Power Grid Frequency and Inertia

The technical reason why wind and solar can never get to 'net zero'

Unless something totally unexpected happens, I won’t be posting anything more until after the holidays. Family time is more important than just about anything else. Merry Christmas and Happy New Year!

In early December, I cross-posted an article that Thomas Shepstone had just published about the problem Australia is having with too much rooftop solar. California is on the brink of similar problems.

I’ll call it a bi-polar problem. When you’re up, you’re up; when you’re down, you’re down. It’s all about having too much of a good thing, or what was thought to be a good thing, and then crashing as an aftermath.

Power generators have to match their frequency and phase to the power grid system. For those not familiar with this, just remember that AC electricity acts in wave form. Here’s a crude sketch I made to illustrate electrical waves that are in phase vs out of phase. To learn more about frequency, read Kilovar1959’s excellent article. For more about phase, read his post in the same series Here.

When out of phase, the generated electricity will be fighting against the current (even in a literal sense) instead of being able to add power to the grid.

All the generators on the grid have to be in synch. When there’s a mismatch, bad things can happen, so the grid has to be protected with automatic relays and switches to trip circuit breakers on the out-of-synch power source, to protect the rest of the grid. Kilovar1959 describes what could happen the minute an out of synch generator is connected:

“Closing a breaker out of sync is like closing a breaker with the wrong phase rotation. There will be massive current flows, the generator will instantly try to pull into sync. This can explode circuit breakers, evaporate bus work, rupture stator windings, twist or break shafts, shear turbine blades, and has even ripped generators right off the foundations. Not good!”

In a really bad situation, there’s a risk that the contagion could spread. If our power system gets to the point where it’s dominated by unreliable sources that are unable to synchronize their frequency and phase during extreme situations, the domino effect could take down the whole grid. And with a predominance of ‘renewables’ during a prolonged period of no wind and little sun, it might take a very long time to get things up and running again.

Note: I’m strictly a layman; not an engineer. I took a course in electricity and magnetism, and got a bachelors degree in physics, but hey, that was 60 years ago. I might be just a little bit rusty! If my explanation needs correction, please let me know with a comment.

With conventional power stations like coal, gas, nuclear, or hydro, the massive turbines and generators have a lot of rotational inertia. Like a giant flywheel, fluctuations of input or load demand cannot easily change the speed of rotation. The stability caused by inertia gives the needed second or two for the controls to adjust to maintain synchronization. For more about inertia, read Kilovar1959 Here.

But with wind and solar, there is no similar inertia. Synchronization has to be done with digital controls. Voltage, frequency, and phase has to be managed by inverters. The inverters have limitations, and therein lies the problem. Supply has to match demand. Inverters have limited capability to maintain the needed synchronization when the grid is highly stressed by large differences between supply and demand. It’s a technical problem, as we’ll see.

The reason I have wanted to learn about this is because in Australia and California rooftop solar has been so heavily subsidized that it has become extremely widespread, and is supplying a very large proportion (80% in South Australia) of the electrical power during midday on summer days that are clear and sunny, but zero power at night (except some from batteries). Powermag.com reports “In California, more than 2.7 TWh of renewable energy—primarily solar—was curtailed last year” - curtailed means wasted - way too much when not needed; not enough when it is needed.

So far, the gap has been bridged by traditional sources of ‘dispatchable’ power coming on line as needed. But as a larger and larger proportion of grid power comes from wind and solar, and as more and more conventional power sources are shut down, we are approaching a dangerous situation and are almost bound to have rolling blackouts or worse, sooner or later. Or something worse yet - someone else controlling your solar panels - keep reading.

I found a scientific paper about the inadequacies of inverters to solve this problem. Inverters change the DC power from solar panels to AC. They have sophisticated semiconductor circuits that are supposed to take care of the synchronization problem since solar panels don’t have rotational inertia.

I don’t pretend to understand the technicalities in the scientific paper, and even the abstract is not easy reading. But I think I get the gist – there are times when grid inertia isn’t really solved even by the newest inverters – we still need, and probably will always need, the real rotational inertia associated with large turbine generators. Fasten your seat belt for the next paragraph and bear with me…

A lifetime of 1.57 years for a solar system of course is worthless, and even a lifetime of 11 years is unacceptable, compared to conventional power plants that can last for many decades. I assume that replacing inverters is not cheap and there is the downtime to consider also.

Here’s a little less technical info, from engineering.com “Conventional thermal (like with steam turbines) power generation provides excellent frequency regulation, combining the inertia of very heavy rotating components with the ability to rapidly control power by adjusting the supply of steam to the turbine. This is equally true of both fossil fuel and biomass-fired power plants. In large power stations, the damping provided by the rotating inertia means that a significant change in frequency will take more than one second to occur, which is sufficient time for adjustments in the steam supply to compensate for the change.

To maintain frequency stability, the rotational speed of every generator connected to the national grid must by synchronized. This can be extremely challenging. As electrical demand varies within the grid, it affects the electromagnetic load on the generator and subsequently the input mechanical power required to maintain a constant speed. If the power is not controlled to account for this, the speed of rotation will change, and the grid frequency will fluctuate.

As wind, solar and other distributed and renewable sources are beginning to replace large, centralized power stations, it is becoming more difficult to achieve frequency stability. There are two major reasons for this. First, there is now a far larger number of small generators, many of which are not directly operated by the grid. This makes controlling them far more complex. Second, most of these small generators provide no inertia, meaning that a much more rapid control response is required to effectively maintain frequency stability.” emphasis mine - wait till you hear what Australia is doing about this..

“…Solar panels can be rapidly switched on and off to provide good frequency regulation. However, because solar panels are dispersed very widely and are not directly operated by the grid operator, achieving this kind of control is especially challenging…” emphasis mine - can you see where this is going? read on.. from the article that Thomas Shepstone posted 12/6/24:

“The body responsible for keeping the lights on in Australia's biggest electricity grids wants emergency powers to switch off or throttle rooftop solar in every state to help cope with the daily flood of output from millions of systems.

In a report released on Monday morning (Dec.2nd), the Australian Energy Market Operator said "emergency backstop" powers were urgently needed to ensure solar installations could be turned down — or off — in extreme circumstances…”

So, it’s gotten to the point where your rooftop solar in Australia will soon be under someone else’s control - and they’ll be able to turn it on or off in “extreme circumstances”. My suggestion? Disconnect from the grid if you can afford to, or at least have a portable generator and spare cans of gas for it and your internal combustion car. I have both a Generac generator and a small (600 Watt) off grid solar system for a little emergency backup.