From pv magazine USA
Things fail with age, whether they be cars, phones or inverters. Alencon, an optimizer manufacturing company out of Hatboro, Pennsylvania has published a white paper looking at the failure rate of central inverters used in aging 600 V solar plants.
The inverters in question are five to 10 years old and are beginning to fail. And not in isolated cases here and there, but with “a high degree of regularity,” according to the study.
Inverter failure is not the issue, as many project owners reserved cash during project development in anticipation of significant repairs down the line – inverters included. The process has been complicated by factors tied to the relative youth of the solar industry when these projects were planned and built.
Most paramount, 600 V inverters are not nearly as popular as they were five to 10 years ago and the majority of today’s major manufacturers no longer make them. Additionally, many companies that were leading manufacturers at the time do not exist anymore, meaning that direct hardware replacements are now impossible.
In replacement, string inverters are the logical choice, as they provide a similar power rating, but they lack the isolation transformers present in central inverters.
The sum of these factors leads to two hurdles in replacing old 600 V central inverters: voltage mismatches between old PV strings and new inverters, and grounding mismatches between old PV technology and the now transformerless string inverters.
Older PV components fail to generate enough voltage to turn on and maintain the operation of the 1,000 V or 1,500 V replacement candidates, and many of the projects require grounding. Again, innovation creating obsolescence, as string inverters these days are ungrounded or “floating.” These floating inverters can form the 480 V output needed to interact with the grid without the need of a grounded isolating transformer that older projects accounted for and required.
With these challenges in mind, Alencon developed two potential solutions that incorporate the use of the company’s String Power Optimizer and Transmitter (SPOT), a piece of hardware that uses galvanic isolation technology to optimize the existing technology to new inverters. SPOT uses an internal, high-frequency isolation transformer to convert power from DC to AC, back to DC. In a sense, SPOT takes the place of the isolating transformer and maps the voltage differential between the 600 V solar panels and the 1,000/1,500 V inverter.
The white paper offers one solution for SPOT installation between the existing PV strings and combiner boxes and one for installing SPOTs after the combiner box. Installing SPOT between the combiner box and the existing PV panels solves the aforementioned voltage mismatch and grounding challenges, while also allowing for string-level maximum power point tracking (MPPT). The downside to this solution is that, in some applications, it would require significant field rewiring.
Installing SPOT behind the combiner box trades string-level MPPT for array-level MPPT, which the company claims leads to faster installations and lower balance of system costs. This is the solution that Alencon sees as particularly advantageous for aging residential installations, where PV is installed on the roof and the inverters are on the ground.
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