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Comment by zanderwohl | Hacker News Reader

This may be a stupid question, but I've always heard to never plug a generator into an outlet - what makes these safe enough that you can?

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The biggest problem with plugging a generator into an outlet is you need a double ended cord. If you use everything correctly they work fine, but there are too many ways things can go wrong. The most obvious one is you plug everything in, start the generator, and the trip over the cord on you way back inside, thus sending the live male end flying toward you. Once in a while a kid will see these dual male plugs and plug them in - lots of ways things can go wrong here.

There is also the problem of backfeeding, but this is more theoretical - even if you know to limit your power use, your neighbors won't and so your generator will trip its own breaker. Linemen are taught to either treat power lines like they are live, or to short them to ground - it can happen they don't do this and you kill a linemen, but still not as big a worry as it is made out.

They make generator "inlets" that sidestep all these problems when used correctly. Most common I've seen is a block so you can't have the inlet and the main breaker both on at once.

> There is also the problem of backfeeding, but this is more theoretical

it is not theoretical from the POV of power company employees working on their lines. the power goes out, they go to fix the equipment or wiring, and if the inverters connected to the grid have not shutdown, there could be 10s of kW to even MWs being backfed into the system they are working on.

that's why in the US at least, grid-tied inverters are required to have protection against this. if they do not see power from the grid, they shutdown (and rather quickly).

Yep, and they see the power by trying to increase/decrease the frequency of their own output and sensing the resistance from the grid.

Hang on. You didn't address the reason they gave in the half sentence your quote skipped. How are those backfeeders going to keep going when there's way way more load than they can possibly supply?

> if they do not see power from the grid, they shutdown

I realize this is a complete noob question but I'm still curious: with enough "backfeeders" could it not in theory be possible that the backeeders see each other's output and keep operating?

Say, a few houses at the end of the line, each with very little electricity consumption (no-one's at home), but with panels and inverters going full tilt, something like that?

Apparently the anti-islanding features in the inverters prevent this kind of situation. They need several minutes of clean power to start back up, and do other checks.

Yes, this is a concern with some older inverter designs. They use phase-locked loops which are meant to lock to the grid frequency but if you and your neighbours on the feeder are generating and consuming a roughly equal amount of energy and the feeder loses its connection to the wider grid, the loops can lock to each other and form a stable island. How long this would last in practice is not really known, it would probably be rare for it to happen at all and even rarer to stay stable for more than a few minutes but it has been demonstrated so it could have become an issue. Newer inverter have specific anti-islanding protection to detect situations like this and disconnect if it happens.

(the specific case of very little consumption but panels and inverters going full tilt doesn't work as this would lead to the frequency rapidly increasing, exceeding inverter set points and the local grid collapsing with all the inverter tripping off. The danger scenario is closely matched load and generation, like a real grid)

Thanks, anti-islanding protection must be a fascinating topic. You could look for voltage, frequency, phase anomalies but you also have to balance between being safe and having too many false positives. I'm not sure I'd want to be a line worker knowing there may be many inverters, from various manufacturers and with varying levels of quality, in every direction.

Perhaps a more robust solution would be for the power company to install remote-switchable circuit breakers at various places of the grid. Use some sort of wireless IoT protocol to listen for "on/off" signals and to report back "I'm now on / I'm now off!". Then the power company can disconnect parts of the network by themselves, without trusting that every inverter, generator and battery will do the right thing.

Again, all linemen are taught to short the wires or they are alive. All grids have regular places where two different lines meet and there is a switch at those points where in the worst case is they can cut the down line close a switch that normally is open and they can back feed a line from a different one and get power to people who are on each side of the break and then they can come back later to fix the break. These switches are miles apart, which means that you cannot be sure while you're working on the line someone else didn't go to that switch and change it.

Also, people have been back peeking the grid from their home generators for decades by now. Solar and inverters are a different variation of that attack, but it is nothing new. Any utility will have procedures because they can't guarantee that's not happening and we learned years ago.

I'm not saying it never happens, only that correct procedure already makes it not an issue

The inverter box contains circuitry that cuts power output within some specified number of milliseconds if unplugged rendering the prongs "cold".

Thanks. I would have asked Claude or something but I know so little about this topic that I wouldn't have been skeptical enough of its answer.

