5 ms·

Generators are spinning machines with mechanical power applied to the shaft via some type of prime mover such as a gas turbine. The gas turbine has a throttle w

Generators are spinning machines with mechanical power applied to the shaft via some type of prime mover such as a gas turbine. The gas turbine has a throttle which controls how much mechanical power it makes, and how much electrical power the generator makes. When the power applied to the generator shaft by the turbine is greater than the load placed on the generator by people’s lights and heaters, the turbine and generator speed up until the gas turbine backs off the throttle a bit to balance power out of the turbine in to the generator and power out of the generator. If people turn on more lights and heaters and power out of the generator is greater than power in, the imbalance is made up from the rotating machinery’s inertia and it slows down. If the imbalance isn’t corrected by opening the throttle on the gas turbine it slows down until it stops rotating. If the throttle is already wide open and demand exceeds supply the machines slow down until they stop and nobody gets any electricity, so they have to have some blackouts to reduce the load so that demand is not greater than supply. Nothing should blow up though.

WJW 6y ago
This is entirely correct, I would like to add that every single grid-attached power generator I have ever seen has had ample safety devices that would safely disconnect and shut off the generator well before physical damage could occur. Something "blowing up" is only possible if every possible safety device failed.
- rhodozelia 6y ago
  true. generally if frequency is greater than 62 Hz or less than 57 Hz generators will be allowed to disconnect immediately to avoid damaging themselves. Offhand I am not quite sure what the damage from off-nominal frequency operation is though - inductance is jwl - any impedances with a lagging reactive component will vary proportionally with frequency. a decrease in frequency will cause an increase in current. I imagine off-nominal frequency operation causes something to get hot.
  - mschaef 6y ago
    > Offhand I am not quite sure what the damage from off-nominal frequency operation I can think of at least a couple potential issues: * If the grid frequency is lower than the frequency the machine is trying to turn, it's possible for the generator to jump forward and mechanically slam forward to the next pole. * Some turbines have problems running too far off rated speed. (IIRC, torsional resonance issues.)
    - rhodozelia 6y ago
      Resonance slightly above rated speed I have heard of. Pole slipping, a loss of synchronism, i would associate more with transient events such as large losses of load or generation that cause big changes in power flows as opposed to a sustained operation at 58 Hz. Loss of synchronism or pole Slipping should be protected against by out of step protection elements?
      - thu2111 6y ago
        I think their assumption is that if the grid collapses completely it will take place in a short period of wild frequency fluctuations as generators trip out, demand swings wildly and so on, so you could get very large frequency distortions very quickly, perhaps too quickly for some safety equipment to kick in? Or some equipment can disconnect itself "safely" in the sense of not putting humans in danger but may still sustain damage to itself. I guess it's also quite hard to regularly test this sort of safety mechanism end to end, so there's surely a fear that it wouldn't work.
        mschaef 6y ago
        > if the grid collapses completely it will take place in a short period of wild frequency fluctuations as generators trip out, demand swings wildly and so on, so you could get very large frequency distortions very quickly, perhaps too quickly for some safety equipment to kick in? Or some equipment can disconnect itself "safely" in the sense of not putting humans in danger but may still sustain damage to itself. My general read on this is that generation can trip off more easily than most kinds of load, so when reserves are low, they need to be more explicitly proactive about taking load offline. That In case a generator trips offline that consumes all the remaining responsiveness and takes the grid to a collapse. I assume what a total collapse looks like is