[time-nuts] Line Frequeny Stablity

Magnus Danielson 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 
different matter.

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 
consistent picture.

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 
facto islanding.

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.
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