Page 28 - North American Clean Energy March April 2015

solar energy

Choosing a Central Inverter

Utility-scale solar considerations

By Louis Lambruschi

Image 1. Image 2.
Image 3. Image 4.

WITH THE EVER-GROWING NUMBER OF CHOICES of central inverters in the solar energy Image 1. A typical two-megawatt (MW), 1000-volt, outdoor-rated central inverter
industry, it’s important to recognize there are diferences between the players—some Image 2. Networked busbar temperature sensors allow remote monitoring of critical connections
more subtle than others. What may seem like an attractive initial purchase price, for Image 3. Inverter rack showing liquid-cooled, ield-replaceable phase power modules

instance, can prove false economy when analyzing an inverter’s long-term performance Image 4. Replaceable inverter phase power module
and features.
Typically designed for utility-scale solar ields, a central inverter can operate at higher
voltage levels and handle up to two megawatts (MW) of power. All inverters perform the

basic function of converting direct current (DC) to alternating current (AC), but some
provide additional beneits by virtue of their hardware and their ability to be programmed.
to the factory” a non-option). Since service will be performed outdoors in unpredictable
weather conditions, the process should be quick and relatively easy.
Design details
Even though many inverters make the claim of having a modular design, it’s worth

Most central inverters are engineered for direct, outdoor mounting. he ideal scenario is to considering just how modular they are. If a module requires heavy equipment to change,
truck in the unit, mount it to a pad, and connect it to a transformer; though sometimes it’s along with removal of extensive wiring harnesses, it might not lend itself to a very quick
skid-mounted with a transformer. Either way, the inverter will often be subject to extreme change after all.
weather and environmental conditions over its lifetime.
Evaluate the mean time to replace (MTTR) for all critical components, including IGBT

Some designs include forced-air ventilation, using ilters and hoods to clean the air before modules, inductors, capacitors, and fuses. Also, consider what level of support is required
it’s used to cool internal components. his can result in a lower initial cost, but ongoing to replace parts. Ideally, they should be installable by site maintenance personnel without
maintenance is the trade-of. Air-breathing units will have ilters to change periodically. requiring a service call to the manufacturer. All of these points should weigh into a central

Plus, water/glycol systems come with their own requirements.
inverter decision, as they factor into the total cost of ownership (TCO).
Compressor-based air conditioning allows for the more sensitive electronics to be sealed
from the environment, but it carries a relatively high rate of power consumption. Another Conclusion
inverter option comprises of a sealed design, using refrigerant in an evaporative cooling As mentioned, the basic function of any solar inverter is to convert DC from the solar

(two-phase) coniguration. his option provides for a high degree of internal cleanliness, as panels to AC for connection to the grid. “Smarter” inverters will do this, but can ofer
usually only the low-maintenance condenser assembly is exposed to the elements. Without additional beneits related to grid quality, such as fault tolerance and grid support.
a compressor, the evaporative cooling system tends to use less energy to run, employing a Firmware and programmability in smart inverters can allow for lexibility of control
small, low-low rate pump.
and operating autonomy. If they contain the algorithms for real and reactive power

Yet another factor to consider is the eiciency of the cooling system. When evaluating management, they can also eliminate or reduce the responsibility of external site
an outdoor inverter package, consider what percentage of the inverter’s throughput will be management by the utility. Be sure to consider the inverter’s tolerance to grid disturbances,
consumed by blowers, air-conditioners, or pumps.
as well as its course of action when a disturbance occurs. At minimum, it should include
low-voltage ride-through (LVRT), high-voltage ride-through (HVRT), frequency ride-

Maintenance considerations
through (FRT), and should be able to provide volt-amp reactive (VAR) support during these
When it comes to maintenance issues, users tend to agree that less is more. Any aspect of events.
maintenance that can be eliminated, or at least automated, is desirable. All major central One last item to bear in mind is the level of long-term support from the manufacturer. A
inverter components should be easily accessible from the outside, rather than requiring central inverter is a big investment, and with a life expectancy measured in decades future

technician entry into the enclosure. his not only simpliies the maintenance process, but support is critical. With the playing ield consisting of manufacturers with a wide range
also makes it safer for the technician.
of longevity and experience, it’s worth checking into a company’s background, inancial
It’s also important to enquire about the level of personal protective equipment (PPE) stability, and capabilities. Consider the depth of engineering support, local sales oices, or
requirements for individuals who will be working on it. Although infrared (IR) camera authorized distributors, and the experience of ield service teams.

viewing ports can be useful for onsite troubleshooting, internal temperature monitors that
are strategically mounted inside where the critical electrical connections are made, can
Louis Lambruschi is the marketing services and E-business manager for the Energy Grid Tie
save time and money. hese monitors are always on the alert for anomalies, which can be Division of Parker Hanniin Corporation.
transmitted through the inverter’s diagnostic system and remotely reported to an operator.

Servicing a solar inverter is another topic to research before committing to a speciic Parker Hanniin Corporation | Energy Grid Tie Division
design. Obviously, an inverter should be designed with ield service in mind (and “return
www.parker.com/gridtie

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MARCH/APRIL 2015