 FREEDGUY wrote:
...what happens in a storm outage situation where 10's of thousands of users are "OFF " during "non curve" times ?? |
Consequence of storm outage (or any outage event) is unrelated to how the system responds to demand change.
The short answer is, that when there's ANY large-scale astable action (a storm takes out a generating facility, a transmission line or substation goes down, etc), there's a chain of events that occurs that impacts basically everything across the system. IF the components WITHIN the system are working properly, AND there's enough generating faciilities that can be properly managed, the system will 1) isolate failed segments and 2) restabilize itself around the remaining generation, transmission, and load.
In order to understand how the system responds, one has to understand how the components (both generator, and load, and the grid) operate. I'll do my best to make it simple, but it ain't:
An AC generator running, develops two power characteristics... one is VOLTAGE, the second, FREQUENCY. There are two devices which are necessary to CONTROL those characteristics... Voltage is controlled by a REGULATOR (which varies field current, which modulates field voltage via closed-loop feedback), Frequency is controlled by a GOVERNOR, which modulates throttle (regardless of wether it's an internal combustion engine, a combined-cycle diesel/turbine, a fossil-steam turbine, or a nuclear-steam turbine, or a hydroelectric turbine) to maintain a proper output frequency.
When a generator has no load, the only energy that is used, is what is requred to keep the generator spinning at governed speed, keep the field excited to reach regulated voltage, and operate the prime mover's housekeeping (like... cooling, lubrication, etc). That means, a minimum amount of input energy (fuel) is required just-to-run, and once that point is met, adding an electrical load will cause input energy consumption to increase, hopefully in direct proportion to output demand, but things are never perfectly efficient.
Imagine you have one large electric motor, and one large generator powering it. Ignoring all the startup issues, once running, the two will remain basically engaged such that the electric motor, with no load, will present basically no load (aside from friction and cooling, right) to the generator. Add a load to the motor, and the generator will 'feel' it. When a load is engaged, the voltage sags, and the
generator engine becomes loaded, and slows, so the governor starts
adding throttle to maintain proper speed. The REGULATOR brings up field
current to maintain voltage. As RPM comes up, voltage rises TOO (voltage is a function of magnetic field intensity AND rotor speed through the field, so voltage rises as frequency increases, or as engineers say, V/Hz relationship). Rising voltage makes the REGULATOR back off on field current, which reduces the prime mover's torque load, causing frequency to rise higher, until governor cuts off input energy, which causes RPM to start falling, which causes voltage to fall which causes regulator to raise field current...
See this viscious circle? This is the realm of GOVERNANCE... trying to maintain a stable system, amidst inherently unstable circumstances. [this is why there's a difference between tractor governors, and combine governors, and generator governors]
Let's say that motor, is actually 10,000 electric motors, all on the same trio of wires, and instead of one generator, there's 10 much larger generators, all on the same trio of wires. That's the equivalent of 1000 motors per generator, right?
Now we get to the part of how to understand how AC generators can 'share' a same 'line'...
When you have several loads, and several generators, the generators HAVE to be SYNCHRONIZED. It's like having a row of five-six engines, all in a line, with a clutch on the back of the first engine, driving the front of the second, which drives a clutch to the third, then fourth... They ALL have to be synchronized, and they do this using those three WIRES.
In a bunch of clutched engines, if you apply a load at ANY point, it will have an immediate load on the crankshafts of ALL engines, but if NO engines respond with more throttle, the crankshaft speed will fall. If SOME respond, the speed may not fall as fast, and if all respond, it may come back up, and overshoot the 'governed speed'.
in an AC grid, you have a BUNCH of generating plants, all coupled together, and as loads come on, they ALL 'feel' the load, but like the long line of engines example, the only ones that respond, are the ones that ADD more throttle.
Let's say, in that long line of say, 10 engines, you got 1 huge, and 9 tiny ones. They're all hooked together, pulling a load. The huge one is the ONLY one with a governor, and it is running at some particular speed. The load is about 85% of what that generator can pull, while the rest is being pulled by all the tiny ones. You have all the tiny ones' throttles pinned wide open. They will NOT overspeed, because doing so means they're trying to spin the crank against the load FASTER than the big huge generator wants to spin. When that happens, the really big generator's throttle closes, which causes THAT engine to reduce it's throttle. The added load starts bogging down the smaller generators (after all, they're tiny), and the big generator's governor brings it back up to speed.
