Photo courtesy of Ingeteam
Utility-scale solar, which commonly uses central inverters, is a whole different ballgame from residential and commercial solar. Central inverters face different demands from those facing string and microinverters.
“Commercial-grade equipment is often applied to utility-scale installations, yet these solutions are not designed for the requirements of these installations,” says Kleber Facchini, product manager at Eaton.
Ryuta Ray Saka of TMEIC says that residential and commercial-scale inverter manufacturers are challenged with a different set of expectations than utility-scale inverter manufacturers.
“As the name indicates, micro-inverters are evaluated for the performance of ‘micro’ power management including the effects of shading losses, module thermal mismatch and module soiling losses,” Saka says. “Central inverters are challenged with “macro” or large-scale power management features, such as plant-level reactive power response and dynamic power curtailment.”
There are several points that central inverter manufacturers have to keep in mind, according to Josephine Tsen, product manager of utility-scale and large commercial segments at Schneider Electric’s Solar Business. These include reliability, number of MPPT inputs and serviceability.
“A central inverter must be reliable because if it goes down, a large portion of the plant’s power generation is down, too,” Tsen says. “If a central inverter is down, having a strong service network and ability to fix it quickly is crucial.
“Also, string and microinverters have many MPPT inputs for higher MPPT efficiency, while central inverters mitigate this disadvantage with innovative MPPT algorithms and a high conversion efficiency,” she continues. “Lastly, the sheer size of central inverters makes them difficult to swap out, so manufacturers always have to keep serviceability in mind in their designs.”
Higher penetration of solar in the United States is creating new challenges for the industry and central inverters, according to Mark Goodreau of Solectria Renewables.
“Utilities are calling for smart inverters that can improve grid stability and help mitigate power quality issues caused by high penetration of PV systems on distribution feeders,” Goodreau says. “Adding smart inverter capabilities, such as voltage regulation and fault ride-through, will free up the grid for more PV installations and allow the U.S. market to continue to grow.”
Another challenge as solar grows and plant sizes increase, according to Peter Gerhardinger of Nextronex, is increased scrutiny on the affect inverters will have on the distribution lines.
“These studies add cost and time to the interconnect-agreement process,” Gerhardinger says. “It may be interesting to develop a generic solar inverter model that could be used by all manufacturers.”
Lou Lambruschi, marketing services and E-Business manager at Parker Hannifin, sees a challenge with increasing options on the market.
“With the growing number of inverter brands available, customers are more carefully examining the value offered in a design and considering the long-term cost of ownership rather than a low initial purchase price,” he says.
Because utility-scale solar plants are playing a more important role toward grid stability, Juan Luis Agorreta of Ingeteam’s R&D Solar PV Division says that central inverters are critical.
“They need to be much more reliable because they deal with greater power flows, so an equipment failure would mean a higher amount of energy wasted,” he says.
Reliability can be measured by mean time before failure (MTBF), but more importantly, as Parker Hannifin’s Lambruschi points out, there is the question of mean time to repair (MTTR).
“While a component failure is a relatively rare event, if it takes hours or days of downtime to repair, productivity will suffer,” Lambruschi says. “It’s advantageous to use a design with as much modularity as possible, allowing repairs to be done rapidly and efficiently.”
Ingeteam’s Agorreta says that international standards are useful in unifying criteria that may apply in different countries.
“For this reason, we believe that it would be very helpful to develop a kind of international reliability standard,” he says.