[ntp:questions] Converting from Y-m-d h:m:s

unruh unruh at wormhole.physics.ubc.ca
Mon May 17 00:56:07 UTC 2010


On 2010-05-16, Uwe Klein <uwe_klein_habertwedt at t-online.de> wrote:
> unruh wrote:
>> On 2010-05-16, Uwe Klein <uwe_klein_habertwedt at t-online.de> wrote:
>> 
>>>Terje Mathisen wrote:
>>>
>>>>Marc Leclerc wrote:
>>>>
>>>>
>>>>>Hi,
>>>>>
>>>>>I am looking to stay on true time, the real issue here in those
>>>>>installation is not realy having the perfect time but that all the
>>>>>device in the field have the same time. There are new ways to acheive
>>>>>this such as IEEE 1588 but since no one is ready for this using GPS
>>>>>device is our closest friend. But it's funny you asked, soon we will
>>>>>tackel simple synchronous switching of breaker and then we'll have to
>>>>>keep up with the AC cycles
>>>>
>>>>
>>>>This is actually somewhat easier:
>>>>
>>>>You only need 8 or 10 ms accuracy to do this, in order to determine the 
>>>>start of each 50 or 60 Hz cycle, then you phaselock your switch to the 
>>>>AC cycles.
>>>
>>>There is a (massive?) hitch.
>>>The grid 24h average is held to the nominal frequency.
>>>
>>>Shortterm frequency is load dependent.
>>>
>>>so you need to sync all boxes to the _same_ zerocrossing in a
>>>"variable frequency" domain with the help of a "true time" source.
>>>not easy.
>> 
>> 
>> Well, it cannot be very dependent, since loads are all shared these
>> days. That means that all providers have to have exactly the same phase
>> or your get cross feeds(generator A dumps its current into generator B
>> without having gone through any houses along the route. See the massive
>> east coast power blackout where one generator system lost phase
>> cascading to automatic shutoffs of the whole east coast)
>
> In a (widearea or not) homogenous AC grid the frequency goes down if
> net energy is taken out and goes up when energy is pushed in.
> ( energy stored in the grid is proportional to f and U )
>  From a powerstation viewpoint you directly control input "torque" (energy in/out, phase)
> and only in result the frequency.
>
> > And the easiest way is to hold absolute phase against an external time
> > standard.
>
> You don't, you target to match average frequency to the reference frequency.
> ( Thats why AC referenced clocks have good longtime but bad shorttime timekeeping.)

As I said, that is not good enough. If you have a generator in Niagra
Falls and another in James Bay both feeding New York, if the phases
differ even by a small amount that is a huge amount of power sloshing
back and forth between Niagra and James bay doing nothing but heating
wires and burning out generators. You want to make sure that those two
have exactly the same phase (to much better than a degree) at all times.
Long term averages are not good enough. You can burn out systems ( but
more likely trip breakers and send nuclear reactors into emergency
shutdown mode) if your phases are mismatched. And that can take days to
bring back up again.
 If your generator is the only one on the circuit, you can be sloppy,
but now adays, noone's is isolated. They are all part of huge grids.

>
> Controlling a widearea AC grid is quite a bit more involved
> than meets the eye on first blush.

It sure is. And phase control is part of that. 

>
> uwe
>




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