I think the dominant question here is, "I have this machinery that produces electricity, and it's hooked up to the grid. Why can't I also hook it up to the ground, and -- during periods of negative prices -- send the electricity into the ground instead of the grid?" Nobody charges you to ground your wires.

The ground is a poor load and also there's a finite amount of energy you can dump into it because every watt that goes in gets turned into heat, you'd liquify the ground very quickly at the multi MW scale.

Well, I don't know. Let's think about how you would turn off solar panels. I remember hearing about solar panels (~10 years ago?) that would actually break under very-sunny-but-zero-load conditions, so I did some research using queries like 'solar panels without load' and am led to think that this isn't usually a huge problem (with regular/modern panels?). But physically speaking, unless you cover the solar panels with a blanket, the amount of incoming sunlight won't decrease, and when you aren't turning that into electric power, it'll turn into some more heat.

If your solar panels are on an open circuit with no way to generate electric power, I guess the amount of heat produced in the panels would be roughly equivalent to the heat produced by other dark objects -- which may or may not be too hot. If you keep turning the sunlight into power to route some of that energy into the ground, that may be a lot if you're doing that in a single spot, but I don't think it would be impossible to come up with a workable grounding system.

That said, I'd be very interested to hear how a large solar farm like this turns off their panels.

Likely if they need to most of them can turn the panels away from maximum sunlight and that would take care of the turning off/full light no load problems. Same with the molten salt tower systems, just just the aim on the mirrors.

We routinely dump lightning bolts right into the ground and they go well beyond multiple megawatts. They're very brief, but if lightning took ten seconds to strike, it would still provide several dozen megawatts of power. How quickly would we be melting the ground at the multi-MW scale? How many lightning bolts would it take before the ground around my house melted and flowed downhill?

Lightning bolts do in fact melt the ground (see fulgurite), and they take at most about 0.2 seconds.

Yes very localized and for a very short amount of time and most of the energy radiated away into the surroundings, including the atmosphere

At that point, isn't it cheaper, safer and easier to turn off the machine? The machine being on has no value, only producing energy being consumed on the grid has value.

The value of the machine being on is precisely that you avoid the costs of turning it off and then turning it back on. Those can be considerable, depending on what machine you're talking about.

How does excess electricity impact the frequency?

It's not really excess electricity, but excess energy, and it's an artefact of how traditional power generation works: You generate electricity by creating rotation and using that rotation to drive a generator. Now, it's basic physics (conservation of energy) that if you add energy into a rotating system, it will spin faster and faster with every bit of energy you add, because the energy is transformed into kinetic energy of the rotation. The only way to prevent that speeding up of the rotation is to take energy out of the system--which, in the case of an electric generator you would usually do by connecting it to an electric load, i.e., the grid. But if demand is too low, you are removing energy from the rotation at a lower rate than it is added back from the mechanical drive side, so the net energy in the rotation increases and thus the speed of the rotation increases and thus the frequency of the electricity output increases.

Essentially, electric load is what brakes electric generators, if you take off the brakes but keep the motor running, they'll speed up.

My guess is that load acts as an inertial mass that adds drag to the turbines. Think of cycling downhill at the same power used when going uphill: you’ll spin like a crazy fool

I thought voltage needs to increase because a differential is required to add power to the network.

Hmm has anyone experimented with variable frequency transmission and solid state frequency synthesizing power transformers?

So you might be able to increase all the turbines from 50 to 70. That will absorb the excess power for a moment. Then you're maxed out and still need to shed just as much power as before.

So it doesn't help in any significant way, you have to set up a more complicated grid with very expensive transformers, and it requires you to design all your turbines to be twice as strong.