Generators must synchronize with the grid. Huge spinning rotor masses that will experience tremendous forces to coerce them into matching an RPM that corresponds to the grid's frequency.
Frequency is also impacted by load: the greater the load on the generator the more torque required at its input shaft to maintain the same RPM. If the generator's input engine is already at max torque then RPM must decrease all else equal. That in turn requires that every other generator on the grid also slow down to match.
When a huge chunk of generating capacity disappears there isn't enough power feeding the remaining generator input shafts (all else equal) to maintain RPM so the grid frequency must drop. That tends to destroy customer equipment among other problems.
Generators are motors and motors are generators. If the capacity disappears too quickly the grid _drives the generator as a motor_ potentially with megawatts of capacity all trying to instantly make that 100 ton rotor change from 3600 RPM to 2800 RPM or whatever. Inertia puts its $0.02 and the net result is a disintegrating rotor slinging molten metal and chunks of itself out while the bearings turn into dust.
Protective equipment sees this happening and trips the generator offline to protect it. Usually the coordinating grid entity keeps spare capacity available at all times to respond to loss of other capacity or demand changes. This is also the point of "load shedding": if spare capacity drops below a set level loads are turned off.
If spare capacity is not maintained or transmission line choke points present problems then capacity trip outs can cause progressive collapse as each generator sees excessive load, trips, and in turn pushes excess load to the next generator. If your grid control systems are well designed they can detect this from a central location and command parts of the grid to "island" into balanced chunks of load/capacity so the entire grid does not fully collapse.
Of course when you want to reconnect the islands it takes careful shifting of frequency to get them aligned before you can do that.
If all generators collapse you end up in a black start situation that requires careful staging lest more load than you expected jumps on the grid (maybe due to control devices being unpowered or stuck somewhere), triggering a secondary collapse.
Caveat: not a grid engineer so I may have gotten some of this wrong but hopefully it helps anyone who wonders why load shedding exists or how a grid can "collapse" and what the consequences are if you don't do those things and just let it ride.
Am grid engineer. You nailed it. It can get incredibly complex modelling this stuff, and reading all the armchair observers banging on about 'single points of failure' is amusing.
My professor did that for Bonneville Power Administration. I worked on a tiny piece of his modelling software. A short writeup of it https://ciex-software.com/fmcmx.html
Oh yeah. These posts are the limit of my knowledge. Real-world power plants, switchgear, and grid management is way way more complex! And if you want to talk about "hard real time requirements".... it doesn't get much harder or real time than keeping the grid going :)
Thanks to you, the operators, and the linemen out there keeping it all going. When it works correctly no one notices.
An interesting side effect of that is one can use the grid frequency to coordinate emergency power response - individual nodes (batteries, peaker plants, etc.) can react directly to the frequency measurement with generation or load, thus stabilizing the grid. Too much energy is equally an issue. Usually it's called fast frequency response these days.
Good point... oversupply lightens the load on the generator meaning not as much angular momentum is converted to magnetic flux then to an electric field. Unopposed the torque is able to increase the rotor's speed which directly determines grid frequency.
My grid tie solar system does exactly what you say. It monitors the frequency of grid power and matches it dynamically. There are defined parameters for how out of spec it can get and for how long. I don't recall the exact numbers but imagine 0.2Hz for 100ms, 0.5Hz for 1ms, 1Hz for 500ns. Same thing for voltage though that allows a much wider range.
In CA all grid tie solar also requires communication with the utility (through the manufacturer) with a backup connection source (Internet and LTE in my case). This is so if the grid is nearing capacity or going unstable the utility can command the inverters to allow a wider band of voltage and frequency. The last thing the grid needs in an unstable scenario is everyone's solar panels tripping off at the same time.
Technically the utility can also command the panels to stop production if there is an excess supply but they are limited in how long and how often they can do that.
Fun fact: The interconnect operators used to keep track of the average frequency over time and would run the grid slightly fast or slow to ensure the grid averaged 60Hz over time. This allowed clocks and such to maintain time by relying on the grid. That is no longer the case though. I think they still roughly aim for a 60Hz average but if they're behind by 0.01Hz over the past week they no longer run the grid at 60.05Hz for a while to "catch up".
Fun fact number 2 I just learned recently: Southern California used to be on 50Hz! That's right, the USA had split cycle just like Japan. Most of the country on 60Hz, SoCal on 50Hz. Right after WW2 they made the switch apparently. I guess a lot of stuff was dual frequency capable at the time but the utility provided assistance where required.
Fun fact number 3: Ever wonder why we have 110/220 or 115/230 or 120/240? Because every local utility picked their own standard: 110 (from Edison's DC system carried over into AC world), 115, or 120. It was not until relatively recently that we really standardized on 120/240 (+/- 5% which is 114 - 126 but with brief excursions allowed). That's why some old appliances might say 110 or 115 on them.
