DC decisions: Large-scale PV wiring trends are leaving behind DC combiners, cables

inverter pad

Closer to the grid connection, interrupt ratings are higher, which is why SolarBOS recommends fuses instead of breakers.

All large-scale PV projects will involve a ton of modules and only one grid connection, so all of the aggregation decisions made in between comprise a big chunk of a profitable project pitch. We asked two major eBOS suppliers for the trends leading the way in large-scale PV wiring in 2018, and both of them point to huge reductions in traditional DC wiring and an emphasis on fuses.

Goodbye DC combiners

The most significant DC wiring trend for Jason Whitaker, president of Shoals, has been the migration away from the traditional combiner paradigm to harness assemblies, which is a big step toward simplification and reducing the amount of DC cable needed. The Big Lead Assembly (BLA) is a prime example.

“What previously had been the feeder cable, running from the inverter to the combiner, has now become the entire solution in itself: The BLA, a large conductor, tailored for each site, in which all the PV circuits are combined as they branch off from the BLA at the optimal electro-mechanical positions within the array,” he says. “This allows for a streamlined, plug-and-play installation.”

Pre-paralleling PV strings and incorporating in-line fuses can further reduce the amount of cabling needed.

“If the ILF [in-line fuse] saves more than 25 ft of PV cable, it has paid for itself,” Whitaker says. “ILFs can significantly reduce both the amount of cable required within a PV array and the number and/or size of combiners deployed.”

On a typical tracker installation with combiners and component harnesses (ILFs), a 39 percent reduction in DC string wiring can be expected. Then, factor in the BLA, and that number moves to about a 51 percent reduction. Note: Shoals also recently partnered with Array Technologies on a solution tailored to its tracker system, which improves those economics even more.

“Optimization of the electrical and mechanical systems together will result in a decreased installation time while simultaneously increasing system reliability,” Whitaker says.

All that, coupled with significant labor savings will result in the lowest installed cost solution. The BLA not only provides value from a CAPEX perspective, it also offers significant savings in OPEX when you consider the increased reliability and the elimination of components that require operations and maintenance (O&M).

Shoals’ Big Lead Assembly

Diagram of the streamlined design of Shoals’ Big Lead Assembly combined with in-line fuses.

Hello AC combiners

The philosophical shift in the industry from central inverters to string inverters in larger-scale applications has big implications for eBOS decisions. What was typically all DC aggregation is a big mix of DC and AC. Coel Schumacher, CTO of SolarBOS, sees this as a chance to install fused AC combiners and recombiners instead of breakers.

“Breakers are great for applications with variable loads — plugging too many devices into an outlet, a hair dryer in a bathroom. In a solar application there aren’t any variable loads or hair dryers, so if an overcurrent protective device [OCPD] trips, there is a real problem, and resetting the OCPD is the least of your concerns,” he says. “Breakers degrade with every use, so the more times they are used as a disconnect, the more likely they are to nuisance trip at some later time when they shouldn’t.”

Four trends leading ground-mounted solar in 2018 from IHS Markit

As you get closer to the grid connection, interrupt ratings are higher. Fuses offer higher interrupt ratings and are more cost effective at these higher ratings than breakers.

“When a fuse is replaced, it will operate as it did when the original was new. A breaker on the other hand, after being reset, may not,” Schumacher says. “Breakers can also be more difficult to replace when they do eventually fail.”

Remember that every connection in a design is a source of contact resistance which generates heat, so one way to reduce potential for failure is to reduce those contact points. An example: Some systems are designed to connect the output lug to the copper bus bar using neutral bars attached with fasteners. This would mean one or more connections than is necessary and relies on small fasteners. Instead, SolarBOS uses a distribution block that directly connects to the incoming string wires and the output lug.

“Similarly, our positive bus bar typically connects to the next component directly with a bolted connection rather than using additional busbars or unreliable clamps,” Schumacher says. “Not only do all of these connections perform better, but there are fewer of them, which reduces the number of connections that need to be checked in maintenance efforts.”

