Governors and Phase-Synchronization Are Important
August 22nd, 2008
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Just because this came up recently in a few comment threads which were exchanged recently, I thought I would share some interesting images I came across not long ago. These relate to the importance of governing and controlling the rotation and power of generators on an electric grid.
Modern electric generators are generally of the three-phase alternating current variety. Alternating current systems are much different than simpler direct current systems in that they cannot simply be connected any witch way with no concern for any kind of synchronization. In an alternating current system the phase of all the generators and the transmission lines must be properly synchronized and must remain constant within a small margin. If the phase of a power system drops or increases it can damage equipment or cause the generating stations to loose synchronization.
There are ways of altering the phasing of grid sources, using inductive and capacitive elements to act as delays, but it remains critical that the generators are all spinning at the proper frequency. In the US, Canada and much of Asia this is 60 Hz and in much of the rest of the world it is 50 Hz. One of the interesting things about induction generators is that they operate in much the same way as motors. When a generator is connected to the grid it can be spun up in sync with the electricity of the grid. In order to generate power the generator has torque applied to it and this in turn creates more inductance and thus power in the system. One thing that is key, however, is that the torque and spin rate of the generator must be properly regulated. If the rotors of the generator are pushed too quickly they will start to spin faster than the phase of the grid. If they are not pushed hard enough they can slow down.
The resistance a generator encounters actually varies depending on the load on the grid. If the load is high the generator will tend to slow down and more torque is required to keep it spinning at the correct rate. If not then the generator will slow down no longer be generating electricity effectively. If the load drops and the system does not compensate for this it can cause the generator to over spin and this is generally even worse. The phase miss-match can lead to greater stress on the system than it is designed for. Even worse, while the phase of the grid is fighting to keep the generator at a constant rotation rate, the turbine is trying to speed it up. This would be like trying to fight an electric motor by forcing it to turn at a rate it is not adjusted to. The mechanical stresses could be enormous.
Okay… enough with the boring talk and now to the pictures
I don’t know the entire story behind what happened, but apparently in 1949 there was a major failure of the control and governing system at the number two powerhouse of the Tennessee Valley Authority’s Ocoee dam in Tennessee. The generator experienced an “overspend” event and the proper safety systems did not kick in to reduce the water flow or stop the system. The system passed its design criteria and the electrical and mechanical systems were no longer in sync.
And…
Well, the whole rotor, armature and turbine shaft assembly broke loose and launched itself through the wall of the powerhouse…
(Click on the image to enlarge)
So to make a long post short, keeping the grid and all the generators properly synchronized, phased and governed is kinda important.
This entry was posted on Friday, August 22nd, 2008 at 11:40 pm and is filed under History, Misc. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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August 23rd, 2008 at 12:27 am
A generator’s reactive power capabilities (known as ‘leading’ and ‘lagging’ capabilities) are technical characteristics of the the generator. Operators are paid for reactive power capability and usage to help manage the issues mentioned in the lead article.
To obtain the reactive power utilization payment, a generator must produce or consume reactive power. The production and consumption of reactive power is instructed by the grid controller on an individual generator basis in accordance requirements.
Note that it is not just the production of reactive power that is required of a generator. The ability of a generator to consume reactive power when instructed to do so assists in maintaining system voltages within upper limits during low demand periods.
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August 23rd, 2008 at 12:57 am
DV82XL said:
You’re right, of course. I guess I might not have said everything in a completely clear way because I was trying to say a lot about the systems in as short and quick a statement as reasonably possible and at the same time be somewhat simple.
Anyways, what you see is the result of what happens when a generator’s governors and the gate controls fail to throttle properly and keep the generator running right. I think what happened here was basically that the load may have not been high or something and the system was running at a high throttle forcing it way way past the rotational speed it should have been at and in opposition to the fields.
Like I said, I’m not 100% sure on the circumstances, but clearly the governing and control mechanism failed and this is what happened.
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August 23rd, 2008 at 1:06 am
What ever the reason, those images underline just how much energy that is being trafficked in these plants. God, it must have been something to have been there (and not get hurt).
By the way this sort of reactive moderation service cannot be supplied by wind generators, but they do consume a lot because of the variability issue.
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August 23rd, 2008 at 1:31 am
I don’t know the circumstances either but this kind of thing can definitely happen because of the huge forces at work. If you had a generator running and then you suddenly threw it onto an unphased power source there could be an enormous mechanical shock or if the turbine was pushing hard and fighting to push the rotors faster than the grid frequency with enough power then you could snap the rod right off.
One thing I have heard of is sudden load shifts or sudden cutoffs physically destroying machinery and generators but this is still an impressive image.
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August 23rd, 2008 at 1:48 am
Take a look at the website of Dynamic Demand This is a U.K. firm that is pushing for controllers to be installed on inductive loads that will monitor line frequency and by starting and stopping the load in response to changes and supply second-to-second balancing of supply and demand to the grid. It can also help black-start recovery; restarting after serious power outages.
The idea was actually patented by an American inventor, but was picked up by these guys when the patent lapsed.
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August 23rd, 2008 at 2:15 am
Interesting photos, but in general, just as a rule of thumb, this is the kind of thing we make an effort to avoid.
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August 23rd, 2008 at 3:47 pm
DV82XL said:
That is an interesting idea but my one concern with it is the whole idea of shifting responsibility for a stable and working power grid from the generators to the end consumers, especially the small ones kinda smacks of the whole distributed generation BS.
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August 23rd, 2008 at 4:21 pm
drbuzz0 said:
Yes there is that, but dynamic demand load control like this is just a very elegant way to deal with black starts, which are always an issue if the power is off in a district for any length of time and every compressor on a thermostat wants to load in at the same time. That it helps the grid maintain frequency discipline is just a bonus.
Looks like the whole thing can be implemented in a single chip with an active switch; shouldn’t break the bank Think of it like cat.converters on cars – it helps everyone without being a great benefit to the individual vehicle.
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August 24th, 2008 at 10:40 am
Engineering Edgar said:
A bit of an understatement?
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August 26th, 2008 at 1:34 am
DV82XL said:
My first thought would be “Oh crap.. I hope I don’t loose my job”
and my second would be “How do I make this seem like it was NOT my fault”
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