It's not so much being unplugged as it is sensing whether the mains power is down. If it is, they stop backfeeding immediately, just like hardwired solar inverters do.

If this weren't done, there would be risk of electricution to anyone working on wiring in the house or on the utility distribution network.

I commented a little too quickly and left the nuance out. Your answer is much better!

They have to sync up and feed power in phase with the grid when its up as well, in the case of net metering setups.

Even without net metering.

Yes, the "balcony" style setups, sync to the power outlet of the house, which could be the grid, or it could augment a generator when the grid is down.

It's also important for the male plug prongs to not be live when unplugged.

But how do they do it technically? Some countries require anti-islanding devices for grid connected solar, these are around $1000. That alone is much more expensive than the $300 plug in panels.

The inverter can sniff the grid power at frequent intervals to determine its voltage and phase.

It outputs at the same phase and frequency as the grid, and with a slightly higher voltage (think of a ball rolling down a slope from a higher voltage to lower voltage).

If the grid is down then it stops the inverter for a minimum safety period (some minutes iirc)

And the panels synchronize to the grid's frequency; a generator doesn't have any logic to synchronize to the grid's frequency.

Skip to about 10 minutes into this video: https://practical.engineering/blog/2022/12/5/what-is-a-black...

Furthermore, the danger of backfeeding is if you do it in a power outage and you start to power your neighbors' houses. It'll overload your generator and create a hazardous situation for anyone working on the wires. (This is why you're supposed to install either a generator switch or interlock. I have one that only allows me to energize when my main breaker is turned off.)

Fancy power electronics in them that make sure they only send power to the outlet while the grid is energizing it too.

Generators are typically grid-forming. They can (and will) be out of phase with the grid power, and will backfeed and create hazardous conditions upstream if the grid is out (and your main breaker is still on.) To use a generator safely you need a transfer switch (or similar) to make sure only one source of power can be active at a time.

Solar inverters (esp for this use, but also in general) are grid following; they'll use the existing frequency and support it, and if the grid goes off they'll go off too and do no power-off backfeeding.

The inverters are thus broadly safer to be plugged in to systems that are otherwise unprepared.

Generators could be made grid-following (many are now inverter-based), but for most users that would be silly, since the supplemental power would be more expensive (and louder!) than from the grid.

In addition to the electronics that others mentioned, they also have limited amperage. That way even if you are pulling from your mains power and the solar at the same time, your circuit hardware shouldn't melt or catch fire.

Unless you have five of them. Which people will absolutely foolishly do.

Generators are synchronous machines which means they can produce a useful reference frequency without any external source as long as they are spinning at a reasonably correct RPM. This RPM is easy to enforce with mechanical means. This independent operation is also what makes them so dangerous. There is no control signal from the grid back to the generator.

It can be quite safe to plug a generator into an outlet if you take precautions to properly island the system. I had an interlock on my breaker panel that would let me turn on a 50A backfeed breaker if my main breaker was switched off.

These sync with the grid and also have safety mechanisms to avoid backfeeding if the grid is down (or it's unplugged).

The back feeding problem is actually identical for plug-in and regular inverters. When the grid goes down, they keep generating power, creating a disconnected 'island'. All inverters are designed to be incredibly unstable in this mode, quickly desynchronizing from each other.

UL 3700

tl;dr Plug-in solar inverters must power off within a second if they stop detecting the grid's sine wave.

Generators have no such protection and will happily backfeed through the transformer outside your house to energize power lines. Line workers are careful, but there have been deaths. After hurricanes seems the most common based on a quick search.

So it's not a stupid question, indeed you should never plug a generator into an outlet. The double-ended "suicide cord" which allows you to do that has that name for a reason. (It can be fine to plug a generator into an inlet circuit, if the electrical panel has a proper interlock that physically forces disconnect from the utility before connecting the generator inlet.)

Yep, if you're gonna redneck the generator after a hurricane, you'd wanna disconnect your house from the grid. One trick in more rural areas is to have your power meter hooked up to one panel, and then your house panel be a "sub panel" to this main panel. Now you can shutdown your main panel and still redneck your house.

The better way (and legal way) to do this is to have a generator plug plug installed, this way when you switch over to generator your property is disconnected from the grid.

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