From pv magazine Global
Researchers from the Bern University of Applied Sciences have conducted an online survey to investigate the “time to failure” (TTF) for residential inverters. They have found that 34.3% of the devices experienced their first failures after 15 years.
“I would say this failure rate is acceptable, even good,” researcher Christof Bucher told pv magazine. “One assumes the inverter must be replaced once in the lifetime of a PV system.”
The survey includes inverters from Fronius, Huawei, Kostal, SMA, SolarEdge, and Sputnik. However, the scientists said that the analyzed devices are not representative of the current PV market, as they were purchased many years ago.
“The oldest inverters investigated in this study are from the early 1990s,” they said.
The survey includes data from 1,195 PV systems with 2,121 inverters and 8,542 optimizers. The biggest share of the inverters used in these systems had power ratings between 10 kVA and 15 kVA.
“A large part of the data was collected with the help of people who know the history of the PV systems well,” the scientists explained, adding that incomplete, unanalyzable data records were deleted. “The survey gave respondents the opportunity to add their own comments to the technical data.”
In order to analyze the TTF, the research group used the Kaplan-Meier estimator, which is commonly used to estimate the survival function from figures that are censored, truncated, or have missing values. The survival curve shows which percentage of the population is still alive after how many years.
“The Kaplan-Meier estimator can be adjusted for different influencing factors, for example to the year of commissioning, the inverter manufacturer or the nominal power ratio,” the researchers explained.
The scientist said the survival curve considers how many inverters are still under observation in the corresponding year of operation. That could help assess when a certain percentage of the inverters will have their first expected malfunctions.
“However, because only little data is available for old inverters, the uncertainty in the survival curve increases with increasing inverter life,” said the scientists.
The survey showed that more than 65% of the inverters did not present yield-relevant faults by their 15th year of operation. The scientists said the TTF mainly depends on the manufacturer and a device’s location, with outdoor installations having shorter TTF rates. They said the overall reliability of an inverter is proportional to the reduction in the number of power electronic components. In inverters with power optimizers, the first fault often occurs earlier than in devices without optimizers, according to the scientists.
“The inverters and optimizers in this study represent a broad, albeit not representative, cross-section of the inverters and optimizers used in PV systems on buildings in Switzerland and Europe,” they explained.
When they first announced the survey in July, the researchers said the causes of the failures and defects had not been precisely recorded. They said at the time that the “the rule of thumb that inverters should run without faults for around 15 years” had been confirmed by their findings.
The research project will run until the end of 2025 and extended to other systems.
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