mechanical or electronic? someone said that electronic clocks can be fooled by glitches, something which could be cured with a decent filter....
The guys that run the national grid will run it fast when the load lightens to compensate.
~20 years ago I was in the cooontrol room at Benmore Hydro station (NZ's biggest at that time, I think still the biggest one on the grid) they had two clocks one was synchronised to an atomic standard (WWVH?) the other ran from the mains. I think at the time I saw it the discrepancy was two seconds
** The frequency never really changes, the phase merely drifts up and down around the centre value.
That is - on a high res frequency meter, the effective AC supply frequency varies from say 50.1 to 49.9 over a couple of minutes then drifts back again.
** All the alternators right across the Eastern half of Australia are phase locked when in use.
Most power up my end of the state has to be fed into a local Hydro station to be "Cleaned up" I don't how this is done but I suspect they might run one alternator as a motor and pump water back to the top of the dam to run back through the other turbine and alternator.
A guy I know who has a power station ticket tells me that in the 70's and 80's their reputation depended on keeping the frequency accurate through the day in sync with the reference chronometer. These days the alternators are run at their limits to try and keep up with modern electrical household loads so they run slower.
No, the frequency of the mains does change, as the generators speed up and slow down in response to the changing load. If a big load is suddenly applied the frequency will drop until the control system on the turbine compensates, and increases the power to turbine to increase the frequency.
In a AC power system the only energy storage you have is the spinning mass of the turbine/generator. If the load is too high the generator will slow down, and if the load is too light, the generator will speed up until the control circuit compensates. The problem with large base load power stations is that for thermal reasons they can not change there power instantly. They can only ramp up / down at a specified rate, and can take several hours to ramp from half to full load.
On the other hand gas turbine and hydro don't have the thermal constraints of a large coal fired station, and can supply the peak loads more quickly.
It can fall well below 49.9 for several minutes if there is a problem with an alternator. In Tasmania where the grid is much smaller, the frequency deviations are even greater, and it not uncommon to fall to
49.5 or less for several minutes, but they still maintain an average of
50.000 Hz
The standards require accumulated time deviation be within +/- 5 seconds on the east coast, and +/- 15 seconds on Tasmania.
The phase angle of each generator varies depending on its contribution and distance to the load. It is important for power system stability that the phase angle from one end of the power grid to the other does not exceed 180 degrees.
You obviously have no experience in power systems. The frequency of the mains can and does vary.
Five events in August 2005 alone
22:00 hrs 14 August Loy Yang A unit 2 tripped from 530 MW Min Freq = 49.810 Hz, below 49.85 for 338 seconds
22:00 hrs 17 August Loy Yang A unit 4 tripped from 310 MW Min Freq = 49.835 Hz, below 49.85 for 20 seconds
20:29 hrs 20 August Eraring unit 4 tripped from 660 MW Min Freq 49.740 Hz, below 49.85 Hz for 458 seconds
06:08 hrs on 24 August No contingency or load event Min Freq 49.844 Hz, below 49.85 Hz for 18 seconds
10:26 hrs on 30 August Loy Yang A unit 2 tripped from 458 MW Min Freq 49.828 Hz, below 49.85 Hz for 36 seconds
No, it not a "phase lock", as the phase between the alternators is not locked, and does not have a fixed relationship. The phase between the alternators does vary depending on the load.
No, the big generators at the power stations, and the big power lines than connect them. Still, I remember studying them in third and fourth year at university, something I believe that you missed out on.
Please explain what YOU mean by well below? I know you snipped them because you can't stand being wrong, but I would 49.74 Hz well below
49.9Hz in the context of power system frequency control, and it stayed below 49.85 for 458 seconds.
No, Phase Locked = no change in the PHASE ANGLE between the units. As there is a change in phase between alternators depending on the load, they are not phase locked. Again, something you might have learned about if you finished studying.
I had one of these when I was a kid. The minutes "cards" looked like a miniature Rolodex. ;-) A really odd thing about it was that the hours had a double set of cards (i.e. 1 1 2 2 3 3 4 4). At about 20 minutes after the hour when the minute would change, the hour "card" would also flip at the same time exposing the duplicate "card". I never understood why they did this. Since the hour card had to flip at precisely the time that the "59" minute card flipped, it wasn't like there was some unsolvable timing problem that required extra cards. I guess they did it to just make the hour column Rolodex fuller for easier flipping. It usually flipped around 20 minutes after the hour, but not always the exact same minute.
Maybe thats it .. it was to compensate for inaccurancy in the lining up of the 2 sets of numbers .. I remember these clocks tho .. great fascinating things .. had one when i was a kid .. till it stopped running .. motor must have died or damaged its gears perhaps .. remember pulling it apart and having these half-number cards scattered EVERYWHERE .. loved it :)
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