> On Feb 9, 2017, at 4:31 PM, Poul-Henning Kamp <[email protected]> wrote: > > -------- > In message > <4fbdd81ddf04fc46870db1b9a747269202916...@mbx032-e1-va-8.exch032.ser > verpod.net>, "Thomas D. Erb" writes: > >> I was wondering if anyone was familiar with this proposal, is this >> a uncoupling of line frequency from a time standard ? > > The interesting thing about this is that all research and experiments > (for instance on the danish island Bornholm) indicates that the only > way we stand any chance of keeping future AC grids under control in the > medium term is to lock the frequency *hard* to UTC. > > Its a very interesting topic. > > In the traditional AC grid power is produced by big heavy lumps of > rotating iron. This couples the grid frequency tightly to the > power-balance of the grid: If the load increases, the generators > magnetic field drags harder slowing the rotor, lowering the frequency > and vice versa. > > This makes the grid frequency a "proxy signal" for the power balance, > and very usefully so, because it travels well and noiselessly through > the entire AC grid. > > The only other possible "balance signal" is the voltage, and it > suffers from a host of noise mechanisms, from bad contacts and > lightning strikes to temperature, but worst of all, it takes double > hit when you start big induction motors, thus oversignalling the > power deficit. > > Where the frequency as "proxy" for grid balance reacts and can > be used to steering on a 100msec timescale, you need to average > a voltage "proxy" signal for upwards of 20 seconds to get the > noise down to level where you don't introduce instability. > > The big picture problem is that we are rapidly retiring the rotating > iron, replacing it with switch-mode converters which do not "couple" > the frequency to power balance. > > For instance HVDC/AC converters, solar panel farms, and increasingly > wind generators, do not try to drag down the frequency when they > cannot produce more or drag the frequency up when they can produce > more power, they just faithfully track whatever frequency all the > rotating lumps of iron have agreed on. > > As more and more rotating iron gets retired, the grid frequency > eventually becomes useless as a "proxy-signal" for grid balance. > > Informal and usually undocumented experiments have already shown > that areas of grids which previously were able to run in "island" > mode, are no longer able to do so, due to shortage of rotating iron. > > One way we have found to make the voltage a usable fast-reacting > proxy for grid power-balance, is to lock the frequency to GNSS at > 1e-5 s level at all major producers, which is trivial for all the > switch-mode kit, and incredibly hard and energy-inefficient for the > rotating iron producers. > > The other way is to cut the big grids into smaller grids with HVDC > connections to decouple the frequencies, which allows us to relax > the frequency tolerance for each of these subgrids substantially. > > This solution gets even better if you load the HVDC up with capacitance > to act as a short time buffers, but the consequences in terms of > short circuit energy are ... spectacular? > > (It is already bad enough with cable capacitance in long HVDC > connections, do the math on 15nF/Km and 100.000 kV yourself.) > > All these issues are compounded by the fact that the "50/60Hz or > bust" mentality has been tatooed on the nose of five generations > of HV engineers,
Umm… you left out the 25Hz power system that runs across the road just a few miles from here :) Some of these ideas take a *long* time to die. It’s been happily doing it’s thing for over a century. > to such an extent that many of them are totally > incapable of even imagining anything else, and they all just "know" > that DC is "impossible”. Except we’ve had HVDC distribution running around for many decades and it seems to work quite well. Indeed that’s your point here. > > In the long term, HVDC is going to take over, because it beats HVAC > big time on long connections, and it is only a matter of getting > semiconductors into shape before that happens. That however, > is by no means a trivial task: It's all about silicon purity. … or silicon carbide purity or something even more weird and (so far) poorly understood. Bob > > > -- > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > [email protected] | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by incompetence. > _______________________________________________ > 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. _______________________________________________ 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.
