Electrical outages. cyber attacks ?

There is an unfortunate effect here, where in the interests of energy efficiency, the large induction motors are now almost exclusively being fed from some sort of VSD or inverter drive, and the star-dela type starter of yore is very much on the wane. Now this tends to remove the inertial loads that do indeed switch from motoring to generating as the supply frequency dips. A VSD has no such effect  being a rectifier and some switching transistors. Actually if anything it may exacerbate an overload related dip, if the electronics tries to correct by  increasing the conduction angle, and therefore the average current, to maintain a constant horse power input, if the supply voltage droops - this is like a negative resistance, in that the current rises as the voltage falls, and is exactly the opposite of what is needed for stability.

So because of these loads that break the link between overload and falling frequency, we are far more vulnerable to the fact that much of our generation is also just inverters, this is the case on solar farms, and all wind turbines of significant size.

The upshot is that looking only at the frequency deviation may no longer be a safe proxy for state of national load and other parameters should be used instead or in addition.

There is an unfortunate effect here, where in the interests of energy efficiency, the large induction motors are now almost exclusively being fed from some sort of VSD or inverter drive, and the star-dela type starter of yore is very much on the wane. Now this tends to remove the inertial loads that do indeed switch from motoring to generating as the supply frequency dips. A VSD has no such effect  being a rectifier and some switching transistors. Actually if anything it may exacerbate an overload related dip, if the electronics tries to correct by  increasing the conduction angle, and therefore the average current, to maintain a constant horse power input, if the supply voltage droops - this is like a negative resistance, in that the current rises as the voltage falls, and is exactly the opposite of what is needed for stability.

So because of these loads that break the link between overload and falling frequency, we are far more vulnerable to the fact that much of our generation is also just inverters, this is the case on solar farms, and all wind turbines of significant size.

The upshot is that looking only at the frequency deviation may no longer be a safe proxy for state of national load and other parameters should be used instead or in addition.