It’s quite common to focus on power quality (PQ) events such as voltage sags, interruptions, and disturbances and then conclude that these events are causing electronic equipment to misoperate. It’s an understandable conclusion as these events can be easily recorded, dissected, and then properly mitigated on the system. However, there have been cases where these PQ events would occur with the same severity, and the facility’s equipment would either misoperate or unexpectedly continue to operate normally. Let’s take a look at one of those situations.
It was a hot summer this past year, and I imagine a lot of the solar panel owners were producing energy to subsidize the grid and receiving credits for doing so. Unfortunately, this was not the case for one residential solar customer. The prior year, their utility bill was in the thousands of dollars range. Their original inverter misoperated that year, which prompted a contractor to replace it with a new one. The failure cause of the original inverter was unknown; however, the newly installed inverter continued to randomly misoperate through the end of last year and into this year. There were other inverters of different types and sizes installed on the same electric utility distribution circuit in proximity to this residence, but they did not misoperate in tandem with this customer’s inverter. This supported the idea that the problem was localized only to this dwelling. The contractor eventually installed a power logger and was able to capture the moment when the inverter started to misoperate. What they found at the output of the inverter was that one of the phases appeared to float briefly and thus the inverter could not sync with the utility distribution system. This caused the inverter to shutdown randomly during operation. Upon further investigation, the contractor identified a loose connection in the fused AC disconnect between the inverter’s output and the electric utility’s revenue meter. After the contractor corrected this deficiency, no additional random misoperations occurred.