In the utility grid, you have several hundred large powerplants, and several thousand smaller plants. Any plant that connects to the grid, synchronizes itself to the grid's waveform, and once engaged, attempts to increase voltage, and frequency of the grid. in doing so, larger generating plants start reducing throttle, which causes more load to come to the 'new' plant. Eventually, the new plant finds itself running at full-wide-open.
Engineers refer to this as 'infinite load' circumstance... effectively, the input power of a prime mover is at it's limit... like, you're climbing a mountain in a diesel truck... you eventually find a gear that will maintain a constant climb, while your foot throttle is totally planted to the floor. The load (the grade) now determines how fast you'll go, because you simply have no more crankshaft power available.
Small generating plants that want to fire up and connect to the grid, and 'sell' their power, must synchronize, then connect in, and once in, add throttle, to try to 'push' the line frequency, and line voltage higher. They'll advance the throttle either to some proportion of capacity to which they want to sell to the grid (basically, taking it from someone else's load).
The 'gotcha' is that this system works well if, and only if, there is:
1) plenty of plants that can run econcomically at PARTIAL loading
2) power sources on the line which are stable... not susceptible to being started or shut down without plenty of forewarning.
3) plenty of plants which can start rapidly, and 'pick up' lots of load when a station or line segment has been knocked out.
This is where most people are mislead:
When you have unstable power SOURCES, those power sources' lack of stability, cause a cascading domino-effect that destabilizes the grid.
A wind turbine, for instance, is an 'unstable' source... it depends on wind, which might blow constantly for days, or even weeks, until a storm front finally arrives, and as winds go from a reasonable speed, up to an extremely dangerous speed, the wind turbine must 'shed' itself and shut down... feather it's blades, turn sideways to the wind, so that it doesn't self-destruct. When this happens, it usually isn't just one turbine, it's literally HUNDREDS... and it all will happen over the course of say... five to ten seconds.
When that happens, power flowing in one direction on grid interconnections suddenly reverses direction, desabilizing the grid. Small generating plants must respond rapidly in order to pick up the dropped load.
(this is where I explain the 'gotcha'...)
Every power plant has a 'minimum' input, just to EXIST. A fossil steam plant (i.e., Coal) needs property, and a security guard, a bunch of insurance, property taxes, groundskeeping and environmental compliance, just to BE THERE. A Nuclear plant requires even MORE, because it's a nuclear site... It takes so much more, that the ONLY way to operate a nuclear plant, is to have it running at 100% load ALL THE TIME. NOBODY will run one at partial, because the cost to run it at partial load, vs. full, is basically the same. Nobody will EVER operate a nuclear-steam generating plant at partial, they also would not just 'shut it down' when there' was plenty of 'other' power competetors out there... it would, at that point, be a clear and total loss in terms of economic performance.
A fossil-steam plant, however, will consume less fuel at a partial, than full setting... and when shut down completely, will require very little (security, taxes, and insurance, and a little maintenance to maintain readiness), and when demand requires, they can be fired up in anticipation, or respond FAIRLY quickly to help 'pick up' a dropped load.
Unfortunately, they won't be immediate... so we will have to keep SOME of them ready, and running at a low loadpoint, so that they're ready for high demand circumstances. "SOME" meaning... enough to pick up a considerable load... Like... about 200% of whatever the 'volatile' source is capable of (a 1 megawatt loss will need at least 2 megawatts of backup ready to 'pick up' for the sudden loss [*because a high-magnitude loss results in unstability that requires considerably more input capacity, just to stabilize*])...
The alternative, is to try to replace all the fossil-steam with solar and wind turbines... solar that generates full capacity, even in overcast skies and the dark of night... and wind turbines that generate full power, even when there's no wind, partial wind, or extremely excess wind...But there's no such thing as solar that generates in the dark, or wind that generates during doldrums, or stays operational and safe in a 120mph+ storm, and doesn't fall flat-on-it's face after the storm passes.
So with all this in mind, the circumstance that we will see in the future, is that when a storm or disaster causes some load destabilization event to occur, grid segments will go down, substations in the segments that TRY to recover will fry, 'reliable' generating systems will become 'islanded', and large quantities of people totally unrelated to an affected area, will be without power.
Quality will plummet, cost will skyrocket, and we'll all be blissfully happy with the result... Or we'll be frozen or cooked to death... starving, without water... 'enjoying a primitive lifestyle', but that's what we call it 'progress'.
Edited by DaveKamp - 23 Apr 2022 at 1:31am
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!! Kamps post made perfect sense!! Why couldn't YOU make a reply like his ?? Oh, wait, he's not in your TTPP club 