— Solar Builder magazine

SolarBOS expands design services for large solar projects

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SolarBOS has expanded its Design Services team to deliver additional services to customers in the large commercial and utility sectors of the solar industry. These services include wire layouts, takeoffs, detailed wire schedules and eBOS optimizations for systems larger than 1 MW.

“Providing detailed wire schedules helps ensure that every length is correct”, says product manager Zuzana Piras. “Manufacturing overmolded harnesses and wire bundles in house results in SolarBOS quality and short lead times.”

SolarBOS improves designs by leveraging experience from wire layouts on hundreds of sites and by using custom software to generate takeoffs from site-specific plans. SolarBOS manufactures 2-string and higher string count overmolded harnesses for all module types, including First Solar Series 6.

— Solar Builder magazine

11 things to know about the SolarBOS AC Combiner

SolarBOS introduced the industry’s first fused AC combiner for string inverter applications in September 2016. Since then, SolarBOS engineers and product managers have refined and improved the product offering, having supported solar projects combining as many as 36 string inverters with a single AC combiner. SolarBOS now says its AC Combiner line will complement all string inverter applications.

SolarBOS AC Combiner highlights

SolarBOS AC Combiners are easily customizable for wide range of projects. We have some info on it in our Jan/Feb issue, but there are 11 specs to note. solarbos ac combiner

Bi-directional fuses. Fuses are bi-directional and can be back fed. Breaker panels are typically designed for load applications, when used “backwards” for supply equipment the breakers must be back feed capable.

Inverter isolation. Many string inverters include load break disconnecting means. This allows the use of non-load break fuseholders to isolate inverters and realize a significant cost savings.

Output Bussing. Custom output busses allow direct and convenient connection to transformers.

Outdoor rated NEMA-4 enclosures. SolarBOS AC Combiners can be mounted on their back reducing shading and racking requirements.

Interrupt rating. Fuses offer high interrupt rating as standard, commonly 200kAIC. Breaker cost increases drastically as interrupt ratings increase.

Supply vs load application. AC Combiners are designed specifically for supply as opposed to load applications. Breakers are good when there are variable loads (when you plug in a hair dryer and trip a breaker, the ease of resetting it very convenient). In solar applications, loads are not variable and sources are current limited.

Reduced PPE requirements. Current limiting fuses reduce arc flash energy by a factor of 5-10 and the level of PPE required for servicing.

Reduced Maintenance. Fuses do not require annual testing. Breakers require testing at regular intervals.

100% operation and reliability. Breakers degrade every time they trip or are used as a disconnect. Fuses do not.

OCPD coordination. OCPD coordination ensures the correct OCPD trips if an overcurrent event takes place. “Correct” refers to the OCPD most appropriate and closest to the fault. This is easily ensured with fuse coordination ratios. Breakers require a more complicated study. Trip curves are specific to each breaker model (as opposed to common across a fuse class) and complicate breaker replacement.

— Solar Builder magazine

Details on new AC Combiners, Recombiners from SolarBOS

SolarBOS, a balance of system product developer and manufacturer for the Solar Energy Industry, is debuting a new set of AC Combiners and Recombiners. These SolarBOS AC Combiners and Recombiners have been designed specifically for PV systems to facilitate combining multiple string inverter AC outputs more effectively than breaker panels.

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Key highlights

• Outdoor rated NEMA-4 enclosures. AC combiners can be mounted on their back, thus reducing shading and racking requirements.

• Better performance at elevated ambient temperatures. Breakers nuisance trip, particularly at high ambient temperatures. Breakers are commonly oversized to avoid this and cause oversized, more costly conductors. This is not required with fuses.

• Reduced PPE requirements. Current limiting fuses reduce arc flash energy by a factor of 5-10 and the level of PPE required for servicing.

• Reduced Maintenance. Breakers require testing at regular intervals. Fuses do not require testing.

