Time to revisit standalone solutions to solve rapid shutdown system design

Rapid shutdown probably feels like old news at this point, but the product market is just now hitting its stride. Initially, the NEC 2017 and 2020 code changes directly favored the technology of certain module-level power electronic brands, and early versions of inverter-agnostic rapid shutdown devices that paired with string inverters were buggy, causing spikes, nuisance trips and a generally bad rep among installers. Thanks to some new players and advances in standardization and testing, that is now the old news. The rapid shutdown product landscape in 2021 offers a much wider selection of lower cost, inverter-agnostic options for installers to know about.

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Compliant RSD will have either a Photovoltaic Rapid Shutdown Equipment (UL PVRSE) or a Photovoltaic Rapid Shutdown System (UL PVRSS) listing under UL 1741. UL PVRSE is for RSD components that function independent of other equipment in the system, whereas the UL PVRSS designates a system (the RSD component + inverter). Typically the UL PVRSS indicates the inverter brand has incorporated a transmitter from the RSD brand (with the two being tested together to meet the designation).

Either label is typically acceptable to the AHJ so long as all of the RSD components — including the PV inverter or charge controller — are labeled as one or the other, but installers have gravitated much more to products in the PVRSS category. The perception is PVRSE is a lesser-than designation, which is true when that component communicates over the DC line to speak to a transmitter embedded within the corresponding inverter. A system-level listing is really needed in that case.

This perception led IMO Automation to announce a flurry of PVRSS designations for its FireRaptor RSD product in 2020, even though its system doesn’t require an inverter transmitter.

“This was more about marketing than anything — tackling that resistance in the market,” says Joe Covington, GM of IMO Automation “It’s not a situation where code requires it or a situation where UL requires it. It’s an effort on our end to say we hear you saying that there is a lack of trust in the market with these devices. This is an additional step of assurance.”

The FireRaptor is an operational independent RSD system that plays nice with every inverter and doesn’t communicate over the DC power line. FireRaptor is inherently compatible with string inverters because it doesn’t interact with or require any hardware changes inside the inverter to operate. Three devices make up the FireRaptor. The shutdown unit connects at the panel level (up to two at once) with MC4 connectors (which are then connected the same as in a string installation). There is a 24-volt, 71-in. power interface cable that connects to the first FireRaptor unit, and the downstream 24-volt initiator switch, the third component, is mounted near the inverter.

“Again, we are not a MLPE device, we’re not harvesting power, and we’re not having any interaction with the inverter. We have an outside power source and act as a pass-through unit. Power lines connect to each FireRaptor, supplied by the initiator switch, with the homeruns going back to the inverter location, operated simply by a solid-state switch,” Covington says.

Shutdown is initiated three ways and happens within a second in each scenario (with no on-board capacitance issues). 1) Pushing the initiator button. 2) Losing AC power. 3) Hitting an ambient temperature above 185 degrees F on the roof.

In addition to UL PVRSS, IMO went one step further — a step that is difficult for MLPE devices to take — and tested to the still-unpublished AFCI-compliance UL standard to ensures FireRaptor did not introduce an AFCI trigger within a UL PVRSS system.

“In discussing AFCI interference concerns with UL we decided to have them go ahead and implement their testing against interference to address these hurdles and concerns that have sometimes presented themselves in the market,” Covington says.

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C&I assurance

NEP commercial RSD

On the commercial side, the RSD solution from Northern Electric Power (NEP) can work as a separate system or integrated within an inverter. It functions as a totally autonomous closed loop activation system consisting of a PLC signal transceiver, RSD units and an outdoor enclosure that holds the PLC signal transceiver and an optional monitoring gateway that goes adjacent to the inverters. Each RSD unit can connect with panels in 1:1 or 4:1 ratios, but 3:1 and 2:1 versions are coming shortly (possibly available as you read this). And if preferred, the NEP RSD can be inverter-integrated too, currently possible with CPS inverters. So, lots of configuration options depending on your install preferences or LCOE targets.

NEP’s solution also has a thermal shutdown triggered by either the default temperature or one set by the installer.
“If there is a fire, this will activate thermal auto rapid shutdown, well ahead of smoke or firemen. We consider that a safety enhancement,” says Ed Heacox, CEO of NEP.

A unique feature for NEP’s solution is its two-way PLC communication instead of one-way like most others. This provides some cool enhancements for both safety and functionality. At the most basic level, the two-way communication means it can send a signal back from each RSD unit to a low-cost monitoring tool assuring that the system functioned and the shutdown was successful.

That monitoring tool can also be helpful in diagnosing issues in the worst case scenarios.

“If there is a situation like [the Walmart solar rooftop fires], and no one has any idea where it started or what caused it, our data monitoring system would allow for that data to be clear,” Heacox says.

The closed loop system also allows for a service mode in which a voltage can be seen on the module and IV curve tracing can be done, which is handy at commissioning or during an annual PM. With typical rapid shutdown setups you’d need to disconnect the cables.
Last but not least for many big time commercial installers out there, NEP offers custom cable assemblies to meet even the most stringent AHJ standards for mating connectors.

“Some customers are getting criticized by AHJs for using different brand connectors from the panel to these devices. Even if they are both MC4s, some of the AHJs insist they must be mated to the same brand connector, not just type. So, for about a 10 percent cost premium, we’ll configure and OEM connector mate to match the installer’s PV panel connector type.”

This needs a six- to eight-week lead time, but getting brand-on-brand connectors — without requiring a jumper — is a pretty big deal as it both reduces the connector risks as much as possible while also eliminating schedule issues caused by an AHJ surprise.

“We’re looking to get a bigger piece of the market, so we’re looking to go that extra step for the installers that value it.”

Bottom line: You have options


Tigo TS4-A-2F graphic

There is obviously more out there than the FireRaptor and NEP too.

Tigo now has UL PVRSS certifications with 30 inverter models, bringing its certified inverter total to more than 200 inverters from 17 different companies. Most recently, the company passed the milestone of 75 GW of Reclaimed Energy. Tigo measures the incremental energy provided as a result of using their optimizers. This hard data allows customers to monetize the impact of optimization and calculate an objective ROI, the only company to do so. Also, the company announced a $20 million investment round that will be used to improve upon existing products and develop next generation solutions that maximize returns for PV customers. A portion of the new funds will be used to efficiently scale.

APsystems boasts a new solution too that’s smaller and lighter than most other systems, uses less power (<1 W), has a lower minimum operating voltage (7.7v) and, importantly, its RSD-S-PLC Noise Spectrum Density is distinctly separate and away from AFCI detecting noise spectrum range, the company says. This is meant to prevent unwanted AFCI nuisance tripping from string inverters.

We point all this out because as long as module-level shutdown is in the code, having options from multiple brands is a good thing. Maybe you’ve been in a situation where the project was designed and set but a few parts weren’t available. Or maybe cases where purchasing managers wanted to switch vendors but can’t because of the reliance on a specific product feature. Options can help keep your businesses moving forward rather than being hung up on limitations from one specific supplier.

Plus, if the job doesn’t require the extra electronics for shade mitigation, a standalone RSD system is a more economical choice for your customer.

“I think the feeling of the need for optimization is going away because it’s not needed for a lot of installations. And nowadays most of your inverters will give you the information your optimizers have been giving as they become more intelligent,” Covington notes. “So, we’re seeing people move away from optimization unless it’s extreme shading or is implicitly needed.”

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