every generator tripping offline in a matter of a few seconds or minutes. Hopefully, though, there'd be enough ability to disconnect transmission lines to isolate the failure to a specific part of the grid. Back in the 2003 NY blackout, the failure was isolated to the northeast because NJ disconnected its grid. The last couple steps of this timeline are illustrative: https://en.wikipedia.org/wiki/Northeast_blackout_of_2003#Sequence_of_events https://en.wikipedia.org/wiki/Northeast_blackout_of_2003#Seq... * .., * 4:11:57 p.m. The Keith-Waterman, Bunce Creek-Scott 230 kV lines and the St. Clair–Lambton #1 230 kV line and #2 345 kV line between Michigan and Ontario fail. * 4:12:03 p.m. Windsor, Ontario, and surrounding areas drop off the grid. * 4:12:58 p.m. Northern New Jersey separates its power-grids from New York and the Philadelphia area, causing a cascade of failing secondary generator plants along the New Jersey coast and throughout the inland regions west. * 4:13 p.m. End of cascading failure. 256 power plants are off-line, 85% of which went offline after the grid separations occurred, most due to the action of automatic protective controls. I know people that were working at PJM when that happened, and they tell stories of essentially continual announcements on the PA system announcing various emergency states they needed to respond to. Which they did. > I guess it's also quite hard to regularly test this sort of safety mechanism end to end, so there's surely a fear that it wouldn't work. Generators trip all the time. In fact, one of my first jobs was working at a TX based utility company analyzing the frequency disturbances that occur when generators trip offline. You can learn a lot about the way the grid responds by looking at the plot of frequency over time right around a trip.
itronitron 6y ago
A fun science demonstration is to take an old-timey hand crank electrical generator, crank it by hand with nothing attached to the power output, then attach a light bulb and crank it a second time. The amount of resistance resulting from the light bulb using the generated electricity is remarkable. It is much harder to turn the crank with the light bulb attached and that really drives home that electrical devices really do use a lot of energy.
amluto 6y ago
It’s a bit more complicated than this. A lot of big generators are induction generators, and the associated electronics can make a tradeoff of voltage and frequency. In a grid setting, the frequency will be maintained at 60 Hz plus or minus not very much. Everything is, hopefully, arranged to maintain the correct voltage and frequency. The actual mechanism behind this have always struck me as arcane and somewhat fragile, especially in light of modern loads that don’t act at all like resistors under varying voltage conditions.
- rhodozelia 6y ago
  are you referring to modern wind turbines and solar farms which have their output converted through power electronics? I have literally never seen a large induction machine used as a generator, every machine larger than 300 kW I have seen has been a synchronous machine. If I've read about 600 different power plants, 598 of them have had synchronous machines. Where in the world, what application, and in what age of installation are engineers choosing induction machines + power electronics over synchronous machines?
  - thu2111 6y ago
    I think it's indeed a reference to wind and solar. There's a discussion of the problems posed by loss of system 'inertia' here: https://www.drax.com/technology/shock-absorbers-keeping-grid-stable/ https://www.drax.com/technology/shock-absorbers-keeping-grid...
jasonwatkinspdx 6y ago
An important detail here is that as imbalances happen it pulls the grid frequency around. Bad things can happen to various systems if the frequency goes too far out of range. Yes in theory all equipment should be able to trip fail safe, but in practice a power grid has a ton of inertia and there are limits to how you can engineer around that. Even if the grid blacks itself off safely with minimal equipment damage, bringing it back up is hugely more complex than just resetting the breakers on your house/apartment.
floxy 6y ago
For a different rabbit hole to explore: https://www.wired.com/story/how-30-lines-of-code-blew-up-27-ton-generator/ https://www.wired.com/story/how-30-lines-of-code-blew-up-27-...

