Weekend learn: Elusive longevity

From pv magazine 11/2022.

When Jens Heinzler enters a photo voltaic farm, he typically hears a shrill whistling noise from a distance. “A horrible noise,” says the managing director of Sun2Energy, a service supplier for maintenance and technical management of photovoltaic methods. The noise comes from string inverters and whereas the precise defect isn’t at all times clear, Heinzler can inform that string inverters making this type of noise will quickly need to be replaced.

Such crops, whose operators have signed a upkeep contract with Heinzler, went on-line between 2006 and 2020. “Many are actually inflicting hassle,” he says, and it’s extra typically the inverters, slightly than the modules, which can be the supply of this hassle. For string inverters, a malfunction typically equals the tip of the system’s life. And operators ceaselessly want to switch all the inverters in a mission earlier than the interval of feed-in tariff ensures expires.

Heinzler and his workforce are usually not alone: inverters symbolize the Achilles heel of solar energy crops by way of malfunctions. A 2020 information survey by the National Renewable Energy Laboratory (NREL) exhibits that in the USA, about 43% of upkeep calls on PV methods are attributable to inverter malfunctions, and 35% of measured yield losses are attributable to inverter failures. In the identical research, modules are chargeable for solely 2% of upkeep calls and just one% of yield loss. With figures like these, the query arises as to the explanation inverters are such a weak level within the chain. Specialists looking for a solution to this query wish to develop higher predictive fashions and longer-lasting inverters.

Inverter lifetimes

A workforce from the Bern University of Applied Sciences evaluated inverter failure surveys from 1,280 PV methods with 2,151 inverters. Small non-public rooftop methods to massive ground-mounted methods with central inverters had been all represented within the Swiss research. After 15 years, half of the crops present inverters with their first yield-relevant errors.

Nevertheless, there are additionally vital variations between gadgets. Producer, set up web site and the design of the inverter all play an vital function in terms of longevity. For instance, inverters put in indoors noticed their probability of survival drop from 100% to 75% after 13 years. Units that had been put in exterior, largely unprotected from the climate, reached the identical survival likelihood after simply six to seven years. Nevertheless, research writer Christof Bucher additionally says that different correlations could have favored this very clear outcome.

What the scientists again up right here with numbers is in keeping with Heinzler’s observations; whistling inverters are sometimes put in in free-standing assemblies with gadgets totally exposed to the sun. “If the inverters will be mounted below the modules, they’re not less than considerably protected,” Heinzler says. That’s a bonus, he says, as a result of warmth clearly impacts the gadgets. Inverters are speculated to be protected by derating parameters that cut back inverter output in response to temperature limits. Throughout scorching summer time days, these are shortly reached, and inside parts endure higher stress.

Temperature change

“Getting old is favored by environmental components akin to intense warmth or excessive humidity,” says Todd Karin of the PV Evolution Labs (PVEL) testing laboratory in California. A very widespread defect in energy semiconductors is bond-wire lift-off, typically brought on by intense warmth. Semiconductors are soldered to a circuit board manufactured from a fabric sandwich of copper and ceramic. Throughout thermal biking, the supplies increase to totally different levels. The bonding wire to which the semiconductor is linked can not stand up to these actions and subsequently breaks. This results in quick circuits and arcing contained in the inverter.

The fixed change from scorching to chilly can even trigger the ceramics to start to delaminate, Karin continues. This results in impaired conduction of the semiconductor’s warmth and, in excessive circumstances, to quick circuits. Moreover, warmth can result in degradation of the silicone or ceramic insulators within the semiconductor socket. This leads to electrical breakdown with quick circuits, arcing, and hearth hazards.

Precisely how the environmental influences trigger degradation within the parts continues to be unclear and is being investigated, for instance, in Halle, on the Fraunhofer Institute for Microstructure of Materials and Systems, or IMWS. In a mission that started in 2020, a gaggle is working alongside inverter makers together with SMA, Electronicon, and Merz Schaltgeräte. They’ve supplied returned tools and damaged gadgets used below actual situations.

Degradation

The workforce is taking a better have a look at the faulty parts to know the causes and getting old mechanisms. As well as, the researchers are finding out working inverters which have been in use for a few years below real-world situations or have undergone accelerated reliability testing. To characterize the degradation mechanisms, they use strategies akin to non-destructive acoustic microscopy or lock-in thermography. “We get the parts from totally different climates despatched to us by the producers,” says Sandy Klengel, group chief for analysis of digital system integration at Fraunhofer IMWS.

Klengel and her colleagues match the collected getting old information with utilization parameters and data of environmental situations, asking questions akin to was it typically scorching on the web site? Or, was the inverter typically operating at partial load? In lots of circumstances, comparable information is recorded by the inverters on the identical time.

“On this means, conclusions will be drawn about totally different getting old mechanisms attributable to monumental warmth or excessive humidity,” says Klengel. The objective of the analysis mission is to develop higher predictive fashions for inverter degradation and reliability. Klengel is reluctant to make sweeping statements concerning the causes of part failures. The research haven’t but been accomplished and it’s too early to completely decipher the info.

Moisture-sensitive

However Klengel reveals this a lot: “Movie capacitors are significantly inclined to moisture,” noting vital variations between the producers of the capacitors right here as properly. As well as, producers are engaged on new designs for capacitor housings and encapsulation methods. However different parts are additionally affected by getting old mechanisms; chokes, circuit boards, energy semiconductors and all the pieces else in an inverter leads to the lab.

