Just a few short years ago, a large solar PV project could be considered anything well north of 100 megawatts.

It’s a testament to how quickly and dramatically the global solar industry has grown that a large PV plant today is now measured in hundreds of megawatts, and increasingly, gigawatts.

Traditionally, developers of large-scale utility solar power plants always used large central inverters for a very understandable reason. “In the past, lower capex leaned toward central inverters for large projects,” said Sham Ramnarain, chief engineer of Huawei FusionSolar Smart PV Solution for North America. “With today’s innovations and mass production in string inverters, that advantage now leans toward string inverters.”

The use of string inverters in U.S. projects over 5 megawatts in size is expected to grow more than fourfold over the next five years, according to GTM Research. In India, the use of string inverters for large-scale solar projects jumped from about 1 percent in 2016 to 9 percent in 2017, according to consulting firm Bridge to India. They project that that share will expand to 30 percent by 2020.

Unlike central inverters, the same string inverter can be used on projects ranging in size from just 1 megawatt to 1 gigawatt (of course, the number of devices needed will differ dramatically). That sort of flexibility is especially important in today’s ultra-competitive marketplace.

“Today, developers and [engineering, procurement and construction providers] have to bid more projects and they have to bid more variations of projects. Customers are interested in looking at not only traditional and poly modules, but also bifacial and split cells, and there’s always interest in looking at fixed-tilt versus tracking,” said Bates Marshall, vice president and general manager of Huawei FusionSolar Smart PV Solution for North America, who notes that Huawei typically defines megaprojects as anything above 200 megawatts.

“In the past you had to use different inverters for different project designs, and you’d have a file drawer full of designs and optimizations and tradeoffs,” added Marshall. “From a velocity and flexibility standpoint, giving EPCs the ability to rely on a string inverter as one universal building block that doesn’t change at scale is very appealing.”  

As much as flexibility is important, this transition to string inverters for solar megaprojects is primarily being driven by economics. Even before Section 201 tariffs were imposed, EPCs and developers faced enormous pressure to drive down project costs.

The tariff decision has only exacerbated those pressures, forcing EPCs and developers to scour the designs and equipment used in projects for savings. Just a few years ago, the capital expense of string inverters was a financial deal-breaker. “You’d get a quote for the inverter and lay out the preliminary design and bill of materials. And when you looked at it in Excel, the stark reality of capex was staring you in the face,” said Marshall. “The typical three-phase string inverter was twice the price.”

Thanks to a massive ramp-up in production — led by Huawei, which GTM Research ranks as the largest inverter manufacturer in the world — the capex advantage of legacy central inverters has all but disappeared.

Cost savings related to balance-of-systems outlays are also making string inverters more attractive for large projects. Because central inverters were long the standard for large solar developments, EPCs lacked a design methodology suited for string inverters. Huawei has developed a block design approach that divides projects into 3- to 4-megawatt sections, each integrated with a medium-voltage transformer.

The result of developing and encouraging best practices around design has driven down cost. “We have pulled out 3 to 4 cents per watt with block design and technologies like our cluster rack and integrated transformer to consolidate components, reduce wiring and improve quality,” said Marshall.

In the North American market, EPCs generally walk away from projects once they’re built. But the long-term project owners reap the O&M cost benefits of string inverters; Marshall estimates a net present value economic benefit resulting from O&M savings of between 5 and 7 cents per watt.

The use of cloud computing, artificial intelligence and data analytics also drives reliability improvements and cost reductions with megaprojects. String inverters can collect enormous amounts of data that can be analyzed in real time to pinpoint potential problems.

“We can do things like smart IV curve monitoring to provide a very granular, string-level characterization of underperforming strings,” said Marshall. “It tells you that you have a string with broken modules or a string with partial shading. It differentiates one from the other, and your technician can get a text [message]...saying, 'This string has shading, so bring the weed whackers to take care of it.' No central inverter has this capacity.”

All of these advantages that come from the use of string inverters in megaprojects are important because they deliver the lower levelized cost of energy that is required in today’s utility-scale market. “They provide better economic outcomes that are necessary to meet today’s more aggressive PPA requirements,” said Marshall. “That is where the rubber meets the road.”