6 ms·

"Every 1800 RPM generator has to make 30 extra turns." I'm having trouble wrapping my my head around this one. If the goal is maintaining a steady output, shou

"Every 1800 RPM generator has to make 30 extra turns." I'm having trouble wrapping my my head around this one. If the goal is maintaining a steady output, shouldn't they keep revolving at the same rate, without worrying about what day the "current" second (no pun intended) belongs to? After all, I'm assuming they don't "catch up" after leap days, they just run for an extra day at the same output level. Why would leap seconds be different? Put another way, what do they need to spread those extra 30 turns over a 4 hour catch up period? Weren't they given an extra second in which to take care of them?

XorNot 10y ago
The power grid doesn't work if disparate phases aren't synchronized. On a very large scale this is why HVDC is used, but on smaller scales its still more cost-effective to coordinate with GPS time since it's atomic-clock backed.
- dredmorbius 10y ago
  This is the first I've heard of HVDC, and it's apparently a thing. However from my quick read, it's 1) still a frequency-based current, so you're not eliminating the requirement to synchronise generation, and 2) the advantages are in areas other than frequency synchronisation -- e.g., avoiding capacitance load in long-distance undersea cables.
  - fnj 10y ago
    What? WHAT? DC does not have a "frequency".
    - dredmorbius 10y ago
      Damn, I guess it's been too long since I've done (or thought about) circuit theory. But yeah, the general idea of HVDC is a frequency-free current: http://www.cleanlineenergy.com/technology/hvdc/how http://www.cleanlineenergy.com/technology/hvdc/how And now I'm trying to think my way through it: There is definitely DC with no frequency component, e.g., battery circuits. You've got a cathode, an anode, and a flow between them. I'm trying to sort out if DC can have a frequency component. I seem to recall that rectified circuits can operate by basically (pardon crude metaphors) chopping off the negative side of the phase. You'll have a wave pattern, but it's all going in one direction. Pushed through sufficient capacitance or impedance and you'll likely smooth that out as well. Which is getting me to wonder just how a DC generator works.
      - nicky0 10y ago
        There is no DC generator as such, just a converter from AC to DC.
        dredmorbius 10y ago
        Um, isn't a dynamo a DC generator? https://en.wikipedia.org/wiki/Dynamo https://en.wikipedia.org/wiki/Dynamo It's described (and matches my memory) of producing a "pulsing direct current". The commutator, a physical switch, maintains the polarity of current by breaking and switching between generator coils.
        dmoy 10y ago
        iirc (been awhile since EE so could be wrong): Dynamo is basically an AC gen with some crap in front to make output look DC. "Pulsing DC current" is actually ac with >50% of the duty cycle zeroed out. Again, could be very wrong here my memory is fuzzy as hell.
        y4mi 10y ago
        wikipedia seems to agree with you, if i read this right. > A dynamo is an electrical generator that produces direct current with the use of a commutator. >> A commutator is a moving part of a rotary electrical switch [...] that periodically reverses the current direction between the rotor and the external circuit
        dredmorbius 10y ago
        If I'm remembering and understanding correctly: the commutator swaps the + and - ends of the energised coil. So you're not actually zeroing out the inverted end of the wave, you're swapping its connections to the circuit. Where an AC generator coil applies a positive and negative voltage to a single lead, the DC generator swaps leads as the current reverses. I'm glad I'm not the only fuzzy one here....
