[time-nuts] Line Frequeny Stablity
magnus at rubidium.dyndns.org
Wed Apr 5 03:52:32 EDT 2017
Ok, quick intro to the frequency steering.
There still remains rules that say that network frequency should be on
average 60 Hz on the US grid. (Yes, there is proposals to remove it, but
it is still effective.)
Since the generation (let's talk traditional here not to make things
more complex than they need to be for the first overview) is from
generators, essentially big rotating lumps of iron, the balance between
load and generation causes the frequency change. If you have more load
than generation, the frequency will lower while if you have more
generation than load the frequency will go up. Essentially, if you
undergenerate, you would need the rotating energy of the lumps of load
to deliver, but that reduces their speed and if you underconsume the
energy goes into the rotation of the lumps.
Now, by monitoring the frequency you can steer the balance, askning
hydropower to increase or decrease production to balance the shift of
load. The operators have a fair clue on how the day will proceed as
people wake up, industry starts, workday, industry closes down, people
get home etc, so there is a basic pattern there to give a clue, but they
monitor it and balance it.
By also balance the phase, you can know how much you lag behind and
needs to run up by running the frequency high. This require spending
energy by increasing production compared to the load. Now, by being
smart you do that when you have low load, so that you don't have to
spend as much energy to achieve it, but never the less.
Then you have to manage your reactive energy, the VAr, which is a
Breakers have several form of catastrophic protections in them, among
those if the frequency goes bad. Turns out that the frequency monitoring
of breakers gives so diverse readings such that for post mortem
analysis, they provide bogus values. They learned this the hard way
after the North-Eastern Blackout. When they threw out all the
traditional frequency readings, the PMU data that remained painted a
The detailed monitoring of PMU gives much more data, also illustrates
forced oscillation, inter-area-oscillations etc. which makes the phase
wobble in interesting ways, and when analyzed gives good clues about
problems in the network.
An even more "fun" scenario is when the network runs into islanding,
since the link between areas is to weak to keep the frequency at the
same rate, i.e. the link is to weak to support the load, so one part has
overload and goes down in frequency while the other have overproduction
and gos high in frequency, which you can see by the way that phase
starts to deviate between the networks, and that before you have the de
The islanding illustrates the need of the links to be strong enough so
that generators synchronize, or should we say syntonize to be correct
with terms, that is, they have the same rate.
The four islands that you identified do their own independent frequency
steering, but they exchange power. The generation-load thing still
happens, but phase/frequency decoupled. HVDC cables achieve the same thing.
Anyway, phase monitoring has become a very good tool for so many of
these measurements, and that requires a common "reference" phase and
that is GPS. That helps to monitor the phase and frequency of the grid
so that it can be controlled.
A peculiarity of the field is the ROCOF - Rate Of Change OF Frequency.
This is what we call linear frequency drift. Looking on those numbers
give you a good hint where you are going.
Until recently, photoelectric would not provide any of the rotating iron
properties, but the increase popularity of it now requires it to start
to have such properties for the stability of the system.
On 04/04/2017 11:28 PM, Thomas D. Erb wrote:
> Thanks for the info.
> So that tells me how data is recorded - but not how the frequency is kept stable ?
> Is the line frequency now directly tied to GPS clock - with no drift ?
> Thomas D. Erb
> tde at electrictime.com<mailto:tde at electrictime.com> /
> Electric Time Company, Inc.
> Office: 508-359-4396 x 117 / Fax: 508-359-4482
> 97 West Street Medfield, MA 02052 USA
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts