ASK THE EXPERTS: AC-Coupling Oddities

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The OutBack GTFX series is one of several AC-coupling-capable, battery-based inverters on the market.

I need your assistance in resolving an issue with my Enphase 215 microinverters. I have four of them installed in a mini-grid using an OutBack GTFX3048 inverter capable of AC coupling. One morning, the microinverters failed to “wake up.” I disconnected the PV cable to one of them, and when I reconnected it, they all started producing again. It happened exactly like this the next day—in the morning, there was no production until I turned off the AC switch briefly and restored it. What is happening?

Simon Edache • via homepower.com

This problem may have multiple causes and solutions. Without having access to the system and the ability to run tests, it isn’t easy to determine why your system is malfunctioning. One of the first steps would be confirming the lack of power production, versus a communication problem.

Another possibility could be an issue with the voltage that the microinverters are sensing and their inability to lock into the battery-based inverter’s output. If the AC output from the battery-based inverter is outside of the acceptable range for the micros, they will simply wait for a proper voltage before turning on. But it is curious that a hard reset on either the AC or DC side of the microinverters solves the issue, as that shouldn’t impact the battery-based inverter’s voltage.

Since you mentioned OutBack inverters capable of AC-coupling, I asked Phil Undercuffler at OutBack for some troubleshooting suggestions. He recommended verifying the wiring of the remote-operated circuit breaker, which is a control device specific to AC-coupled installations that lets the grid-direct inverters “see” the AC voltage required for operation. If this breaker isn’t wired properly, it could remain in the “off” position, even though it should be on.

It is necessary to make sure all of the system components are compatible for an AC-coupled system. The OutBack AC-coupling solution is designed to work with multiple grid-direct inverters. This helps manage battery charging without requiring you to wire your own ad-hoc relay system. In addition, some grid-direct inverter manufacturers restrict the use of their inverters in AC-coupled systems. This is less of a concern than it has been in the past, but still worth verifying with your grid-direct inverter manufacturer, who might also have clues as to why your microinverters aren’t waking up.

You also need to consider all the power sources present. If an engine generator is connected, you need to install controls that won’t allow the grid-direct inverter(s) to connect to the generator’s output.

Ryan Mayfield • Renewable Energy Associates

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