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.
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
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.
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
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.
(I'm a software engineer with no formal EE education, so "it's complicated, go read a book first" is also an acceptable answer)
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.
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.
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.
Unless I'm missing something of course. Which is entirely possible.
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.
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.
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 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.
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.
This incident involved high voltage lines which are a different problem. Taking these down requires a bucket truck and specialized tools and training. Firefighters have a lot of training but most of us stay away from electricity. Not to mention they would essentially have to have a specially trained crew that is trained and educated regularly for what are once in a decade type incidents.
Scenes like this are an extremely rare situation, more then likely why it was highlighted in this group. Firefighters enjoy looking at cases like this and thinking about how they would deal with it in their jurisdiction.