Ever since ground-mount 1,500-V systems were mentioned in the 2017 National Electrical Code, manufacturers have been working hard on 1,500-V-rated solar panels, inverters and everything in between. Higher voltage systems allow installers to condense systems while achieving the same power output.
Section 690.7 in the 2017 NEC established for the first time that ground-mount systems can have a maximum voltage of 1,500 V. Large utility-scale systems had already started shifting to 1,500 volts in the years prior to this code because of different standard requirements, but the updated code opens the possibility of 1,500 volts for smaller utility-scale projects and the high-voltage string inverters that suit that market well.
The “2017 International Technology Roadmap for Photovoltaic (ITRPV)” highlighted the trend of system voltage increasing from 1,000 volts to 1,500 volts. The study found that from 2020 onwards, the 1,500-V market will be greater than 30%, and that voltage will reach a market share of greater than 50% beginning in 2025.
What is 1,500 volts?
An increase of 500 volts allows contractors to condense systems because each inverter can process more energy. More panels can be connected in series to make longer strings. Fewer wires are needed. Fewer inverters are needed because they can accept more power. But a 1,500-V system can only work if all components are rated to perform at 1,500 V.
Most solar panel manufacturers have begun updating their panels used in utility-scale projects to 1,500 V. Jeff Juger, director of business development for JinkoSolar, explained that solar installers will still need the same number of total panels to reach the intended wattage in a 1,500-V system, just fewer strings of panels. For example, if a module’s voltage at open circuit (Voc) is 45 VDC, then a 1,000-V system allows 22 modules (1,000/45) in a string whereas a 1,500-V system allows 33 modules (1,500/45) in a string.
The cost of 1,500-V modules is essentially the same as lower-voltage panels, though slightly different materials are used in production. Juger said the only issue with these upgraded panels is the increased opportunity for potential induced degradation, or PID.
“High voltage can create risk of PID, where ions migrate from the cells to the module frame, resulting in current leakage,” Juger said. “Jinko was the first to offer a framed PID-free module and has had no issues with PID in its 1,500-V modules.”
Current drives cost
Higher-voltage systems are cheaper because fewer materials are needed, from big things like inverters to small things like wires and disconnects. This is because current, which is inversely proportional to voltage, will go down as voltages go up. Conductor sizes can decrease because the current is less, and costs go down with conductor size.
“The adage that we have here is current drives cost,” said Eric Every, product manager at Yaskawa – Solectria Solar. “The higher that you make your voltage, the less actual commodity material that you need to purchase.”
Every said going up from a 1,000-V to 1,500-V system just means making the wire insulation a little bigger, while going down in voltage and current requires more actual copper material within the wires. Using more copper is expensive in the case of multi-megawatt projects, so using less copper with 1,500-V systems is a welcome option for large-scale solar developers and installers.
Big systems, big gains
1,500-V cost savings are greatest when deployed at a large scale. That’s why 1,500-V central inverters have become the dominant, and basically exclusive, choice for new large utility-scale installations in the United States, while 1,500-V string inverters are just now entering the market to serve smaller utility projects.
“With this shift to 1,500 volts, you get more powerful inverters,” said Carlos Lezana, marketing and communications for solar for Ingeteam, which manufactures both string and central inverters. “If you develop [a 100-MW] power plant with a 1,500-V central inverter, you will need less units [and] less inverters than if you do it with 1,000-V inverters.”
Fewer central inverters means less labor and fewer combiner boxes needed to consolidate wires in these high-voltage installations. Fewer inverters also means fewer technicians needed to fix problems after commissioning.
“When you have less inverters, then all those costs, those labor costs, are reduced,” Lezana said.
Although moving to 1,500 volts comes with many benefits, there are downsides too. More voltage passing through fewer central inverters means more power loss if an inverter fails.
“Obviously if you have less inverters, it’s because each one of them is managing more power, so if one goes down, then you lose more power,” Lezana said. “All PV inverter manufacturers have a failure rate. You have to make sure that your suppliers, or the company that is in charge of the operations and maintenance, is going to be very fast in the response, because you want to have your power plant operating at full power all the time, as many hours as possible.”
Enter 1,500-V string inverters
1,500-V string inverters entered the utility-scale market about a year ago but make more sense for smaller, community solar projects.