        planteen 10y ago
        Take a DC motor, attach a load resistor, and spin it at constant speed. Look at the output voltage on a voltmeter. You will get DC output.
  - nicky0 10y ago
    From Wikipedia (top of 2nd paragraph) "HVDC allows power transmission between unsynchronized AC transmission systems"
- brandmeyer 10y ago
  This (unfortunately, commonly-held) misbelief is solidly in Not Even Wrong territory. Parallel AC sources with inductive impedance are naturally synchronized to each other. A phase error between them produces a torque error that is naturally self-correcting.
  - Animats 10y ago
    In a power grid, the AC generators are basically synchronized and in phase. But there's some tweaking on phase to control which way power is shipped. Inductors and capacitors are switched in and out to induce small phase shifts and drive power in the desired direction. If you want to know more about this, there's online training called "PJM 101", from the power grid operator for the northeastern US. HVDC systems are connected to AC systems using rectifiers and inverters. An interconnect point is a switching power supply. A very large switching power supply. The output DC isn't perfectly clean DC and the output AC isn't a clean sinusoid; converters have to approximate and try to filter. Wikipedia has a nice explanation, with good pictures.[1] [1] https://en.wikipedia.org/wiki/HVDC_converter https://en.wikipedia.org/wiki/HVDC_converter
dlgeek 10y ago
A lot of systems use the 60hz (or 50hz depending on location) signal from AC power as a way of keeping time. It's used enough that power companies will actually adjust the frequency minutely at night in order to make up for any mistakes made during the day. (Which in turn makes it so reliable that more systems use it to track time, and so on...) I guess grandparent post is saying power companies go ahead and feed in extra cycles for the leap second. It's not something I'd heard of, but it wouldn't surprise me.
- metafunctor 10y ago
  I remember reading that the frequency at any given time is usually somewhere between 49.9Hz and 50.1Hz (and similarly around 60Hz I'd imagine). So for a 50Hz clock, it's not very accurate at all. However, I just found that the _average_ frequency over a length of time should be exactly 50Hz, so it is, in fact, good for keeping time. More here: https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html
- kalleboo 10y ago
  I remember hearing that Analog TV stations were also used for timekeeping since they had a very stable clock for the blanking interval
  - Animats 10y ago
    US analog TV (all three classic networks) used to be synchronized to an atomic clock in NYC. This allowed switching from one source to another without frame tearing. Separate from that, there was a scheme for sending time of day in the vertical retrace interval, to set clocks and VCRs. Few devices received that, although I once encountered a hotel TV with a built-in analog clock that did. (The clock was wrong; although it could set itself automatically, it had to be told manually which channel provided a time signal. That's probably why this feature never caught on. I was bored enough to figure out how to set the thing.) US telephony used to be synchronized to an atomic clock in New Jersey. The TDMA multiplexing was kept in sync so that frames could be transferred synchronously without buffering. If the clock connection was lost, T1 to T3 lines would get slightly out of sync, and occasionally drop or add an 8-bit frame. This is unnoticeable in audio, but an error for modems. But neither of these was a time count. It was just a square wave.