“That transistors, or IGBTs [insulated-gate bipolar transistors], have faults, is one thing we observe much less,” says Heinzler, who doesn’t level at one failure sample both, noting issues vary from repetitive self-shutdowns and energy loss to capacitor points. From his expertise, capacitors and communications tools are cited as the commonest points.

Usually, sure patterns will also be recognized based mostly on producers and years of manufacture, which helps with troubleshooting specific points. Batch numbers are additionally key: if the manufacturing dates of the identical inverter sort are a very long time aside, it might be that the producer makes use of totally different suppliers, and the error sample modifications. To be on the protected aspect when shopping for inverters or assessing the chance of installations, consumers ought to subsequently have a look at the inverters’ invoice of supplies, says Klengel of Fraunhofer IMWS. There are inverters in the marketplace with excessive reliability and a 20-year guarantee in addition to resistant capacitors, she provides.

A number of failures

String and central inverters have totally different failure modes. PVEL’s Karin experiences that string inverters have as much as 30 occasions as many parts as central inverters, vastly rising the susceptibility to failure. As well as, inverters with increased energy conversion are more and more shrunk in dimension. The calls for on string inverters are subsequently monumental, he says. “These days, you wish to set up string inverters with 150 or extra kilowatts of energy with two staff and with out an auxiliary crane,” says Jens Heinzler.

The issue with increased energy density is heat dissipation. Karin, subsequently, sees added worth in giving information analysts from a upkeep service supplier entry to numerous temperature sensors within the inverter. Just about all energy semiconductor modules would have such sensors in-built. However not all inverter producers be sure that finish prospects have entry to the info. This information may even be used to foretell the failure or borderline operation of an IGBT.

Design and structure

It’s not solely the producers’ design that impacts the service lifetime of inverters but in addition the way in which during which they had been put in. Heinzler explains an excessive case with a PV plant constructed with 270 put in inverters, every with a capability of simply 9 kW. Even with a failure price of simply 1%, he says, you could change tools a number of occasions a 12 months. Barely extra highly effective gadgets would vastly cut back the variety of parts.

Central inverters, alternatively, would have the longest life expectancy however would additionally require extra upkeep. For Heinzler, the explanations for the upper life expectancy lie in the truth that central inverters have subtle air flow methods and the inside is generously designed. Overheating is never an issue. However a protected set up web site additionally has a constructive impact on service life, says Heinzler. This safety is also supplied by a form of protecting constructing for string inverters.

One other side that results in an earlier finish of life for inverters, in response to each the Bern research and Heinzler’s expertise, outcomes from a widespread development towards value optimization. If the DC generator is ready to be overloading by 40% or 45%, he sees extra frequent malfunctions in operation than in methods the place the oversizing is just 10% to fifteen%. “If I at all times drive my automotive at 220 km/hour, it would break down quicker than if I at all times run at a cushty velocity of 120 km/hour,” Heinzler says.

Effectivity trade-off

Preventative measures for producers may see them dimensioning the ability semiconductors extra generously, which prevents bondwire lift-off, explains Karin. Nevertheless, it makes the gadgets each dearer and fewer environment friendly.

Many shortcomings originate from attempting to optimize mission prices. Heinzler talks about how cable runs will be reduce in half when mission designers use a Y connector to place two strings on one MPPT [maximum power point tracking] enter. However that isn’t with out issues, he says. In a single case he noticed, this set up methodology was deemed nice by the producer’s information sheet, but Heinzler says he discovered that the gadgets had been extra susceptible to failure. The particular yields of the models additionally different far more.

When Heinzler and his workforce eliminated the Y connectors, the susceptibility to interference decreased. His conclusion: “It’s a must to maintain a greater eye on the steadiness between development and working prices. Particularly as a result of anticipated working section of 20 to 30 years. If you wish to save a number of extra meters of cable right here and save a number of extra plugs there come hell or excessive water, you’ll find yourself paying the invoice.”

Anybody who contains the anticipated prices for restore or substitute of the inverters within the worth won’t construct one thing like this, says Heinzler. He appeals to a “sound electrical engineering sense” when planning the methods. This contains not an excessive amount of DC overloading and that inverters are taken out of direct daylight. This will increase the life expectancy of the inverters.

Life expectancy

Formulating an correct life expectancy for gadgets is comprehensible from a enterprise standpoint, however it’s not with out its issues, Karin says. His impression is that producers are gearing as much as produce parts with a life expectancy of 15 to twenty years. “Designing inverters means discovering a nice steadiness between a sturdy, dependable product and one that’s means too costly,” he says. “Due to this fact, the idea that an inverter lasts 10 to fifteen years turns into a self-fulfilling prophecy.” If everybody within the business makes the identical assumption, producers have little incentive to make their merchandise extra strong and sturdy.

However, producers have been complaining for years that initiatives are too fixated on capex and thus choose towards higher-quality parts. And but the business has moved on. Expertise marketing consultant DNV produced a white paper noting that 15 years in the past it was widespread to switch an inverter each 5 years. DNV estimates that the life expectancy of recent gadgets for ground-mounted methods is now simply over ten years, which is mirrored in guarantee intervals.

Steady enhancements are being made, and Klengel suspects that additional progress will be made in proving and guaranteeing the longevity of inverters. She notes that the vitality business is inserting increased calls for on proving the reliability of parts, which drives improvement. Klengel believes that competitively priced inverters with a life expectancy of 30 years, as is the case with photo voltaic modules, are technically possible.

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