As part of the solar industry’s attack on costs, we're seeing a push to improve inverter performance and quality.
“For solar, you have a DC input and you need to change that into AC power the grid can accept,” explained Intertek Global Business Leader Troy Hewitt. “It is the job of the inverter to do that.”
Wind turbines also require inverters, Hewitt said, but a turbine generator’s typically “wild” AC voltage and frequency must be rectified as DC before the inverter transforms it back to AC for the grid.
“Most of the pure-play inverter companies are in the solar market,” Hewitt said, while turbine manufacturers tend to make their inverters in-house. Intertek tests “everything from the single-panel, less-than-200-watt microinverters to multi-megawatt wind turbine inverters. They all have to comply with the same safety and grid connection standards.”
The bulk of Intertek’s inverter testing business is the basic tests for safety and grid connection, Hewitt said. “The U.S. safety standard is UL 1741. It references, for grid interoperability, the IEEE 1547 and 1547.1 standards. There are at least six safety tests. There are maybe twelve to fifteen grid connect tests.”
Grid connect tests “verify that the inverter will detect abnormal grid conditions and respond appropriately. Abnormal conditions can be things like high or low grid voltage, high or low grid frequency, or islanding, where there is a grid outage -- maybe a tree falls on a wire.”
An inverter, Hewitt said, “has to be able to detect abnormal grid conditions and shut off in the prescribed amount of time. That can be anywhere from two seconds to 160 milliseconds, depending on the severity of the abnormality.”
If the inverter fails to stop sending power into the grid during a grid outage, Hewitt said, it could endanger the repair person.
Intertek does two tests for high voltages, two for low voltages, two for high frequencies, two for low frequencies and, for each, there is a test to see if the inverter responds quickly enough. And there is a test to see if it responds quickly enough to islanding.
There are also tests for power quality and interferences.
“When you convert DC to AC, the wave form does not exactly match the grid wave form,” Hewitt explained of the inverter’s power quality function. “The inverter must transform the wave form harmonics to within 5 percent of the grid’s harmonics.”
Surge and radiated immunity tests “verify the inverter’s ability to cope with electromagnetic interference.”
The basic UL 1741 safety testing begins with a construction evaluation, Hewitt said. Next come “temperature tests, together with a maximum voltage determination, a dielectric test to determine the integrity of the unit after it’s heated to its maximum operating temperature, and verification that after the application of temperature, nothing gets hotter than it is supposed to, which could lead to shock or fire or breakage.”
There are also input and output tests to verify the inverter’s factory ratings.
Finally, there are “abnormal tests that could be destructive to the unit if it does not behave properly,” Hewitt said. It could burst into flames or throw sparks that cause a cloth placed over the unit to catch fire or explode or dangerously throw off a piece, Hewitt said. “Most of the equipment we test is pretty boring, but occasionally it gets exciting.”
Costs for the tests vary. Testing a microinverter is “in the $35,000 range” and takes two to three months if there are no surprises. Testing a multi-megawatt inverter, excluding extra fees that might be necessary for such a large unit, is “in the $90,000 to $100,000 range” and takes six to nine months.
“Inverter testing is not as far down the bankability road as module testing, but it is the same sort of thing: more and more severe tests,” Hewitt said.
Intertek does efficiency testing that goes beyond the basic standards requirements because it is often a prerequisite for incentives, Hewitt said. “And if a financier, developer or project owner has a power purchase agreement or a production-based incentive, they want to make sure the efficiency is there over the life of the unit.”
There is always some power loss to heat: “96 percent or 97 percent efficiency is good. Typical efficiencies are 91 percent to 95 percent. Anything under 90 percent, in a solar application, is bad, unless it is a unique configuration.”
To further measure bankability, Hewitt said, “we look at manufacturing quality control. We go through grid-connect testing in a more severe way. We may do 5,000 on-off cycles under different abnormal conditions.”
There are also more rigorous power quality and surge tests that are “more representative of what a unit will see over its lifetime,” Hewitt said.
Finally, a software check certifies, Hewitt said, “that, if a change to the software was made in the field, a change control process was followed and the inverter can remain in compliance with requirements.”
Bankability testing is “about $30,000 above and beyond that for the basic safety and grid connect testing,” Hewitt said.
This is the first in a new GTM series on inverter testing. It follows an ongoing series on module testing which includes: Testing and Ranking Solar Module Quality, What Do Solar Module Test Procedures Prove?, A More Realistic Take on Solar Module Testing, and The Solar Industry Module Testing Gold Standard.