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Comment by Trisell | Hacker News Reader

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.

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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.

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