Actually, these days, in the widely-connected continental area of US & Canada (with exceptions like Texas), the frequency will be much closer than +/- 0.5 Hz. Power systems will start "islanding" and shedding load if frequency goes outside say +/- 0.1 Hz. It's very important for the power system frequency to stay stable - energy flows and therefore revenue flows depend on it. The power utilities are absolutely anal about tracking frequency and maintaining it precisely.
It turns out that an AC power line is almost entirely inductive in nature. This means that the power flow has little to do with the voltage, more or less all about the phase angle difference between the ends. Actually, a voltage difference generally means a reactive power flow, which is counter-productive, so voltage is maintained very constant as well. At the transmission line between cities, you see the bird's eye view aggregate of power consumption - and utilities will put systemic voltage support at various points to ensure that voltage is well matched. In order to keep the system stable, power utilities must maintain strict frequency control. In the case of Whitehorse YT, they are not connected to the continental grid and therefore are not subject to the same controls. The power is probably generated by diesel, coal, or perhaps hydro power. It is much more difficult to maintain frequency control under these conditions - a single big load (like a mill) "dumping" because of some disturbance, can change your load base substantially, throwing the frequency way out of whack. You can't instantaneously change the speed of the motor or turbine shaft, so you have to slowly bring the system back in line. Here in Manitoba where I live, we have hydro-electric generation way up at the north end of the province supplying something like 90% of our power, with most of the load in the south, separated by, hmm, something like 800 km. Plus, we actually export *a lot* of power into the north-central states. In the 1960s, Manitoba Hydro built an AC-to-DC converter up north and a DC-to-AC converter down south, and ran the 800 km with a 500 kV DC line. There are lots of benefits to this, but one of the biggest ones is that the phase & frequency of the output of the DC-to-AC converter can be controlled on a cycle-by-cycle basis - it's done with thyristors (huge SCRs, used to be mercury-arc but now almost all solid state), so there are no rotating shafts. This gives our power system excellent stability. In fact, Manitoba Hydro can modulate the output of the DC-to-AC converter to help stabilize the system from other, external disturbances. The possibilities are limitless. Of course, the downside is that the loss of that DC line to the north leaves us up a creek, and that happened about a decade ago when a cyclone went through and took it down. What an exciting time that was! The frequency of the AC system up north from the generators to the AC-to-DC converter station is not well controlled. If we dump a big load in the south, the DC system changes almost instantly to adjust, but then there's nowhere for the energy to go, so the northern AC system goes way up in frequency. Conversely, if we add a big load in the south, it can go down, but then that's generally planned, and more controlled. They say you don't run your clock on the frequency up north. In fact, nobody does, because it's high voltage transmission, and there isn't enough population to justify putting a distribution substation in there - some folks complain that they can SEE the transmission line, but they are forced to run on diesel generated power. It isn't a conspiracy, it's pure practicality and economics - you can't just drop a wire and plug in your stove! OK, maybe I digress, sorry. What I do find interesting is that the Yukon folks don't have multiple cross-correlated timing systems checking each other. If the frequency were to go very low (say 50 Hz) it will cause significant dissipation in power transformers and the like, perhaps even damaging them. It turns out that flux in the core of said transformers is related to V/f, so if the frequency drops over 10%, you get a substantial increase in flux, and if you are close to the saturation of the core, well, let's say that makes for not-so-happy customers. Heavens, designing such systems is what I (used to) do. I could close my eyes and see the design - it wouldn't be that difficult! Maybe I should give them a call. So when I see derogatory comments directed at the Yukon power folks, be careful - they might be guilty of not having a backup time tracking system, perhaps even of not tracking it automatically. But, it's much more difficult when you are not connected to the continental grid! Regards, Dean Weiten. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
