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Comment by davidbanham | Hacker News Reader

We can easily contact someone but they can’t do it remotely. The power company needs to come on scene. Isolating power is a lot more complex than it used to be because of solar installations. We can never be sure that every building’s solar is properly isolated from the grid.

In these cases where people need to be evacuated from a vehicle that’s live due to power lines there are protocols for doing so, it’s just more risky. We need to ensure they jump from the vehicle so they’re never touching the vehicle and the ground simultaneously. We also need to get them to shuffle to make sure there’s no step potential between their feet. Neither is easy to guarantee with a member of the public probably panicking.

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>We can never be sure that every building’s solar is properly isolated from the grid.

I believe it's part of requirement for solar installation to stop feeding power to the grid in case there is no power from the grid.

Yes, and this is why grid-tied solar installations go out when the power goes out. You need a battery backup for energy independence. This catches a lot of first-time solar owners by surprise if they don't do their research; they think that solar will mean they don't lose power if there's a daytime power outage, but unless they also get a battery they lose it anyway.

Would a battery really sove that though? If regulations say that anything feeding the grid should shut down if it detects an outage of incoming power (I'm still perplexed as to how you'd measure that on the same conductor, but it's not really my field), wouldn't a battery feeding the grid have to do the same. And if you have the cut-off "upstream" of your house (so your house can still get power from the battery, but the grid doesn't), then why couldn't you do that just with the panels and no battery? Obviously a battery would be better, but in terms of cutting power in an outage, I can't see a difference.

When the grid goes down the panels power the house if there's electricity demand, or the battery if there's not. The reason this doesn't work without the battery is that in any closed circuit, electricity production must equal electricity demand at all times. The grid can aggregate millions of households and then has peaking power plants to adjust electricity production to meet electricity consumption. Residential solar panels have no such mechanism - if it's the middle of a summer day and the only appliance you have plugged in is your computer, then that computer is getting your ~4kW of power and is gonna fry. Add a battery and any excess power instead goes to charge the battery.

When the battery is full, the grid is off, the house isn't drawing anything, and the panels are still producing, then the circuitry does some trickery to just disable the panels. (I think the inverters actually feed the power back 180 degrees out-of-phase so it cancels out the other power coming from the panels, but the details are beyond me.) Note that the panels are actually disabled at this point, though - they're effectively producing nothing, not just producing less. That's why this doesn't work without the battery - you can break a circuit entirely to stop power from flowing, but as long as the circuit exists without energy storage, power in must equal power out.

I imagine it's two separate inverters, a solar inverter that cannot function without grid power and a separate battery inverter. The two feeding the house circuit via a break-before-make transfer switch reduces the chances of accidentally back-feeding the grid because the isolation switch didn't work.

You may also be restricted from exporting back to grid at all if there is battery involved by some utilities.

It's part of the requirements, but that doesn't guarantee it will happen. Equipment can fail to operate correctly (or be misconfigured) and you still need to verify that the power is correctly isolated before doing work on potentially live conductors.

That's IEEE 1547.

There's a firewire/IEEE1394 joke in here somewhere.

We can never be sure that every building’s solar is properly isolated from the grid.

Is this a new problem? Standby generators seem pretty common in the SF Bay Area - you can tell when the power goes out by the sound of generators in the neighborhood. I'd think that there are at least as many sketchy generator installs as solar installs.

Generators isolate themselves from the grid (unless they're installed extremely irresponsibly.) Part of the point behind solar is that it feeds power back to the grid because the operators are the only ones with the infrastructure to deal with the excess.

Grid tie inverters do the same, no power company would allow them otherwise.

Generators don't isolate themselves, a code compliant generator will have a transfer switch that makes it impossible to backfeed the grid.

The problem with generators is that anyone can buy one for a few hundred dollars and hook it up to backfeed their house without a transfer switch because a transfer switch is expensive to install and "I'll never forget to turn off the utility breaker before I turn on the generator"

At least you need to get approval (and inspection?) from the power company before you can utilize net metering with solar, but anyone with basic electrical knowledge (or access to Youtube) and $20 can rig up a suicide cable to backfeed the grid from their generator.

While that's true most generators are only rated for a few KW and most circuits they're hooked up to the bad, backwards way are several hundred KW i.e. their local grid that has been disconnected. So they would trip their breakers immediately and go offline or else stall or melt.

Unless I'm missing something of course. Which is entirely possible.

If everything is working perfectly, then sure. But maybe that cheap 30A generator isn't running well enough to trip its 30A breaker and it's trying its best to backfeed 20A of current into the grid. Or maybe the other 5 neighbors on his pole transformer tripped over to generator or solar, so that one guy is able to power his pole transformer while the lineman is trying to work on it, or maybe he can't figure out why the breaker keeps tripping, so he keeps resetting it, only to have it trip again a second later, meanwhile he's sending surges of current into the grid.

Utility workers will normally ground de-energized conductors before they work on them, but they need to do it perfectly each time to protect themselves from backfeeding.

You can turn a generator off, and power in lines ends. Solar is different in that there is no way to safe a panel. If it’s receiving sunlight it’s generating power. That’s a pretty different paradigm then any other electrical system currently used on buildings.

Inverters should be equipped with automatic anti-islanding, which causes the inverter to shut off when it no longer detects power from the grid supplier (there are a few different ways to detect this). A lot of inverters built for grid-tie applications can't function without a mains connection anyway because of the way they track the frequency (unity power factor output).

