Inverter Innovations In Utility-Scale PV Applications
As sizable economic investments,large-scale photovoltaic power plants require components that canreliably deliver a given project’s promised energy yield throughout itsservice lifetime. For inverter manufacturers, addressing this rapidlygrowing segment of the market demands a departure from their approachesto the residential PV market.
Multi-megawatt PV installationstypically include hundreds or thousands of strings that must bemanaged, notes Dr. Leo Casey, chief technology officer at Satcon."Factor approximately eight acres of panels per production megawatt asstandard, and it is easy to see the multiplier effect of dependentcomponents and the proliferation of critical decision points in a givensystem that must be managed optimally in order to achieveprofitability," he adds. On the DC side, this increased scaleequals greater complexity in energy-harvest processes – providing richopportunities for improvement, but also introducing newgrid-integration, remote-command and control issues on the AC side.
Althoughmany inverter manufacturers have recently rolled out new utility-scaleproducts with increasingly high power outputs, Peter Gerhardinger,chief technology officer at Nextronex, believes the bigger-is-betterapproach may have its limits and that the industry has yet to firmlyestablish optimal inverter-sizing guidelines for large projects.
Inany case, "We believe it makes sense to always have multiple inverterson a solar farm to improve reliability and minimize tare losses," hesays. "Using a distributed approach, where the solar array – or, invery large fields, portions of the array in 1 MW increments – ishard-wired to a DC bus and the inverters are switched in and out asneeded can significantly reduce installation cost, improve low-lightenergy harvesting, and improve reliability and uptime."
Accordingto Casey, 1 MW represents the maximum individual output forcost-effective inverter design. Beyond this point, increased costs forcopper and other materials will exceed any benefits received.
VerenaArps, technical sales support manager at SMA America, says that forlarge-scale PV projects, utilizing a three-phase central inverter willusually be more cost-effective than incorporating a string concept.
"Formedium-voltage connection, high-efficiency designs utilizing inverterswithout integrated transformers allow for direct connection with anexternal medium-voltage transformer, which lowers costs and results inincreased efficiency," she adds.
Inverter efficiency remains animportant consideration for utility-scale PV projects, but with manyunits hovering around 95%-98% efficiency, some manufacturers believeinverters are approaching their efficiency ceiling.
"Eventually,there are practical limits that make higher efficiency gains uneconomicuntil new technologies become available," says Tucker Ruberti, directorof product management at PV Powered.
Dr. Michael Seehuber, CEOof REFU Electronik GmbH, anticipates additional utility-scale inverterefficiency gains of approximately 1% over the next two to five years.In addition, he believes manufacturers must expand the voltage windowand range of operation for their inverters to accommodate large-scaleprojects.
Overall design improvements, new semiconductors andnew switching technologies have resulted in recent increases ininverter efficiency, says Arps. However, "As efficiency gains becomesmaller, increased attention will be given to uptime, system controland service," she predicts.
Solar plant control as a contributorto overall grid stability is likely to be placed under particularscrutiny 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 inverteris equipped with SunIQ, a monitoring and controls platform thatincludes tools to manage integration into the electric grid, saysMinesh Shah, renewable systems platform leader.
"A utility orgrid operator can send commands to the solar plant to specify themaximum output of the solar facility," he explains. "Upon receiving thepower output command, SunIQ coordinates the output of all the invertersof a solar power plant such that at the point of interconnection, theoutput of the facility does not exceed the request."
Throughoutthe company’s history in the wind turbine industry, Shah has observedthe evolution of both Federal Energy Regulatory Commission (FERC)requirements and sophisticated plant-level controls to properlyaccommodate the large amounts of wind energy that have entered the gridover the years.
For solar, current FERC requirements are similar to those of pre-2003 wind power, he says.
Toaddress future regulatory needs, PV Powered is currently using a U.S.Department of Energy and Sandia National Laboratories award to researchadvanced utility command and control tools, as well as systemintegration advancements, from the inverter perspective.
Meanwhile,given the immense amounts of production at stake in a utility-scale PVinstallation, inverter manufacturers strive to continuously increaseinverter uptime and ensure any in-field failures can be addressedimmediately.
"Inverters are the system component with the mostbuilt-in intelligence, so we are able to pinpoint problems when theyoccur," notes Seehuber, adding that inverter-identified issues may ormay not originate from the inverter itself.
The data-richinformation portals, mobile status updates and other modern systemmonitoring resources that are now commonly available for system ownersand operators often feature extensive inverter diagnosticscapabilities. GE’s SunIQ, for instance, includes a solar SCADA systemthat troubleshoots on an individual inverter level.
"Efficiencyis incredibly important, but so are reliability, productive lifetimeand time to repair, maintain and install," states Ruberti. "Each ofthese factors contributes to the lifetime cost per kilowatt of output."
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