XorNot 10y ago

The power grid doesn't work if disparate phases aren't synchronized. On a very large scale this is why HVDC is used, but on smaller scales its still more cost-effective to coordinate with GPS time since it's atomic-clock backed.

dredmorbius 10y ago
This is the first I've heard of HVDC, and it's apparently a thing. However from my quick read, it's 1) still a frequency-based current, so you're not eliminating the requirement to synchronise generation, and 2) the advantages are in areas other than frequency synchronisation -- e.g., avoiding capacitance load in long-distance undersea cables.
- fnj 10y ago
  What? WHAT? DC does not have a "frequency".
  - dredmorbius 10y ago
    Damn, I guess it's been too long since I've done (or thought about) circuit theory. But yeah, the general idea of HVDC is a frequency-free current: http://www.cleanlineenergy.com/technology/hvdc/how http://www.cleanlineenergy.com/technology/hvdc/how And now I'm trying to think my way through it: There is definitely DC with no frequency component, e.g., battery circuits. You've got a cathode, an anode, and a flow between them. I'm trying to sort out if DC can have a frequency component. I seem to recall that rectified circuits can operate by basically (pardon crude metaphors) chopping off the negative side of the phase. You'll have a wave pattern, but it's all going in one direction. Pushed through sufficient capacitance or impedance and you'll likely smooth that out as well. Which is getting me to wonder just how a DC generator works.
    - nicky0 10y ago
      There is no DC generator as such, just a converter from AC to DC.
      - dredmorbius 10y ago
        Um, isn't a dynamo a DC generator? https://en.wikipedia.org/wiki/Dynamo https://en.wikipedia.org/wiki/Dynamo It's described (and matches my memory) of producing a "pulsing direct current". The commutator, a physical switch, maintains the polarity of current by breaking and switching between generator coils.
        dmoy 10y ago
        iirc (been awhile since EE so could be wrong): Dynamo is basically an AC gen with some crap in front to make output look DC. "Pulsing DC current" is actually ac with >50% of the duty cycle zeroed out. Again, could be very wrong here my memory is fuzzy as hell.
        y4mi 10y ago
        wikipedia seems to agree with you, if i read this right. > A dynamo is an electrical generator that produces direct current with the use of a commutator. >> A commutator is a moving part of a rotary electrical switch [...] that periodically reverses the current direction between the rotor and the external circuit
        dredmorbius 10y ago
        If I'm remembering and understanding correctly: the commutator swaps the + and - ends of the energised coil. So you're not actually zeroing out the inverted end of the wave, you're swapping its connections to the circuit. Where an AC generator coil applies a positive and negative voltage to a single lead, the DC generator swaps leads as the current reverses. I'm glad I'm not the only fuzzy one here....
      - planteen 10y ago
        Take a DC motor, attach a load resistor, and spin it at constant speed. Look at the output voltage on a voltmeter. You will get DC output.
- nicky0 10y ago
  From Wikipedia (top of 2nd paragraph) "HVDC allows power transmission between unsynchronized AC transmission systems"
brandmeyer 10y ago
This (unfortunately, commonly-held) misbelief is solidly in Not Even Wrong territory. Parallel AC sources with inductive impedance are naturally synchronized to each other. A phase error between them produces a torque error that is naturally self-correcting.
- Animats 10y ago
  In a power grid, the AC generators are basically synchronized and in phase. But there's some tweaking on phase to control which way power is shipped. Inductors and capacitors are switched in and out to induce small phase shifts and drive power in the desired direction. If you want to know more about this, there's online training called "PJM 101", from the power grid operator for the northeastern US. HVDC systems are connected to AC systems using rectifiers and inverters. An interconnect point is a switching power supply. A very large switching power supply. The output DC isn't perfectly clean DC and the output AC isn't a clean sinusoid; converters have to approximate and try to filter. Wikipedia has a nice explanation, with good pictures.[1] [1] https://en.wikipedia.org/wiki/HVDC_converter https://en.wikipedia.org/wiki/HVDC_converter

dlgeek 10y ago

A lot of systems use the 60hz (or 50hz depending on location) signal from AC power as a way of keeping time. It's used enough that power companies will actually adjust the frequency minutely at night in order to make up for any mistakes made during the day. (Which in turn makes it so reliable that more systems use it to track time, and so on...) I guess grandparent post is saying power companies go ahead and feed in extra cycles for the leap second. It's not something I'd heard of, but it wouldn't surprise me.

metafunctor 10y ago
I remember reading that the frequency at any given time is usually somewhere between 49.9Hz and 50.1Hz (and similarly around 60Hz I'd imagine). So for a 50Hz clock, it's not very accurate at all. However, I just found that the _average_ frequency over a length of time should be exactly 50Hz, so it is, in fact, good for keeping time. More here: https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html
kalleboo 10y ago
I remember hearing that Analog TV stations were also used for timekeeping since they had a very stable clock for the blanking interval
- Animats 10y ago
  US analog TV (all three classic networks) used to be synchronized to an atomic clock in NYC. This allowed switching from one source to another without frame tearing. Separate from that, there was a scheme for sending time of day in the vertical retrace interval, to set clocks and VCRs. Few devices received that, although I once encountered a hotel TV with a built-in analog clock that did. (The clock was wrong; although it could set itself automatically, it had to be told manually which channel provided a time signal. That's probably why this feature never caught on. I was bored enough to figure out how to set the thing.) US telephony used to be synchronized to an atomic clock in New Jersey. The TDMA multiplexing was kept in sync so that frames could be transferred synchronously without buffering. If the clock connection was lost, T1 to T3 lines would get slightly out of sync, and occasionally drop or add an 8-bit frame. This is unnoticeable in audio, but an error for modems. But neither of these was a time count. It was just a square wave.