monitoring data. But the most tangible benefit to the installer is the faster, simpler installation a substring optimizer could offer.

Most substring optimizers are capable of connecting at least two solar modules in series, thus immediately reducing the installed rooftop electronics by half. Some can connect as many as nine modules in series, resulting in a smaller fraction of required connections and installation time. The substring optimizers are typically connected to each other in parallel and then combined on a home run to the inverter’s DC ports.

Fewer parts vs. fine-grained control

Faster installations and fewer connections are great features on installation day, but in the long term, fewer installed parts could mean fewer parts that can fail in the field, and installers of MLPEs know well the reality of having installed thousands of small array-level electronics.

Substring optimizers can reduce the warranty risk a solar installer takes on, at both the small and large scale. Small commercial projects and DC microgrids, in particular, can benefit from the associated savings and reduction in module electronics; arrays that use 1/9 as many power optimizers are optimized in every sense of the word, and that makes a big difference when hundreds of modules are in play.

Of course, MLPEs can offer some of the most fine-grained control of the array. These optimizers have shipped in large numbers due to good product design that has lightened the work required to get them rigged up on each module. But where installation time and soft costs are concerned, the quick, simple installation of the substring approach can add value.

Designing for flexibility

Installers and customers like that MLPEs offer the ability to isolate individual modules, but in practicality, it could be a high price to pay. Do the added equipment costs and heightened probability of an MLPE failure offset the need to ensure each single module is invulnerable to shading on its adjacent module? Do PV modules really fail at a greater rate than MLPEs?

With substring optimizers, PV array designers have the flexibility to build small, asymmetrical strings of modules to overcome complex rooflines and shading. One substring might be eight modules in a flat, open area of the roof, while another substring might be four modules designed around the shade pattern of a chimney. And, like MLPEs, substring optimizers that source current are able to provide maximum powerpoint tracking to ensure the highest-possible energy harvest.


Substring optimizers connected on a DC bus offer monitoring at the device level, allowing customers to see overall energy harvest and their best-performing areas of their array.

The inherent limitation of substring optimization is that when a module fails, a substring device won’t target the problem as definitively as an MLPE will. However, the substring optimizer will show a marked decrease in performance, allowing the installer to target the substring of modules, helping to avoid the string inverter guesswork of checking every module,  while not significantly increasing the time to identify the affected module in the substring.

On the other end of the spectrum, there’s the inherent limitation of MLPEs when it comes to monitoring – is it really essential to report module No. 14’s decreased production due to pollen, snow or other natural hazards to solar performance? For installers that receive phone calls from anxious customers of MLPE-based systems, there might be such a thing as too much information.

Cost implications

MLPEs have come down in cost over the last few years, but even some early-to-market substring optimizers can cost a bit less than the equivalent number of MLPEs they replace. That’s attractive on its own, but the real savings come in the reduced soft costs of working with substring optimizers. When you replace as many as eight or nine optimizers, you’re creating the potential to speed up the associated time to install the solar optimizers by as much as 89%. Furthermore, the design concept of substring optimizers connected to DC-bus-based inverters can allow for a faster commissioning time at the end of the installation, which means no more waiting around while dozens of MLPEs are pinging the inverter. With the reduced risk of fewer installed parts, installers are less likely to make return trips to the job site to perform swap-outs of failed electronics.

What’s next for Rapid Shutdown?

Article 690.12 has gotten a major overhaul with NEC 2017. Rapid Shutdown, itself, has been reworked and further defined within the 2017 version of 690.12. As a function of these sizable updates and as a measure of compromise between fire service and solar industry advocates, the new Rapid Shutdown requirements will not be enforced until January 2019.

Nonetheless, installers making the wholesale shift to substring optimizers will want to ensure that these devices are compliant with downstream code changes in order to avoid switching to new technology again within several years. The good news is that substring optimizers have several clear options for satisfying the next set of Rapid Shutdown requirements.

Substring optimizers will satisfy NEC 2017 updates to Rapid Shutdown, provided they can meet one of the listed criteria for compliance. These include listing and labeling the PV array for Rapid Shutdown compliance; limiting controlled conductors within the array boundary to 80 V or less within 30 seconds of Rapid Shutdown initiation; or installing a nonmetallic PV array with no exposed wiring and array more than eight feet from any grounded metal part.

Of the available options, the second, limiting voltage to 80 V or less within 30 seconds of a Rapid Shutdown event, is the most easily achievable, provided the connected inverter uses a global shutdown command over powerline carrier that quickly drops the voltage of the optimizers at the array. It’s likely that many substring optimizers will also be listed for Rapid Shutdown compliance via Nationally Recognized Testing Laboratories in advance of the 2019 compliance target.

Looking ahead

PV integrators will need to increasingly abide by Rapid Shutdown, and as a result, a key advantage in reducing soft costs and warranty implications comes with making those installations faster. The DC-bus-based inverter architecture using substring optimizers is an approach poised to meet these needs, while giving customers flexible system options like battery integration and smart operational features.

Substring optimizers, in turn, are positioned for growth and are likely to supplant MLPEs as the latter continue to experience some quality limitations. As solar PV continues to trend toward wider adoption, installers will look to technology solutions like substring optimizers to help them manage installation times and minimize equipment and costs, thus streamlining residential and commercial projects.