Comment by anonymousiam | Hacker News Reader

Comment by anonymousiam | Hacker News Reader

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It's a long story. That solar plant was basically a UPS because the neighborhood where I bought a house had terrible power, and yes, they were just for the server. The root cause of the power problem was a 50-year old 4kV "last mile" which was finally upgraded to 16kV a few years before I took down the system. Power there is great right now, but maybe not for much longer because California keeps shutting down power plants. The NG plant less than a mile away will be gone next year. San Onofre is gone now too (https://www.sandiegouniontribune.com/business/story/2022-08-...). I hope to sell the place in CA soon.

I now have another house elsewhere with 4kW of solar on the roof. I would add more, but for two reasons:

1) There might be enough space up on the roof for another 2-3kW, but it's close.

2) The local laws governing grid-tie are such that I can't just add more panels. I can put up another separate system, but with a separate meter. I'm concerned that two separate systems on the same grid point trying to sync to the grid may have problems where they interfere with each other. I have read about such problems.

Regarding point #2) Absolutely yes that can be (and is) a problem for exactly the reason you say in terms of synchronising 2 (or more) PV setups to the local grid and (I would imagine) have the facility for emitting volumous clouds of magic smoke.

Fortunately the solution would appear to be use of a GPS central time clock that is fed to the 2x PV arrays individualy... if you run your own local time server you can 'skew' the clock signals to each array to get them in sync with each other and then in to a dummy load of spare lead-acids or a capacitor bank (if you want to get fancy and don't mind the 'large gray box' emitting smoke and launching itself off the wall from time to time).

Then you sync your 'mini-grid' to their slightly larger mini-grid.

Oh and when synching - never forget the old-school 4-light-bulb trick as a visual confirmation that things are synched correctly before tying in to the grid. Oh and fuses, large ones. With spares.

If you can find a friendly H.V. engineer or Linesman they might be able to tell you what clock source 'their grid' uses (GPS, Network time etc etc) which makes synching to the local-ish grid a whole lot easier if you have the exact same setup. I think the main thing is to make sure you have the dummy-load ready to feed/smooth what can be at times be a large inrush/outrush till the load equalizes.

Obviously, always seek out advice from someone that knows what they are talking about and has knolwedge of local laws, quirks of the grid etc.

Thanks for the reply. I got a few years of experience doing power systems in 1980, which came in handy when I did the CA PV system in 2003. (My embedded/web development skills also came in handy when designing/deploying the solar M&C subsystem, because no such products existed at the time.)

I'm somewhat skeptical that GPS synchronization is the real answer here. The problem is synchronizing the phase of the PV system to the grid so they can work together to power the load, allowing excess PV power to be sold back into the grid when not needed locally. To synchronize, one needs to control the phase of the PV system so it always slightly leads the phase of the grid. Determining the phase of the grid is the challenge. With two or more locally grid-tied PV systems, you cannot know the phase of the grid with much certainty because each PV's take on the grid phase is the aggregate of the actual grid phase and the phase of the other PV systems. Positive feedback and phase oscillation are likely.

Using GPS to synchronize with the grid assumes that the grid's phase is stable, but my observations over the past few years have revealed that today's grid has much less short-term stability than the grid of old. I'm assuming that the lower phase stability today is caused by the recent massive adoption of grid-tied PV arrays. So instead of using GPS time to guess the current grid phase, you really need to know what it is (locally). Aside from placing grid phase monitors on nearby houses, I don't know how to do this.

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