As sizable economic investments, large-scale photovoltaic power plants require components that can reliably deliver a given project's promised energy yield throughout its service lifetime. For inverter manufacturers, addressing this rapidly growing segment of the market demands a departure from their approaches to the residential PV market.
Multi-megawatt PV installations typically include hundreds or thousands of strings that must be managed, notes Dr. Leo Casey, chief technology officer at Satcon. ‘Factor approximately eight acres of panels per production megawatt as standard, and it is easy to see the multiplier effect of dependent components and the proliferation of critical decision points in a given system that must be managed optimally in order to achieve profitability,’ he adds.
On the DC side, this increased scale equals greater complexity in energy-harvest processes – providing rich opportunities for improvement, but also introducing new grid-integration, remote-command and control issues on the AC side.
Although many inverter manufacturers have recently rolled out new utility-scale products with increasingly high power outputs, Peter Gerhardinger, chief technology officer at Nextronex, believes the bigger-is-better approach may have its limits and that the industry has yet to firmly establish optimal inverter-sizing guidelines for large projects.
In any case, ‘We believe it makes sense to always have multiple inverters on a solar farm to improve reliability and minimize tare losses,’ he says. ‘Using a distributed approach, where the solar array – or, in very large fields, portions of the array in 1 MW increments – is hard-wired to a DC bus and the inverters are switched in and out as needed can significantly reduce installation cost, improve low-light energy harvesting, and improve reliability and uptime.’
According to Casey, 1 MW represents the maximum individual output for cost-effective inverter design. Beyond this point, increased costs for copper and other materials will exceed any benefits received.
Verena Arps, technical sales support manager at SMA America, says that for large-scale PV projects, utilizing a three-phase central inverter will usually be more cost-effective than incorporating a string concept.
‘For medium-voltage connection, high-efficiency designs utilizing inverters without integrated transformers allow for direct connection with an external medium-voltage transformer, which lowers costs and results in increased efficiency,’ she adds.
Inverter efficiency remains an important consideration for utility-scale PV projects, but with many units hovering around 95%-98% efficiency, some manufacturers believe inverters are approaching their efficiency ceiling.
‘Eventually, there are practical limits that make higher efficiency gains uneconomic until new technologies become available,’ says Tucker Ruberti, director of product management at PV Powered.
Dr. Michael Seehuber, CEO of REFU Electronik GmbH, anticipates additional utility-scale inverter efficiency gains of approximately 1% over the next two to five years. In addition, he believes manufacturers must expand the voltage window and range of operation for their inverters to accommodate large-scale projects.
Overall design improvements, new semiconductors and new switching technologies have resulted in recent increases in inverter efficiency, says Arps. However, ‘As efficiency gains become smaller, increased attention will be given to uptime, system control and service,’ she predicts.
Solar plant control as a contributor to overall grid stability is likely to be placed under particular scrutiny this year as utilities ramp up their involvement in solar, raising concerns about overproduction, underproduction and inverters' roles in ensuring a stable grid.
In an effort to meet utilities' and developers' concerns, GE Energy's new utility-scale solar inverter is equipped with SunIQ, a monitoring and controls platform that includes tools to manage integration into the electric grid, says Minesh Shah, renewable systems platform leader.
‘A utility or grid operator can send commands to the solar plant to specify the maximum output of the solar facility,’ he explains. ‘Upon receiving the power output command, SunIQ coordinates the output of all the inverters of a solar power plant such that at the point of interconnection, the output of the facility does not exceed the request.’
Throughout the company's history in the wind turbine industry, Shah has observed the evolution of both Federal Energy Regulatory Commission (FERC) requirements and sophisticated plant-level controls to properly accommodate the large amounts of wind energy that have entered the grid over the years.
For solar, current FERC requirements are similar to those of pre-2003 wind power, he says.
To address future regulatory needs, PV Powered is currently using a U.S. Department of Energy and Sandia National Laboratories award to research advanced utility command and control tools, as well as system integration advancements, from the inverter perspective.
Meanwhile, given the immense amounts of production at stake in a utility-scale PV installation, inverter manufacturers strive to continuously increase inverter uptime and ensure any in-field failures can be addressed immediately.
‘Inverters are the system component with the most built-in intelligence, so we are able to pinpoint problems when they occur,’ notes Seehuber, adding that inverter-identified issues may or may not originate from the inverter itself.
The data-rich information portals, mobile status updates and other modern system monitoring resources that are now commonly available for system owners and operators often feature extensive inverter diagnostics capabilities. GE's SunIQ, for instance, includes a solar SCADA system that troubleshoots on an individual inverter level.
‘Efficiency is incredibly important, but so are reliability, productive lifetime and time to repair, maintain and install,’ states Ruberti. ‘Each of these factors contributes to the lifetime cost per kilowatt of output.’