For example, with a 20-MW power plant, EPCs could use five or six 1,500-V central inverters or hundreds of 1,500-V string inverters. The choice would come down to overall cost and serviceability, but it’s clear that large utility-scale projects might still stick with central inverters.
“If you’re doing 100 MW, I’m not really sure it makes sense to use string inverters,” Every said. “We’ll bid a project on it, but likely the customer will choose the centrals because they’ll be able to have that granularity.”
The serviceability of string inverters in general makes 1,500-V string inverters an attractive option for smaller utility-scale systems.
If one fails, only one string is affected and can easily be swapped for a new unit without much delay. Central inverters are still cheaper than string in terms of cents per watt on large utility-scale projects, but string may win out for simpler serviceability, especially when higher voltage raises the stakes.
“The recovery is much faster on string. You just do a simple replacement with spare units that might be on site. A replacement can be done in half an hour, so it’s a fast recovery,” said Ed Heacox, general manager of CPS America.
Solectria has chosen to approach the 1,500-V market with only string inverters because of this fact. The company is considering its 1,000-V central inverter a legacy product now–not recommending it for new designs.
“When we decided to produce a 1,500-V product, we said, ‘Hey, we’re going to have a really hard time with this O&M issue if we go with a central inverter. All of our customers really like the serviceability of that string inverter, let’s just do that,'” Every said.
Wood Mackenzie Power & Renewables’ “Global PV Inverter and MLPE Landscape: H1 2018” found three-phase string inverter shipments grew 59% year-over-year and exceeded central inverter shipments by nearly 7 GW from 2015 to 2017. It found that 2018 was the beginning of increased adoption of string inverters in the United States because of new 1,500-V string inverter models.
“We’re finding that a greater percentage of the projects that our customers are doing are implementing them with a string inverter topology as opposed to central inverters. And every year that percentage seems to go up,” Every said.
Solectria has traditionally focused on C&I projects between 100 kW and 20 MW, with most of them being 1- to 5-MW commercial rooftop or small ground-mounted projects. Its new 1,500-V string inverter, which will begin shipping in December 2018, will target the “I” part of C&I–industrial, distribution-connected, community solar, corporate procurement-type projects.
“If it’s a comparison between 1,500-V and 1,000-V [string inverters], the cost savings are real for the 1,500 volts,” Every said. The lower build cost that results from less labor in the field, less terminations and less wire can help installers win more bids.
“You get a better cost per watt in your modules, you get a better cost per watt when you look at the pipe and wire in the system,” Every said.
Danger: High Voltage!
Increased voltage means increased safety consequences for workers if something goes wrong. But Heacox has faith in industry codes and standards.
“The standards and regulations account for higher voltages, so theoretically there should not be any increased safety risks related to this,” Heacox said. “I do trust that, actually. People know more energy is more risky.”
John Drummond, CPS America applications engineer, adds that installers must be sure to use 1,500-V-rated equipment, from wire insulation to voltmeters and everywhere in between. Installers must also remember that working space clearances are different for 1,500-V projects.
“The typical working space clearances for 600 and 1,000 volts are not applicable to 1,500 volts. You have to use taller fences, keep things spaced further apart, add extra lighting, stuff like that,” Every said.
Although Drummond and Heacox do believe 1,500-V technology is safe, it is still bound to the ground. Drummond said he foresees a future where 1,500-V systems can go up on roofs too, but it would require industry acceptance.
“Really early on…these types of applications were ‘behind the fence,’ and I think just as we shifted from 600 to 1,000 volts, it’s the same kind of hesitancy to shift to 1,500 volts for all applications,” Drummond said.
Every does not see 1,500 volts ever being allowed on residential rooftops due to safety concerns, but he said 1,500-V commercial rooftop solar could be a future possibility. However, the long strings that come with 1,500 volts would be limiting when designing rooftop array configurations. Putting 26 to 28 panels in a straight line is not always easy.
“In 1,000-V [rooftop systems]…you get a little more flexibility with how you lay out your rooftops because your strings are shorter. You’ve got to run a couple more wires, but at least you can pack up the roof a little bit better,” Every said.
High-voltage solar systems bring flexibility and cost savings to solar installers, and options will continue to expand as more innovative 1,500-V solar equipment enters the market.