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

unruh unruh at wormhole.physics.ubc.ca
Sun May 16 11:29:05 UTC 2010


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)
And the easiest way is to hold absolute phase against an external time
standard.
>
> uwe




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