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Comment by etimberg | Hacker News Reader

Yes, North America is not a single synchronous grid. All of it operates at 60Hz, but interconnections between the regions are done using either HVDC or VFTs because electrical signals from both regions would not be in-phase (i.e. have synchronized peaks)

https://en.wikipedia.org/wiki/Western_Interconnection has details on how the Western Interconnection works.

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If two interface points take different-length paths from the origin, they might be out of phase at the interfaces. If I'm doing the math right, a path 250 miles longer would be 90 degrees out of phase.

There are many factors that affect the phase angle. Path length is one, so is the impedance of the lines (a line with double the X will get the same phase offset in half the length). Other equipment will have phase offsets such as transformers where the phase offset is a function of the winding configuration. There are event specialized transformers just for this purpose https://en.wikipedia.org/wiki/Quadrature_booster

I wonder what the actual advantage of a synchronous grid is.

You can transfer power from one side to another without the extra hardware expense and losses that a HVDC connection imposes.

Non-synchronous interconnects are more expensive, inefficient, and complex than synchronous ones.

Historically cost. Building a non-synchronous grid is much harder because you need equipment that can correct for phase differences in your grid. This equipment used to be very large and very expensive, Mercury-arc valves are some of equipment that used to be used (they provide AC to DC conversion) [1].

But modern power electronics are so cheap and efficient that High Voltage DC transmission lines are becoming increasingly common, driven by the fact the High Voltage DC is more energy efficient that High Voltage AC (you can push more power down the same cable, with less energy lost to resistive loads because DC doesn't suffer from the skin effect [2]).

With more High Voltage DC transmissions lines it becomes easier to run non-synchronous grids, because the DC transmissions acts as natural breaks between synchronous grids. Running a non-synchronous grid tends to be a little easier because you no longer need to worry about strange phase variances caused by multiple transmission lines of different lengths injecting phase delays into your network.

[1]: https://en.wikipedia.org/wiki/Mercury-arc_valve [2]: https://en.wikipedia.org/wiki/Skin_effect

That's sort of what I was thinking. Seems easier to coordinate a HVDC interties than a synchronous grid. I also keep coming back to the thought that long east west and north south interties would be beneficial when you have large percentages of solar and wind.

You can provide frequency control in one part to keep another part stable. For example if you have a part of your grid with a high level of renewables (for example, South Australia), system stability can be provided by large intertia generators in other parts of the grid (in our example, coal power stations in Victoria).

Take away the interconnect, and the islanded part of the grid is no longer stable, requiring the renewables to be switched off and replaced with gas and diesel generators. This happened after a storm earlier this year:

https://www.afr.com/companies/energy/sa-to-remain-cut-off-fr...

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