• 100% operation and reliability (as opposed to breaker degradation). Breakers degrade every time they trip or are used as a disconnect.

RELATED: How to select the right combiner box for your next solar project 

• Interrupt rating. Breaker cost increases drastically as interrupt ratings increase. Fuses offer high interrupt rating as standard, commonly 200kaic.

• Supply vs load application. Solar systems are current limited supplies, not loads. Resettable breakers for variable loads do not add value.

• Bi-directional fuses. Breaker panels are typically designed for load applications. When used “backwards” for supply equipment, the breakers must be back feed capable. Fuses work in both directions and back feeding is not a concern.

• OCPD coordination. OCPD coordination ensures the correct OCPD trips if an overcurrent event takes place. This is easily ensured with fuse coordination ratios. Breakers require a more complicated study. Trip curves are specific to each breaker model (as opposed to common across a fuse class) and impede breaker replacement.

• Factory pre-wiring (optional). Reduced field installation time due to pre-terminated and custom length input conductors.

• Load or non-load break inverter isolation. Many string inverters include load break disconnecting means. This allows the use of non-load break fuseholders to isolate inverters and realize significant cost savings.

• Output Bussing. Custom output busses allow direct and convenient connection of transformer and AC recombiner.

— Solar Builder magazine

1,500-volt systems to trend in 2017: Here’s what you need to know

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SolarBOS combiner

Large-scale solar projects are heading into yet another sea change: the 1,500-volt PV system (Vdc). The idea behind the voltage increase is the same now as it was during the move from 600 volts to 1,000 volts — further reduce installation costs and increase profitability by reducing the number of inverters and other BOS components required.

“But there is a difference this time,” notes TJ Kanczuzewski, president of Inovateus Solar located in South Bend, Ind. As a solar EPC and distributor, Inovateus Solar was one of the first solar companies to be introduced to 1,000 Vdc from 600 Vdc and has seen these types of technology transitions before. “Today’s 1,500 Vdc are more sophisticated systems than we’ve ever had before.”

Advanced Intelligence

Kanczuzewski relays his experience designing projects with Schneider Electric’s new Conext SmartGen 1,500-Vdc inverters, which can record and store operations and service history, as well as upload all of this data and self-diagnostics to the cloud.

“The Conext system also offers 30-year service life and a minimum of 15 years before the first major service,” he says. “So, this time, these systems are very intelligent as well as powerful.”

And this is just the beginning as all of your favorite inverter brands start to launch and ramp up production on their product lines. At the 2016 Solar Power International, Yaskawa-Solectria Solar, one of the most widely installed brands, pre-announced its 1,500-Vdc, utility-scale string inverter, the SLX 1500 line. These inverters will be available at various power levels and AC voltages, but adding in its Wireless Mesh network eliminates the need for communication wiring, reducing communications and BOS cost.

“In addition, the Wireless Mesh proves to simplify commissioning, has robust/secure networking, advanced grid functionality, superior asset management and improves response time,” says Natalie Holtgrefe, senior marketing manager for Yaskawa-Solectria Solar.

RELATED: How optimizers bridge the gap to 1,500-volt PV systems 

But is it safe?

Regardless of ancillary benefits, being an early adopter to such a step up in voltage carries risk.
“There is a lack of understanding in the industry concerning incident energy and arc flash risk. This is true for 1,000-Vdc systems and clearly becomes more important in 1,500-Vdc systems,” says SolarBOS CTO Coel Schumacher.

BOS equipment provides overcurrent protection and disconnecting means used for system operation and maintenance and must be accessible to personnel. Due to the nature of photovoltaic installations, there are a significant number of sources that aggregate in BOS equipment, and a series of devices are typically used to achieve this.

“While it is possible to isolate these devices, the long runs between them make it inconvenient as well as difficult to isolate a device by a means within line of sight,” Schumacher continues. “If the device is not completely isolated, portions of the equipment remain energized. This poses an arc flash risk and yet there is no consensus on how to evaluate that risk, much less how much risk there truly is.”

