Many of you are asking what is backfeed and why it is dangerous?
In normal grid operation it is not an issue, it is called export. A premise exports power onto the grid and gets paid for it. In an outage situation, the system islands and does not export. In both cases everyone is good.
Up until a couple of years ago backfeed was rare, 99+% of interconnections were grid following inverters that needed a grid signal to operate, and shut down when that signal went away. A few times there were reports in dense clouds of inverters of them staying active by seeing each other as the grid. But rare.
Sometimes you would see it from portable generators, but they too were rare.
Two of the fastest growing trends are low cost portable generators, and grid forming inverters. Properly installed and used both are fantastic. Improperly installed they are dangerous. The funerals from improper installation don't make even the local news and it takes work to find them, but they happen.
Backfeed happens during outages and it becomes dangerous when that energy goes backwards through a step down transformer and into the distribution primary. Now all you need is a cable laying on the ground and a good ground path. Unfortunately humans standing in a puddle make great ground paths.
Utilities with AMI systems have the basis of of finding and locking out ground paths if the meter can report during an outage (and many cannot).
In one 20,000 premise outage more than 30 backfeeds were found when the data was run by data scientists. Many local inspectors are not trained or sensitive to back feed potential in grid forming inverters and many installers also are not trained in proper installation. This is a gap that need to be closed before it becomes national news.
Yes the best of the grid forming inverters will prevent backfeed, if they are set right.
Yes, transfer trip switches cost money and need to be checked every 3 to 5 years for function.
We need to make sure that inspectors and installers are properly trained and that both generators and solar are properly installed. Utilities need to do the work to find backfeed. But of course in most locations, the only thing you can do to someone who does not have a properly installed system is to hit them with a fine for code violation. Now try finding that back up generator.
Oh, to make it easier, even though they are illegal in many states Amazon will sell you double male extension cords (suicide cords as they are called in the industry).
If you work on outages (and there will be many soon), don't assume it is safe, don't assume it is de-energized, even if you opened and tagged the disconnects - wear your PPE, yes, I know it is hot and sweaty and slows you down, but the life you save may be yours.
Doug Houseman
Expert Member
Top Contributor