There can be a danger to first responders on the DC side of the inverter, which may still have a high DC potential from the panels (string voltage can be 100vdc and up). Fortunately, code in a lot of areas (in general areas that have adopted a 2014 or newer NEC) requires a "DC Rapid Shutdown" feature on new solar installations. In these systems there is a small module on each panel that detects the loss of a signal from the inverter and disconnects the panel from the string, eliminating the shock hazard from the panel cabling. This way the panels just get hot instead of producing power. That's only going to be on newer installations but at least NEC requires a big "DC Rapid Shutdown" sticker on the inverter if so. These systems can also be useful to avoid "cold start" overvoltage problems.

Beside automatic disconnects, every solar installation I've seen (Boston area) has a readily accessible grid disconnect colocated with the inverter on the side of the house. As in, I could walk up to anyone's house and disconnect their solar from the grid with no special tools. (I know they all have automatic disconnect as well, because by and large the majority are the same brand -- SolarWave. Not sure about microinverters though.)

My understanding is that inverters are required (by electrical code among other things) to disconnect from the grid when grid power is lost. Malfunctioning (or illegally installed) inverters are possible but shouldn’t be a common problem.

Yes, but in firefighting, a lot of the time we are up on the roof, cutting holes in the attic to let heat out of the building. The invertor will shutdown power to the inside of the building, but the line from the solar panel to the invertor is still hot. That line can be cut through as in a lot of installs it can be run through the attic. This line is never discharged but is always hot. As well as all of the lines between the panels. These can ground out in the building and cause electrical charge in objects that would be 'off' if it was a traditional power hookup that all power to the building can be shutoff from cutting the power grid building service. If one of those lines is cut, and a firefighter sticks a pike pole into the attic and touches that wire it could electrocute him just as readily as if the building was still energized by the power grid.

I wonder how effective grid isolation/'anti islanding protection' is, when the 'island' is big enough, and contains multiple grid tied solar installations.

It probably comes down to how they detect a grid outage.

But I can think of some scenarios where the solar inverters can see each other on the grid, and assume the grid is still up

I think they usually look at the frequency and magnitude of the voltage, if it is outside of say 57.5 to 62 Hz or 80-120% of nominal voltage the inverter probably decides it is no longer connected to the grid.

It is possible, but extremely unlikely, that a distributed generation island gets lucky that there is enough generation to meet the load on the island and it can respond fast enough to keep frequency and voltage within range when the island forms.

Dumb question, but how do you measure the voltage on a conductor if you're also putting out that same voltage on that same conductor? For the frequency I can imagine you could probably detect a frequency shift with an independent oscillator, but even then, since the inverter's sine wave is generated electronically, I can't imagine it would follow the usual laws that rotary generators do (frequency changes when supply and load are mismatched).

(I'm a software engineer with no formal EE education, so "it's complicated, go read a book first" is also an acceptable answer)

It is a good question and I don’t design inverters so I don’t really know.

I imagine an inverter with one solar panel behind it can’t muster enough oomph to back feed a sizable chunk of distribution network - if it tried it would not be able to maintain nominal voltage. Say the load of the neighborhood is 500 kW, that’s 1200 A at 240V, which the inverter can’t supply unless there is 500 Kw of solar panels behind it. The 500kw load at 240V looks like a 0.2 ohm resistor to the inverter - essentially a short. Actually looking at the impedance of the network is one technique protection relays use to detect line faults.

Maybe the inverter can’t sustain the nominal voltage when feeding essentially a short so the voltage will be less then nominal and then it knows it is operating outside or normal conditions.

I can’t imagine a scenario in which the inverter could maintain nominal voltage at its terminals with a 0.2 ohm load looking out, if there is 240V across 0.2 ohm then there is 1200A through the 0.2 ohm.

In summary despite putting out voltage it will not be able to maintain the desired terminal voltage.

For frequency I understand most grid tie inverters somehow follow the grid frequency as opposed to generating their own.

I'm surprised fire departments don't have equipment for emergency grounding of power lines, e.g. something that creates a ground connection and some form of launcher that throws a steel cable and/or mesh across the line.

I know this exists for subway systems with a third rail, I'll edit if I find the video.

Or insulating mats for evacuation, potentially with steel conductors underneath them to avoid step potential.

> We can never be sure that every building’s solar is properly isolated from the grid.

This sounds like a risk that needs to be balanced against other risks. If the fire on that bus had got to the point that it was clearly going to burn the passengers alive, it surely would have been better to allow them to walk over a possibly-electrified area? That's assuming that the responders wouldn't have needed to walk over it though.

Yep, absolutely. Chances are if there's a going fire there won't be any isolation at all before the occupants need to be moved. In that case you try and extract them from the vehicle without letting them be shocked. Never touch the car and the ground at the same time. Shuffle away from the incident rather than walking to limit step potential.

Presumably it doesn't work to just short the power cables together and let the overcurrent protection trigger?

I had no idea. Are there more protocols for live vehicles? Just so I know what to do!

Ideally, stay in your vehicle and wait for help to disable the power. The vehicle's frame will protect you as long as you stay inside the car.

If it's necessary to leave the vehicle (e.g. due to fire), you have to avoid being in contact with the vehicle and the ground at the same time. You have to do a funny looking hop into the air.

Source: Corny energy company safety video https://youtu.be/fLVzvMTgGDY?t=168

That video has higher production value than most TV these days!

I'm pretty sure these videos are produced to he corny so that they're memorable.

thanks!

Very interesting. I hadn't considered the challenges solar could introduce to isolating power. Thanks for the answer.

There's a similar problem with people using male-to-male cords (suicide cords) on their generators too? Plug a portable generator into your house this way and now it's pumping out electricity and not purely consuming it.

It should be obvious that before you do so you should turn off the main disconnect. Otherwise you're electrifying the line for utility workers and also powering (half of) your neighbor's house backwards through a 20A circuit from your little generator, which won't work out well...

It should be obvious, but I suspect it isn't too many people... common sense isn't common.

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