SolarBOS offers BOS equipment including combiners and recombiners with various options for circuit count, current ratings, OCPD and disconnecting means. On the AC side of the inverter, SolarBOS offers configurable switchgear that is necessary for string inverter implementation.

Yaskawa-Solectria Solar’s DISCOM 1500 string combiners offer various options that make design and safety easier for installers, including MC4/H4 connectorized wire whips, compression lug studs and heavy gauge bus bars.

Eaton, which has extensive experience in managing DC circuits in other high voltage DC environments such as battery storage systems, rail systems and steel mills, reminds us that the call for 1,500-volt safety extends to the equipment, too.

“At 1,500 Vdc, there is substantially higher voltage stress on the solar modules, which can make modules more susceptible to potential induced degradation (PID),” says John Vernacchia, segment manager for renewable energy at Eaton. “Only a few years ago, PID had a disastrous effect on many solar projects. As developers look at using this higher voltage technology, caution should be taken to use PID-resilient solar modules and to use grounded arrays. Past experience has shown that floating arrays are significantly more sensitive to PID due to the negative voltage bias placed on the solar modules.”

Eaton’s 1,500-Vdc inverters will employ a new proprietary DC design concept that replaces manual DC disconnects with DC contactors to improve both control and enhance operator safety.

Eaton’s 1,500-Vdc inverters will employ a new proprietary DC design concept that replaces manual DC disconnects with DC contactors to improve both control and enhance operator safety.

Reducing BOS costs

As dazzling as the new capabilities are, the potential BOS cost reductions are just as enticing for an industry constantly having to prove its economic worth. GTM Research estimates that overall project costs can be reduced by three to five percent by moving to 1,500-Vdc systems, realized mainly through reduced installation time and fewer components.

SolarBOS’s Schumacher says the 1,500-Vdc projects they’ve done are acting as flagship installations to prove the benefits of higher voltage systems. In general, he says, BOS equipment becomes more energy dense and cost effective at 1,500 Vdc.

“In addition 1,500-Vdc systems lend themselves to higher AC voltages [600 Vac or more], which helps to reduce AC conductor and switchgear cost,” Schumacher says.

Eaton developed its Crouse-Hinds series DC collection system, Sunnector, to reduce costs and installation time in these utility-scale solar projects. This system can help reduce labor and material costs by 15 percent on average, according to Vernacchia, in 5 MW and larger-scale, grid-tied solar projects that use fixed-tilt ground-mount racking designs.

A key here is using aluminum for long-distance runs, but still incorporating copper connections to the PV modules. This way contractors are able to use standard copper connectivity and tools, while project owners are able to reduce costs by taking advantage of lower cost aluminum wire.

Yaskawa-Solectria’s SLX 1500 line and Wireless Mesh network eliminates the need for communication wiring, reducing communications and BOS cost.

Yaskawa-Solectria’s SLX 1500 line and Wireless Mesh network eliminates the need for communication wiring, reducing communications and BOS cost.

So, where are we?

Holtgrefe says Yaskawa-Solectria Solar is seeing considerable demand for 1,500 Vdc in utility-scale projects. However, 95 percent of demand in the C&I space is still for 1,000-Vdc products. Her expectation is for the C&I space to move toward 1,500-Vdc systems at a slower rate of adoption than utility-scale.

“We’ll need to educate our customers about this new offering and help them understand the value,” Kanczuzewski says. “Code standards will need to be revised in some areas, and different utilities may have their own guidelines, so installers will need to make sure that 1,500 Vdc is compliant or show how 1,500 Vdc is becoming the new standard. We can always go back to 1,000-Vdc systems if our customers require it, but we hope we’ll be able to transition quickly.

“Inovateus went through the same type of customer education and standards review with the transition to 1,000 Vdc, but we expect that 1,500 Vdc will overcome any hurdles and become the norm within the next two to three years.”

Chris Crowell is the managing editor of Solar Builder.

— Solar Builder magazine