How to maximize large-scale PV site value with string design

CPS Wirebox Installed before Inverter Body

CPS America inverters come with a separable but integrated wiring box where all of the conductors go in and out.

Service models and technology at the string inverter level is vastly improved from where it was a few years ago. Earlier this week we looked at the value of choosing string inverters over central inverters for projects upward of 20 MW, thanks to 1,500-volt architecture, and then the nuances of approach among virtual centralized string and distributed string.

Choosing string means taking advantage of its positives as much as you can, which means design flexibility and maximizing uptime. If a string inverter or two has to come off line for maintenance, only a fraction of the generation of the system is affected compared to the large chunks connected to a central inverter.

Doubling down on this advantage, for distributed string projects, CPS America inverters come with a separable but integrated wiring box where all of the conductors go in and out. Sold together, every inverter comes with its mated wire box. Four bolts hold the wire box to the inverter body — they slide together with guide pins and one large electrical connector — leaving no wires to play with between the inverter and the wire box.


“If there is a problem with the inverter, 99 percent of the time, it will be in the inverter not the wiring box,” says Sarah J. Ozga, product manager North America for CPS America. “And what takes so much time when swapping out inverters is disconnecting the wires to remove it. So with the integrated but separable wire box, you never have to touch a wire — just leave the wires and conduits installed in the wire box, disconnect the inverter and slide in the new one in.”

An RMA swap of the inverter body takes only 15 minutes with this approach from CPS. Suddenly, having them dispersed all over the site doesn’t seem as daunting.

There are communication advantages across the market too that improve O&M while simplifying commissioning. Flex Gateway, for example, is a communication card added to just one inverter in the communication daisy-chain of up to 70 inverters. When the inverters are connected to a third-party monitoring system or even simply a router, the CPS Service team can remotely access the inverter for troubleshooting, to change the settings and even update firmware on the connected inverters. This will reduce truck rolls and get systems up and running faster than before.

— Solar Builder magazine

Saving costs with large-scale string inverter design, part 1

CPS 60kW Ground AZ

String inverters are now a staple of the commercial and industrial and small utility-scale segments, which was solely the domain of central inverters once upon a time. The trend started about six years ago when string inverters souped-up to 1,000 volts and developers and EPCs saw the value in chasing the higher efficiencies, multiple MPPTs and greater energy harvest rewards provided by distributed string architecture.

But two years ago, the shift to an even lower cost 1,500-volt architecture started, and the math shifted right back to central inverters because 1,500-volt three-phase string inverters weren’t available.

“A year and a half ago, string inverters were about 8 cents per watt and central plus combiners were 6 cents per watt, so that seemed cheaper,” says Ed Heacox, GM, CPS America. “Central plus combiner boxes seemed cheaper.”


But string inverters have souped-up again, and that economic story has flipped again. Instead of string vs. central, the discussion is changing to distributed string vs. “virtual centralized” string. In the Solar Builder Inverter Buyer’s Guide this year, you’ll see a bunch of string inverters in the 100 to 125 kW range, and those that aren’t rated at 1,500 volts will be soon. The cost is now closer to 5 to 6 cents per watt, with central inverters still sitting at a cost per watt similar to two years ago. That aforementioned 3 to 5 MW cap is about to be a thing of the past. CPS has its 1,500-volt product coming out in June and says it is rocketing past that 3 to 5 MW sweet spot.

“We are having regular discussions about projects 20 to 30 MW in size now when before, that was extremely rare,” says Sarah J. Ozga, product manager North America for CPS America. This could possibly go as high as 100 MW in the not-too-distant future.

Now, all costs being equal, some will still gravitate to central inverters because of operations and maintenance preferences: Lots of walking or driving all over to address each O&M issue spread across a 30 MW site, and god help you if the site was mapped incorrectly. String inverters can feel unwieldy if you’re unprepared for them.

“A lot of time O&M depends on the developer or EPC’s personal experience with inverter reliability,” Heacox says. “Those who had a lot of downtime on central, are for sure leaning to string. But those who have had great experiences don’t feel they need to change for projects larger than 5 MW. But some see that if they go down the string path, there is more interchangeability among suppliers with relatively similar products — and the engineering and workload need for swapping out string inverters is a lot easier than reengineering a 3 MW power station.”

In part 2, we will look at two different string solutions that offer the lowest cost path and meet any O&M preferences you may have. We will also dive into this in MUCH greater detail in this upcoming free webinar. Sign up here.

Utility-Scale String Design

Wed, Jun 20, 2018 2:00 PM EDT

When designing a large site one consideration is String or Central. Both have well defined benefits. Historically, the large utility-scale sites have mainly relied upon central inverters. Now a third option, the Virtual Central, is paving the way for string inverters into this space. In this webinar, we will discuss the benefits and disadvantages to both the distributed and centralized string architectures and how the design choice affects installers, developers and site owners. Sign up here.

— Solar Builder magazine

SMA’s Sunny Boy-US series achieves UL 1741 listing

SMA America announced the Sunny Boy-US series as the first residential inverter to achieve UL 1741 SA (Supplement A) listing, which represents a significant step in creating a more stable grid and providing the best technology for the residential market.

