Comment by mauvehaus | Hacker News Reader

Comment by mauvehaus | Hacker News Reader

Parent Full thread

De jure, I think they're pretty free to do with them what they want. De facto they leave the batteries pretty fully charged most of the time.

AIUI, they'd have to have a disconnect somewhere upstream of us to let us back feed the grid during an outage, otherwise they couldn't safely work on the lines. And they'd need a way to enable and disable back feeding remotely[0] to allow them to work on anything between that hypothetical disconnect and us.

At the moment, none of that exists, so only we benefit from the powerwalls. It's also 34kwh in the pair of batteries. That wouldn't add up to too many household-days if you started adding households. You'd have to do some fancy switching to not overwhelm the relatively few powerwalls in an outage.

For context, based on our usage, we're at about three days of backup just for us [1]. It doesn't take much imagination to come up with a scenario where that happens where we are. We are the very last pole.

[0] Which in turn requires connectivity. The cell network here is sparse, to put it mildly. We just got fiber, but that's hanging on the same poles as the power. The fail-safe mode for no grid and no connectivity is to not backfeed, but at that point, you're back where you are today.

[1] It's the fridge, mostly.

You don't need internet for everything... by definition these systems are connected to the grid by wires which can transmit a limited signal. Utilities in countries such as NZ have managed load for decades by turning hot water heaters in customers' homes off and on remotely with ripple control units which transmit a signal over the power line itself. [0]

[0] https://en.wikipedia.org/wiki/Zellweger_off-peak

This is correct, an added benefit is if the power poles are actually down then the communication over the powerlines would be down, but that’s fine as power couldn’t be sent over the lines anyway in that case. What I wonder about/have concerns in regards to, is as someone else mentioned the relays required to turn on and off backfeeding in terms of electrical Utility workers having to de-energize a line to perform work. I’m not too sure that these workers can entrust their lives to the reliability of various (multiple) relays to not sporadically switch back on once the line has been confirmed as de-energized (in other words depending on the relay not to malfunction and re-energized while the workers are working upon the line )

I assume there will be some types of tools to monitor / alarm the workers for exactly this scenario (ie something they can attach to the line while it is being worked on and if it becomes re-energized an alarm will sound).

Generally grid backfeed capability for these systems is predicated on the presence of a stable voltage and frequency already on the grid - this is both a safety measure and a practical necessity, as the inverter needs to synchronize with the grid frequency.

Just to add the terms: a "grid following" matches the grid's voltage/frequency, so it only works when connected to the grid. A "grid forming" system can set its voltage/frequency independently and can only work off-grid

It sounds like mauvehaus is talking about a hybrid system that has both grid following and grid forming capabilities: switching to grid-forming as it disconnects. But that means that you can't power your neighbors.

I wonder why it is that real world systems don't use a GPS receiver to obtain a 1 Hz signal that's synchronized to the GPS clock and then generate a local 50/60 Hz clock for grid synchronization.

I'm just an EE with no experience in real power systems, so this may be absurd to someone who knows how this stuff works.

I didn't take power systems either (EE that went CE), but even if your clock was accurate and the system was tuned wouldn't you still likely be out of phase? I don't remember enough about the whys (like, does the type of load modify the frequency? gone down the memory hole) but I remember a guest lecturer mentioning that the actual frequency frequently had small deviations from nominal, too

Yeah that's what the synchronization is for - the precise frequency doesn't matter except for consumer devices that use it as a clock signal. If the hot line is at 120V and you're a quarter cycle out of phase at 0V, all of that energy is just going to flow directly to your circuit and destroy something. Since the frequency represents the spinning of large turbines, a power plant going online out of phase is essentially slamming their turbines into a massive external force (things go boom).

If you use GPS-based synchronization you have a reference of when zero crossings should occur. This does not mean that you have to output exactly that waveform.

Once you have a reference you can use a control algorithm. like a PID, to adjust the phase of the grid that you are generating locally.

If you sense that the grid is running out of phase from what your GPS clock says is the true reference you can increase/decrease your power output a bit to increase/decrease its frequency and catch up with the phase error.

It would be analogous to a type 2 PLL. The 1 Hz GPS clock would be the reference clock, the 60 Hz grid would be the "PLL output" and the VCO would be the power turbine, or AC inverter.

Ok, potentially stupid question, but if you're already using the grid as a control signal what do you need the external clock for?

From the comment I replied to:

> Generally grid backfeed capability for these systems is predicated on the presence of a stable voltage and frequency already on the grid.

You are using the reference clock to know precisely how misaligned the grid is and you use the reference to make small corrections to the generator power or inverter phase to restore phase alignment.

If you have an external stable reference clock (GPS) you know what the grid *phase* should be and it should be easier for network of multiple small inverters to keep the grid stable and to bring it up by themselves if there's a large blackout.

Bless, thank you for helping me take that one full circle.

It doesn't help under normal operating conditions.

It takes roughly 16ms at the speed of light to cross the US. 16ms is also coincidentally one entire cycle at 60Hz. So, who should sync to GPS and at what phase?

In addition, when the grid begins to get overdrawn, the phase begins to slide due to the physical nature of generators.

Global sync doesn't really matter; local sync is what's important.

Back in the 90s, I interacted with a home intercom system that ran purely on the electric wires. Just plug in the devices into any electric wall sockets, and they just worked. Voice wasn't too bad either.

Question: when an actual power outage happens, does the powerwall backup take over immediately without loss of power to the household? Or if not how long does it take to switch the grid interlock? Just wondering if this would be suitable as a UPS for my computers.

It's entirely transparent. The app will notify after the power has been off for some amount of time to avoid nuisance notifications for flickers or brief outages.

Reply on news.ycombinator.com