Fun fact number 4: 120/240 is a backwards compatibility hack. It was too late to change to 240 and 120 is (for physical reasons) better for electric lighting applications (thicker less fragile filament for same light output). How to solve this? Change your MV-LV transformers to 240 but center tap them. Instead of Line-Neutral you provide customers Line 1 - Neutral - Line 2. Connections across L1/L2 give you 240 volts, connections across L1-N or L2-N give you 120 volts! Everyone's happy! There is a NEMA plug standard for low-amp 240V. It has both blades horizontal (looks like the unimpressed smiley face). I wish it were more popular in kitchens for boiling water and such.
If you have two AC generators connected to a load and one of them shifts is phase any amount ahead of the other one, the second one becomes a motor. Power flows in proportion to the difference in phase angle.
Take a little hobby DC motor and hook it to a battery. It will spin. Now hook it to a small LED and turn the shaft. The LED will light up. In fact hook both up. The battery will spin the motor and light the LED based on its voltage/charge state. But if you start spinning the shaft you will begin taking over from the battery. Spin even faster and the battery will start charging. Stop trying to spin the shaft and watch the LED dim and the motor slow down as the battery takes over again.
Fundamentally electromagnetism is a unified force.
A changing electric field induces a magnetic field: the electricity creates a magnetic field inside the motor-generator causing the rotor's electromagnet or permanent magnet to spin to align with that field. For a DC motor halfway through the rotation the polarity switches causing the rotor to keep spinning to align with the new magnetic field (AC doesn't need this because it is varying on its own). The motor-generator is operating in "motor" mode here because the magnetic field changes first and the rotor is trying to catch up to it ultimately consuming energy.
Now spin a motor's shaft putting energy into the system. The rotor has a magnetic field either because its an electromagnet or a permanent magnet. You are spinning the rotor shaft so by definition the magnetic field coming off the rotor also spins. Thus the stator sees a continuously varying magnetic field which - you guessed it - induces an electric field in the stator windings.
In reality this is a continuously varying effect based on whether the rotor's motion is leading or lagging the magnetic field (really it is trying to lead or lag but never gets far otherwise it would burn up).
The short version is a generator at "idle" but synchronized is a motor. The grid is providing the power to spin that motor (or the attached engine/steam turbine is providing just enough rotation of the shaft to keep the generator synchronized but no more).
As the generator begins to "provide" power more torque is put into the shaft's rotation. This causes the magnetic field to want to lead the electric field, transferring energy into the electric field. The entire grid strongly resists the shaft actually spinning faster though. The mechanical energy that wants to make the rotor go faster is siphoned off by the magnetic flux and electric fields. Electric field steals energy from the magnetic flux. Reduced magnetic flux steals energy from the rotor's angular momentum to replenish itself (if it couldn't the rotor would slow down). The rotor gets angular momentum from the connected source of torque like a steam turbine. The Cycle of Life ... or something.
If you suddenly cut the transmission lines and took no action all that torque would overspeed the generator very quickly though because no electric current could flow so the electric field just builds up (stealing energy from the rotor) then gives the energy right back as more magnetic flux (speeding up the rotor). This would let the rotor just go faster and faster while also heating everything up.
Frequency is also impacted by load: the greater the load on the generator the more torque required at its input shaft to maintain the same RPM. If the generator's input engine is already at max torque then RPM must decrease all else equal. That in turn requires that every other generator on the grid also slow down to match.
When a huge chunk of generating capacity disappears there isn't enough power feeding the remaining generator input shafts (all else equal) to maintain RPM so the grid frequency must drop. That tends to destroy customer equipment among other problems.
Generators are motors and motors are generators. If the capacity disappears too quickly the grid _drives the generator as a motor_ potentially with megawatts of capacity all trying to instantly make that 100 ton rotor change from 3600 RPM to 2800 RPM or whatever. Inertia puts its $0.02 and the net result is a disintegrating rotor slinging molten metal and chunks of itself out while the bearings turn into dust.
Protective equipment sees this happening and trips the generator offline to protect it. Usually the coordinating grid entity keeps spare capacity available at all times to respond to loss of other capacity or demand changes. This is also the point of "load shedding": if spare capacity drops below a set level loads are turned off.
If spare capacity is not maintained or transmission line choke points present problems then capacity trip outs can cause progressive collapse as each generator sees excessive load, trips, and in turn pushes excess load to the next generator. If your grid control systems are well designed they can detect this from a central location and command parts of the grid to "island" into balanced chunks of load/capacity so the entire grid does not fully collapse.
Of course when you want to reconnect the islands it takes careful shifting of frequency to get them aligned before you can do that.
If all generators collapse you end up in a black start situation that requires careful staging lest more load than you expected jumps on the grid (maybe due to control devices being unpowered or stuck somewhere), triggering a secondary collapse.
Caveat: not a grid engineer so I may have gotten some of this wrong but hopefully it helps anyone who wonders why load shedding exists or how a grid can "collapse" and what the consequences are if you don't do those things and just let it ride.