[time-nuts] Line Frequency standard change - Possible ?

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Feb 9 16:31:51 EST 2017

In message <4FBDD81DDF04FC46870DB1B9A747269202916B42 at 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, 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".

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.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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