WJW 6y ago

This is entirely correct, I would like to add that every single grid-attached power generator I have ever seen has had ample safety devices that would safely disconnect and shut off the generator well before physical damage could occur. Something "blowing up" is only possible if every possible safety device failed.

rhodozelia 6y ago
true. generally if frequency is greater than 62 Hz or less than 57 Hz generators will be allowed to disconnect immediately to avoid damaging themselves. Offhand I am not quite sure what the damage from off-nominal frequency operation is though - inductance is jwl - any impedances with a lagging reactive component will vary proportionally with frequency. a decrease in frequency will cause an increase in current. I imagine off-nominal frequency operation causes something to get hot.
- mschaef 6y ago
  > Offhand I am not quite sure what the damage from off-nominal frequency operation I can think of at least a couple potential issues: * If the grid frequency is lower than the frequency the machine is trying to turn, it's possible for the generator to jump forward and mechanically slam forward to the next pole. * Some turbines have problems running too far off rated speed. (IIRC, torsional resonance issues.)
  - rhodozelia 6y ago
    Resonance slightly above rated speed I have heard of. Pole slipping, a loss of synchronism, i would associate more with transient events such as large losses of load or generation that cause big changes in power flows as opposed to a sustained operation at 58 Hz. Loss of synchronism or pole Slipping should be protected against by out of step protection elements?
    - thu2111 6y ago
      I think their assumption is that if the grid collapses completely it will take place in a short period of wild frequency fluctuations as generators trip out, demand swings wildly and so on, so you could get very large frequency distortions very quickly, perhaps too quickly for some safety equipment to kick in? Or some equipment can disconnect itself "safely" in the sense of not putting humans in danger but may still sustain damage to itself. I guess it's also quite hard to regularly test this sort of safety mechanism end to end, so there's surely a fear that it wouldn't work.
      - mschaef 6y ago
        > if the grid collapses completely it will take place in a short period of wild frequency fluctuations as generators trip out, demand swings wildly and so on, so you could get very large frequency distortions very quickly, perhaps too quickly for some safety equipment to kick in? Or some equipment can disconnect itself "safely" in the sense of not putting humans in danger but may still sustain damage to itself. My general read on this is that generation can trip off more easily than most kinds of load, so when reserves are low, they need to be more explicitly proactive about taking load offline. That In case a generator trips offline that consumes all the remaining responsiveness and takes the grid to a collapse. I assume what a total collapse looks like is every generator tripping offline in a matter of a few seconds or minutes. Hopefully, though, there'd be enough ability to disconnect transmission lines to isolate the failure to a specific part of the grid. Back in the 2003 NY blackout, the failure was isolated to the northeast because NJ disconnected its grid. The last couple steps of this timeline are illustrative: https://en.wikipedia.org/wiki/Northeast_blackout_of_2003#Sequence_of_events https://en.wikipedia.org/wiki/Northeast_blackout_of_2003#Seq... * .., * 4:11:57 p.m. The Keith-Waterman, Bunce Creek-Scott 230 kV lines and the St. Clair–Lambton #1 230 kV line and #2 345 kV line between Michigan and Ontario fail. * 4:12:03 p.m. Windsor, Ontario, and surrounding areas drop off the grid. * 4:12:58 p.m. Northern New Jersey separates its power-grids from New York and the Philadelphia area, causing a cascade of failing secondary generator plants along the New Jersey coast and throughout the inland regions west. * 4:13 p.m. End of cascading failure. 256 power plants are off-line, 85% of which went offline after the grid separations occurred, most due to the action of automatic protective controls. I know people that were working at PJM when that happened, and they tell stories of essentially continual announcements on the PA system announcing various emergency states they needed to respond to. Which they did. > I guess it's also quite hard to regularly test this sort of safety mechanism end to end, so there's surely a fear that it wouldn't work. Generators trip all the time. In fact, one of my first jobs was working at a TX based utility company analyzing the frequency disturbances that occur when generators trip offline. You can learn a lot about the way the grid responds by looking at the plot of frequency over time right around a trip.

itronitron 6y ago

A fun science demonstration is to take an old-timey hand crank electrical generator, crank it by hand with nothing attached to the power output, then attach a light bulb and crank it a second time. The amount of resistance resulting from the light bulb using the generated electricity is remarkable. It is much harder to turn the crank with the light bulb attached and that really drives home that electrical devices really do use a lot of energy.

amluto 6y ago

It’s a bit more complicated than this. A lot of big generators are induction generators, and the associated electronics can make a tradeoff of voltage and frequency. In a grid setting, the frequency will be maintained at 60 Hz plus or minus not very much. Everything is, hopefully, arranged to maintain the correct voltage and frequency. The actual mechanism behind this have always struck me as arcane and somewhat fragile, especially in light of modern loads that don’t act at all like resistors under varying voltage conditions.

rhodozelia 6y ago
are you referring to modern wind turbines and solar farms which have their output converted through power electronics? I have literally never seen a large induction machine used as a generator, every machine larger than 300 kW I have seen has been a synchronous machine. If I've read about 600 different power plants, 598 of them have had synchronous machines. Where in the world, what application, and in what age of installation are engineers choosing induction machines + power electronics over synchronous machines?
- thu2111 6y ago
  I think it's indeed a reference to wind and solar. There's a discussion of the problems posed by loss of system 'inertia' here: https://www.drax.com/technology/shock-absorbers-keeping-grid-stable/ https://www.drax.com/technology/shock-absorbers-keeping-grid...

jasonwatkinspdx 6y ago

An important detail here is that as imbalances happen it pulls the grid frequency around. Bad things can happen to various systems if the frequency goes too far out of range. Yes in theory all equipment should be able to trip fail safe, but in practice a power grid has a ton of inertia and there are limits to how you can engineer around that. Even if the grid blacks itself off safely with minimal equipment damage, bringing it back up is hugely more complex than just resetting the breakers on your house/apartment.

floxy 6y ago

For a different rabbit hole to explore: https://www.wired.com/story/how-30-lines-of-code-blew-up-27-ton-generator/ https://www.wired.com/story/how-30-lines-of-code-blew-up-27-...