SMA America inverters

SMA has completed functional and safety testing with the Sunny Boy-US series of inverters according to the published UL 1741 SA Standard, and Sunny Boy inverters now available for ordering will enable customers to comply with the new standard.

California and Hawaii are the first states to mandate that inverter manufacturers meet the UL 1741 SA requirements of the UL 1741 standard, with compliance required in September 2017. Since it is likely that other states will follow suit, SMA has chosen to upgrade its inverter technology to meet the standard almost a year ahead of schedule.

RELATED: How California’s Rule 21 inverter requirements expand grid capacity, limit energy (revenue) generation 

“Residential solar continues to grow at a rapid pace, and the impact on utilities is significant. Collectively the industry is looking for the best, most effective long-term solutions for a modern grid,” said Sven Schreiber, executive vice president of the Residential business unit at SMA. “SMA recognizes the importance of UL 1741 SA, and has opted to add the necessary functions and features to our inverters immediately. We believe that providing inverters certified to this standard now will save our customers time and money later.”

While the UL 1741 SA listing is not yet mandatory in all 50 states, anyone who wants to comply with the standard can choose SMA inverters knowing they will provide the right features and functions should additional states adopt the same requirements in the future.

Although it is not yet mandatory, SMA believes that complying with the UL 1741 SA requirements as soon as possible is a responsible decision that ensures optimal grid health and stability, while also providing customers with the best possible inverter.

The Sunny Boy-US series includes the following models:


Don’t leave yet! Check out our 2017 Inverter Buyer’s Guide.

— Solar Builder magazine

String inverter market trends: Forecast shows 15 percent CAGR up to 2024

String inverter market size is expected to cross $7 billion by 2024, as per the latest report by Global Market Insights, Inc.

Rising demand for continuous and reliable power sources owing to growing population and urbanization will propel global string inverter market share over forecast period. Strong demand trends for off grid electricity will also play a significant role in industry evolution, with over 800 million of total population having poor or no access to electric grid infrastructure as of 2015.

What’s driving demand?

Fronius string inverters optimizers

Major industry players cited in the report include SolarMax, Huawei Technologies, Sungrow Power Supply, Fronius International (pictured), KOSTAL Solar Electric, Growatt New Energy Technology, Schneider, Ingeteam, Ginlong Technologies, Advanced Energy Industries, KACO New Energy and Solaredge Technologies.

Reliability, high efficiency and accessibility are some of the parameters which will drive the global string inverter market. Compact size, light weight, ease of installation and ability to increase power density are some of the other features which may positively favor the industry landscape.

Declining component cost coupled with low ongoing maintenance will positively influence string inverter market growth. Advancement of technology has reduced the three-phase inverter price by over 20% from 2013. Cost competitiveness will increase its adoption among residential and commercial users.

Global industry revenue generated from on-grid will exceed $5 billion. It finds application in small to medium scale industrial & commercial complex and large-scale utility projects. Increasing adoption of decentralization system is set to drive standalone system demand.

The global revenue generated from up to 10 kW string inverter market for 2015 was over $225 million and is predicted to witness strong growth owing to growing rooftop solar system demand from residential and commercial consumers.

In the U.S., programs including Renewable Energy Production Incentives (REPI) have been introduced to encourage the adoption of renewable energy technologies which will positively impact demand. REPI program offers incentive payments to new renewable energy facilities that generate and sell electricity.

For Europe, UK string inverter market size is set to witness growth over 18% by 2024. Shifting trend towards sustainable energy may favor the industry growth. The electricity generation through renewable resources reached from 64.7 TWh in 2014 to 83.3 TWh in 2015.

— Solar Builder magazine

Report: String inverter market to reach $3 Billion by 2021

In the latest “String Inverter Market – Global Forecast to 2021” report, available Research and Markets, the expectation is for the string inverter market to reach $3 billion by 2021.

Fronius string inverters optimizers

Reasons behind the numbers

String inverters are used in residential, commercial & industrial, and utilities. The string inverter market is expected to be driven by decreased balance of system cost, removing systems monitoring capabilities, and easing installation and customization.

On the other hand, shading one panel reduces the efficiency of panels, along with heat losses due to the large size of the inverter, is the major restraint of the global string inverter market, the report states.

RELATED: Multiple PV manufacturers integrate new cell-string optimizer technology from Maxim 

Segments trending

With regard to system type, the off-grid segment is estimated to grow at the fastest CAGR. Though off-grid technology is cheaper, the report authors say the string inverter application is not very cost effective, which restricts the installation of string inverter at a large scale, such as three-phase string inverters.

In coming years, with declining solar module, inverter, and overall solar systems prices, off-grid string inverters are expected to gain momentum.

Asia-Pacific is expected to be the largest market for string inverters, with China and India being the biggest markets in the region. It is estimated to dominate the global market during the forecast period, owing to government targets, policy support (tax incentives and RPOs), incentives such as feed-in tariffs, and tenders & competitive bidding in the region.

Here’s a link to the report.

— Solar Builder magazine