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

Uwe Klein uwe_klein_habertwedt at t-online.de
Sun May 16 11:58:48 UTC 2010


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.)

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

uwe




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